Subject: DSC throughput ... explained?

From: Stupendous Man

Submitted: Mon, 11 Sep 95 21:31:40 EDT

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.

  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)
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