SMTO Electronic Newsletter - Volume 1, Number 1 (July 1997)

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TABLE OF CONTENTS

• 1.0 INTRODUCTION
• 2.0 SMTO STAFF
• 3.0 CURRENT STATUS OF FACILITY
• 3.1 BOLOMETER RECEIVER SYSTEMS
• 3.1.1 BOL-1300
• 3.1.2 BOL-870
• 3.1.3 BOL-350
• 3.1.4 FOUR-COLOR
• 3.2 HETERODYNE RECEIVER SYSTEMS
• 3.2.1 SIS-230
• 3.2.2 SIS-345
• 3.2.3 SIS-490
• 3.2.4 SIS-660
• 3.2.5 NASMYTH OPTICS UPGRADE PROJECT
• 3.2.6 CLOSED-CYCLE RECEIVER PROJECT
• 3.3 SPECTROMETER SYSTEMS
• 3.3.1 EFFELSBERG AOS
• 3.3.2 NASA 4 x 1 GHz AOS
• 3.3.3 AOS-3/4 (1 GHz)
• 3.3.4 AOS-5 (250 MHZ)
• 3.3.5 FILTER BANK SPECTROMETER
• 3.3.6 SPECTROMETER STABILITY
• 3.4 PRINCIPAL INVESTIGATOR INSTRUMENTS
• 3.4.1 MARTIN 460 GHz
• 3.4.2 MPIfR 810 GHz
• 3.4.3 CYCLOPS
• 3.4.4 CHIRP SPECTROMETER
• 3.5 MULTI-FEED INSTRUMENTS IN DEVELOPMENT
• 3.5.1 ARRAY-345
• 3.5.2 ARRAY-480 + MACS
• 3.6 TELESCOPE CHARACTERIZATION MEASUREMENTS
• 3.6.1 POINTING MEASUREMENTS
• 3.6.2 INCLINOMETER MEASUREMENTS
• 3.6.3 REFRACTION CORRECTION
• 3.6.4 PHASELESS-HOLOGRAPHY
• 3.6.5 SATELLITE HOLOGRAPHY
• 3.7 BUILDING AND ENCLOSURE
• 3.7.1 ACCOMMODATIONS
• 4.0 TELESCOPE REPAIR AND MAINTENANCE
• 4.1.1 BACK-UP STRUCTURE DAMAGE
• 4.1.2 CAMAC SYSTEM
• 4.1.3 CAMAC POWER SUPPLIES
• 4.1.4 UNINTERRUPTABLE POWER SUPPLY
• 5.0 COMPLETED SMTO PROJECTS
• 5.1.1 CONTROL SOFTWARE
• 5.1.2 DATA ANALYSIS PROCEDURES
• 5.1.3 SAMBUS-VME INTERFACE
• 5.1.4 CHOPPING SECONDARY FREQUENCY GENERATOR
• 5.1.5 250 MHZ AOS STEERING LO UNIT
• 6.0 SUMMER SHUTDOWN IMPROVEMENTS
• 6.1.1 ENCLOSURE WHEEL BOXES
• 6.1.2 WEATHER STATION
• 6.1.3 TELESCOPE/BUILDING CONTROL SYSTEM
• 6.1.4 RECEIVERS
• 6.1.5 SPECTROMETERS
• 6.1.6 AUTOMATIC CALIBRATION
• 6.1.7 ELECTRONICS LAB
• 7.0 FUTURE SMTO PROJECTS
• 7.1.1 OPTICAL POINTING
• 7.1.2 DOOR AND ROOF SEQUENCER
• 7.1.3 RECEIVER LO/IF SWITCH BOX
• 7.1.4 HARMONIC CHECK FEATURE
• 7.1.5 UNIVERSAL LOCAL OSCILLATOR
• 7.1.6 PROJECTS ON-HOLD
• 7.1.7 CAMAC REPLACEMENT
• 8.0 MISCELLANEOUS
• 8.1.1 ON-GOING SAFETY REVIEW
• 8.1.2 ACCEPTANCE TEST PROTOCOL
• 8.1.3 DOCUMENTATION
• 9.0 WEATHER STATISTICS
• 10.0 VISITORS AND OBSERVERS:
• 11.0 CONCLUSION

1.0 INTRODUCTION

This is the first of what we hope will become a biannual Electronic Newsletter which describes the current activities at the Submillimeter Telescope Observatory (SMTO). This issue covers the first six month period of 1997.

The SMTO is a collaboration between the Max-Planck-Institut fur Radioastronomie (Bonn, Germany) and the University of Arizona's Steward Observatory (Tucson, Arizona). The observatory, consisting of the 10-meter "Heinrich Hertz Telescope" (HHT) and the "Eugene Frazier Facility" co-rotating enclosure, is located on Mount Graham in southeast Arizona at an altitude of 3200 meters. First light at the wavelength of 1300 microns occurred in February 1994 and at 350 microns in December of the same year. Regular visitor observing began in January 1996. The surface accuracy, as determined by 38 GHz satellite holography, is currently set at about 20 microns.

The majority of the HHT's facility sub-mm instruments have been contributed by the partner institutes. The SIS receivers have been provided by the Steward Observatory Radio Astronomy Lab (SORAL), headed by C. Walker, and by the MPIfR Submillimeter Lab, headed by R. Guesten. The latter group has also supplied the facility acousto- optic spectrometer backends. The bolometers were developed at MPIfR-Bonn by E. Kreysa. Funding for these projects is independent from the SMTO's budget and is provided either directly by the partner institutes or through external funding agencies.

The SMTO passed a critical milestone in early 1997 with the beginning of routine operation with a full suite of bolometric and heterodyne instruments available for use by the astronomical community. Excellent millimeterwave weather occurred during the winter months of February and March with numerous periods where sub-mm observations were possible. The telescope pointing proved to be adequate for most of this time. Although continued long-term improvement is required in several areas, (such as pointing, spectroscopic baseline stability, receiver/telescope calibration, user-friendliness, etc) good scientific results of publishable quality are being achieved.

As of early June, the telescope entered its yearly shut-down period. During the initial months the telescope will be used to carry out further system tests before being taken out of operation for mechanical modifications. It should be back in operation by mid September with the official 1997/98 observing season beginning towards the end of October.

2.0 SMTO STAFF

The interim Director of the SMTO is Buddy Powell, who also serves as the Associate Director of the Steward Observatory. A Steering Committee consisting of R. Hayward, R. Mauersberger, B. Peters and R. Warner has been assisting in the day-to-day management of the facility. On September 1st, Dr. Tom Wilson from MPIfR-Bonn will take up duties as the new SMTO Director. The leadership provided by a full-time Observatory Director will be crucial in setting the facility's long-range scientific goals as the SMTO enters the operational phase of its life.

The operation of the facility is handled by three support units. The "Operations" unit is managed from Tucson by R. Warner and consists of three site managers (D. Ashby, D. Officer, and B. Stupak) and one machinist (J. Casas), the duties of which are primarily based at the site. Astronomical support for visiting observers is provided by the "Scientific" unit. This group, often referred to as the "Friends of the Telescope", is headed by B. Peters and includes H. Butner and D. Muders. Two additional "FofT" positions will be filled in September by P. Gensheimer (formerly MPIfR-Bonn) and S. Platt (formerly University of Chicago). Much of the software development for the facility is also provided through the efforts of the scientific staff. The third support group, the "Technical" unit, is managed by the B. Hayward and includes two technicians (R. Esterline and G. Holmberg). An open position for a Cryogenic-Microwave engineer will be filled by F. Patt (MPIfR-Bonn) this fall.
The SMTO staffing levels during the last six months have been stable with the exception of the recent departure of one of the FofT's (J. McMullin, who left to join the NRAO GBT project). By this fall, the observatory will be fully staffed for the first time in a long while. Each of the SMTO support units, the Operations, Scientific and Technical Services, have been chronically under-staffed. The facility is now well on the way to recovering from the departures of many long-standing personnel over the last couple of years and the subsequent loss of corporate knowledge which resulted. This, coupled with the fact that much of the staff is still new to the project, has had a serious impact on the overall Program Plan for implementing improvements at the telescope as well as the level of support which the staff has been able to provide for visiting astronomers. However, enthusiasm and morale are high, and as solid experience is gained and staffing levels reach their full complement, the prospects for achieving and maintaining the necessary expertise to make the HHT a world-class facility are truly excellent.

3.0 CURRENT STATUS OF FACILITY

Much has been accomplished during the 1996/97 observing season in spite of the chronic man-power shortage. Numerous instrument packages from the partner institutes were delivered and commissioned. There have been many small but vital improvements in the on-going effort to increase performance, reliability and usability of the system as a whole. Nonetheless, some significant equipment problems remain to be solved before the HHT can come into its own as a front-line research facility. The following sections summarize the current status of the SMTO, especially the HHT's suite of scientific instruments. Also included is a description of the modifications, upgrades and repairs made to the telescope since the start of 1997. Major projects which are currently underway are summarized, as are significant new initiatives planned for the future.

3.1 BOLOMETER RECEIVER SYSTEMS

3.1.1 BOL-1300 : The MPIfR 1300 micron bolometer proved to be the work-horse receiver during the first half of the 1996/97 season and while used extensively for pointing measurements, it was also utilized for several astronomical projects. This prototype/test receiver demonstrated excellent reliability and required very little maintenance. With the arrival of the Four-Color Bolometer in February, the 1300 micron system has since been decommissioned but remains available on-site as a back-up instrument.

