Web-based discussion

McMahan and Simkin will work together to establish a threaded news-server maintaining a ``hypermail archive" at the SAC site. This will allow SAC members to use Netscape or another Web browser to enter discussions, prevent numerous cross-posting of detailed material, and maintain a single mail archive site.

Review of telescope requirements draft doc.

1. The next issue addressed was tracking drift open loop (i.e. without a guidestar.) The term discussed was identified as a very low-frequency flexure of the telescope as a whole between the guider and image focal plane, independent of the specific instrument.
2. It was decided to put as a goal a flexure rate of <0.01 arcsec per minute. This does not include instrument flexure, which will be part of the instrument image-smear budget. Higher frequencies will be treated in the TBD AO budget.
3. The separation between flange and focal plane was reset from 150 to 200 mm, to accomodate CCOB and the needs of other IR instruments. Blanco noted that this increases the moment on the flanges, but not significantly.
4. It was agreed to put as a goal a capability within the guider to do a periodic photometric measurement on the guidestar, to monitor transparency variations as a function of wavelength. This would allow tie-ins with extinction measurements made by an automated photometric telescope at the site. Most of the time, the guider would run unfiltered for max. sensitivity, or broadly filtered to roughly match the observation wavelength (refraction differences would be computed.)
5. Gemini compatability was raised again. Simkin noted that the OWG had concluded on the basis of manpower and efficiency that there should be no more than 4 instruments mounted at once. Blanco noted that if you went to 2 instruments at each Nasmyth port, then that implied that ALL instruments would be fed with a minimum of 4 mirrors. With the cube, there are 2 straight-through paths, presumably for the spectrometers which likely have additional fold optics, and the imagers which work off 4 mirrors. It was agreed that the cube concept minimized optics.
6. The space envelopes for the instruments were discussed, based on 3d sketches provided by Blanco at the Web site. Cecil noted that on the 3-instrument cluster side there was more than 1.5m of space from the focus on the straight-through path. This would accomodate say 80 cm to a collimator, allowing a 5 cm beam for our f/16 input. The volume-phase high-efficiency, holographic transmission gratings Barden (NOAO) has been working with have fringe densities of 1200-5000/mm, which means R up to 100,000 in certain fiber-fed schemes. You would still have 0.6m of space after the collimator to put in the grating, camera, and focal plane. Baldwin ``noted with delight" that the straight-through port was actually a little bit bigger than the Blanco Cass. cage. So, the space here looked adequate but of course the relevant SAC committee would verify this for other possible instrument configurations.
7. The SAC agreed to specify a requirement of a minimum acceptable instrument payload of 1500 kg on each Nasmyth port, with a goal to go to the full Gemini mass. Elias had suggested that the latter would bump up slightly to 2200 kg, so this has become the goal. If there is no compelling engineering reason to limit to 1500, then go to 2200 kg automatically. Otherwise, the SAC would then debate the trades. The SAC felt that accomodating the larger mass should remain as a possible trade with increased aperture, TBD at a later date. Elston felt that the harder driver on the mass limit was the cluster side. There, three 400 kg 4m instruments that would meet our image spec was a challenge but doable with a 1K detector but 700 kg was more like what you needed as you moved to IR mosaics. An imager or spectrograph weighs about the same, with a lot of dead weight simply to avoid flexure. 700 kg instruments would imply 2100 kg for the 3-instrument port.

Simkin began by noting that the current task list did not prioritize tasks. Issues like e.g. instrument interfaces, needed to be defined before we discussed imagers. We needed specific reporting dates for each of the tasks; we need to decide if the SAC merely sets the science requirements for the instrumentation then solicits bids, or if the committee designs the instrument. Cecil agreed that the list needs to be prioritized by our next telecon. At this stage he is trying to make the list comprehensive and to get various people to agree to various committee assignments.

It was felt that the SAC committees should use the science drivers as articulated at the Michigan meeting to define overall requirements in their instrument category, even if the drivers appear to require several different spectrometers or imagers. Activities that could be accomplished on the Blanco will be ranked low on SOAR. The core capabilities of instruments will be driven to exploit SOAR's superb images (including spectral images.) Synoptic programs will also be favored.

The overall goal for the Nov. meeting is to adopt a formal set of core requirements for the instrument complement of SOAR. Technical details would be developed after the meeting as we continue our technology evaluations with the now-agreed-upon science requirements and instrument interfaces. (We would shoot for a review of this information at another face-to-face meeting in the Spring, perhaps in Florida.)  The Nov. meeting would be open to non-SAC members, but to preserve ``discipline" Simkin urged the SAC to put everything down on paper first for circulation so that data are properly supported and referenced. We don't want shoot-from-the-hip commentary, just facts at this stage. Cecil would set deadlines so that report content would be frozen and deposited in the SAC Web site on Nov. 5. This would allow 10 days for reports to be assimilated and the first round of discussion by email.