# Subject: GRIM throughput calculation

## From: rhoads@astro.Princeton.EDU

## Submitted: Wed, 29 May 1996 12:26:36 -0400 (EDT)

## Message number: 16
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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).
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