3.1.2 BOL-870 : The MPIfR 870 micron bolometer, when time and weather permitted, was primarily used early in the observing season for pointing measurements although it did successfully perform some flux density measurements of nearby galaxies (Lesch at al) and young stellar objects (Guertler et al). Except for a problem in November with its He-3 valve, which was quickly repaired by E. Kreysa (MPIfR-Bonn) during an emergency visit to the SMTO Tucson Labs, this receiver demonstrated good reliability and performance. It too has been decommissioned following the arrival of the Four-Color but remains at the telescope in a serviceable condition for back-up use.

3.1.3 BOL-350 : As an interim measure to provide high-frequency coverage at the HHT until the Four-Color system could be made available, the MPIfR delivered a 350 micron bolometer last December. Unfortunately, thermal problems inside the dewar were encountered following the shipment of the instrument which resulted in a serious degradation in the system's sensitivity. Although the MPIfR commissioning team felt that the problem was on the verge of being solved, it was decided that the short time remaining in an already tight schedule would be better spent by improving the telescope pointing model with additional measurements at 870 microns, thus allowing some real astronomical observing to be carried out. Coverage of the 350 micron window was thus postponed for a few months until the Four-Color was commissioned.

3.1.4 FOUR-COLOR BOLOMETER : The MPIfR Four-Color Bolometer was shipped back to Bonn after the end of the 1995/96 observing season for modification of its optics and the addition of improved anti-reflection coatings on its 1300, 870, 450 and 350 micron channel filters. It was returned to the HHT in February of this year for recommissioning. Due to the conflicting schedules for members of the MPIfR team, there was only enough time for the 1300 and 350 micron channels to be tested on the sky. Subsequent measurements by the SMTO staff have since been made at 870 and 450 microns, thus allowing the pointing offsets for all 4 channels to be determined. As the opacity was very good during this period (Tau(225) < 0.07), a 350 micron beam map on Mars was also achieved. The beam appears to be close to the diffraction limit (ie: 8") and looks symmetric with very low sidelobe response. It was successfully used in coordinated continuum measurements of Comet Hale-Bopp as well as determining flux densities of several YSO's (Guertler, Jena). Since the Four-Color was shipped to the US under an international carnet, it must be returned to Germany this summer and, after some minor modifications (which may include the addition of an improved feedhorn for the 450 micron band), it will be re-imported in time for the beginning of the 1997/98 observing season.

3.2 HETERODYNE RECEIVER SYSTEMS

3.2.1 SIS-230 : The SORAL 230 GHz single-channel receiver was resurrected in mid September 1996 after spending the summer at the telescope in an uncooled state. It developed a "cold" leak shortly after and the dewar was sent out to Infrared Labs in Tucson for repair. During that time, upgrades were made to the mixer bias circuit, HEMT power supply and LO dump design. The receiver underwent its formal Acceptance Test at SORAL on November 17th and was subsequently installed at the HHT. Double-sideband receiver temperatures between 60-70 K were achieved across the 210-275 GHz tuning range. The recent modifications resulted in a noticeable improvement in its total power stability. The main beam efficiency on Jupiter was found to be about 76%. The receiver was granted provisional acceptance as a facility instrument pending the delivery of full documentation and further telescope tests. As the telescope was without any spectrometers at the time, no spectral line measurements could be performed until February 1997. It has since been used for many astronomical observations, most notably for spectral line measurements on Comet Hale-Bopp. In general, the receiver is simple to use and easy to tune. It has performed reliably, with the exception of the dewar leak in September and a failed cold-head in February. Its most serious problem is a tendency for its IF passband to become unstable when the mixer is tuned for optimum sensitivity. This is caused by resonances in the match between the tuned SIS junction and the HEMT amplifier which even the 10 dB of isolation from its cooled ferrite isolator cannot totally eliminate. Unless care is taken, the HEMT amplifier can become marginally unstable and may even run into oscillation. Under these circumstances, unless the mixer is "detuned" sufficiently, the resulting spectrometer baselines will likely be of poor quality and the noise will not integrate down as the inverse square of time. This is particularly true for Position Switched observations. This process typically degrades the receiver's sensitivity to about 100K. SORAL plans to replace the existing junction this summer in order to move the band of optimum performance from 230 to 245 GHz. Such a change may also improve the resonance problem. When the MPIfR 660 GHz facility receiver arrives next season, a certain amount of reshuffling of the receivers currently located on the left-hand nasmyth will have to take place. This will mean that the SIS-230 will have to be moved and a new optics layout will be required. As all of these events will result in significant changes to the SIS-230 receiver package, it has not yet been accorded status as a full facility instrument.

3.2.2 SIS-345 : The MPIfR 345 GHz dual-channel, semi-automated receiver was returned to Germany after the 1995/96 observing season for the refurbishment and upgrading of several of its sub-systems. One of the primary goals was to provide new low-noise, wide-band tunerless mixers. While these full-height waveguide mixers achieved double sideband temperatures of less than 100K across a 305 to 375 frequency range, they unfortunately exhibited a 2 GHz ripple arising from internal reflections within the junction channel. Although much progress was made in reducing the amplitude of the ripple (down to less than 20K ptp), the MPIfR decided to utilize the backup tunable mixer design in order to ensure the best possible sensitivity. The formal Acceptance Test was held in Bonn on December 18th where it was granted provisional approval. As the receiver was not configured exactly as it would be at the telescope, with various bias and monitoring modules required for the automated tuning of the receiver not being hooked up, full evaluation of its LO, IF and remote tuning algorithms was necessarily delayed until the commissioning run. A comprehensive collection of documentation and circuit drawings were provided. The receiver was delivered to the HHT in January, having survived the trip with the exception of one of the mixer micrometers being slightly damaged in transit. As this was the first full shake- down this season of the HHT system, especially for the AOS's and the rest of the telescope control software, the German commissioning team experienced more than their fair share of problems, with CAMAC faults being close to the top of the list. However, both channels achieved a T(DSB) of better than 125K across the 320-365 GHz band. Total power drift scans on Saturn yielded a 24.4" unconvolved beam in both elevation and azimuth. As with the SIS-230 receiver, the SIS-345 also has the tendency for its IF to resonate at the band edges when the mixer is tuned for optimum sensitivity and may become unstable, with Channel 1 being the more problematic of the two. The HEMT in Channel 2 was replaced with a spare amplifier which seemed to help alleviate its symptoms somewhat. Again, the interim solution is to "detune" the mixer, thus sacrificing sensitivity to obtain improved baseline stability. As this is a dual-polarization receiver, it becomes particularly tricky to tune both mixers for both good sensitivity and good stability. There is no mechanism to differentially adjust the LO power to each mixer so a compromise must be reached when attempting to tune for dual-channel operation. This usually leads to a 10-20% degradation in sensitivity over that of an optimized single-channel. For this reason, the SIS-345 has been used almost exclusively as a single-channel receiver over the last few months. In May, near the end of the observing season, the dewar developed a "cold"leak, and while the receiver could be kept operational (with sufficient warning from the observer), the problem eventually developed into a "warm" leak. The dewar was returned to Bonn in June where, after being disassembled, was quickly repaired by the firm CryoVac. The rest of the receiver's electronics were also returned in order for further upgrades to be carried out. To address the resonance problem, the MPIfR are considering the installation of new HEMT amplifiers based on the Caltech design which might preserve the receiver's current 1-2 GHz IF bandwidth. Failing this, cooled Pamtech isolators could be inserted between the mixers and its current HEMT's, with the unfortunate penalty of reducing the IF bandwidth to less than 750 MHZ. There are also plans to modify the optics package so that the LO power to the mixers can be individually adjusted. As these modifications will result in significant modifications to the receiver, the SIS-345 has not yet been accorded status as a full facility instrument. The MPIfR are still hoping to upgrade the mixers with a fixed-tuned design, thus making the HHT one of the few sub-mm telescopes with a dual-channel receiver utilizing tunerless mixers at this frequency. Not only would tunerless mixers simplify the tuning process (ie: 4 fewer micrometers) but would result in an inherently more reliable receiver. A new design using reduced-height waveguide is currently under development in Bonn. It is hoped that the new mixers might be ready for next winter's observing season.

3.2.3 SIS-490 : The SORAL 490 GHz single-channel receiver was granted provisional approval during its November 8th Acceptance Test in Tucson. The receiver was installed at the HHT during the following week and achieved T(DSB) values of < 150K across the range of 430 to 480 GHz. An unfortunate resonance at the upper edge of the tuning range degrades its performance to about 225K at 490 GHz. The main beam efficiency was found to be about 47% on Saturn and 45% on Mars. Since its delivery, there have only been a limited number of tests to characterize its spectroscopic performance. In January, the SIS-490 was removed to free up space for the MPIfR 480 GHz prototype system. During this time, a new lower-loss window was installed on the dewar. In early March, SORAL modified the pick-off mirrors on the nasmyth flange so that the 345 and 490 GHz receivers could co-exist opposite each other on the nasmyth platform. One interesting feature of the optical design is that it incorporates a low-loss polarizing grid that allows for simultaneous 230 and 490 GHz observations. There has been no attempt yet to exploit this capability as there is only a single room-temperature IF system which must be shared between the two receivers. The SIS-490's commercial phase-lock system malfunctioned in March and was returned to Tucson for repairs. The SIS-490 can be difficult to tune for the uninitiated. Of particular concern is its RPG Physics frequency multiplier (x6) which utilizes 4 backshort tuners. Unfortunately these are not repeatable enough to allow the use of standard look-up-tables and must be tuned in an iterative pattern while monitoring the mixer current. The multiplier, Gunn oscillator and mixer must all be adjusted in concert - 8 micrometers in all. This, along with the complicated power up/down procedures required in order to avoid damaging the multipliers, resulted in many of the SMTO staff being nervous about attempting to tune the "Medusa" without receiving comprehensive and rigorous training. During the summer of 1997, SORAL intends to upgrade the mixer to improve the matching at the high-end of the band by installing a new junction with a different type of tuning stub design. As with most SIS junctions used by SORAL, these devices come from JPL and, through a collaborative arrangement, are mounted in the SORAL mixer blocks by Jacob Kooi of the Caltech Submillimeter Observatory (CSO). SORAL has also acquired approval for funding from SO to build a second IF system. Additionally, the multiplier will be returned to RPG Physics to be upgraded with improved micrometers which are much more repeatable. Improved protection circuitry will also be added to the multiplier bias supply. SORAL has volunteered to redesign the optics layout of the combined 230/490 GHz system so that they and the MPIfR 345 and 660 GHz receivers can all be accommodated on the left-hand nasmyth. As all of these events will result in significant changes, the SIS-490 receiver has not been granted full acceptance as a facility instrument and retains its provisional status.

