Minutes to APO 3.5m User's Committee Phone Conference, September 18, 1995 Attending: Julie Lutz, Rene Walterbos, Bruce Gillespie, Ed Turner, Michael Strauss, Don York, Chris Stubbs, Lew Hobbs Three basic items to discuss: 1. The current status report of the telescope from Bruce Gillespie, posted as apo35-general message #15. 2. Ongoing engineering done by Chris Stubbs and colleagues to assess the optical performance of the telescope. 3. Discussion of scheduling of fourth quarter. Bruce Gillespie circulated a report (apo35-general message 15) of what was accomplished during the recent engineering push, and where the telescope stands at the moment. The high points are: - Azimuth drives now seem to be working reliably. - Stalling Nysmith rotator motor now seems to be working. - Enclosure drive is rather erratic, and continues to be a source of worry. - One of the three motors which drives the secondary (tilt and focus) started to miss steps Sunday evening. It is not clear how long this problem has actually been happening; it is possible that this is related to some of the pointing problems that have come up recently. This was worked on on Monday (editor's note: I observed Monday night, and secondary motor was fine; telescope was in proper collimation.) - Recollimation of secondary was done. - Reflectivity of mirror surfaces measured at 60% (product of all three mirrors, not each individually; this is at 6000 A). It was 73% when everything was perfectly clean. This certainly affects throughput calculations. - Primarily with observations with Ed Kibblewhite's CHAOS instrument, a variety of resonances have been discovered in the support structure. The strongest of these is at 20 Hz, with a typical amplitude peak-to-peak of 0.5" (which, when added in quadrature to the intrinsic seeing, adds 0.1" to the PSF width). This resonance is driven by the bit rate at which one of the stepper motor sends out commands. However, analysis shows that if that bit rate is taken off of resonance, the total power in vibrations does not diminish significantly. A 10 Hz oscillation of several arc-second amplitude is seen when the telescope points into the wind. - The enclosure wheels remain a serious concern, although they have not gotten obviously worse. It is possible that the manufacturers did not harden the steel properly, making them liable for repairs. A new wheel will arrive in mid-October. We discussed the throughput of the DIS. Rene believes that his measurements and those of Jim Gunn are in fact in agreement. Jim measured the throughput of the spectrograph optics, while Rene measured that of the whole system: telescope, optics, grating, and slit. Ed reported that the gravitational lens monitoring program, which has taken direct images with DIS every other night since the telescope came on line, shows no evidence of a general decline of throughput with time. In imaging mode, the throughput is 15-20%, similar to what is found with the DSC (see recent postings to apo35-dsc from Michael Richmond). Chris Stubbs has compared DIS images with images of the same field from the Tololo 4m; he says we're getting 75% of what we expect. He suspects possible scattered light problems. One clue is from DSC data in hand, Michael Richmond (apo35-dsc #2) finds that 14\% of the light from a bright star is between radii of 5.5" and 96". There was strong support for plans to measure the throughput of all the instruments on a regular basis, and making the results available to the APO community. The guider continues to have problems; Spectrasource continues to deliver unreliable electronics and software. The current plan is to swap out the 1024x1024 chip currently there, and send it back to them for electronics fixing, while we use a 512x512 chip with the resulting smaller field of view. The pixels are 0.14"; it would make a lot of sense to put in a focal reducer and thus increase the field of view. People asked whether it would make sense to throw away the existing guider and start over with another more reliable vendor; this was estimated to cost $15-25K, money which is not clearly budgeted. But Chris told us that the guider in fact does seem to work, if given a bright star; it guided correctly to a few tenths of an arcsecond in one hour, while an unguided exposure drifted by several arc-seconds in the same amount of time. Chris Stubbs and his colleagues at UW have put in a tremendous amount of work to characterize the optics of the telescopes. They are writing a series of reports, which will be distributed widely very soon. The made a poor man's Hartman mask by closing the mirror covers and opening the ports to the individual Nysmith foci (seven in all). They found an enormous (~1') translation of the images as a function of focus; could this be related to the problems the support motors for the secondary were having? With these seven apertures, the individual rays come only within 1" of a common focus, which is definite proof of figure errors in the optics. Suspicion weighs most heavily on the secondary. Chris is planning to build a real Hartman mask, and continue similar tests. He will rule out problems with the figure of the primary by doing the Hartman tests at prime focus. Chris has also looked at the time evolution of the pointing model; it has drifted all over the place! This could conceivably be related to the problems with the secondary drive motors. We discussed the high fraction of engineering time, and how we will know when it is time to slack off. No conclusion was reached. We discussed the fourth quarter schedule, and the issue of how to schedule in general. There was concern from UW about the amount of time being spent in instrument changes; it would be good to limit it to no more than one instrument change per night. The people at NMSU felt strongly that time was being given in overly fragmented pieces, and this should be avoided if possible. In particular, as higher-ranked proposals get scheduled first, the lower-ranked proposals are "fit into the cracks", and are often broken up into little pieces. We discussed various ways to make the scheduling more efficient. One plan that had some support was to have the APO director schedule the 70% of the top proposals from each institution, and then give the remaining 30% as blocks of time to each institution to schedule themselves. This caused much discussion. Bruce Gillespie pointed out that this was closely related to the issues of queue scheduling, and service observing, which we might consider in the future. Michael Strauss re-emphasized the strong desire from Princeton to have the throughputs of each of the instruments measured and characterized. The next meeting will be Monday, October 9, at noon EDT. -Michael Strauss APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 18 in the apo35-general archive. You can find APO the archive in /u/strauss/apo/mailer/apo35-general on astro.princeton.edu APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-general@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO