There has been some concern that the 3.5m and GRIM II have a higher thermal background than expected. On the engineering run last week, Mike Ledlow and I measured the background in the K filter as a function of airmass. We find a good linear fit from 1 to 2 airmasses of K background = 4180 + 2330 X DN/s/arcsec^2 This measurement was taken at an air temperature of about 275K. Using the zeropoint of 22.24 derived on the same night, this translates to background of 12.8 mag/arcsec^2 at 1 airmass. We can compare the instrumental and atmospheric contributions to the background at 1 airmass using the measurements quoted by Wainscoat & Cowie (1992, AJ, 103, 332) for UKIRT and the UH 2.2m, also at 275 K: APO 3.5m UKIRT UH 2.2m instrument 13.2 12.9 13.8 mag/arcsec^2 atmosphere 13.8 14.1 13.9 mag/arcsec^2 On the whole things don't look too bad. The 2.2m measurements indicate that there may be some room for some improvement, especially since the coatings on the 2.2m were apparently not especially clean at the time the background was measured. The additional 0.6 mag of emission reduces our 3.4m aperture to 2.6m. The camera on the 2.2m had an optimized cold stop that blocks out the high emissivity central hole in the primary; I don't know if GRIM II does. We also roughly determined the background at Ks from single measurements at 1 and 2 airmasses. We find Ks background = 2300 + 2240 X DN/s/arcsec^2 Using the zeropoint of 22.37 derived on the same night, this translates to 13.2 mag/arcsec^2 at 1 airmass, about 0.45 mag fainter than at K. This is similar to the difference Wainscoat and Cowie find between the K and K' filters. Alan Watson APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 35 in the apo35-grim archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-grim/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-grim@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO