# Subject: DSC throughput ... explained?

## Message number: 5 (previous: 4, next: 6 up: Index)

```  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.

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
```