Jim Annis and I went over some numbers today, and we seem to be able to make sense of the measured DSC throughputs, at least roughly. Start with photons entering a cylinder 3.5-m in diameter, at the head of the telescope. a. secondary blocks some light 10% ? leaving 90% of original b. mirrors aren't perfect, bounce 60% 54% c. CCD chip has moderate QE 40% 22% Earlier, I posted messages that claimed 15-18% of the incoming photons were detected on the DSC chip, inside a radius of about 100 arcseconds. I assumed that the gain of the camera was 2.0 electrons/DN in those calculations (one of the stated assumptions). I have since learned that the gain was set to be 2.5 electrons/DN. If it actually was 2.5, then my calculations were incorrectly low by a factor of 2.5/2.0 = 1.25. Thus, d. measure a throughput 15-18% x 1.25 = 19-23% So, if the CCD chip isn't a terrific one, we can explain the measured throughput. Recall that the measurements were made in the R/r' band. I took a guess that the secondary obstructs 10% of the full aperture -- please correct as necessary. Michael (and Jim) APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 5 in the apo35-dsc archive. You can find APO the archive in /u/strauss/apo/mailer/apo35-dsc 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-dsc@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO