News of interest to SPIcam observers: The installation of the new NA2 baffle during summer shutdown has considerably improved the flatness of calibration images taken off the mirror covers. Although twilight flats are still preferable and superflats taken from object images are best of all, mirror cover flats are now a desirable alternative when the other kinds are not possible. Mirror cover flats will also be slightly redder than the twilight sky or the night sky, and may therefore be more appropriate for photometry of red targets through broadband filters. Drawbacks: the lamps are quite faint in the bluest filters and will not reproduce night-sky fringing patterns in the reddest filters (i', z', I); only object superflats from the night sky can compensate for fringing. Thanks to the new baffle, the illumination pattern of mirror cover flats now has very little dependence on rotator position, except in the outer corners of the image. Observers planning aperture photometry on small objects near the center of the field should not have to worry about rotator position for mirror cover flats. Observers doing surface photometry on larger objects or aperture photometry of multiple targets throughout the field may want to take flats at multiple rotator positions and combine them into a master flat. This does require coordination with the observing specialist, since we have to be very careful about having the mirror covers closed and motors active at the same time -- so be sure to communicate your plans clearly to the observing specialist. Here are some suggested exposure times that should give 20K-30K counts for the Sloan filters. We do not have a current set of exposure suggestions for MSSSO or narrowband filters. filter lamp exposure ---------------------------- u' Brt Qtz (900s gives about 1400 counts) g' Brt Qtz 5s g' Dim Qtz 300s r' Dim Qtz 90s i' Dim Qtz 60s z' Dim Qtz 90s Acknowledgments to Bill Ketzeback for extensive testing. Russet. APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 849 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