SAC Minutes

The SOAR SAC met face-to-face for the first time in Tucson on the weekend Nov. 15/16. Attendees: Baldwin, Cecil (chair & minutes), Diaz, McMahan, Simkin, & Steiner (substituting for Dottori). Elston was absent. Moretto & Blanco represented the project office. Presentation by Code (WIYN) on day 2. The aim of the meeting was to refine science drivers for the telescope, discuss progress of the various sub-committees on organizing the science mission, and define the strawman instrument suite. To schedule the meeting close to the GNIRS design review, the SAC met after the Board. In future, the SAC will meet beforehand so that issues that develop can be forwarded to the Board via the Project Scientist in a timely fashion. In the following, issues that came up at the meeting and that were resolved afterwards are reported in [ ].

Summary resolutions: The SAC

  1. is concerned about possible erosion of the instrument budget. Instrument funds should not be identified as the project contingency. The telescope should come in at a maximum of $23M even if the aperture has to shrink below 4m. The SAC accepts that observer support infrastructure, calibration hardware/software, instrument control computers, and half of the Project Scientist's salary are valid expenses for the instrument budget.
  2. feels that there is incentive for the project to reduce construction costs but no corresponding reward to develop solutions that reduce operating expenses; the Board should explore a way to do this. Contractors need to be rewarded for solutions that will maintain telescope performance with the sort of minimal support model that was discussed by the OWG; they should demonstrate that image quality and observing efficiency can be maintained at this level. In particular, they should adopt common, forward-thinking  top-level hardware to interface to the TCS. Software should be coded and documented to computer industry IEEE standards.
  3. recommends that the TCS hardware be state-of-the-art and not a copy of the Gemini design which is based on obsolete hardware (VME); the emerging compactPCI standard was mentioned.  CTIO requests top-level look&feel compatibility, but this will suffice.
  4. recommends that there be an external technical committee to review major contract specifications for completeness; simply giving a spec. to the vendor might not convey the importance of minimizing overall operational complexity because vendors do not use or normally maintain telescopes. It was also felt that this committee would develop more of a memory of issues as they developed throughout the project than would be the case if we simply populated design reviews with one-time participants.
  5. recommends that the project aggressively explore AO with the expectation that additional funds will be found by the partnership to implement it in early operations. The goal is to have the system under construction as SOAR enters commissioning. Serious efforts to raise the necessary funds should therefore begin now, and design work should commence in about 1 year (the same timescale appropriate for the instruments.) Instrument designs should include provision for upgrade to AO when this makes sense scientifically.
  6. developed consensus on project science goals. The refined Science Requirements draft will be released to the partnership for comments in early Dec.
  7. To implement this science, the SAC has selected the following 5 instruments. They will be subjected to further study by sub-committees to sharpen the science goals. The aim is not to develop instruments by committee.

Actions:

  1. project the likely limitations of fibers & IFU microlenses (Diaz, Cecil, Baldwin)
  2. is these a broad science case for spatial resolution provided by multi-IFU's vs. multi-slits? Quantify target density & desired spatial multiplex (McMahan, Simkin, Diaz, Elston)
  3. establish maximum R in optical & IR where SOAR instruments would be readout vs. background noise-limited, general detection limits from simulations (Simkin, Elston)
  4. settle on AO correction order for SOAR, quantify sky coverage goals & examine if we should skip an NGS system and go straight to an LGS solution if feasible (Baldwin, Diaz)
Dates and upcoming events: Day 1 began with a summary by Cecil of his presentation to the SOAR Board a few days before. He then turned to general instrument issues in a Draft SOAR Instrument Philosophy: Next the SAC reviewed the various draft reports that sub-committees had generated (and which are available in the SAC area at the Web site.)  The need for collimation in both imagers was debated. It was recognized that reimaging was required in the IR anyway, so a dispersive element should be incorporated if  it did not limit the field of view. McMahan argued that the optical imager should be a simple instrument comprising a filter wheel in the converging beam, possibly feeding a mini-mosaic (4K^2) to cover a 5' field w/ sampling appropriate for tip/tilt.  Cecil felt that the results soon to come from recollimated optical systems like the Taurus Tunable Filter would speak for themselves and that the SOAR community would be clamoring in a year for this sort of imaging spectrophotometry in the nominal imager.  The cost would be optics, two narrow-gap etalons at $60K each, and increased scattered light from the extra glass.

