Subject: concern about low throughput of DIS

From: rwalterb@NMSU.Edu

Submitted: Fri, 4 Aug 1995 09:53:42 -0600

Message number: 2 (previous: 1, next: 3 up: Index)

						August 4, 1995


Various NMSU users of DIS have alerted me that they are quite
concerned about the apparent low throughput of DIS, and I would like
to ask other users if they have experienced similar
problems. Specifically, please let me know if you have done any
througput calculations, using observations of standard stars or other
calibrated sources.

Initial anectodal evidence for low throughput has been placed on more
solid footing now that we have reduced spectra of standard stars. The
througput we get is about 5% in the red, and a little worse in the
blue. This appears to be at least a factor two worse than one might
hope for. The throughput estimate is after taking into account the
effect of the atmosphere, so it includes all of the telescope and
detector losses. I think that the major uncertainty we stil have is
that it is difficult to prove beyond doubt that the standards fell
exactly in the slit, even though the observers tried to make sure of
this by careful imaging of the stars just before the spectra were
taken. But we all know that things in DIS (used to) move around a bit
from time to time, so there remains this concern. However, I think we
now have at least two separate calibrations which both indicate low

Let me briefly describe a specific result. The standard star Feige 15
was observed on January 25 through a 2" wide slit, in low-res
mode. From the pre-spectrum image it was determined that only about
60% of the star light went through the 2" slit (so we did take this
into account). The exposure time of the spectrum was 70", so tracking
should have been good in such a short period. Integrated red and blue
observed fluxes were calculated from extracted 1-d spectra by summing
pixel intensities over the correct wavelength ranges. These fluxes (in
ADUs) were compared to the fluxes expected, given the attenuation by
the atmosphere for the appropriate airmass, the telescope area
(correcting for blockage by the secondary) and exposure time. The
resulting throughput estimates are 5% in the red, and 3.4% in the

Please let me know if you have questions about this, similar concerns
(preferably backed up by hard evidence), or evidence to the contrary.

Rene Walterbos

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