3.2.4 SIS-660 : This facility instrument is being developed by the MPIfR. It is currently undergoing system tests in Bonn and is expected to be delivered to the HHT for commissioning during the second half of the 1997/98 observing season. Details on its performance will be provided to the user community as they become available.

3.2.5 NASMYTH OPTICS UPGRADE PROJECT : The arrival of the SIS-660 will result in four receivers, the SIS-230, 345, 490 and 660, all vying for space on the lefthand nasmyth. As this new receiver is slated to be installed in the position currently occupied by the SIS-490 GHz receiver, SORAL has undertaken (in consultation with MPIfR-Bonn) to redesign the optics layout, not only to accommodate the move of the 230 and 490 instruments to the far end of the nasmyth platform, but to exploit the opportunity that this forced reshuffling provides to enhanced the spectral-line capabilities of the HHT as a whole. The new optical arrangement will allow the following multi-frequency observing modes to be implemented:

• Single-Polarization 230 GHz
• Single-Polarization 490 GHz
• Dual-Polarization 345 GHz
• Dual-Polarization 660 GHz
• Single-Polarization 230 & 345 GHz
• Single-Polarization 230 & 660 GHz
• Single-Polarization 490 & 345 GHz
• Single-Polarization 490 & 660 GHz

Unfortunately, because of the peculiarities of the optical layout, simultaneous 230 & 490 GHz operation won't be possible. However, plans are underway to provide both the SIS-230 & 490 receivers with a common SSB filter with the unwanted sideband terminated on a bucket of liquid nitrogen. Physical constraints preclude such a capability for 345 & 660 operation. SORAL plans for the various rotating mirrors used to select between the numerous receiver combinations to be under computer control. The nasmyth mirror drive and the SSB translation stages will also be automated. The SMTO will provide the switch boxes for feeding the LO microwave reference to the selected receiver and for distributing the receiver IF's to the desired facility spectrometers. If funded, it is hoped the full system might be ready for installation during the second half of the 1997/98 observing season.

3.2.6 CLOSED-CYCLE RECEIVER PROJECT : The original intent of this Steward Observatory project was to replace the current 230 and 490 GHz facility receivers with a single integrated cryogenic package. Combining the two frequency bands would free up an extra position on the left-hand nasmyth flange as well as reduce the logistical and operating costs associated with the current hybrid LHe systems. The design would incorporate NRAO-style "inserts", each containing the individual mixers, vacuum windows and HEMT amplifiers. The basic dewar is now complete and has been successfully cooled by the JT compressor to 4K. A limited number of thermal capacity tests have been carried out. As time has progressed, however, more ambitious plans for the JT-fridge have been suggested. SORAL has presented a preliminary plan which would provide tunerless dual-polarization mixers in the 230, 345 and 490 GHz bands and would incorporate much of the LO and IF systems from the current generation of facility instruments. Discussions are currently taking place on how SO will proceed with the project.

3.3 SPECTROMETER SYSTEMS

3.3.1 EFFELSBERG AOS : In an attempt to address the lack of back-end capability at the HHT early in the 1996/97 observing system while the 1 GHz and 250 MHZ AOS's were being refurbished in Bonn, the MPIfR offered to send their Effelsberg "Traveling" AOS as an interim measure. This 800 MHZ bandwidth system arrived in mid November. The initial installation went relatively smoothly, with few major obstacles encountered with either the hardware or the new VAX interface software written by the SMTO staff. A serious problem, however, was uncovered towards the end of the commissioning period when it was found that the spectral data was being contaminated by the AOS's internal PC microcomputer accessing the dual-ported system memory at the same time as the CCD data was being written. It is assumed this problem arose as an aftereffect of new software revisions which the system had undergone following its last successful trip abroad and before its installation at the HHT. Although it was determined that this problem could be worked around for Position Switched observations, the solution for Wobble Switching would have been somewhat more difficult to achieve. On the hardware side, while the AOS demonstrated excellent stability, with the RMS noise integrating down with the square root of time for at least 1000 seconds, it was found that channel sidelobe response had "wings" at about the 20% level. This was a curious problem as no such effect had been seen when the instrument was used at Effelsberg or Chile and is likely due to some misalignment in the optical path which occurred during shipment. It was decided, with the agreement of the MPIfR, that it would be difficult to address all these problems before the scheduled arrival of the MPIfR 1 GHz AOS's and that the system would be held in reserve. With the facility spectrometers now in place, the Traveling AOS was returned to Bonn in April where the Digital Group (lead by W. Wiedenhoever) are planning an extensive upgrade of its electronics with a VME microcomputer system.

3.3.2 NASA 4 x 1 GHz AOS : SORAL and Photonics, Inc. continue their collaborative effort to modify the NASA-funded 4-bank spectrometer for operation as a radio astronomy back-end. Changes have been made to the optics and an improved video amplifier board has been designed. The laser is also in the process of being replaced. It is hoped that the upgraded spectrometer will be returned to Tucson in the near future for further laboratory tests.

3.3.3 AOS-3/4 (1 GHz) : Following the end of the 1995/96 observing season, both of the MPIfR 1 GHz AOS's were returned to Bonn for refurbishment. The prime emphasis was to improve the overall system stability so that deep integrations on weak sources could be more readily achieved. The MPIfR has invested about several tens of thousands of DM in upgrading these units. They had been essentially re-engineered with new CCD's, lasers, and beam collimators. A new water-cooled, temperature-controlled rack was purchased to contain the optical packages. About 3 man-years of effort have been devoted to the task. In the lab, stability tests have demonstrated Allan Variance times of close to 1000 seconds (remember that the stability of the sky is usually something on the order of several tens of seconds). The 1 GHz wideband spectrometers were returned to the HHT in January. Both units survived the journey and were installed with few problems (the most serious being the pump in the chiller unit which began to over-heat and had to be replaced). Allan Variance tests indicated a stability time within a factor of two of that measured in the more benign laboratory environment. The RMS noise has been found to integrate down with the square root of time for at least 10,000 seconds. One serious problem encountered following the commissioning of the AOS's was the calibration of Wobble Switched spectra, which, depending on the source and the observing parameters chosen, resulted in line strengths which were up to 50% weaker than those found using Position Switching. For a short while this imposed one of the most serious scientific limitations on the science which could be achieved at the HHT. The inability to beam switch, which is highly desirable for deep integrations towards compact sources, prevented observers from achieving the best possible spectrometer baselines. The SMTO staff established, using a test "tone" controlled by an RF switch which could be slaved to either the Signal/Reference or Blank timing waveforms, that the AOS's were inappropriately integrating for a short period during the Blank Time cycle. With advice from our colleagues at the MPIfR-Bonn, it was eventually diagnosed as an incompatibility problem in the polarity in the timing signals.

3.3.4 AOS-5 (250 MHZ) : As with the 1 GHz AOS's, the 250 MHZ High-Resolution Spectrometer was sent back to Bonn at the end of the 1995/96 season. It too has undergone a major refurbishment, although work has proceeded on it at a lower priority than the wideband spectrometers. Its upgrade has proved to be somewhat less straight forward than the other units, primarily because of problems with the its Bragg cell which displayed indications of degraded efficiency and instability and was subsequently returned to the manufacturer (Marconi) for repairs. More recently, the VAX used at the MPIfR development lab in Bonn died. As its CAMAC interface is essential for the testing of the spectrometer, another computer had to be found. This delay had an adverse impact its schedule and as a result, the Hi-Res AOS will not be delivered to the HHT until early this fall for commissioning.