It was recognized that AO would benefit the optical waveband by reducing the FWHM vs. the near-IR where FWHM would not  decrease much from the tip/tilt value but Strehl would double and so  benefit spectroscopy.  The ideal AO for SOAR might turn out to be an adaptive M2 to correct all foci.  However, if we have  to choose which focus to correct, the SAC recommended AO for the 3-instrument cluster rather than the Gemini port.  There was discussion about the need for an LGS vs. NGS.  It was agreed to adopt the project goal of an LGS because it opened up a significant part of the sub-K sky.

An informal quote from Laplacian in Hawaii for a curvature wavefront sensing system based on an n-element bimorph mirror was discussed. For the AO report, Baldwin et al. will continue to develop costs and performance differentials for 16 and 35-element correctors and a PUEO concept.  The goal is to ensure that whatever the concept identified, it be upgradeable to a laser.  Baldwin & Cecil later discussed the Gemini approach w/ Simon Morris (DAO), Doug Simons (Gemini), and Mark Chun (recently hired Gemini AO Scientist).  [Morris felt that extragalactic spectroscopy on a 4-m AO system would not be competitive w/ Gemini.]

After lunch, spectrometers were discussed.  It was noted that the SOAR partnership has an unusually large representation of stellar astronomers. Hence it was agreed that a high-throughput, bore-sight spectrometer had priority.  If  throughput is ruthlessly maximized, this instrument would compete favorably on point sources with Gemini-GMOS.  Diaz and Baldwin will work to refine the performance trades in wavelength coverage and spectral R vs. throughput.  Another goal is to minimize cost.

The rest of the day was consumed with discussion of the instrument characteristics to enable partner science.  This culminated in the list at the head of this report.

Day 2 began with a briefing by Moretto on the telescope optics.  The field curvature of the optics will be changed slightly from a classical RC to match the Gemini field curvature.  All images remain diffraction limited (at 1 micron) over 7.'5-diameter with bare optics.  The annular field 7.'5-15' diameter would be accessed by spectrometer fibers (if such an instrument is provided) as well as guide probes and wavefront sensors (WFS). There was a some confusion as to how suitable these images are for guiding and WFS.  [The astigmatism introduced by not following the RC curvature is within the Airy disk & does not alter the guidestar centroid.The active optics mode is to diagnose the mirror figure with a star on-axis, then monitor the figure differentially with a star in the guide field. Thus initial astigmatism in this field is easily compensated for.]

Moretto then showed refractive and reflective correctors that had been developed during the SWG phase to speed up the system from f/16 to f/10.  Both concepts had design constraints that might no longer apply.  Relaxing these would lead to considerable simplifications and fewer optics.  Moretto, Blanco & Cecil will be revisiting the corrector optics in a few weeks.  Moretto will also examine recollimation in the imagers to see how scattered light can be minimized, to retain the option for  spectral capabilities within the ``imagers".

Art Code (WIYN Scientist) then presented an overview of the decision process that WIYN followed during its recent evaluation of the desirability of an AO retrofit.  They ultimately decided on tip/tilt alone, although they expect an eventual upgrade to AO. The reason is that WIYN is a wide-field telescope , and AO programs are narrow field. Art noted that the process was tedious because there is no WIYN Observatory Director to say ``make it so".  He urged us to get one in place before the Project Manager rides off into the sunset (before commissioning). He outlined how WIYN is incorporating their tip/tilt unit as a reimaging mirror on the A&G unit on WIYN.  There are two funding programs in NSF that are appropriate. Their proposal includes funds for an IR imager. He gave Baldwin an early draft of their proposal which relates more to the atmospheric properties over the site than the science drivers.

Next there was a discussion about SOAR's approach to AO.  To summarize, the strategy is to gather info to develop an AOB-concept that can be upgraded to a laser. In a year or so, assess the progress on lasers (principally the Livermore effort @ Keck) and adaptive M2 (Steward). Baldwin will work with Moretto to monitor these efforts. Site conditions relevant to AO will be measured by Gemini above Pachon starting in mid-January. Many balloon launches and SCIDAR measurements are planned. Hopefully El Nino will not skew the results.

