SMTO Electronic Newsletter - Volume 1, Number 2 (April 1998)

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TABLE OF CONTENTS

• 1.0 INTRODUCTION
• 2.0 SMTO STAFF
• 3.0 CURRENT STATUS OF FACILITY
• 3.1 CONTINUUM RECEIVER SYSTEMS
• 3.1.1 FOUR-COLOR BOLOMETER
• 3.2 HETERODYNE RECEIVER SYSTEMS
• 3.2.1 SIS-230
• 3.2.2 SIS-345
• 3.2.3 SIS-490
• 3.3 SPECTROMETER SYSTEMS
• 3.3.1 AOS-A/B (1 GHz)
• 3.3.2 AOS-C (250 MHz)
• 3.3.3 FILTER BANK SPECTROMETER
• 3.3.4 SPECTROMETER STABILITY
• 3.4 FRONTEND / BACKEND OBSERVING MODES
• 3.5 PRINCIPAL INVESTIGATOR INSTRUMENTS
• 3.5.1 CYCLOPS
• 3.5.2 MPIfR 2 mm BOLOMETER ARRAY
• 3.5.3 MPIfR 810 GHz SIS RECEIVER
• 3.5.4 CfA HOT ELECTRON BOLOMETER RECEIVER
• 3.6 TELESCOPE CHARACTERIZATION MEASUREMENTS
• 3.6.1 POINTING MEASUREMENTS
• 3.6.2 INCLINOMETER MEASUREMENTS
• 3.6.3 REFRACTION CORRECTION
• 3.6.4 SATELLITE HOLOGRAPHY
• 3.7 BUILDING AND ENCLOSURE
• 3.7.1 ACCOMMODATIONS
• 3.7.2 WORK-SPACE
• 4.0 TELESCOPE REPAIR AND MAINTENANCE
• 4.1.1 CAMAC SYSTEM
• 4.1.2 MISCELLANEOUS FAULTS
• 5.0 COMPLETED SMTO PROJECTS
• 5.1.1 LINE POINTING
• 5.1.2 AUTOMATIC CALIBRATION
• 5.1.3 DATA ACQUISITION AND ANALYSIS PROCEDURES
• 5.1.4 TELESCOPE CONTROL PANEL
• 5.1.5 INTERLOCK CONTROL & STATUS MICRO
• 5.1.6 ELEVATION INTERLOCK
• 5.1.7 NASMYTH LIGHT-TOWERS
• 5.1.8 TIME BASE DISTRIBUTION
• 5.1.9 FE/BE INTERFACE
• 5.1.10 LO INTERFACE
• 5.1.11 SMTO TUCSON LAB
• 5.1.12 SMTO VEHICLES
• 6.0 CURRENT PROJECTS
• 6.1.1 RECEIVERS
• 6.1.2 SPECTROMETERS
• 6.1.3 SOFTWARE
• 7.0 SUMMER 1998 SHUTDOWN
• 7.1.1 ENCLOSURE WHEEL-BOXES
• 7.1.2 OPTICAL POINTING
• 7.1.3 REMOTE CONTROLLED TERTIARY MIRROR
• 7.1.4 LOFT COMPRESSOR
• 8.0 FUTURE SMTO PROJECTS
• 8.1.1 CLOSED-CYCLE RECEIVER PROJECT
• 8.1.2 UNIVERSAL COLD LOAD
• 8.1.3 UNIVERSAL IF SYSTEM
• 8.1.4 UNIVERSAL LOCAL OSCILLATOR
• 8.1.5 REMOTE CONTROLLED NASMYTH LIGHT-TOWERS
• 8.1.6 CAMAC REPLACEMENT
• 9.0 MISCELLANEOUS
• 9.1.1 DOCUMENTATION
• 9.1.2 WEB PAGE
• 9.1.3 ETHERNET LAN
• 9.1.4 MICROWAVE LINK
• 9.1.5 PUBLICATIONS
• 10.0 WEATHER STATISTICS
• 11.0 VISITORS AND OBSERVERS
• 12.0 CONCLUSION

1.0 INTRODUCTION

This is the second issue of our biannual electronically distributed Newsletter which describes recent activities at the Submillimeter Telescope Observatory (SMTO). This issue covers the period from August 1997 through to mid-April 1998.

The SMTO is a collaboration between the Max-Planck-Institute for Radioastronomy (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 recent 37 GHz holography with the LES-9 satellite, is currently set at under 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 Heterodyne Receiver Group, headed by R. Guesten. The latter group also supplied the acousto-optic spectrometers. The continuum receivers were developed by the MPIfR Bolometer Group, headed by E. Kreysa. Funding for these instruments is independent from the SMTO's budget and is provided either directly by the partner institutes or through external funding agencies.

In early June 1997, the telescope entered its yearly shut-down period. During the summer months numerous improvements were carried out, most notably being a very successful holography run in September which resulted in a significant improvement to the HHT's surface accuracy.

The SMTO began the 1997/1998 observing season with its facility bolometer system in place and the two lowest frequency heterodyne instruments (ie: SIS-230 and SIS-345) available for use by the astronomical community. The SIS-490 receiver was installed in January. The weather to date has been better than last season's with numerous and lengthy periods ideal for sub-millimeter observations. The telescope pointing proved to be adequate for most of this time. Although we continue to be plagued with occasional glitches in the telescope control system and the calibration of some spectral lines may be suspect, good scientific results of publishable quality are readily being achieved.

There have been several notable successes with Principal Investigator (PI) instruments at the HHT this season. The first occurred in February with the installation and first astronomical observations of the MPIfR 7-element bolometer array operating at 2 mm. The second was a Hot Electron Bolometer from the Harvard-Smithsonian Center for Astrophysics which was tested on the telescope in March. The first astronomical spectra ever measured with an HEB system were taken in the 690 and 810 GHz bands.

2.0 SMTO STAFF

On September 1^st, Dr. Tom Wilson (MPIfR-Bonn) took up duties as the new SMTO Director. The staff would like to offer our thanks to Buddy Powell (Associate Director of the Steward Observatory) who acted as interim Director for the last year and a half prior to Tom's arrival.

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 Supervisors (Dave Ashby, Doug Officer, and Bob Stupak) and one machinist (John 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 Bill Peters and includes Harold Butner and Dirk Muders, along with Paul Gensheimer (formerly MPIfR-Bonn) and Steve Platt (formerly University of Chicago), both of whom joined the SMTO in September. The third support group, the "Technical" unit, is managed by Bob Hayward and includes two technicians (Robert Esterline and Gene Holmberg) and Ferdinand Patt (formerly MPIfR-Bonn) who returned to the SMTO in October to take up the position of Cryogenic-Microwave engineer. Susan Lake is the Program Coordinator and is assisted by a part-time student, Karen Gilbert. In April Platt left the SMTO to join the Snow and Ice Research Group at the University of Nebraska. We wish him well in his future endeavors. The SMTO has already begun a search to fill the vacant FofT position.

The observatory was fully staffed for the first time in a long while and allowed us to provide both a Site Supervisor and Friend of the Telescope at the HHT on a 24-hour, 7-days a week basis. With the return of the 3 room "Engineer's Residence", we can now accommodate reasonably large teams of visiting observers as well as technical support teams from Tucson. Enthusiasm and morale remain high, and as solid experience is gained, 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 last year's Summer Shutdown and the first half of the current observing season. The month of October was quite hectic as the telescope was put back together following the holography run and numerous instrument packages from the partner institutes were delivered and commissioned in quick succession. These include the Four-Color Bolometer, the SIS-230 and SIS-345 facility heterodyne receivers, and the high-resolution AOS. The Texas Filter Bank Spectrometer was successfully resurrected and is now fully operational. The SIS-490 receiver returned to the HHT early in the new year. There have also been many small but vital improvements in the on-going effort to increase performance, reliability and usability of the system as a whole.

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 summer of 1997. Major projects which are currently underway are summarized, as are significant new initiatives planned for the future.

3.1 CONTINUUM RECEIVER SYSTEMS

3.1.1 FOUR-COLOR BOLOMETER : The MPIfR Four-Color Bolometer was shipped back to Bonn temporarily last summer to satisfy US import regulations. It returned to the HHT in the fall for re-commissioning. This bolometer is the system of choice for continuum observations and has 4 channels centered at 1300, 870, 450 and 350 microns. The changeover between channels typically takes less than 10 minutes. The weather during the commissioning run in mid-October started out surprisingly good (the opacity at 225 GHz was around 0.08), but unfortunately degraded by the time the entire system was ready and only the 1300 micron wavelength could be checked out on the sky. Observations of standards indicate the Noise Equivalent Flux Density (NEFD) at both 1300 and 870 microns is about 0.6-0.8 Jy/sec^(1/2). The SMTO staff continues to characterize the higher frequency bands as time and weather permit. There is some evidence that the optics may be under-illuminating the telescope since the measured FWHM beamwidths appear to be about 25% larger than expected. To date, the Four-Color has primarily been used at 1300 um for determining the telescope pointing model. However, early in the season it was used with the CYCLOPS polarimeter PI instrument (see below). It has also been used to look for dust around nearby main sequence stars (Butner) and it has been used at 870 microns for mapping nearby galaxies (Wielebinski et al). Other programs in the works include measurements on cold dark cores (Muders et al) and Orion cores (Wilson).