3.3.5 FILTER BANK SPECTROMETER : This is a Steward Observatory project involving the resurrection of the Filter Bank Spectrometer (FBS) system formerly used at the University of Texas's Millimeter-Wave Observatory (MWO). This spectrometer has 3 bandwidth modes: 256 channels of 1 MHZ, 256 channels of 250 KHz and 128 channels of 62.5 KHz. It will provide a back-up to the MPIfR AOS's and, should it prove to have the desired stability, would be available for observing proposals which require deep integrations to detect extremely weak spectral lines. Its narrow-band mode also enhances the HHT's capacity for measuring cold, dark clouds and maser lines. Although the instrument operated successfully at the MWO for a number of years before being closed down in the mid 1980's, its data acquisition system was deemed to be obsolete and has been replaced with a modern VME microcomputer with an Ethernet link to the Telescope Control computer. The SO Technical Division has provided the man-power for developing the new micro and its software. The micro also contains the first VME interface to the observatory's SAM-Bus data distribution system, thus allowing it access to the Time Stamp and Subreflector Sync & Blank signals. SMTO staff have supplied the software for the VAX interface as well as a new frequency translation system to convert the output of the various HHT receivers down to the 600 MHZ center frequency required by the FBS. This IF Processor (IFP) is designed to handle the standard IF band of 3-4 GHz as well as IF bands at 1-2 GHz, 2-4 GHz, and 4-6 GHz. The spectrometer was installed at the HHT in early February and used a temporary down-converter unit so that software development and system tests could proceed unhindered while long-lead RF and microwave components were on order. Because of operational constraints, this project has essentially been a background task, with work on the FBS usually only possible during System Test / Staff Time. Numerous difficulties have been encountered and resolved, including problems with data formats between the VME and the VAX; timing jitter on the integration & read-out sequence due to the latency effects in the VxWorks multi-tasking operating system; and "contaminated" data occurring in the first dump of each scan. Perhaps the most subtle problem experienced were glitches in the channel data arising from the commercial (Greenstreet) analog-to-digital converter. These were resolved in June and astronomical observations indicate that line strengths between all three filter banks and the AOS's are in good agreement in both Position and Wobble switched modes. Allan Variance times of between 50 and 100 seconds seem to be achievable, which was certainly far better than the AV-Time of the June sky at 230 GHz (eg: < 10 sec). Further system tests are planned after improvements are made to the FBS's thermal equilibrium (it is currently sitting directly under an air-conditioning vent). Mechanical stability is also a concern. The FBS racks sway back-and-forth quite distressingly when the building moves, which will likely have adverse effects on Position Switch observations. This may also be a significant contributor to the numerous dead channels which tend to come and go over time, most of which are due to poor electrical contacts as both boards and components work themselves loose from their sockets. Dead channels will be an on-going problem. The IFP's test features will allow us to automate the detection of suspect channels so that they can be flagged and subsequently ignored until properly repaired. It is hoped that the FBS will be available as a facility instrument this fall. Comparison tests with the MPIfR 250 MHz AOS should prove interesting.

3.3.6 SPECTROMETER STABILITY : In general, the MPIfR 1 GHz AOS's (ie: AOS-3 & AOS-4) seem to be working well, although there have been occasional incidences of poor baseline stability. These are being investigated by the SMTO staff. It must be noted, however, that there are many effects which can lead to unwanted baseline structure other than instability or non-linearities inherent to the spectrometer itself. Fluctuations in the atmosphere are obviously a dominant source of instability. Other potential sources are air currents near the receiver (which may cause the mylar diplexers in the optical path to vibrate), movement of the IF cables between the nasmyth platform and building (which may cause a variation in the standing waves which are seemingly unavoidable in such systems), amplifiers in the IF chain which may be inappropriately operating near saturation (which will result in incomplete baseline subtraction), and so on. These effects can often be difficult to decouple from the those arising within the spectrometer itself without, especially when working with radiometric systems which achieve such wide (ie: 1 GHz) bandwidths. It has been established, however, that both the SIS-230 and 345 receivers can exhibit unstable IF passbands if they are tuned for optimum sensitivity (due to resonances in the match between the SIS junction and the HEMT amplifier). Under these circumstances, the baselines of Position Switched spectra will be particularly degraded. Additionally, there is 10 MHZ ripple that seems to randomly come and go which is common to both wideband AOS's and has been seen in both the SIS-230 and 345 receivers. This would seem to suggest that it arises in the front-end optics, although the 9.6m path length between a receiver on the lefthand nasmyth and the secondary should be result in a ripple closer to 15 MHZ. As time permits, the SMTO staff will continue to characterize all of the HHT's spectrometer systems, with particular emphasis on stability, linearity and dynamic range. Other tests planned, once the high-resolution AOS and the FBS are operational, are simultaneous observations with the 1 GHz AOS's so that a direct comparison of baseline stability can be made. As these narrow-band spectrometers have bandwidths of 25% (or less) than that of the wide-band units, the demands they make on passband stability are obviously much less rigorous.

3.4 PRINCIPAL INVESTIGATOR INSTRUMENTS

3.4.1 MARTIN 460 GHz : This PI instrument belonging to B. Martin (SO) was the only heterodyne receiver available to the HHT during the first few months of the 1996/97 observing season and was used for phaseless-holography measurements. It also served as the front-end for the Chirp Transform Spectrometer (see below) observations of Comet Hale-Bopp. The receiver uses a mixer design developed for the Smithsonian Submillimeter Array (SMA). It is well matched over a broad IF bandwidth of 4-6 GHz. It has been used at the HHT across a tuning range of 330-492 GHz and achieves a T(DSB) of less than <140K.

MPIfR 480 GHz : In January, the MPIfR-Bonn brought a prototype 480 GHz receiver to the HHT for telescope tests during the commissioning run of the SIS-345 and 1 GHz AOS's. This PI instrument, built at the Submillimeter Lab (Rolf Guesten, Head), provided a test-bed for evaluating various key components of the gestating MPIfR 460-490 GHz 16-element array receiver, including the fixed-tuned mixer, 2-4 GHz IF system, local oscillator chain and single-sideband filter. The SORAL SIS-490 receiver was removed from its position to free up space on the left-hand nasmyth. The receiver yielded slightly better performance on the telescope than in the lab due to the colder temperature of LHe at the 10,000 foot altitude. It achieved a double sideband receiver temperature of 80 to 120K across its 430-500 GHz tuning range. The measured beam map appeared diffraction limited and undistorted. The 2 GHz wide IF output was translated in frequency so that it could be accommodated by the two MPIfR wideband AOS's. Its LN2 terminated SSB filter seemed to perform well, reducing both the spectral lines and noise contribution from the unwanted sideband. Unfortunately, between the pressures of time and limited amount of sub-mm weather available during January, there was not opportunity to fully characterize the entire system.

3.4.2 MPIfR 810 GHz : A second MPIfR prototype receiver, designed by F. Schafer, was also brought to the HHT for telescope tests during the January commissioning run. This 810 GHz system utilized a double- slot, open-structure mixer. It was mounted on the left-hand nasmyth in the location normally reserved for the SIS-230 receiver. It achieved a T(DSB) of less than 900K. During a short period of excellent weather (Tau(225 GHz) around 0.05), total power measurements were performed on Mars, yielding a 13" beam. When deconvolved, this results in a diffraction limited beam of 9". There was little evidence of any beam distortion, which strongly suggests the recent accidents with the backup structure have had minimal adverse effect. Some spectral observations were made, using the SIS-345 receiver to perform line pointing and then changing over to the 810 GHz receiver for CO(7-6) measurements. The telescope pointing was initially stable at 3-4" for the first two weeks of the German SIS-345 commissioning run but then worsened to about 10" for the last week, which is a disaster for a beam of this size. It was difficult to find CO(7-6) point sources and only those with broad emission could be detected. This would suggest that, for the time being, one day a week should be set aside to do a pointing run. However, any spectroscopic success at 810 GHz should be considered a major achievement at this stage of the HHT project, especially considering the anomalously poor weather during the month of January.

3.4.3 CYCLOPS : This PI instrument was built by J. Glenn as part of his PhD dissertation at SO and was specifically designed to perform polarimetry measurements of magnetic fields in star forming regions. It uses a waveplate made of Rexalite inserted in the beam of one of the HHT's facility bolometers. During the observing run in the fall, the MPIfR 1300 micron single-color bolometer receiver was used. The Four-Color bolometer was utilized in a subsequent run in March. A PC-based signal processing system is used to acquire and log the data. The instrumental polarization from telescope has been found to be about 0.2%. No elevation dependence is seen over the 20-45 degree range.

3.4.4 CHIRP SPECTROMETER : The first astronomical observing program of the 1996/97 season was carried out by an international consortium (JPL, MPIfA, Caltech, UM, SO, SMTO) using a prototype Chirp Transform Spectrometer (CTS). The Principal Investigator is Mark Hofstadter of JPL. This novel high-resolution spectral processor achieves 4100 channels over a 180 MHZ bandwidth with a channel resolution of roughly 44 KHz. Developed as part of NASA's MIRO instrument, a microwave radiometer and spectrometer built by the US, Germans, and French, its small size is specifically advantageous for space-based operation. The production version of the CTS is intended for launch on ESA's Rosetta mission to Comet Wirtanen. Using the Martin 460 GHz PI receiver while at the HHT in October/November 1996, the CTS group obtained first detections of the CO J=4-3 line in Comet Hale-Bopp. The spectrometer returned again for a second successful run in March/April 1997.

3.5 MULTI-FEED INSTRUMENTS IN DEVELOPMENT

There are a couple of multi-feed receivers which are currently under development at the partner institutes which will see use on the HHT in the near future. There is not room in this Newsletter to describe them in detail but a brief overview is given below (hopefully the instrument builders at SORAL and MPIfR can be persuaded to provide a comprehensive review in a subsequent issue).

