Subject: Improved 3.5m Throughput!?

From: elt@astro.Princeton.EDU

Submitted: Mon, 27 Jan 1997 23:30:59 -0500 (EST)

Message number: 119 (previous: 118, next: 120 up: Index)

I had hoped to have clear and definitive throughput measurements to
announce shortly following the end of the recent shutdown in order
to test the hypothesis/hope that re-aluminization of all three of
the telescope's optical surfaces would produce a great improvement.
The bad news is that it has not been possible to obtain such data so
far, primarily due to a lack of photometric conditions.  (Indeed,
there has been almost constant mediocre-to-bad weather since the beginning
of January.)

The VERY good news is that there is strong eveidence suggesting
that the re-aluminization did produce the sort of large improvement
we hoped to achieve.  The two main bits of evidence are as follows:
1) On the night of Jan. 15, just at the resumption of science operations,
   SPICAM was used to make some very quick and restricted (in terms of
   filters used) observations of a Landolt standards field.  This data
   has been reduced by Chris Stubbs using the same procedure he and his
   group used to measure SPICAM's QED before the re-aluminization.  A
   copy of his report of the results of this analysis is attached below.
   It indicates a 70% improvement in throughput in the R band with
   something like a 10% uncertainty.  These data were obtained during a
   "photometric interval" in a less than pristine night, but are probably
   OK, if limited.
2) Anecdotal evidence from many experienced users since the end of the
   shutdown reveals a wide spread impression that sensitivity has
   dramatically improved.  Indeed, in so far as I am aware, everyone who
   has used the telescope in reasonably good conditions, with whatever
   instrument, has noticed the improvement.  Rough estimates of the gain
   have ranged from 50% to a factor of three.  Dan Long, our most
   experienced Observing Specialists is quoted as being virtually certain
   that there has been a big change.

The weather forecast for the next several nights indicates hope of the
clearest skies since the end of the shutdown.  We plan to continue to
try to obtain definitive throughput measurements at multiple wavelengths
and with all major instruments.  It may be necessary to pre-empt some
science operations for this purpose.

Assuming that the greatly improved throughput is confirmed, we will
next address the issues of monitoring and maintaining it in the long
term future.

Ed Turner


>I have taken a quick peek at the images taken under allegedly
>photometric conditions on Jan 15, to test throughput.  THe
>frames have not been flat-field corrected, so it's really a
>crude estimate, but I get (to about 15% accuracy) a
>value for integrated flux from an R=20 star of
>205 e/sec
>This is to be compared with a pre-aluminization value of 120 e/sec,
>and is frighteningly close to the calculation in my memo of Oct 18
>that predicted 212 e/sec from a well-behaved system.
>This is really good news.  Evidently we're up by a factor of 1.7
>in R, perhaps even better in blue light.   We win back the shutdown
>time in a few months if this is true!
>I still would like to take a full set of characterization frames
>in multiple passbands at the earliest opportunity.
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