3.2 HETERODYNE RECEIVER SYSTEMS

3.2.1 SIS-230 : The SORAL 230 GHz single-polarization receiver was returned to the HHT in mid-October for re-commissioning. It underwent several modifications in the SORAL labs over the summer. Most significant was the replacement of its SIS junction in order to move the band of optimum performance from 230 up to 245 GHz. The receiver routinely achieves double-sideband receiver temperatures between 60-80 K across its 210-275 GHz tuning range. The main beam efficiency on Saturn was found to be about 77% and the beam was 35.3" in azimuth and 37.0" in elevation.

In general, the receiver has performed reliably and is relatively easy to tune. It still exhibits a tendency for its IF passband to become unstable when the mixer is tuned for optimum sensitivity, presumably due to resonances in the match between the tuned SIS junction and the HEMT amplifier. Unless care is taken, the HEMT amplifier can become marginally unstable and may even run into oscillation. In order to avoid instability problems, the SMTO staff recommends that the mixer be "detuned" slightly so that a reasonably flat IF passband is obtained (this is easily done by monitoring the "real-time" AOS video display). Strong peaks in the passband often result in poor spectrometer. This is particularly be true for Position Switched observations.

Recent tests at the telescope indicate that the SIS-230 exhibits unequal sideband response (at least at the CO(2-1) line). Line amplitudes in the upper sideband appear to be about 25% greater than those in the lower sideband. The Director recommends that observers should, where possible, scale their line measurements to a standard calibration spectra (eg: Orion KL) until the SMTO staff have the opportunity to characterize the receiver's sideband ratio more accurately.

The SIS-230 has seen use in numerous astronomical projects over the intervening months. The next major hurdle is to properly align the receiver so that its beam is exactly coincident with that of the SIS-490. This will allow dual-frequency observations can be carried out. The SMTO intends to grant the SIS-230 full acceptance as a facility instrument in its present condition.

3.2.2 SIS-345 : The MPIfR 345 GHz dual-polarization, semi-automated receiver was returned to Germany in the summer for the repair and upgrading of several of its sub-systems. The cold-leak which occur late in the 1996/97 observing season was successfully repaired by the firm CryoVac, although it unfortunately required the dewar to be largely disassembled. The optics package was modified with the addition of a rotating grid at the output of the quadrupler so that the LO power coupled into each mixer can now be individually adjusted. This feature allows the junction current in both mixers to be optimized to achieve good sensitivity in dual-channel mode. Finally, the cooled 1-2 GHz amplifiers were replaced with new HEMT's from Caltech. This was done in an attempt to address a resonance problem in the IF similar to that experienced with the SIS-230 receiver. In order to maximize the IF bandwidth, the original design for the SIS-345 did not utilize cooled L-Band isolators which, while providing 10 dB or more of isolation between the mixer and HEMT, would unfortunately reduce the IF bandwidth to less than 750 MHz. The hope was that the Caltech self-balanced HEMT's would achieve better isolation than the older standard HEMT's yet preserve the 1-2 GHz passband.

The SIS-345 returned to the HHT in late October. While using the MPIfR beam pattern measurement system to co-align the orthogonally polarized channels, the commissioning team discovered that the beams no longer had the same shape as had been measured back in Bonn. Rather than the canonical 11 dB edge taper, one beam had acquired a 25 dB illumination and the other about 8 dB. The dewar was opened up and the position of one of the cooled lenses was adjusted slightly. This plus additional tweaking of the mirrors external to the dewar resulted in a taper of about 12 x 16 dB for Channel 1 and 18 x 18 dB for Channel 2. Both channels achieved a T(DSB) of between 80-140K across the receiver's 320-375 GHz tuning range. Once the receiver was finally tested on the sky, beam efficiencies on Mars and Saturn of 45-50% were found for both beams. The beam size (El x Az) on Mars (4.5") for Channel 1 was 22.1" x 21.0" and for Channel 2 was 23.2" x 23.6". The beams were aligned on the sky to within 5 arcsec.

The IF passband associated with Channel 1 tends to have rather large ripples in the IF passband and can exhibit strong broad resonances at times, even with the new self-balanced Caltech HEMT's. Channel 2's tuning is usually more benign but both channels have occasionally demonstrated sharp spikes in the IF which are indicative of the HEMT's running into oscillation. Again, "detuning" the mixers to obtain improved baseline stability is recommended, albeit at a slight cost in sensitivity.

While the beam efficiencies seemed somewhat low, the most troublesome aspect of the receiver is that the line strengths can vary, depending on the sky frequency, by a factor as low as 0.5 and as high as 2. The receiver also exhibits unequal sideband response. There is also some evidence that the line-intensities seen in Channel 2 may vary with time. The SMTO staff are attempting to investigate these effects, as the observing schedule and weather permit, and to more fully characterize the calibration of the receiver. Tests are underway or planned to determine exactly how the line intensities are modulated across the tuning range and between the mixer sidebands. The results of these tests may help determine what is causing the line intensity variation and whether it is possibly due to some short path length reflection in the optics. In the mean time, the Director recommends that observers should, where possible, scale their line measurements to a standard calibration spectra (eg: Orion KL) until the SMTO staff are able to get a better handle on the receiver's calibration.

In spite of the problems with line calibration, the receiver has seen extensive use in numerous astronomical projects since October. The SMTO intends to grant the SIS-345 full acceptance as a facility instrument in its present condition.

3.2.3 SIS-490 : The SORAL 490 GHz single-channel receiver was installed at the HHT in early January 1998 after undergoing several improvements back in the Tucson labs. A new bias box has been provided for the frequency multiplier. The mixer has received a new SIS junction with a different type of tuning stub design to improve the receiver's sensitivity at the upper end of the band. 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). The fluorogold IR blocking filter was replaced with one made of Zytex allowing lower RF losses and improved thermal capacity to be obtained.

The receiver now achieves a T(DSB) of 130-180 K across its 435 to 498 GHz tuning range. On Saturn, the beam was found to be 16.5" in both elevation and azimuth. The main beam efficiency on Saturn was about 45%. The forward scattering efficiency on moon was measured at 72%.

The SIS-490 can be difficult to tune for the uninitiated, in particular its x6 frequency multiplier. Although the device was returned to its manufacturer, RPG Physics, to have its 4 backshort tuners upgraded with improved micrometers which are somewhat more repeatable, the SMTO staff still recommends the receiver be tuned in an iterative pattern. While monitoring the mixer current, the multiplier and mixer - 6 micrometers in all - must all be adjusted in concert. This maneuver, along with the complicated power up/down procedure required in order to avoid damaging the multiplier, has not been easy for the faint of heart. However, the SMTO staff has acquired much more familiarity and training with the receiver than was the case last year.

One interesting feature of the optical design of this receiver is that it incorporates a low-loss polarizing grid that allows for observations in the 230 and 490 GHz bands to be taken simultaneously. The first CO(2-1) and CO(4-3) dual-frequency measurements were made in late January. It was found that the beams were off by about 35". This angular separation is obviously too large to allow meaningful 230/490 GHz observations to be carried out. The SMTO staff will attempt to address this problem by adjusting the optical path of the receivers to co-align the beams.

There has only been a limited opportunity to use the SIS-490 in the short time since its delivery, but it seems to be performing reliably. The SMTO intends to grant the receiver full acceptance as a facility instrument.

3.3 SPECTROMETER SYSTEMS

3.3.1 AOS-A/B (1 GHz) : These 1 GHz AOS's were delivered to the telescope in January 1997 by the MPIfR after a major upgrade which significantly improved their overall system stability allowing long integrations on weak sources to be much more readily achieved. The new water-cooled chiller and temperature-controlled rack containing the optical packages has worked reliably and have eliminated the effects from the large temperature variations which typically occur in the Computer Room where the spectrometers are located. Stability tests at the telescope have demonstrated Allen Variance times on the order of 500 seconds (remember that the stability of the sky at sub- mm wavelengths is usually on the order of several tens of seconds). The RMS noise has been found to integrate down with the square root of time for at least 10,000 seconds. In order to reduce confusion, the spectrometers have been renamed, with the former AOS-3 now referred to as AOS-A and AOS-4 now designated AOS-B. The effective channel resolution widths are 1.021 and 0.996 MHz respectively.