3.5.1 ARRAY-345 : This is a facility instrument being built by SORAL. While still relatively early in the design stage, the original concept is for an 11-element array, with 7 of the mixels (mixer- elements) in one polarization packaged in a hexagonal arrangement and the remaining 4 in the opposite polarization interleaved on the sky to achieve a fully sampled grid. The mixers will use a 4-6 GHz IF. The receiver itself will use a closed-cycle JT fridge. To ensure commonality, SORAL intends to copy as much of the control electronics designed for the MPIfR array project as possible. The current schedule call for 7 elements to be ready for testing on the HHT during the winter of 1998/99. The full 11-element system would be ready for the 1999/2000 observing season.

3.5.2 ARRAY-480 + MACS : This is a PI instrument being built by the MPIfR. The receiver has 16-beams with 8 mixels in each polarization interleaved on the sky. The mixers use a 2-4 GHz IF. A Daikin fridge is being used to cool the front-end. The entire receiver will rotate to compensate for the rotation of the image as the telescope tracks the source on the sky. The MPI Auto-Correlation Spectrometer (MACS) will provide the array with a flexible spectral analysis capability. To cover the 16 IF's of 2 GHz bandwidth, MACS will be able to handle 32 banks of 1 GHz (each with 1024 channels), 500 MHZ (2048 channels) and 250 MHZ (4096 channels). The design is based partially on the spectrometer being built for the NRAO Green Bank Telescope. It will use 512 of the Canaris 1024-lag / 125 MHZ correlator chips, the development of which was largely funded by NASA. The Array + MACS project is well along with many of the sub-systems completed or prototyped. Current schedule calls for an observing run of the system in an 8-element configuration at the CSO in spring 1998. It would be installed on the HHT in its full-scale 16-element version for some portion of the 1998/99 observing season.

3.6 TELESCOPE CHARACTERIZATION MEASUREMENTS

3.6.1 POINTING MEASUREMENTS : The effort to increase the accuracy of the telescope pointing is one of the SMTO's top priorities. As it currently stands, one can usually depend upon pointing errors of less than 5". Clearly there is still a great deal of room for improvement. Some recent observers, however, have experienced errors of up to 10". This has usually been traced to a poor determination of the horizontal and vertical offsets for the receiver being used. These must be measured individually for each receiver and their measurement will be relative to the horizontal and vertical offsets of the bolometer used to determine the full telescope pointing model. The recently re-commissioned Four-Color bolometer has a different dewar and mount than the earlier single-color bolometers and consequently is not located at precisely the same point in the focal plane as the Bol-1300 and Bol-870 micron systems were. More importantly, both the facility SIS-230 and SIS-490 GHz receivers were removed during the MPIfR commissioning run in January to make room for PI receivers and the mounts for their nasmyth mirrors were subsequently modified, requiring a re-measurement of their associated offsets. Complicating the situation, the tilt of the azimuth axis, which formerly had varied on the time scale of weeks, suddenly began varying by a few arcsecs each day in late January and early February. The cause of this has not been determined, and it has since gone back to its slower weekly variation. All of these effects, however, caused the determination of the pointing offsets to be less accurate than we would have liked.

3.6.2 INCLINOMETER MEASUREMENTS : Although the residuals after fitting a new pointing model are usually around 2-3 arcsec, the constants continue to change by 2-5" over a time period of about a week. The biggest variation seems to be due to a time-dependent variation in the tilt of the azimuth axis. This is not a unique problem to the HHT - many other radio telescopes have experienced it. It is hoped that by performing tilt versus azimuth measurements on a periodic basis with an inclinometer, the pointing model might be correctable in near real-time. Early measurements with a single Schaevitz inclinometer yielded inconsistent readings and disagreements with the results as determined by the astronomically determined pointing model. It is suspected that the source of the problem may be caused by variations in the ambient magnetic field near the inclinometer. An inexpensive ceramic inclinometer (whose manufacturer claimed was less susceptible to magnetic fields) was tried but this device has refused to work as advertised. A sensitive magnetometer has been used to measure the variation in the localized magnetic field with azimuth at the inclinometer mounting plate. In a further effort to deconvolve this effect, four Schaevitz inclinometers, kindly donated to the SMTO by the Herzberg Institute of Astrophysics (Victoria, Canada), were mounted at 0, 90, 180 & 270 degrees (ie: double qradrature). This arrangement has provided much more meaningful data than the single inclinometer used previously. However there still seems to be a component of the tilt vs. azimuth dependence due to the Earth's magnetic field, some of which may be warping through the elevation fork and causing spurious readings. For what we hope will be a final solution, we have recently ordered a Model 701 dual-axis tiltmeter from Applied Geomechanics ($2.5K). These devices are used by several other observatories, including the JCMT and OVRO which have provided us with information on their experiences. These devices are not susceptible to magnetic fields. When installed and characterized over the next few months, they will hopefully allow us to implement a major improvement in the pointing model.

3.6.3 REFRACTION CORRECTION : The calculation for determining the refraction correction has been a problem in the past mainly because of the influence of inaccurate data from the observatory's venerable Heathkit weather station. The pressure sensor reading is off by at least 10%, the relative humidity sensor is notoriously erratic, and the temperature sensor sensitivity tends to drift. This problem will be addressed by the purchase of a new scientific-grade weather station.

3.6.4 PHASELESS-HOLOGRAPHY : An attempt was made to measure the large-scale accuracy of the HHT's surface in October 1996 using the phaseless-holography technique. The Martin PI receiver was used at a frequency of 408 GHz for this experiment. Due to the relative weakness of the astronomical sources available at these high frequencies, this technique will yield relatively poor spatial resolution. However, it was hoped that it would be sufficient to characterize the telescope's global shape and allow us to determine if there has been any gross deformation arising from the recent accidents with the back-up structure. This technique has enjoyed excellent success over the past few years, particularly at the James Clerk Maxwell Telescope. The weather in the fall, unfortunately, was rather uncooperative but several holography maps or Mars and Saturn were acquired. The data has yet to be fully analyzed. The recent departure of several key members of the holography group has more-or-less put this project on indefinite hold. However, while we may not have any direct measurement of the HHT's current surface accuracy, we do have beam maps made at 350 micron by the Four-Color bolometer and at 490 GHz by the facility SIS receiver. While neither map has been critically analyzed, they both show symmetric beam shapes with little distortion and low sidelobe levels. Beam maps made with both the MPIfR 480 and 810 GHz prototype receivers in January also showed no evidence of distortion down to the 5% level.

3.6.5 SATELLITE HOLOGRAPHY : The demise of the LES-8 has complicated the process of accurately measuring the surface of many of the world's major radio telescopes, the HHT included. The LES-9 satellite, however, is still functioning and recent discussions with Lincoln Labs indicate that there is a good chance that it might be made available for new round of holography measurements this fall. As during previous phase-reference holography runs, we will use the 38 GHz holography receiver developed by NRAO for the 12-meter telescope. Unfortunately its 37.5 GHz Gunn oscillator has died since it was last used. The SMTO is in the process of replacing this device ($1.7K). Assuming our request for satellite time is approved, we anticipate the holography run will require close to one month of telescope time, with the following schedule: one week to remove the subreflector and mount the holography receiver; three weeks of holography and surface adjustment (obviously done in a cautious and iterative manner); one week to reinstall/realign the subreflector (including the tedious horizontal collimation procedure). Depending on satellite accessibility, we would hope to carry out the holography run directly following the summer shutdown, from mid September to mid October.

3.7 BUILDING AND ENCLOSURE

3.7.1 ACCOMMODATIONS : One significant problem during the current observing season was the shortage of bedroom space at the observatory. The facility's five bedrooms have always presented a serious limitation but one which would have been ameliorated somewhat had the Forest Service allowed the Engineer's Residence to be placed back on the summit last winter. Without its 3 additional bedrooms, it was particularly difficult during the commissioning run in January to accommodate the sizable MPIfR team of engineers and astronomers and still provide an adequate mix of on-site SMTO support staff. Fortunately several members of the commissioning team elected to commute to a motel in Safford and our nearby neighbor, the Vatican Advanced Technology Telescope (VATT), allowed us access to its vacant bedroom space whenever possible. As a temporary measure, two extra beds were set up in the SMT's office area. It is obvious, however, that this is a problem which the facility will be facing for the foreseeable future. The long-term solution may be to relocate our Machine Shop to the Large Binocular Telescope (LBT) now under construction and turn the vacated space into additional bedrooms. In the short-term, however, it is hoped that the Engineer's Residence will be permitted back up the mountain this fall now that the most recent lawsuits by the environmentalists have been resolved in our favor.

4.0 TELESCOPE REPAIR AND MAINTENANCE

4.1.1 BACK-UP STRUCTURE DAMAGE : In early December there was a 2nd incident within a year which damaged the telescope back-up structure, this time when the door crane accidentally collided with one of the carbon-fibre CFRP struts. The tube was subsequently removed, returned to Tucson for repairs and re-installed over a four day period. The cause of the accident was due to the fact that the telescope had not been driven all the way up to the zenith. While the door crane has an interlock that will prevent the telescope from being operated unless the crane is completely flush with the door, there is no interlock to prevent the door from being used if the telescope is not above 85 degrees. To avoid future accidents of this nature, the SMTO staff have been advised to use the interior crane for the hoisting of cryogens and gas bottles, thus reducing the reliance on the door crane and putting the telescope and backup structure less at risk. The new concrete loading dock which was added to the front of the SMT building last summer now makes this mode much more convenient. Additionally, there are plans to install a new interlock system which will monitor the position of the telescope and disable the movement of the doors and/or provide an audible alarm if someone attempts to use the crane when the telescope elevation is too low. For future reference, it has also been established that replacement CFRP struts can be obtained from Vertex (Duisburg, Germany) for approximately $5K with an 8 week minimum delivery time.

