Beginning in the near future and not later than October 1, a program of regular Shack-Hartmann observations will be instituted on the 3.5-meter. The purpose of this project is to monitor the telescope collimation. These observations will be carried out by the Observing Specialists and will take approximately 15-30 minutes on each night they are carried out. They will require rotation of the tertiary and must be done after the telescope and enclosure have reached thermal equilibrium, which can take up to several hours depending on when it is possible to start the ventilation of the enclosure and how long it takes to reach a stable ambient night time temperature. The Observing Specialists will attempt to acquire Shack-Hartmann collimation data on the next suitable night if it has been >=3 nights since the previous successful such measurement. During a middle of the night instrument change or at the end of the night (including morning twilight) will probably be the most convenient times to interupt science operations for this purpose. In any case, the Observing Specialists will discuss the timing of the collimation measurement with the affected observer(s) on each night in order to try to minimize the impact on science observations. However, it is important to understand that this critical engineering program is NOT optional; that is, the affected observer(s) may NOT elect to forego the Shack-Hartmann observations for that night due to their impact on the scheduled science program. If an observer or PI believes that his/her program should be exempt from interuption for collimation measurements for some compelling scientific reason, he/she should email me a request for such an exemption along with its justification well in advance of the program's scheduled time. I anticipate granting very few such exemptions. As stated above, the initial cadence of Shack-Hartmann collimation monitoring will be once per 3 nights to the extent weather permits. The results of these measurements will be monitored continuously and it may prove necessary to increase their frequency or possible to reduce it, or even to discontinue the project, depending on what the data show. The time required to obtain the Shack-Hartmann data will also evolve and may be longer than estimated above until a smooth procedure is established and practised. The background of this new engineering effort is as follows: During the conclusion of the summer shutdown, Shack-Hartmann measurements indicated that the telescope optics were capable of delivering 0.3 arcsec FWHM images! This encouraging result is to the credit of the long standing and ongoing image quality campaign and to all of the work that has been done on the support and control of the telescope optics. However, over a period longer than one day and less than three days, the telescope collimation drifted or jumped, it is not clear which, to the extent that its limiting image quality was worse than 0.5 arcsec FWHM. The most likely culprit is believed to be motion (tilt) of the secondary, but that explanaton is not definitely established, and even if it is correct, the cause is not clear. The goal of program described above is to allow a better understanding of the telescope decollimation process. If it is not possible to fix the problem until a new top end is put into service, which seems quite possible, a program of frequent and regular recollimation may be required. The Shack-Hartmann monitoring program will also allow a determination of how this might best be done. No recollimation during the night will be attempted initially however. I appreciate your cooperation with the Observing Specialists in carrying out this important engineering project with minimum impact on science observations and trust that we can all agree on the importance of achieving the best possible imaging preformance. Ed Turner APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 613 in the apo35-general archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-general/INDEX.html 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