3.3.2 AOS-C (250 MHz) : The 250 MHz High-Resolution Spectrometer had been sent back to Bonn at the end of the 1995/96 season for further upgrades. It returned to the HHT in October 1997 and with the exception a semi-rigid cable in the optics package which was damaged in shipment and the replacement of its controller firmware with the most up-to-date version, the re-commissioning went well. The instrument, once it reached a thermalized state in the temperature- controlled rack, achieved an Allen Variance time of about 250 seconds. The resolution bandwidth of each channel is approximately 400 KHz. It has been found that the frequency of the center channel of the Hi-Res AOS tends to drift with time more so than the wideband AOS's do. The SMTO staff continues to perform regular monitoring tests with all three units to determine the correct value of the center channel.

3.3.3 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 (for bandwidths of 256, 64 and 8 MHz respectively). Its narrow-band mode 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. John Hughes of the SO Technical Division was responsible for this part of the project. The micro also contains the first VME board 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 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. It also provides a computer-controlled noise source and comb-line test signal. After overcoming subtle timing jitter problems with the VxWorks multi-tasking operating system and glitches in the channel data arising from the Greenstreet analog-to-digital converter, the FBS has been used routinely for astronomical observations since November. The line strengths between all three filter banks and the AOS's are in good agreement in both Position and Wobble switched modes. Allen Variance times of about 100 seconds seem to be achievable. Improvements in both its thermal and mechanical stability were realized by moving the instrument from the Right Receiver Room to the Computer Room. The occurrence of "dead" channels has declined as the problem of poor electrical contacts with boards and discrete components has been addressed.

3.3.4 SPECTROMETER STABILITY : In general, all four of the SMTO facility spectrometers appear to be performing well. There have been occasional incidences of poor baselines but these tend to be associated with the unstable IF passbands which the SIS-230 and SIS-345 receivers sometimes exhibit when their mixers are tuned for optimum sensitivity. Under these circumstances, the baselines of Position Switched spectra will be particularly degraded. Further improvements are planned by the SMTO staff to eliminate or reduce unwanted baseline structure beyond that arising from possible instability or non-linearity within the spectrometers. These include improvements to the IF chain to ensure against amplifier compression (which can lead not only to incomplete baseline subtraction but modulated line intensities as well). High-quality, flex-stable microwave cable assemblies (costing about $400 each) will be used for making the jump between the nasmyth platform and building. The relative movement between the telescope and the enclosure, and the bending this causes to the IF cables, likely has an impact on baseline stability, especially during position switched observations. The hope is to end up with a spectral line system where the dominant source of instability is from fluctuations in the atmosphere itself.

3.4 FRONT-END / BACKEND OBSERVING MODES

A new Frontend/Backend (FE/BE) interface system has been installed at the HHT which allows the observer to easily switch between the various facility receivers and spectrometers without having to manually re-cable any of the IF signals, as has been the case in the past. The selection of the appropriate IF channel from the desired receiver frontend is now under control of the Station Computer. Two 6-position microwave switches are used to select which of the 1-2 GHz IF signals from the various SIS mixers will be fed to the dual- channel IF Total Power box, translated up to 3-4 GHz and then distributed to the Acousto-Optic and Filter Bank spectrometers. These two IF channels, known as "IF-A" and "IF-B", are independently controlled, allowing a mix and match selection of FE's. The following table indicates the various operating modes which are possible:

   --------------------------------------------- 
  |  Front-end  |     IF-A      |     IF-B      | 
  |-------------+---------------+---------------| 
  |     Rx-1    |  Test Source  |  Test Source  | 
  |     Rx-2    |    SIS-230    |       -       | 
  |     Rx-3    |   SIS-345/1   |   SIS-345/2   | 
  |     Rx-4    |       -       |    SIS-490    | 
  |     Rx-5    |    Spare #1   |    Spare #1   | 
  |     Rx-6    |    Spare #2   |    Spare #2   | 
   ---------------------------------------------

Standard observing modes, for example, would include the SIS-345 receiver with its Channel-1 output on IF-A and Channel-2 on IF-B. When switching over to the SIS-230 frontend, its single-polarization output would be available on IF-A. The SIS-490's output, on the other hand, is obtained on IF-B, thus accommodating 230+490 dual-frequency observations. This new system is expandable and could handle any new single or dual-polarization receivers (with 1-2 GHz IF's). The built- in Test Source mode utilizes a noise diode and is available for performing IF system tests (ie: power leveling, dynamic range, linearity, etc). Note that as we migrate to receivers with IF frequencies other than 1-2 GHz in the future, we will require a more complicated IF switch box design.

The new FE/BE interface also extends to the selection of the facility spectrometers. They are now configured so that each IF channel can be processed by a wide-band spectrometer as well as a narrow-resolution backend at the same time. We normally operate with IF-A feeding AOS-A and FBS in parallel and with IF-B feeding both AOS-B and AOS-C. This spectrometer pair combination gives each IF channel a 1 GHz bank (ie: either AOS-A or B) as well as a narrow resolution bank (ie: either the FBS or AOS-C). This leads to the following standard receiver configuration modes:

       IF-A  = SIS-230 or 345/1    ->     AOS-A + FBS 

       IF-B  = SIS-490 or 345/2    ->     AOS-B + AOS-C

For added flexibility, a 3-4 GHz switch matrix (under manual control, alas) can be used to direct the IF signals to the desired wideband/high-resolution spectrometer pair as shown below:

   ------------------------------------------------- 
  |  Front-end Mode  | AOS-A + FBS  | AOS-B + AOS-C | 
  |------------------+--------------+---------------| 
  |  Dual - Standard |     IF-A     |     IF-B      | 
  |  Dual - Swapped  |     IF-B     |     IF-A      | 
  |  IF-A Only       |     IF-A     |     IF-A      | 
  |  IF-B Only       |     IF-B     |     IF-B      | 
   -------------------------------------------------

This scheme allows us to swap the spectrometer pairs (for example, should one of the units fail) but also allows all 4 backends to be fed with the same signal (useful for test purposes or for operating the high resolution FBS and AOS-C spectrometers in parallel).

Going hand-in-hand with the FE switch, is the automated control of the two HP8671B frequency synthesizers which are used to provide the 8-9 GHz reference for locking the appropriate Gunn oscillator in the LO chains of the various receivers. A microwave switch controlled by the Station Computer is used to select whether the output of Synthesizer #1 goes to SIS-230 or SIS-345. In order to accommodate dual-frequency observations, the reference from Synthesizer #2 goes to the SIS-490.

The end result of all of the above modifications is that the switch over between SIS receivers can now take place rapidly. Under normal observing, no alteration in any of the IF or LO cables is required. If the receivers have been pre-tuned, the only manual adjustment required is the movement of the nasmyth light-tower. And this has now been made much easier with the addition of a linear encoder, which allows the position of the pick-off mirrors to be set accurately.

3.5 PRINCIPAL INVESTIGATOR INSTRUMENTS

3.5.1 CYCLOPS : This PI polarimeter was built at SORAL as part of J. Glenn's PhD dissertation and was specifically designed to investigate the role of magnetic fields in star formation by measuring the magnetic field line geometries in molecular clouds. It uses a waveplate made of Rexalite inserted in the beam of a wideband continuum detector along with a PC-based signal processing system to acquire and log the data. Previous observation runs at the SMT have determined that the instrumental polarization from telescope is less than 0.2% and appears to be elevation independent. In early November, Cyclops returned to the HHT and was used with the 1300 micron channel of the facility's Four-Color bolometer. Although almost half the allocated time was lost to weather or telescope control problems, all the objects on the original target list were each observed down to the desired rms noise level.

3.5.2 MPIfR 2 mm BOLOMETER ARRAY : This 7-element multi-beam bolometer was installed on the telescope in February. It was developed by E. Kreysa and his colleagues from Bonn. Using a 100 mK dilution fridge, this highly-sensitive array actually has 19 hexagonally packed elements but due to the size of the HHT's elevation bearing, it is limited to about 7 pixels at a wavelength of 2 mm. This relatively low frequency was chosen so that the beam of the HHT would be uniquely matched to the sizes of galactic clusters in which the Sunyaev-Zeldovich effect is present. It is hoped that such measurements could lead to a new estimate of the Hubble constant. The array is also an ideal instrument for mapping dust emission from extended molecular clouds. Its installation on the telescope went smoothly, and while the weather in Arizona was not exactly stellar during its 3 week observing period, many successful engineering tests were carried out as were a limited number of astronomical projects. At the end of the run, the dilution fridge and dewar were stored at the HHT and the feedhorn/detector assembly was returned to Bonn. It is expected that it will be scheduled for a second observing session on the HHT at the start of the coming 1998/99 season.