4.1.2 CAMAC SYSTEM : On two separate occasions (mid-December and early January), a serious problem with the CAMAC system prevented the antenna drive microcomputers from operating. In both situations, the fault seemed to arise from a failure in the handshaking between the crate controller and the two Antenna Micros and consequently led to telescope being down for several days. In the earlier case, the fault seemed to vanish during the course of investigating the problem, which suggest it was caused by some mechanical fault in the CAMAC crate or cabling. In the second case, which coincided with a CAMAC power supply failure, several CAMAC cards had to be replaced as well as a Data Highway cable. It was also discovered that the Auxiliary Control Bus (ACB) cable feeding the Antenna Micros had to be modified to disable the interrupt lines. This is puzzling indeed since the Micros are configured in software to ignore these interrupt inputs in the first place. There have also been numerous failures of various CAMAC digital input and output cards. Keeping a sufficient supply of operational spares has been a difficult but critical goal of the SMTO staff. To this end, a great deal of effort has gone into maintaining the CAMAC system. Its unreliability still has the potential to cause serious amounts of telescope down-time. The SMTO staff eagerly await the day when the CAMAC system is replaced with modern VME-based microcomputers interfaced to the Telescope Control computer via its Ethernet link.

4.1.3 CAMAC POWER SUPPLIES : The CAMAC power supplies continue to be an area of serious concern, especially with those supplying power to the Antenna Crate which, at 32 amps is getting uncomfortably close to its maximum (sea level) specification of 38 amps. There were additional failures of several of the power supply units during December and January. Although repaired and modified by SMTO staff, these problems have all too often resulted in the diversion of precious man-power effort as well as forcing us into the dire situation of having no spare units available. A new 50 amp power supply have been ordered to serve as an additional spare with the hope that it will carry us through until such time as the entire CAMAC system is replaced.

4.1.4 UNINTERRUPTABLE POWER SUPPLY : The UPS system showed signs of reduced output capacity in December and was no longer able to provide adequate output power when the input power was lost. All of the batteries were subsequently replaced ($8.6K) and the unit is now functioning properly again.

5.0 COMPLETED SMTO PROJECTS

5.1.1 CONTROL SOFTWARE : Efforts have continued in the attempt to upgrade the system's control software and user interface. As well as numerous small improvements to the system, several important new features have been incorporated. For example, simultaneous observations with multiple spectrometer backends can now be accommodated and the code for "slow" frequency switching has been written and verified during preliminary tests. Dark Current measurements and Noise/Comb-Line tests can now be performed remotely on the AOS's. Johann Schraml (MPIfR) has made significant strides in minimizing the telescope dead-time between sub-scans.

5.1.2 DATA ANALYSIS PROCEDURES : Additional improvements to the data analysis procedures have also been implemented, although more needs to be done. Line pointing has now been installed and tested. This feature will expand the number of pointing sources available for use by the HHT's heterodyne receivers. The on-line display of spectrometer data has now been modified to show the Quotient Spectrum (ie: (S-R)/R) as opposed to the Difference spectrum.

5.1.3 SAMBUS-VME INTERFACE : The Status and Monitor Bus, based on the NRAO 12-meter design, is being used to distribute various time critical signals throughout the telescope (ie: the Chopping Secondary Sig/Ref Integration Period, Blanking Time, Time Stamp, Receiver Ready, Out-of-Lock, etc.). The SAMBUS is interfaced to the Telescope Control computer through the CAMAC system. The goal of this project was to provide the interface between the SAMBUS and other VME microcomputer-based instruments. While initially required for the FBS micro, this capability will be exploited by future VME-based systems.

5.1.4 CHOPPING SECONDARY FREQUENCY GENERATOR : The VME-based Frequency Generator for controlling the Chopping Secondary period has been successfully implemented. This microcomputer, running under VxWorks, was designed and built at the University of Arizona's Research Instrumentation Center. It allows the Signal/Reference Integration Period and Blank Time parameters to be set by the Telescope Control computer (ie: rather than with a manual function generator as was previously done). It is easily expandable and could be used for the current bolometer chopper or the hot/cold load calibration chopper wheel planned for the future.

5.1.5 250 MHZ AOS STEERING LO UNIT : This dedicated frequency synthesizer will provide the MPIfR High Resolution 250 MHZ AOS with a steerable LO for selecting which portion of the 3-4 GHz receiver IF is to be analyzed. This has been done in the past with a commercial signal generator, thus tying up one of the few pieces of general test equipment which the SMTO has available. The hardware design for this piece of facility equipment was done by F. Patt and the programming of the units embedded 8051 micro was carried out by D. Ashby.

6.0 SUMMER SHUTDOWN IMPROVEMENTS

6.1.1 ENCLOSURE WHEEL BOXES : Late last year one of the enclosure wheel-bearings managed to work itself out of alignment and as well as causing it to emit noises of a rather distressing nature, it eventually forced the building so far off center that the carousel actually began to scrape against the outer wall. It was determined that the grouting epoxy used to provide the proper alignment between the wheel box and the track had failed, resulting in the wheel losing its full-surface contact with the track. Unfortunately this also caused excessive lateral stresses which ended up damaging the bearings. In December the wheel-bearing assembly was removed and taken back to Tucson for repair. The wheel was re-ground and both bearings and their seals were replaced. The assembly was returned to the site, installed and re-aligned within two days. The problems encountered with the SMT's wheels are somewhat of a puzzle as the assemblies are similar to those used successfully at the Multiple Mirror Telescope for close to 15 years. Accordingly, Steward Observatory is carrying out a redesign of the these units with the goal of strengthening the bearing support. Two of the four wheel-boxes (those directly under the enclosure doors) will be replaced towards the end of the Summer Shutdown (September/ October). R. Warner has been managing this project. The University of Arizona is covering the $65K replacement cost.

6.1.2 WEATHER STATION : As noted earlier, the inexpensive Heathkit weather station presently being used is neither sufficiently reliable or precise enough to provide the humidity and barometric pressure data required to accurately determine the refraction compensation value. The SMTO has investigated several commercially available meteorological instruments and is in the process of ordering a replacement unit at a cost of about $5K. A PC will be used to collect the weather data and relay it to the Telescope Control Computer. We also intend to have this PC take over the Taumeter data collection function as its PC is getting well on in years. D. Muders has been placed in charge of this project.

6.1.3 TELESCOPE/BUILDING CONTROL SYSTEM : During the 1996 summer shutdown, the Control Desk was totally rebuilt and repackaged in order to improve and simplify telescope operation. New electronics have been added which provide comprehensive status information on the telescope and building drive units as well as allowing for the doors and roof to be opened and closed from within the Control Room. Additional enhancements, primarily to improve the telescope's safety interlock system, will be incorporated over the Summer Shutdown. Much of the effort deals with the installation of new cabling, hydraulic sensors and interlock relays. R. Esterline and H. Holmberg have taken the lead roles in this project.

6.1.4 RECEIVERS : Numerous small improvements to the HHT's receiver systems are planned during the Summer Shutdown. High-quality microwave cable assemblies will be installed between the nasmyth platforms and the building in order to reduce the effects of variations in the IF passbands due to mechanical movement. Additionally, the JCMT has kindly sent us several sheets of Goretex fabric (used in the membrane which encloses this 15-meter telescope) which will be used to cover the elevation bearing aperture to the lefthand nasmyth platform in an attempt to eliminate air currents from vibrating the mylar diplexers used by the various heterodyne receivers. Both of these efforts should help to improve spectrometer baseline stability. Remote control and monitoring of the apex ambient load by the VAX is also planned. Dedicated coax lines will be run from the left and right Receiver Rooms into the Cassegrain Cabin to facilitate beam alignment procedures. Additional lines will also be run from the Computer Room to the left Receiver Room so that the "real-time" displays from the AOS's will be available to aid in receiver tuning. A similar scheme for displaying the AOS display will also implemented in the Control Room. Rack-mounted oscilloscopes will be purchased as part of this system. The one located in the Receiver Room will also be available for displaying SIS mixer I-V and Total Power curves while the scope in the Control Room will also be used to monitor the Subreflector LVDT, Sync and Blank signals. Modifications are planned for the V/F's associated with Drumbeat so that up to 6 Total Power channels can be accommodated (ie: the facility SIS-230, 345-1/2, 490 and 660 GHz receivers plus one extra for use by PI receivers). Finally, a compressor will be located in the Loft and used to drive both the SORAL 230 and 490 GHz receivers, thus freeing up the two smaller compressors currently in use.

6.1.5 SPECTROMETERS : One improvement planned for all three AOS and the FBS systems is to provide remote control of their programmable IF Attenuators and their Dark Current switches. The addition of these features in all of the HHT's spectrometers would enable remote level set adjustment as well as allowing the Dark Current offsets to be determined automatically. Remote control of the attenuators is vital if it becomes necessary to extend the dynamic range of the spectrometers to accommodate the large IF power changes (close to 3 dB) which can occur between a hot load and a "cold" sky.

6.1.6 AUTOMATIC CALIBRATION : At sub-mm wavelengths, doing periodic calibration scans is a necessity, especially in marginal weather. A load calibration procedure will be implemented that allows the computer, by invoking a simple command (eg: "CAL"), to control the apex ambient load and record the sky/ambient values and automatically compute a system temperature. The physical temperature of the ambient load will also be monitored. In the Receiver Room, a remote push-button will be provided to facilitate Cold Load measurements (it current takes two observers to carry out a cold calibration).