Baldwin then summarized the sub-committee report on instrument interfaces. Gemini utility interfaces would be provided at both Nasmyth ports. In addition, two other utilities missed by Gemini seem appropriate: vacuum and dry N2. Calibration requirements were outlined. The project will provide a ``great white spot" for rudimentary photometric calibration of CCD imagery. [Walker (CTIO) confirms that dome flats are good to 0.05-0.1% except in the UV.] IR imagers can use the sky or mirror covers. IR spectrometers will obtain most of their calibration lines from the night sky, but occasional lamp spectra are used for setup. It was recommended by the sub-committee that both optical and IR spectrometers use a simple version of the Gemini calibration unit. This $300K <1 m^3 box [cost from D. Simons, presumably would come from our instrument budget] uses a novel projection scheme to produce a fairly accurate simulation of the telescope beam. Blanco noted that this solution would be about 10x more expensive than what was done on WIYN.

The TCS, and control interfaces in general, provoked considerable discussion. The interfaces sub-committee had recommended a lobotomized version of the Gemini TCS. However, upon further discussion, it was realized that  this would rapidly become even more expensive because Gemini has VME controllers throughout.  These are already obsolete, requiring that SOAR change its electronic basis soon after first light. McMahan noted that compactPCI was the emerging standard for machine control (see http://www.picmg.com for details), and that this architecture had the bandwidth to support video, data, and LAN on a single fiber rather than the multiple channels required by Gemini. The project approach of ``hiring someone in Pat Wallace's group to take what we need from Gemini" was viewed as completely inadequate and would tie us to obsolete hardware. Cecil noted that the Wallace SLALIB algorithms used at the higher level of the TCS were all public domain, and that GUI's are almost self-programming. He has taken the TCS-task to resolve and will report to the SAC in a few weeks. [M3 has a controls group and Cecil will meet with their engineers right after Thanksgiving.  German Schumacher will visit the project from the CTIO controls group for 2 wks at the end of January. Cecil has also talked to Jeremy Bailey at the AAO. Bailey has written a Portable TCS in use at the JCMT and under consideration for the AAT TCS, which embodies the Wallace algorithms in a less hardware-dependent fashion than Gemini so that more of the TCS is generic to all telescopes. Cecil requested a copy of the PTCS and its DRAMA software environment for evaluation. Bailey replied that PTCS  was still under development for the JCMT project. He would be willing to work for SOAR via a contract to the AAO, if time can be found to free him from other projects. Cecil will report on this to Sebring.]

The issue of the TCS triggered a discussion on technical review of project decisions. The SAC recognized that in most cases it did not have expertise to evaluate engineering. As outlined in the first section above, the SAC recommended that project specifications be passed by outside engineers with experience in telescope design to ensure completeness and due consideration of downstream operating costs.

The SAC tentatively agreed to next meet several weeks before the telescope concept design review. Steiner suggested Brazil because the next Board meeting will also be held there. Ground-breaking at Pachon is scheduled a few days later. The SAC responded enthusiastically to this invitation.

Bonus section: GNIRS Design Review

Immediately following the SAC meeting there was a 2-day critical review of the Gemini-N Infrared Spectrometer. A clone of this has been proposed by NOAO to share between SOAR & Gemini-S. It would provide spectral coverage over the wavelength range 0.9-5.5 microns, and over a few square arcseconds of sky (or along a 200" slit.) The exhausting review (printed in 7 volumes!) was attended by Baldwin, Cecil, Diaz, Steiner, and Simkin.

Several issues relevant to SOAR came up: it would take 1 wk to cycle the cryostat from operating to room to operating temperature (5 days cooldown, 2 warmup). (Apparently the dewar can be moved between domes while cold, although this is not preferred.) The review committee recommended that this time be shortened. Two image-slicer concepts were discussed. A reflective slicer was proposed by U. of Durham, and the reviewers recommended that it be examined in detail. It is similar in concept to the 3D design of MPI. See SPIE Vol. 2871, 1295 (97). Two scales would be available on SOAR, spanning 5."2 x 3."6 @ 0."2 increments, and 2."6 x 1."8 @ 0."1 increments. The slicer would be off-axis to not interfere with the bare slit. The next step is for Gemini to approve a study to examine manufacturing and alignment tolerances. Another issue is the availability of a full four-quadrant InSb array from the ALLADIN program. There is confidence that Gemini will have two fully operational ALLADIN arrays by Jan. How many spare arrays will work from the InSb production run is uncertain.