3.5.3 MPIfR 810 GHz SIS RECEIVER : During the last week of the observing run in February with the 2 mm Bolometer Array, the MPIfR 810 GHz prototype receiver was brought to the HHT for its second visit (it was last at the telescope in January 1997). This PI instrument, developed by F. Schaefer, utilizes a double-slot open- structure mixer and achieves a double sideband receiver temperature of less than 900 K. The mixer is cooled by a LHe wet dewar. The receiver was mounted on the Left Nasmyth platform in the location normally reserved for the SIS-230 receiver. The pick-off mirror on the light-tower for the SIS-490 receiver also had to be removed. The numerous storms experienced in the Southwest throughout February resulted in poor weather during its short time (5 days) on the telescope. There were only a few periods where the Tau(225 GHz) dropped below 0.06 and the sky tended to be quite variable. Complicating the problem was the fact that all of the planets except Venus were too close to the Sun to be observed. Not helping the situation either was the fact that the pointing model for the left side was almost a month old, which can be a disaster for a receiver with a 9 arcsec beam. It was thus extremely difficult to get a successful pointing and focus. The situation, however, generated useful discussions on the need to implement better alignment between the mechanical and radio axes.

3.5.4 CfA HOT ELECTRON BOLOMETER RECEIVER : The entire 3 week Steward Observatory period during March was primarily devoted to telescope tests of a unique outside PI instrument. The Harvard- Smithsonian Center for Astrophysics (CfA) installed a phonon-cooled niobium nitride Hot Electron Bolometer (HEB) on the HHT. Superconducting HEB mixers may well become the preferred alternative to SIS systems, especially for the range near 1 Tera-Hertz and above. This particular heterodyne mixer was developed at the CfA Submillimeter Receiver Lab (headed by R. Blundell) and is designed to operate at both 690 GHz and 810 GHz. The CfA HEB system was mounted on the Right Nasmyth using a new receiver platform designed by Patt, who also visited the CfA group in Cambridge the month before to discuss the plans for installation in detail. The receiver uses a LHe wet dewar and has two independent LO chains covering the 690 and 810 GHz bands with a Martin-Puplett interferometer for injecting the LO signal. The LO's can be easily swapped and the mixer itself is tunerless so changing frequencies can usually be done quite quickly. It achieves a double sideband receiver temperature of better than 1200 K at 800 GHz. Unlike an SIS mixer, HEB's are not cursed with Josephson currents and, accordingly, do not require a magnetic field suppression circuit. The one limitation of this particular mixer was its relatively narrow 500 MHz IF bandwidth. Other than discovering that the 690 GHz multiplier had died in shipment (a replacement spare was promptly sent out from Cambridge, MA) and eliminating a few ground loops, the installation went smoothly. Although this run took place only a few weeks after the attempt with the MPIfR 810 GHz receiver had been essentially weathered out, the CfA group fortunately had better luck. Successful detections towards several sources were made at the CO (7-6) line at 807 GHz and the CO(6-5) line at 691 GHz, as well as the CI(2-1) at 809 GHz. These, we believe, are the first astronomical spectra ever reported with an HEB system on a telescope. They are also the first spectral line observations in the 690 GHz window taken at the HHT (the first 800 GHz heterodyne observations occurred back in the spring of 1996 with the University of Cologne KOSMA receiver by Stutzki et al). The SMTO Director has been negotiating with the CfA for the return of an improved version of their HEB system early in 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 continues to be one of the SMTO's top priorities. By and large the pointing has been much more stable this year than last when errors of roughly 5 arcsec were typical. This season, after a new pointing model is established, the pointing is good to about 3 arcsec all over the sky and would usually remain at that level for a couple of weeks and then start to degrade to about 5 arcsec. This improvement may well be due to the influence of the new weather station which now provides more accurate data for the refraction calculation. Of more importance, there is evidence that the wandering of the azimuth tilt has decreased significantly from last year. Unfortunately, since we don't know what is causing this effect in the first place, we really can't say why it should be less this season. Occasionally there are instances of poorer than usual pointing, the cause of which is often found to be some subtle fault in the control system. For instance, a recent instance of large pointing offsets on the order of 10 arcsecs or so was eventually traced to be cable problem which resulted in the signal from the azimuth encoder being severely clipped.

3.6.2 INCLINOMETER MEASUREMENTS : Last year it was found that the pointing model would unfailingly erode over a time period of about a week. The largest contribution seemed to come from 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 and have used inclinometers to characterize the effect by performing tilt versus azimuth measurements and then modifying the pointing model accordingly. To this end, the SMTO staff began a systematic program of tilt measurements using four Schaevitz inclinometers (donated by the Herzberg Institute of Astrophysics, Canada) which were mounted at 0, 90, 180 & 270 degrees (ie: double qradrature). The results were open to interpretation, however, as there seemed to be a component of the tilt vs. azimuth dependence due to the Earth's magnetic field, perhaps due to field lines warping through the elevation fork and causing spurious readings. A new inclinometer, a Model 701 dual-axis "tiltmeter" from Applied Geomechanics ($2.5K), was purchased and installed in October. These devices are not as susceptible to magnetic fields as the Schaevitz units.

The tiltmeter along with the 4 older inclinometers, have allowed us to make comprehensive measurements of the HHT's tilt. Unfortunately, having five times as much data does not necessarily make the problem five times clearer. However, the on-going analysis of the data by Peters has uncovered an artifact in the elevation encoder readings. It is a sinusoid of amplitude 1.5 arcsec and an argument of 2*azimuth. It is probably due to a movement of the fork prongs, one forward and one backward as the antenna rotates in azimuth. These tilts cancel each other at the antenna, but the elevation encoder reacts to the tilt of its tine. The antenna drive then zeros it out, thus introducing a pointing error. The effect has been stable for about a month. We will soon attempt to add a compensating term in the pointing model equation. This is bound to improve the pointing, but will unlikely be sufficient for achieving our goal of one arcsec pointing.

Additional analysis of the data indicates that the one-theta variation measured by the inclinometers does not agree exactly with that deduced from the fit found from an astronomical pointing run. The disagreement between the two measurements is on the level of a few arcsecs and the difference vector systematically points to the northwest. It is likely that there is some other effect in the telescope that is not being properly addressed in the fit and that the current pointing model is absorbing the variation in these terms, thus affecting the overall pointing error. One strategy is to use the inclinometer measurements only as a relative measurement. The tilt terms in the pointing model would then be modified according to how much the tilt of the azimuth axis has changed since the last pointing measurement. This strategy may be adopted pending further study.

3.6.3 REFRACTION CORRECTION : The calculation for determining the refraction correction has been a known source of error because of the inaccurate (and sometimes unreliable) data from the observatory's venerable Weather Station. Muders replaced the Heathkit unit in September with a much more precise unit made by Vaisala ($5K). The Station Computer now polls the new weather station via a serial link to obtain temperature, pressure and relative humidity readings which are incorporated into the pointing model.

3.6.4 Satellite Holography : Directly following the 1997 Summer Shutdown, the SMTO carried out a successful satellite holography run. Unfortunately LES-8, the satellite which was used to measure the surface accuracy of the telescope in previous holography experiments, died in 1995. NRAO-Tucson once again kindly loaned us their 38 GHz holography receiver. It was resurrected by Hayward and modified for use with the LES-9 satellite. This included the purchase and installation of a new 37.5 GHz Gunn oscillator ($2K).

The LES-9 satellite, operated by the MIT Lincoln Laboratory for the US Air Force, is normally used for communications, most notably these days with research stations at the South Pole. It has a relatively high-power, high-frequency transmitter at 37 GHz which makes it an attractive beacon for holography experiments. The electrical power capacity of the satellite, originally launched in 1976, has degraded so much that when the 37 GHz transmitter is activated, the satellite cannot be used for anything else. Fortunately the Air Force still agreed to let us use it for 6 hours a day over a contiguous period of 24 days between September 15th through October 9th. To minimize the inconvenience to the other users of the satellite, the beacon was switched on during the same six hour period each night. This meant that the satellite was always roughly at the same elevation (about 60 degrees). During our last holography run in 1995 with the LES-8 satellite, we were able to make measurements as low as 35 degrees, which allowed more information to be gathered on the telescope's gravitational deformation versus elevation. However, as this part of the LES-9's orbit would coincide with the time that LES-9 was above the horizon at the South Pole and required for their link back to civilization, this part of the experiment was no longer possible.

The HHT's sub-reflector was removed in the middle of September and the Holography Receiver was mounted at the prime focus. The receiver is a dual-channel instrument, with a phase-reference channel looking directly at the satellite with a wide beam-width lens/horn combination. The other channel is illuminated by the primary reflector. The two signals are mixed down to 10 KHz and fed into a custom-built digital signal processor (DSP), where a FFT is performed to yield a narrow filter passband of 500 Hz bandwidth followed by a cross-multiplication of the reference and signal channels in the frequency domain. The NRAO holography software package was adapted for use at the SMT. It generates the 2-D FFT map of the phase distribution across the antenna aperture. Maps of various sizes were made during the run (eg: 33x33, 65x65, 129x129, 161x161) but we tended to concentrate more on maps of 65x65 points as they provided sufficient panel resolution (about 20 cm) yet allowed us to achieve two complete maps per night which gave us a better handle on their repeatability. Peters was responsible for the majority of the data reduction and analysis.