6.1.7 ELECTRONICS LAB : In order to alleviate the cramped quarters in the telescope's Electronics Shop, it has been decided to expand the work-space into the room occupied by the Library/Office. A non-load bearing wall was moved by about 15 feet. As well as providing more work-bench area, it will increase the amount of storage space available for electronic components and test equipment. The manpower effort for this project has come entirely from SMTO staff and is now essentially complete.

7.0 FUTURE SMTO PROJECTS

7.1.1 OPTICAL POINTING : In order to help us better understand the telescope's pointing model, we are investigating the most effective manner for equipping the HHT with an Optical Pointing Telescope. Such a system would also help us address the problem of the small number of sources available for pointing at sub-mm wavelengths. At some point it might conceivably be adapted to provide guide star pointing. D. Officer has initiated a preliminary design study with the intention of adopting a commercial solution, using astronomical equipment developed explicitly for high-end amateur astronomy, such as an 8 to 10 inch Cassegrain telescope along with a CCD camera and PC-based image processing software. It is expected that this project will require supplemental funding.

7.1.2 DOOR AND ROOF SEQUENCER : We have decided to replace the microcomputer used in the control unit which automates the opening and closing of the enclosure's roof and doors. This newly designed 8051-based single-board computer (SBC) will be more capable and expandable than the"Standard Micro" which the system uses at present. This current micro, used by the Steward Observatory over the years, is clearly over-taxed for the expanded role we would like it to perform. The new unit will replace the over-whelming array of status lights on the console display with an alphanumeric LCD display and it will be linked to the Telescope Control Computer via a serial link, thus making the system both more user-friendly and allow "Kronen" to track the status of the myriad of interlocks within the system. The redesigned micro will use FLASH memory so that software enhancements of the control code to be easily downloaded. Daughter boards containing up to 8 opto-coupled sensors will provide a modular solution for monitoring the AC voltages used by telescope's numerous interlocks. The new SBC will likely see use in many new future projects as well as in upgrade replacements of older systems. D. Ashby is responsible for the design and implementation of this project.

7.1.3 RECEIVER LO/IF SWITCH BOX : In anticipation of the new optics layout for the SIS-230, 345, 490 and 660 receivers on the lefthand nasmyth platform, a microwave switch-box will be designed for feeding the 8 GHz microwave reference required for phase-locking their LO systems. The system will also have to be able to distribute two microwave references during dual-frequency operation. Additionally, this new capacity for simultaneous observation between pairs of receivers (ie: 230+245, 230+660, 490+345 and 490+660) will complicate the distribution of the various IF's to the 1 GHz AOS's and the narrow-band spectrometers. The numerous IF signal path options are far to complex to continue with our traditional manual interconnection practice of the easily-damaged microwave cable assemblies. A switching matrix will be required to allow the mapping of 8 possible receiver IF's (ie: the 6 current IF's plus 2 for future expansion) into two backend channels. The Hi-Res AOS will be tied in parallel with AOS-3; the FBS will be in parallel with AOS-4. B. Hayward is in charge of this project.

7.1.4 HARMONIC CHECK FEATURE : This would add the ability to change the HP synthesizer frequency in such a way to select another (but correct) harmonic microwave reference and thereby allow for unambiguous tuning of the Gunn oscillator. It would provide a confidence check that the actual sky frequency being used was correct. A terminal will be installed in the Receiver Room so that the observer can change the harmonic as desired from near the receiver where the phase-lock IF can be monitored during the tuning process. The terminal would also be available for other purposes. The HOST4 PC that controls the HP synthesizers will also be reprogrammed so that it can be directed, through its menu system, to recalculate and output the new frequency settings. B. Peters has the lead responsibility for this task.

7.1.5 UNIVERSAL LOCAL OSCILLATOR : The goal of this project is to build a frequency-agile, spectrally-pure microwave synthesizer which could be used by the HHT's single-beam receivers for their microwave reference signal. It would be able to generate two reference frequencies in order to facilitate simultaneous observations at two different wavelength bands. It would eliminate the need to use expensive commercial synthesizers as is currently the case. The ULO would also incorporate a "fast" frequency switching capability. This project is not yet funded.

7.1.6 PROJECTS ON-HOLD : As a result of the SMT Council meeting held in Bonn in late May, several SMTO projects have been put on-hold due to financial considerations. These include:

• a) Remote controlled Tertiary Mirror : This project would see the tertiary mirror unit outfitted with a remotely controlled rotary table. This would ensure that the tertiary could be precisely positioned. Additionally, the system would provide the ability to select between the left and right nasmyth receiver locations under computer control. This will be an essential future requirement if we wish implement remote observing and dynamic scheduling in order to quickly exploit good sub-mm weather.
• b) Remote controlled Nasmyth Mirror Drives : In order to provide remote adjustment of the nasmyth mirrors, the addition of encoders and computer controlled motor servo units to the existing worm-gears are required on both the right and left side. Such a capability will be essential if we wish implement remote observing in the future. The new layout being designed by SORAL for the left-hand nasmyth includes a provision for automating its light-tower. It is likely that the required funds will come from SO. Providing a remote control feature on the right-hand nasmyth will require a separate funding solution.
• c) Universal Cold Load : This system is still in the definition stage but plans call for a moveable mirror, located in the cassegrain cabin, which directs the beam towards a LN2 cold load. A hot/cold calibration chopper wheel would provide the means for real-time Y-factor and T(Rx) measurements and thus allow for optimized receiver tuning as well as full hot/cold calibration of HHT radiometers.

7.1.7 CAMAC REPLACEMENT : Although there have been no serious failures of the CAMAC electronics since January, there are numerous glitches and faults which have be "papered over" in software. While there are functioning spares for most CAMAC cards, the MULI spare has never worked properly and may require some re-engineering to replace an outdated gate array chip. Should the MULI card fail, "Fahren" will no longer be able to receive its 4 Hz interrupts, thus grinding the antenna drive software to a halt. Another single point of failure is the Highway Driver which provides the interface between the Telescope Control VAX (ie: "Kronen") and all of the CAMAC crates. The spare for this has also never worked in its Q-Bus configuration (although it did work as a UNIBUS device with the old VAX 11/750). Its manufacturer (Kinetic Systems) is attempting to repair it but it appears that the engineers that worked on these systems have long since retired. Between these problems, and the ones noted earlier with the Antenna Crate and the Power Supply units, our CAMAC system is living on borrowed time. Accordingly, the SMTO is actively discussing with the technical groups supporting the Effelsberg 100-meter and the IRAM 30-meter telescopes on how best to replace the CAMAC system used at all three facilities. A meeting of Schraml, Lazareff, Perrigouard, Brunswig, Jessner, Neidhoefer, and Peters was held in early April to outline a plan for retiring CAMAC and estimating the amount of time and money required. As well as supplanting the three CAMAC crates of the Antenna Control system and the numerous custom-made cards which they contain, the CAMAC control and data acquisition electronics utilized by the MPIfR AOS's will also have to be replaced. We are hoping to be able to ride on the coat-tails of the MPIfR Digital Group in their effort to upgrade the Effelsberg Traveling AOS. Addressing the amount of man-power effort required will be a non-trivial exercise. The supplemental funding for this project is estimated to be on the order of $40K for the new VME microcomputers and an additional $20K to replace "Kronen" with a new state-of-the-art workstation once the VAX's antiquated Q-Bus architecture is no longer required.

8.0 MISCELLANEOUS

8.1.1 ON-GOING SAFETY REVIEW : Following the accident which damaged the back-up structure in the fall, a committee has been set up to look into any other potential problems at the telescope and provide recommendations on how to avoid them in the future. The goal is not only to protect the telescope from further damage but to ensure the safety of the SMTO staff and visiting observers. We want to ensure that, in our efforts to achieve properly functioning instruments and software, we haven't ignored aspects of the facility's overall safety. Related to this topic, the SMTO staff is making a concerted effort to improve and formalize the facility's preventive maintenance procedures.

8.1.2 ACCEPTANCE TEST PROTOCOL : A new Acceptance Test Protocol for SMTO single/dual channel Heterodyne Receivers has been formulated. A comprehensive Acceptance Test Check-List has also been devised. The SORAL 230 and 490 GHz receivers as well as the MPIfR 345 GHz receiver have been (provisionally) accepted under the new protocol. Acceptance Test guidelines for future spectrometer systems is also being drafted.

8.1.3 DOCUMENTATION : Major strides have been made in improving both the content and presentation of the SMTO documentation. The "User's Manual" now runs some 200 pages long and is still growing. There are also plans to expand the information available on the SMTO Web Homepage.

9.0 WEATHER STATISTICS

The following table summarizes the opacity measured by our 225 GHz Taumeter during the last two observing seasons. This data has not been corrected for anomalously bad readings, such as sometimes occurs when the tipper looks into the surrounding forest.

