Subject: DIS efficiency

From: Ed Turner

Submitted: Sat, 19 Aug 95 03:05:25 EDT

Message number: 3 (previous: 2, next: 4 up: Index)

I talked to Jim Gunn about the DIS efficiency.  The 4-6% reported by
Rene is definitely too low.  Jim says that he measured it in May
roughly and got something like 15% and felt that that number was
already a factor of 1.5-2 lower than he expected.  It was not
clear to me from our discussion whether a changing efficiency was
a plausible explanation.

On technique, Jim felt it impossible to meaningfully measure
throughput either with a slit or in direct mode through a
filter.  His technique, which he said was standard, is to use
the dispersing element but no slit to make a short exposure on
a bright standard.  This includes the dispersing element efficiency
and does not lose any light to a filter; it also does not lose light
which does not get through the slit for whatever reason.

On further reflection, it does not seem to me that it is quite fair,
relevant might be a better word, to exclude the loses to the slit
(image motion, wings of the psf, differential refraction, whatever)
since one does use a slit for faint objects (the only time efficiency
matters much) and suffers those loses presumably.  Of course, most
such loses cannot be cured by modifications of the spectrograph, an
important point when thinking about the how/who/when/etc of the fix.

Ed Turner
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