Our first measurements found that two panels were 100-200 microns out of position. The location of these panels was not surprising as they were nearest the two struts that were damaged in separate accidents during 1996 when the jib crane collided with the backup structure. The inner ring, however, appeared to still be well aligned after 2.5 years. Some of the outer panels, however, had moved. Whether they were disturbed during the earlier accidents or whether they gradually crept out of alignment since the last holographic experiment, perhaps due to the wobbler's vibration, is uncertain.

Early in the run, we would adjust only the worst one or two panels and then make a new map. Later we were setting dozens of panel adjusters at a time. At the end of the run we had realized a surface which is significantly better than the figure we had in 1995. At that time it was determined that every panel of the telescope's 3 rings was positioned to better than 40 micrometers, with an average of about 25 microns rms over the entire dish. We now are getting between 15 and 20 microns rms overall. The outer ring is still the worst, but is much better than in 1995. It is the hardest to measure, since the illumination of the antenna is lower here, and therefore the signal to noise ratio is at its worst as well. We are able to do much better than was the case in 1995 because a data contamination problem in the holography backend was solved (by Andy Dowd, shortly before he left to join the SMA) which caused crosstalk (at about the 1% level) between the dish and reference channels. The problem was actually corrected a few months after the holography measurements in 1995, but by then the transmitter in the LES-8 satellite had failed.

By the end of the holography run we had reached our repeatability limit. In order to better understand this, we stopped moving the panels a few days before the end of the run and simply repeated maps one after the other. Some maps have 15 micron rms deviations and others have 20 microns rms. Some of the repeatability is not random, and is undoubtedly partly artifacts in the data set which might be removed by better analysis or editing out spurious data. Averaging the last series of maps, we can say that the surface is set to between 16 and 19 microns rms. This is significantly better than in 1995 and is close to the 15 microns specified in the pre-construction error budget for panel alignment. The panels themselves, of course, should be better than 10 microns rms. The SMTO Web Site contains a summary of the holography run along with numerous color maps of the surface accuracy.

We are hoping to repeat these measurements, in September 1998 if possible, to see whether the panels creep with time. The condition of the LES-9 satellite is of major concern. Lincoln Labs are not sure if it will have enough power to operate the 37 GHz transmitter much farther in the future. Assuming our request for satellite time is approved again, we anticipate the holography run will require close to one month of telescope time, with the following schedule: 3 days to remove the subreflector and mount the holography receiver; two to three weeks of holography and surface adjustment, 3-5 days to reinstall and realign the subreflector (including the tedious horizontal collimation procedure).

The SMTO, like many other radio observatories, would like to find another satellite that could be used for holography measurements but we have yet to identify an alternative. What is needed is a satellite that transmits at a frequency above 20 GHz and which is in a high orbit so that it stays above the horizon for the 5 hours it takes to do a measurement.

3.7 BUILDING AND ENCLOSURE

3.7.1 ACCOMMODATIONS : One significant problem during the past observing season was the chronic shortage of bedroom space at the observatory. This has finally been solved, we hope, with the return of the Engineer's Residence to the site in November. The Forest Service finally agreed to allow us to park it beside the telescope and have issued us a permit which must be renewed on a yearly basis. The MGIO staff mounted the residence on a trailer in case it may have to be removed during the summer months. Be that as it may, its three additional bedrooms, coupled with the telescope's five units, have, by and large, satisfied this season's accommodation requirements. It has allowed us to find room for providing full-time coverage by both a Site Manager and a Friend of the Telescope, along with the necessary technical staff from Tucson when needed, plus enough space for visiting observing/commissioning teams.

3.7.2 WORK-SPACE : The expansion of the Electronics Lab on the 2nd floor at the site is now complete. This was achieved by extending the room into the space occupied by the Library/Office. As well as providing more work-bench area, it has been fitted out (thanks largely to the efforts of Esterline) with an excellent supply of electronic components, nuts, bolts, hand-tools, etc.

In the Right Receiver room on the 4th floor, most of the SMTO's microwave test instruments and RF components have been consolidated into one area. A bench has been equipped exclusively for RF work. This complements the bench in the Left Receiver room which is set-up for cryogenic/vacuum work.

4.0 TELESCOPE REPAIR AND MAINTENANCE

4.1.1 CAMAC SYSTEM : The CAMAC control system continues to challenge the SMTO staff with numerous subtle faults. There have been some successes, however. Holmberg was able to repair the spare Kinetic Systems Highway Driver in August. This unit provides the interface between the Station Computer (ie: the VAX referred to as "Kronen") and all of the CAMAC crates. With no functioning spare, the telescope was at serious risk of a single-point failure. While Kinetic Systems was able to successfully upgrade the prime unit from a UNIBUS to a Q-Bus configuration when the old VAX 11/750 was replaced with a VAX Workstation, they were never able to get the spare to function properly. It seems this system is so venerable that the company no longer retains the corporate memory to fix it, the only engineer who was intimately familiar with the system having long since retired. The fix was to blow a new set of programmable gate arrays (PAL's) for the spare. As surprising as it may seem, the company did not have a copy of the version used in the prime unit. They did provide us with enough blank Philips gate array IC's for us to make a new set which proved effective, much to our relief.

While there are functioning spares for most CAMAC cards, the MULI spare has never worked properly. This card is critical as it generates the 250 ms interrupt for the Station Computer. Should the prime card fail, the antenna drive software would grind to a halt. One of the spare's problems was that it insisted on constantly asserting its "Look At Me" status bit. This fault has been fixed by Holmberg but its most serious problem is that it is missing an outdated gate array chip which was apparently so obsolete that it wasn't even provided with the spare card when it was delivered to the HHT. (If anyone reading this knows of a source of 512 x 4-bit PROM's (ie: MMI 6306, Farchild 93446, or National 74S570, please get in touch with us). Our only alternative is to re-engineer the original design with a more modern device.

There are other annoying foibles exhibited by CAMAC but they have occurred with less repetition and reduced severity compared to last year. Quite often the fault magically vanishes during the course of investigating the problem. The most serious has been the Antenna Elevation and Azimuth microcomputers (essentially PDP-11's on a CAMAC card). These micro errors seem to go away for a while when we replace a card or a cable, but invariably tend to come back. Peters has managed to "paper over" many of their effects on the control system in software but its final cure will likely elude us until we have a good opportunity to dig deeper into the system. The fear is that since we don't know the ultimate source of the faults, tearing things apart in an attempt to find it might cause things to get worse rather than better. This is something that would likely be unwelcomed in the middle of an observing season.

A similar intermittent problem occasionally occurs with the AOS Backend Controllers. These have been cured by merely reseating the CAMAC cards. On the plus side, the CAMAC power supplies which had given us so much grief last year, have been well-behaved and reliable (so far) this season.

Keeping a sufficient supply of operational spares has been a difficult but critical goal of the SMTO staff. 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 an Ethernet link.

4.1.2 MISCELLANEOUS FAULTS : A large transformer coil in the Uninterruptible Power Supply (UPS) burnt itself out in October and was replaced by Esterline. The UPS was off line for a few days, but the rest of the telescope seemed to survive life without conditioned power for a short time..

One of the two stepper motors in the Focus Translation stage died in June, likely due to being rained on accidently and was replaced by Esterline in September. The commercial motor controller unit also failed and was replaced by a new unit.

A bug in the HOST1 computer which caused the IF attenuators in the MPIfR Total Power box to drop to 0 dB momentarily whenever they were adjusted to a new setting was fixed by Muders. The defective 11 dB HP attenuator unit in the AOS-B IF unit was repaired by Hayward in January

The Adret 5104 frequency synthesizer which provides the computer controlled 90-120 MHz reference for the phase-lock units in the various SIS receivers began to fail intermittently in December. Hayward prepared an interface which would allow the SMTO's Fluke 6160B DC-160 MHz synthesizer to be substituted as a backup.

5.0 COMPLETED SMTO PROJECTS

The SMTO staff continued to upgrade many of the HHT's software and hardware sub-systems. As well as numerous small improvements, several important new features have been incorporated.

5.1.1 LINE POINTING : Muders incorporated line pointing, starting with the position-switched procedure in October followed by a wobble-switched mode in December. Line pointing significantly expands the number of pointing sources now available for use by the HHT's heterodyne receivers. The fit results can now be automatically applied with the CORRECTIONS command just as with continuum pointing.