   Percentage Time with Tau(225 GHz) Less then the Indicated Value 
 ------------------------------------------------------------------- 
| Month |       Season 1995-96        |       Season 1996-97        | 
|       | <0.06  <0.075  <0.15  <0.30 | <0.06  <0.075  <0.15  <0.30 | 
|-------| -----  ------  -----  ----- | -----  ------  -----  ----- | 
|  Oct  |    1      2      54     96  |    4      6      15     51  | 
|  Nov  |    3      7      48     84  |    7     15      47     86  | 
|  Dec  |    8     17      62     86  |   14     22      45     82  | 
|  Jan  |   22     32      64     94  |    8     12      39     74  | 
|  Feb  |    0      1      16     70  |   17     26      65     92  | 
|  Mar  |   14     22      61     92  |   11     22      72     97  | 
|  Apr  |    6     12      51     90  |    1      2      34     80  | 
|  May  |    0      0      33     90  |    0      1       6     54  | 
|-------  -----  ------  -----  ----- | -----  ------  -----  ----- | 
| Average    7     12      49     88  |    8     13      40     77  | 
 ------------------------------------------------------------------- 

     Note :    Tau(225 GHz)   Precipitable Water Vapor 
                   0.06               1.2 mm 
                   0.075              1.5 mm 
                   0.15               3.0 mm 
                   0.30               6.0 mm

As can be seen, the overall opacity conditions at the HHT during 1996/97 were not dissimilar to the previous year's. For both 1995/96 and 1996/97, really excellent 350 micron weather (where the Tau(225) is less than 0.06, although < 0.075 would be acceptable for some observing programs) occurred about 10-15% of the time. For both seasons, 870 micron weather (ie: Tau(225) < 0.15) occurred about 50% of the time. The month of January tuned out to be the worst mid-season month of 1996/97 while February was the poorest during the previous season.

During the coming season, as receivers become available, the SMTO staff will attempt to accurately determine the conversion factor between the Taumeter measurements at 225 GHz and the actual opacities at sub-mm frequencies. While this value is known for the bands serviced by the bolometers, their wide detection bandwidth makes it inappropriate for comparing with the high frequency heterodyne receivers.

It is obvious, however, that the Tau statistics are highly variable from month-to-month and from year-to-year. R. Mauersberger has analyzed radiosonde data for Tucson which indicates the current opacities are similar to that found during the original site survey in the mid 1980's. It does appear from this data that the site was indeed a superb site in the 1960's and 1970's but over the last 10 years, the amount of sub-mm time has decreased somewhat since that time. Whether this is merely a fluctuation or a trend (ie: the effect of global warming) is not known. While there are some who feel that the site is not as good as Mauna Kea, it must be remembered that Mt. Graham is certainly the best of any established site in North America or Europe and, consequently, the HHT is well positioned to play a significant roll in sub-mm astronomy

One of the advantages of the HHT is that its carbon fibre design allows the facility to be used 24 hours a day while most sub-mm observatories operate only during the night (eg: the JCMT typically operates for only 16 hours and the CSO for only 14 hours per day). Mt. Graham also has much less of a diurnal fluctuation during the daytime. Thus, in terms of on the number of available hours of sub-mm weather during the winter months, the HHT's statistics become much more competitive. Since the MPIfR & SO partner institutes have better access to the HHT than any of the other sub-mm telescope, they are able to accumulate more observing time for any given project.

A key requirement for future operation of the HHT is to ensure that the telescope can respond quickly to the arrival of good sub-mm conditions. Dynamic scheduling is an obvious goal. The 3 week observing block allotted to the partner institutes is much more conducive to dynamic scheduling than our competition's and allows us to react quickly to exploit good weather conditions. However, to fully harness this mode of operation, the HHT requires the following:

• high-frequency receivers which are on stand-by and can be ready to go at a moment's notice
• the ability to switch between receiver's rapidly, including remote control of tertiary and nasmyth mirrors
• receivers which can be easily and quickly re-tuned or which are automated
• the telescope pointing model must be accurate and always up-to-date for every receiver

The suite of instruments at the HHT can also be improved to allow us to maximize the amount of science which can be achieved during periods of sub-mm weather. The addition of SSB Filters to reject the added noise from the unwanted image band will improve spectral line measurements. Multi-element arrays, both for our bolometer and heterodyne receivers, would allow mapping projects to be speeded up considerably.

10.0 VISITORS AND OBSERVERS

Over the past 6 months, the HHT has had many astronomers and technical staff from the partner institutes and other external organizations visit the observatory. The following is a (more-or-less) chronological listing of our visitors:

     Christoph Kasemann       MPIfR     Jan  5 - 23 
     Gert Schneider           MPIfR     Jan  5 - 19 
     Thomas Klein             MPIfR     Jan  5 - Feb 3 
     Rolf Guesten             MPIfR     Jan  9 - Feb 3 
     Johann Schraml           MPIfR     Jan  9 - 19 
     Ronald Stark             MPIfR     Jan 20 - Feb 3 
     Frank Schafer            MPIfR     Jan 25 - Feb 3 
     Peter v.d. Wal           MPIfR     Jan 25 - Feb 3 
     Jaap Baars               MPIfR     Jan 27 - 28 
     Bob Martin               SO        Feb  3-9, 11-15 
     John Hughes              SO        Feb  5 -  9 
     Jeff Rille               SO        Feb 12 - 13 
     Chris Walker             SO        Feb 10 - 11 
     Jeff Capara              SO        Feb 10 - 11 
     Gopal Narayanan          SO        Feb 10 - 11 
     Paul Hartogh             MPIfA     Feb 15 - 19 
     Christopher Jarchow      MPIfA     Feb 15 - 19 
     Lucy Ziurys              SO        Feb 16 - 18 
     Aldo Apponi              SO        Feb 16 - 18 
     Troy Pesch               SO        Feb 16 - 18 
     Ernst Kreysa             MPIfR     Feb 18 - 24 
     Peter Lambertz           MPIfR     Feb 18 - 24 
     Lother Reichertz         MPIfR     Feb 18 - 24 
     Karl Menten              MPIfR     Feb 24 - Feb 27 
     Malcolm Walmsley         MPIfR     Feb 24 - Feb 27 
     Gopal Narayanan          SO        Mar  1 -  4 
     Peter Strittmatter       SO        Mar  6 -  7 
     Chris Walker             SO        Mar  6 -  7 
     Jason Glenn              SO        Mar 13 - 16 
     Yancy Shirley            SO        Mar 13 - 16 
     John Bieging             SO        Mar 16 - 19 
     Connie Walker            SO        Mar 16 - 20 
     Deirdre Hunter           Lowell    Mar 16 - 20 
     Margret Hanson           SO        Mar 20 - 22 
     Hua Chen                 SO        Mar 20 - 21 
     Rainer Mauersberger      SO        Mar 22 - 25 
     Bob Martin               SO        Mar 24 - 30 
     Paul Hartogh             MPIfA     Mar 24-27, Mar 31 - Apr 2 
     Mark Hofstadter          JPL       Mar 26 - 29 
     Joe McMullin             NRAO      Mar 28 - Apr 2 
     Rainer Mauersberger      SO        Apr 2-3, 5-7 
     Joachim Guertler         MPIfR     Apr  5 - 10 
     John Hughes              SO        Apr  8 - 10 
     Tom Wilson               MPIfR     Apr 11 - 14 
     Paul Gensheimer          MPIfR     Apr 11 - 23 
     Gopal Narayanan          SO        Apr 15 - 17 
     Christian Henkel         MPIfR     Apr 18 - 23 
     Rainer Mauersberger      SO        Apr 21 - 23 
     John Hughes              SO        Apr 23 - 25 
     Bob Martin               SO        Apr 28 - May 5, 7-9 
     Mark Hofstadter          JPL       May  2 -  5 
     Paul Hartogh             MPIfA     May  6 -  9 
     Connie Walker            Lowell    May  9 - 13 
     Deirdre Hunter           SO        May  9 - 12 
     John Bieging             SO        May 13 - 16 
     Gopal Narayanan          SO        May 13 - 17 
     Jason Glenn              SO        May 16 - 21 
     Erick Young              SO        May 17 - 19 
     Greg Rudnick             SO        May 19 - 21 
     Jeff Capara              SO        Jun 11 - 12 
     John Hughes              SO        Jun 18-20, 23-25 
     Jeff Capara              SO        Jul  2 -  3

11.0 CONCLUSION

As many of you may know, Bob Martin has decided to leave the Steward Observatory and join the Academia Sinica's Institute of Astronomy and Astrophysics (Taiwan) which has recently become a 15% partner in the Smithsonian Sub-Millimeter Array project. Bob will be a key figure in the ASIAA's efforts to provide two additional elements for the SMA. The staff of the SMTO would like to take this opportunity to thank Bob for his contribution over the years. We wish him well in his future endeavors and hope to see him return often to use the telescope which he worked so long and hard to bring to a successful reality.

We also look forward to the arrival of Tom Wilson in Tucson this August to take up the reins as the new Director of the SMTO. Tom's long experience in the radio astronomy field, in both its technical and scientific disciplines, and the respect he commands from both sides of the Atlantic, will ensure that the HHT will continue to be a productive, world-class facility.

In regards to productivity, we would like future issues of this Newsletter to include a listing of any new scientific papers published or talks presented by our user community. We would appreciate it if you would forward your abstracts to us. We also solicit submissions from the instrument builders at our partner institutes to keep us informed of their development efforts and successes.

The SMTO is ramping up for what promises to be an exciting new observing season. With a full slate of receivers and back-ends, we anticipate that there will be many technical improvements and lots of great science achieved over the next year. Of particular note is the possibility of participating in a 230 GHz long baseline interferometry run with the Coordinated Millimeterwave VLBI Array (CMVA). Haystack Observatory has expressed interest in providing the HHT with a hydrogen maser and Mark III recording terminal. Both of our partner institutes have greeted this proposal with enthusiasm. The next few issues of the "SMTO Electronic Newsletter" will hopefully have much in the way of good news to report.