At Michael Strauss's suggestion, I have done a throughput calculation for the APO + GRIM system based on the zero points I posted earlier today. I obtained an overall system efficiency of 0.32 in J band, and 0.25 in K', both at 1 airmass. The calculations were based on a gain of 4.8 electrons/ADU, a telescope collecting area of pi*(3.5 m / 2)^2, Johnson's table of F_nu for a zero magnitude star in J and K bands, photon energies of hc/(1.25 micron) and hc/(2.15 micron), bandwidths of (1/1.1 - 1/1.4)*c/micron and (1/1.97 - 1/2.32)*c/micron. For the K band, the difference between the Johnson K and Wainscoat/Cowie K' band introduces uncertainty. 0.2 < K efficiency < 0.3 seems safe, but the difference in bands ought to be considered for more precise numbers. I have not checked the literature to see if there is a more recent table of F_nu for zero magnitude objects in the CIT photometry system, and have not considered the possible differences between CIT J and Johnson J. Also, I am assuming that Johnson's table gives flux densities for a zero magnitude star extrapolated to zero airmass; my measurements extrapolated to zero airmass give system efficiencies 0.34 and 0.26 so this is not a big issue. The quoted quantum efficiency of the chip is 70% (according to the manufacturer). I don't know the other efficiency factors involved (reflectivity of aluminum surfaces, transmission of lens elements in GRIM, etc); perhaps someone else could comment on those. Finally, I have taken a quick look at some 18 December 1994 data to check for secular trends. Three standards then yield K' band zero points of 22.20, 22.18, and 22.26 magnitudes, compared with 22.21 from last month, so I conclude that there were no important trends at the accuracy level of my calculations in that period. James Rhoads rhoads@astro.princeton.edu ps- the Johnson reference is in the 1966 Annual Reviews of Astronomy and Astrophysics (vol. 4). APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 16 in the apo35-grim archive. You can find APO the archive on http://astro.princeton.edu:82/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