5.1.2 AUTOMATIC CALIBRATION : The Ambient Load vane at the apex can now be operated under computer control for performing automated load calibrations. At sub-mm wavelengths, doing periodic calibration scans is a necessity, especially under variable weather conditions. In October, Peters provided the new software procedure to control the Ambient Load and compute the channel-by-channel system temperature for the backend spectrometers. Holmberg and Ashby built the hardware interface. The physical temperature of the ambient load is also measured with 5 temperature sensors to determine its average physical temperature and thus improve the T(Sys) accuracy. A further improvement was realized by the addition of a remote control "CALIB" button in the Left Receiver room to facilitate Cold Load measurements so that it no longer requires two observers to carry out a cold calibration.

5.1.3 DATA ACQUISITION AND ANALYSIS PROCEDURES : Many new improvements to the data handling and analysis procedures have also been added. Muders implemented remote control of the Dark Current & Comb Line modes in all 3 AOS's. Peters modified the on-line display to show all the calibration sub-scans (ie: hot, cold and sky) for each backend spectrometer. Peters also modified the control software to handle multiple receiver frontends. The CHEF program now calculates T(Rec) separately for each backend.

5.1.4 TELESCOPE CONTROL PANEL : The process begun in the summer of 1996 to totally rebuild and repackage the original Control Desk was one step closer to completion when a new Telescope Control Panel was installed in October by Holmberg. Since it uses LED based buttons and status indicators, we will no longer have to contend with the nuisance of dead light bulbs.

5.1.5 INTERLOCK CONTROL AND STATUS MICRO : The month of December saw another upgrade of the Telescope/Building control system. A new microcomputer system was installed to monitor the numerous interlocks involved in the opening and closing of the enclosure's roof and doors. This 8051-based single-board computer (SBC), designed and programmed by Ashby, is far more capable than the old Steward Observatory "Standard Micro" which it replaced. It also eliminates the over-whelming array of hard-to-read status indicators on the original Console Rack with a pair of alphanumeric LCD displays. It also provides the manual command switches for the Enclosure "Open/Close" and Ambient Vane "Sky/Hot" control functions. It is interfaced to the Station Computer via a serial link, thus allowing telescope control software to track the status of the myriad of interlocks within the system. There are plans to have the ICS monitor additional interlocks and hydraulic sensors in the future. Since the new micro is easy to program, has an excellent selection of on-board digital, analog and serial capability, and is easily interfaced to "daughter" I/O boards for expansion purposes, it will likely see use in many new SMTO projects as well as in upgrade replacements of older systems.

5.1.6 ELEVATION INTERLOCK : The final elevation limit on the telescope was changed from 89 to 91 degrees in October by Peters, Stupak and Casas after determining what the maximum limits for safe operation were. This enabled the elevation pre-limit to be moved from 87 to 89 degrees which in turn allows one to observe objects lying within this elevation range for the first time.

5.1.7 NASMYTH LIGHT-TOWERS : A linear encoder has been mounted on the Left Nasmyth Light-Tower by Patt which allows the position of the pick-off mirrors to be set with much higher accuracy. Esterline has modified the associated electronic drive units for both the Left and Right Light-Tower adjustment motors for 120 VAC operation and the units have been relocated closer to the receiver platform for ease of operation.

5.1.8 TIME BASE DISTRIBUTION : A new distribution system for the GPS Time Base was installed by Hayward in December. The Station 10 MHz reference signal is now available in the Computer Room, Control Room, and Left and Right Receiver Rooms, with each location having 4 spigots available with an output level of about 0 dBm. This has allowed us to lock the two HP8671B frequency generators (used for generating the microwave reference for the LO's in the SIS Receivers) to the Station Standard. The Adret synthesizer used for the 90-120 MHz PLL reference has been modified to accept an external 10 MHz input (rather than 5 MHz). We eventually intend to phase-lock all of the transfer oscillators used in the IF chain. While the IF Processor used in the Filter Bank Spectrometer already meets this specification, the transfer oscillators used in the 1-2 GHz to 3-4 GHz up-converters are free-running and will have to be replaced (see section 8.1.3). These 5 GHz dielectric resonance oscillators are not terribly accurate (one unit is 700 KHz low) and tend to drift somewhat (on the order of 25 KHz), thus causing slight offset frequency errors in spectral observations.

5.1.9 FE/BE INTERFACE : In order to carry out the needed modifications to implement the new IF interface, the MPIfR Reference Switching box was modified by Hayward to allow the Station Computer to select which Frontend would be fed to the spectrometer backends. This box had the dual-channel, 6-position microwave switches which were needed. It allowed the 1-2 GHz IF's from the SIS receivers (ie: SIS-230, 345 and 490 plus a noise source and several spare channels) to be selected by the VAX through the HOST4 microcomputer. A 1-2 GHz amplifier was added to each IF channel in the box to ensure adequate gain. The dual-channel MPIfR Total Power box is now used to control the level and monitor the IF power for whatever receiver channel has been chosen. The MPIfR 3-4 GHz switch box was altered (its 220 VAC power supply was replaced with a 120 VAC unit) and was relocated in the Computer Room to provide the Backend interface switch function. Prior to this new arrangement, the IF system was running dangerously close to the 1 dB compression point - only about 6 dB down. The dynamic range of the new system has been improved by at least 6 dB.

5.1.10 LO INTERFACE : The MPIfR Reference Switching unit was further modified by Hayward to perform the switching of the microwave reference from HP Synthesizer #1 between the SIS-230 and SIS-345 receivers. HP Synthesizer #2 is dedicated to providing the reference to the SIS-490, thus enabling simultaneous 230/490 GHz observations. A new RF switch was added to allow the selection of which PLL IF Monitor from the various phase-lock boxes (ie: SIS-230, 345, 490, plus a spare) should be displayed on the spectrum analyzers in the Left Receiver Room and Control Room.

5.1.11 SMTO TUCSON LAB : The electronic and cryogenic work-space at the SMTO Tucson Labs has been totally revamped, largely through the efforts of Esterline. Much old and unneeded equipment was surplused allowing the lab to be more efficiently arranged for work benches.

5.1.12 SMTO VEHICLES : The new 1998 Chevy pickup truck was delivered in December. This vehicle had a lift-gate unit installed to facilitate the transport of cryogens.

6.0 CURRENT PROJECTS

Numerous small improvements to the HHT's receiver systems are planned over the coming months. With astronomical observing going on essentially 24 hours a day, it is often difficult to carry out many tasks which might impact on the visiting observer. Accordingly many tasks can be carried out only as time and scheduling permit.

6.1.1 RECEIVERS : We hope to install high-quality, flex-stable microwave cable assemblies between the nasmyth platform and the building in order to reduce the effects of variations in the IF passbands due to mechanical movement. An improved interface for monitoring the mixer voltage and bias currents while tuning the SORAL 230 and 490 GHz receivers is planned. This will largely eliminate the need for a dedicated X-Y plotter on the Left platform. Dedicated coax lines will be run from the left and right Receiver Rooms into the Cassegrain Cabin to facilitate beam alignment procedures.

6.1.2 SPECTROMETERS : Temporary switch boxes were installed by Hayward in the Left Receiver Room and in the Computer Room for displaying the desired "real-time" AOS video bandpass on a rack mounted digital scope to aid in receiver tuning. These will be improved in the coming months. The unit located in the Receiver Room will be modified for displaying SIS mixer I-V and Total Power curves while the scope in the Control Room will be used to monitor the Subreflector LVDT, (ie: chopper position), Sync and Blank signals.

6.1.3 SOFTWARE : An effort to shorten the time it takes to begin a scan will be attempted by speeding up the log file handling procedure. Because of the large number of changes implemented recently, portions of the on-line HELP command descriptions are no longer valid. These will be brought up-to-date as quickly as possible.

7.0 SUMMER 1998 SHUTDOWN

As has been the case during previous summers, several major tasks are planned for the up-coming shutdown, the most notable of which are described below:

7.1.1 ENCLOSURE WHEEL-BOXES : Due to recurring problems with the building wheel-boxes, the Steward Observatory has redesigned these units with the goal of strengthening the bearing assembly. Two of the four wheel-boxes (those directly under the enclosure doors) will be replaced during the Summer Shutdown. In preparation for their installation, the tips of all 170 track bolts were ground down last August by Officer, Stupak and Casas to accommodate the wider track roller in the new design. Warner has been managing this project. The University of Arizona is covering the $65K replacement cost.

7.1.2 OPTICAL POINTING : An optical telescope will be mounted behind the subreflector during the summer. Such a system would help 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. Of more imminent use, however, is the hope that by measuring hundreds of stars over the sky, other variations in the pointing will be become apparent beyond that which is modeled with the current pointing model equation. In the ideal case, if we are successful in aligning the radio and optical axis, we can use Optical Pointing to quickly and accurately determine the rest of the model. It also may be useful for checking the refraction correction. For instance, it is not known whether, in the calculation of the refraction constant, it is better to use the relative humidity as measured at the telescope or that derived from the tipper opacity measurements. A 7-inch Maksutov-Cassegrain was purchased in the fall and a new CCD camera and its associated data acquisition software package will be ordered soon ($6K). We hope to be able to adapt a large portion of the analysis software already developed for a similar system in use at the NRAO. Ashby is heading up this effort.

7.1.3 REMOTE CONTROLLED TERTIARY MIRROR : Rather than outfitting the tertiary mirror unit with a rotary table so that it can be directed between either the Left of Right Nasmyth by remote control, as envisioned in an earlier plan, a new scheme being implemented by Patt and Holmberg will use a much simpler and less expensive design. A motor will be used to drive the mirror clockwise or counter clockwise into a "hard" limit. Strong electromagnets (normally used to hold fire-doors open) will then lock the mirror in place. Because of the time it takes to manually swivel the mirror when switching between Nasmyth platforms, this feature is an essential requirement if we wish quickly exploit good sub-mm weather. The ideal scheme, had we the resources, would be one similar to that used at the JCMT, where rotary tables are used to set the tertiary mirror's position in both rotation and deflection. This would provide an extra degree of freedom in the alignment of the receiver to the telescope optics.

7.1.4 LOFT COMPRESSOR : This large compressor, located up in the Loft (ie: 5th floor), is intended to drive both of the SORAL SIS-230 and SIS-490 GHz receivers, thus freeing up two of the smaller compressors currently in use. The history of this custom-built compressor is somewhat checkered but to keep a long story short, it has never really functioned correctly. It was unable to keep one dewar cooled, let alone cool down two cryostats from room temperature. Patt has investigated the problems encountered with this unit and has made modifications which should improve its performance. We hope to have it in operation at the telescope next season.

8.0 FUTURE SMTO PROJECTS

The SMTO staff are involved in several major long range projects which will significantly improve or upgrade the capabilities of the HHT. While some are actively underway, others are in the preliminary design stage.

8.1.1 Closed-Cycle Receiver Project : The intent of this SMTO project is to build a multi-frequency receiver system in a single integrated 4 K closed-cycle refrigerator. Combining more than one frequency band within a single dewar would free up valuable space on the Left Nasmyth flange. It would also reduce the logistical difficulties associated with our current hybrid cryostats, not to mention the sizable operating cost of liquid helium. The JT refrigerator design is a direct copy of the successful NRAO system used at the 12-meter telescope. The dewar will incorporate NRAO-style inserts, or "rockets", each containing an individual SIS mixer, vacuum window/lens and HEMT amplifier. The rocket for the 1.3 mm band is on-loan from NRAO. Negotiations are underway to obtain state-of-the-art tunerless mixers for other frequency bands as well. The possibility of including a single-sideband filter in the design is being investigated. Patt has been placed in charge of this project. The details of the receiver and its time schedule is still being developed.

8.1.2 UNIVERSAL COLD LOAD : The goal of this project is to provide a "universal" Cold-Load to enable calibration of all of the HHT's various receiver systems under computer control. For accurate measurements in the sub-mm, receiver systems should be calibrated often and in a consistent manner. The SMTO has acquired a small test dewar which is being modified for use as a cold termination load with our spare Model 22 cold-head. Although plans for this project are still in the definition stage, the Cold-Load would be located in the Cassegrain Cabin with a movable mirror (likely placed between the apex and the tertiary) to intercept the beam and steer it towards the dewar. Further optics would shape the beam appropriately. Such a system would accommodate all of the receivers on either Nasmyth side. The addition of a Hot/Cold calibration chopper wheel would provide the means for real-time Y-factor and T(Rx) measurements which would help speed up receiver tuning. Patt is heading up this project.

8.1.3 UNIVERSAL IF SYSTEM : As the HHT evolves with the arrival of new-generation receivers which will provide improved performance and reliability over those currently in place, or entirely new receivers operating in windows which the HHT has not yet been equipped, we are presented with an ever growing number of demands on our IF distribution system. We will have to deal with a mixture of IF bands (ie: 1-2, 2-4 and 4-6 GHz) coming from receivers which can be located on either side of the Nasmyth. While some of these are single- polarization systems, it is expected that most receivers will (eventually) be dual- polarization or will allow dual-frequency operation. This means the IF system will have to have capacity for 2 channels in each IF band. Many of the future receivers will achieve ultra-wide 2 GHz IF bandwidths and we are then faced with the problem of partitioning a single IF up into two 1 GHz bands and feeding them into our dual 1 GHz AOS backends. Depending on the type of astronomical project being carried out, we will likely want to vary the amount of overlap. Line surveys, for example, require very little overlap while extragalactic observations will require perhaps 25% or more overlap to allow the spectra to be knitted together accurately. The 2-4 and 4-6 GHz IF's will thus require a new frequency agile IF mixing scheme to convert them to the standard AOS 3-4 GHz input range. Also, the addition of the 4-6 GHz band will have a serious impact on the AOS up-conversion arrangement we are now using for the 1-2 GHz IF signal bands. The 5 GHz transfer LO currently being used is dead in the center of the 4-6 GHz IF, obviously a situation we would prefer to avoid. Thus a new up- converter scheme is required for the 1-2 GHz band as well. And of course, we desire all of this to be under computer-control, not only to dispense with the need to re-cabling when switching between receivers but to speed up the process to exploit good weather conditions. A paper design is being developed by Hayward which accommodates all the technical concerns noted above and which is modular (to allow us to mix and match IF's for PI instruments) and expandable (it can grow as more receivers are added and as IF bands change).

8.1.4 UNIVERSAL LOCAL OSCILLATOR : The goal of this project is to build a frequency-agile, spectrally-pure RF/microwave reference system which would be used by the HHT's receivers for locking their LO chains. Recent events with the current synthesizers being employed have given us concern over their future viability. The Adret 5104 (90-120 MHz) PLL reference synthesizer has failed intermittently several times this season and the backup Fluke 6160B (DC-160 MHz) synthesizer has died completely. One of the HP8671B 2-18 GHz synthesizers used for the microwave reference is no longer capable of providing a high level output below 6 GHz. Because there is a possibility of participating in the Coordinated Millimeterwave VLBI Array (CMVA), we will require an LO system which has very low phase noise. The current HP microwave synthesizer would be marginal at best for VLBI observations at 230 GHz. As well as being suitable for VLBI, the new system for the Left Nasmyth would generate dual 8-9 GHz microwave reference frequencies, as well as dual 100 MHz PLL references, in order to facilitate simultaneous observations at two different wavelength bands. It would also add a "fast" frequency switching capability. This unit would be mounted on the Nasmyth platform to make the cabling requirements easier. Eventually a single microwave reference sub-system may be required for the Right Nasmyth receiver platform. While the HOST4 microcomputer has performed reliably to date, we would like to replace it with a SMTO "Standard Micro" as part of the general upgrade of the IF and Nasmyth Light-Tower control systems. Hayward and Patt are currently investigating various commercial synthesizer modules before finalizing the design.
8.1.5 REMOTE CONTROLLED NASMYTH LIGHT-TOWERS : In order to provide remote adjustment of the Nasmyth Light-Towers mirrors, encoders and a computer controlled servo units must be added to the existing drive assembly on both the Left and Right side. Such a capability will be essential if we wish to implement remote observing in the future. A step in this direction was taken with the mounting of a linear encoder on the left Nasmyth light-tower this season. We are considering a controller based on a SMTO "standard" micro to implement this feature at some point in the future.

8.1.6 CAMAC REPLACEMENT : 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 last year to outline a plan for the global retirement of CAMAC. 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 $50K for the new VME microcomputers and the replacement of "Kronen" with a new state-of-the-art workstation once the VAX's antiquated Q-Bus architecture is no longer required.

9.0 MISCELLANEOUS

9.1.1 DOCUMENTATION : Major strides have been made in improving both the content and presentation of the SMTO documentation. Version 5.0 of the "SMTO User's Manual" now runs close to 250 pages long and is still growing.

9.1.2 WEB-SITE : The SMTO Web-Site continues to grow. It includes up-to-date information on a variety of useful topics, such as:

• Receiver Specifications
• Spectrometer Specifications
• Holography Results
• Recent News
• Electronic Newsletter
• User's Manual

The SMTO Homepage can be found at http://maisel.as.arizona.edu:8080/smt.html

9.1.3 ETHERNET LAN : We have experienced difficulties recently with our Ethernet Local Area Network (LAN) becoming painfully slow, particularly during nighttime hours. We have exhaustively checked out all of the cabling in our ThinNet link and weeded out numerous bad or suspect cables. However, the primary cause of the problem seems to be from the amount of network traffic generated by our neighbors at the VATT. When they are observing, they tend to run at a very high quiescent level which, surprisingly, causes our system to essentially grind to a halt. The time for a key stroke echo at one of our X- Terminals can sometimes take over 10 seconds. The short-term solution has been to disconnect the HHT from the fiber-optic link to the VATT. Unfortunately this also breaks our link to the outside work making it impossible to access the Internet or the Word Wide Web. The longer term solution will be to add a "bridge" to filter out all traffic from the VATT. We also intend to upgrade the current ThinNet system within our own building to a 10BASE-T connection. Since we are very near the 600 foot maximum specification length for a ThinNet connection, this should help improve speed and reliability, as well as making it more flexible.
9.1.4 MICROWAVE LINK : We had hoped that the new high-capacity microwave data link would be in service by now. This system, which operates at 22 GHz to avoid causing any radio interference with our receiver IF's, uses a relay station on Heliograph Peak to connect us to the MGIO Base Camp with a full Ethernet link (10 Mbps, expandable to 45 Mbps) and up to 24 voice channels, of which the SMT will utilize 3 lines (2 voice and 1 fax). From the Base Camp, a frame relay will then connect us to the outside world. Initially we will have a guaranteed 128 Kbps connection (ie: 1/8th of a T1 link) which is about 10 times faster than our current radio link modem (the standing joke around here is that if you have to get a 10 MByte file to the site, it is quicker to drive up and back from Tucson than to FTP it over the Internet). This can be increased if needed at a later data (ie: we can "buy" more bandwidth). The new system will not only improve the telephone service at the summit, but will allow us to move in the direction of remote "eavesdropping" by observers from afar. It also begins to make true remote observing from the home institutions a real possibility in the future. John Ratje, head of the MGIO, has been the driving force behind this project. However, before the new data link becomes a reality, the contractor, Microwave Bypass Systems, has to overcome a severe local radio interference problem encountered on Heliograph from about a dozen nearby transmitters operating in the same band as their IF conversion chain. The company has had to repackage their RF/IF stages with better shielding. They hope to have the radios installed and complete by the end of April.

9.1.5 PUBLICATIONS : As mentioned in the last Newsletter, we would like future issues to include a listing of any new scientific papers published or talks presented by our user community. We would appreciate it if users of the HHT would remember to 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.

10.0 WEATHER STATISTICS

The table shown below summarizes the opacity measured at the HHT by the MPIfR 225 GHz Tipping Radiometer during the last two observing seasons for the October-March period. As can be seen for the 1997/98 season, conditions ideal for observing at 350 microns (ie: where the Tau(225 GHz) is less than 0.06) occurred about 50% more often than last year (16 vs. 11%). We likely have El Nino to thank for this. The weather during the first four months of the observing season (ie: October through January) was exceptional, with 350 micron weather occurring about 20% of the time. For the same 4 month period the year before, this figure was only 8%. The month of December 1997 was truly impressive, with 350 micron weather occurring 38% of the time. On the other hand, the weather has been much worse during February and March. The first two weeks so far this April, however, have been a vast improvement over last year. Thus, as we have seen from previous years, the weather statistics can be highly variable, not only on a year-to-year basis but from month-to-month as well.

One of the advantages of the HHT is that its carbon fiber 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. When these considerations are taken into account, the total number of hours of sub-mm weather available to the HHT during the winter months compared to Mauna Kea become more competitive.

Percentage Time with Tau(225 GHz) Less than the Indicated Value 

 ------------------------------------------------------------------ 
|  Month   |       Season 1996-97      |      Season 1997-98       | 
|          | 0.06   0.075  0.15   0.30 | 0.06   0.075  0.15   0.30 | 
|----------+---------------------------+---------------------------| 
| October  |   4      6     15     51  |  15     23     48     77  | 
| November |   8     15     47     86  |  12     23     59     89  | 
| December |  15     22     45     82  |  38     48     82     91  | 
| January  |   9     12     39     74  |  14     27     58     84  | 
| February |  18     26     65     92  |   9     20     44     80  | 
| March    |  11     22     72     97  |   9     19     49     90  | 
|----------+---------------------------+---------------------------| 
| Average  | 10.8   17.2   47.2   80.3 | 16.2   26.7   56.7   85.2 | 
 ------------------------------------------------------------------ 

( April        1      3     34     80     20     36     84     94  ) 

              (First 2 weeks of the April 1998 only) 

     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

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. 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 rapidly between receivers, including remote control of tertiary mirror and nasmyth light-towers
• receivers which can be easily and quickly re-tuned (ie: tunerless mixers)
• 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.

11.0 VISITORS AND OBSERVERS

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

  Jeff Capara            SO               Jul  2 -  3 
  Chris Walker           SO               Aug  4 
  Henry Knoepfle         SO               Aug  4 
  John Hughes            SO               Oct  1-3, 13-15 
  Chris Walker           SO               Oct 13 - 14 
  Jeff Capara            SO               Oct 13 - 14 
  Ernst Kreysa           MPIfR            Oct 16 - 21 
  Geoff Ediss            MPIfR            Oct 24 - 31 
  Christoph Kasemann     MPIfR            Oct 24 - 31 
  Jason Glenn            SO               Nov  2 -  8 
  Chad Engelbracht       SO               Nov  2 -  3 
  Craig Kulesa           SO               Nov  3 -  5 
  Chris Walker           SO               Nov  6 -  7 
  John Hughes            SO               Nov  3 -  7 
  Pres. Peter Likens     UAz              Nov  5 
  Peter Strittmatter     SO               Nov  5 
  Buddy Powell           SO               Nov  5 
  Keven Uchida           Ohio State       Nov  8 - 10 
  Gopal Narayanan        SO               Nov 11 - 14 
  Aimee Hungerford       SO               Nov 11 - 15 
  Chris Walker           SO               Nov 13 - 16 
  Andy Nelson            SO               Nov 13 - 15 
  Craig Kulesa           SO               Nov 15 - 22 
  Connie Walker          SO               Nov 19 - 23 
  Deidre Hunter          Lowell           Nov 20 - 23 
  Chris Groppi           SO               Nov 20 - 22 
  Rainer Mauersberger    SO               Nov 24 - 26 
  Thomas Klein           MPIfR            Dec  1 - 10 
  Rolf Guesten           MPIfR            Dec  3 - 10 
  Christian Henkel       MPIfR            Dec  3 - 10 
  Karl Menten            MPIfR            Dec 11 - 17 
  Peter Schilke          MPIfR            Dec 11 - 17 
  Dirk Fiebig            ITA-Heidelberg   Dec 17 - 19 
  Rainer Mauersberger    SO               Dec 17 - 19, Jan 1 - 7 
  Connie Walker          SO               Jan  8 - 12 
  Chris Groppi           SO               Jan  8 - 12 
  Jeff Capara            SO               Jan 12 - 13 
  Henry Knoepfle         SO               Jan 12 - 13 
  Chris Walker           SO               Jan 12 - 14 
  Aimee Hungerford       SO               Jan 14 - 19 
  John Bieging           SO               Jan 14 - 20 
  Craig Kulesa           SO               Jan 16 - 21 
  Gopal Narayanan        Umass            Jan 22 - 28 
  Matt Senay             UMass            Jan 22 - 28 
  Walter Esch            MPIfR            Feb  2 - 17 
  Johannes Gromke        MPIfR            Feb  2 - 24 
  Lothar Reichertz       MPIfR            Feb  2 - 24 
  Ernst Kreysa           MPIfR            Feb  5 - 10 
  Ronald Stark           MPIfR            Feb  6 - 16 
  Frank Schaefer         MPIfR            Feb 16 - 24 
  Karl Menten            MPIfR            Feb 17 - 22 
  John Hughes            SO               Feb 25 - Mar 5 
  Joe McMullin           NRAO             Feb 26 - Mar 1 
  Dana Balser            NRAO             Feb 26 - Mar 1 
  Aimee Hungerford       SO               Mar  2 -  4 
  Ray Blundell           CfA              Mar  3 -  9 
  Jon Kawamura           Caltech          Mar  3 - 16 
  Todd Hunter            CfA              Mar 10 - 20 
  Charlie Katz           CfA              Mar 16 - 22 
  T.K. Sridharan         CfA              Mar 20 - 24 
  Edward Tong            CfA              Mar 23 - 25 
  Richard Wielebinski    MPIfR            Apr  1 -  6 
  Michael Dumke          MPIfR            Apr  1 - 18 
  Christoph Nieten       MPIfR            Apr  2 - 16 
  Roland Kothes          MPIfR            Apr  9 - 23 
  Walter Huchtmeier      MPIfR            Apr 17 - 23 
  John Hughes            SO               Apr 22 - 26 
  Craig Kulesa           SO               Apr 25 -

12.0 CONCLUSION

The SMTO has had a fruitful and exciting observing season to date and it is still far from over. Several significant scientific results have been achieved. Hopefully we'll be able to report on them in more detail at our Web-Site as the analyzed data is officially released.

The next Newsletter will describe the activities which will have taken place over the Summer 1998 Shutdown and will provide the latest information on the HHT's suite of instrumentation for the coming 1998/99 season.