Subject: DIS grating choices

From: Russet McMillan

Submitted: Mon, 4 Mar 2002 15:45:15 -0700 (MST)

Message number: 555 (previous: 554, next: 556 up: Index)

Date: Mon,  4 Mar 2002 15:11:49 -0700 (MST)

Hello all,

User opinions are being solicited to help us decide which gratings to
make available on the new DIS.  If you are a current or potential DIS
user, please read this message and send your opinion to me or to your
institution's User Committee representative before Monday, March 11.

Here's the situation: DIS is being upgraded with new detectors, which
have larger formats and smaller pixels than the old detectors, leading
to greater spectral coverage and better resolution.  The new detectors
also have the same pixel scale, whereas the old detectors (and the
gratings designed to go with the old detectors) had mismatched pixel
scales.  Using the same old grating sets with the new detectors would
produce some rather odd combinations of resolution and spectral
coverage, so we are trying to decide how to arrange the gratings for
the new instrument.

There is an additional incentive to this process; if we can settle on
only two pairs of gratings to be permanently mounted, life will be
easier for everyone all around.  Observing specialists will not be
required to spend an extra hour for grating changes, the scheduler
will not need to arrange DIS programs carefully to avoid grating
request conflicts, and observers will not have to specify gratings in
their proposals or worry (much) about making a last-minute change.
Minimizing the handling of the gratings would also increase their life
expectancy, a significant consideration since we have seen scratches
accumulating on the gratings over the past few years.

Here are the expected characteristics of the old gratings with the new

               LOW Red   LOW Blue  MED Red   MED Blue  HIGH Red  HIGH Blue
Central Wave    8000      4224    7500     4000     8465     5000
 Wave Range     14280     6222    6426     3264     2386     1632
 Blaze Angle     3.43     3.63    6.53     6.90     20.57    17.45
 Lines / mm       150       300      300       600      830.8      1200
Resolution      7.0/px  3.05/px  3.15/px  1.6/px  1.17/px   0.80/px

Clearly, the wavelength range of the old low-resolution red grating is
excessive for the new detectors, extending well beyond the dichroic
cutoff at 5350 A and the red sensitivity limits of the detector.
There are two grating arrangements which I think are optimal, but
there is also a third possibility which some observers might prefer.
The most obvious choice for a new low-resolution set would be the old
medium red grating and the low blue grating, which are reasonably well
matched and offer full spectral coverage.  The old high-resolution
pair work reasonably well together, but another possible option is
high red with medium blue, which would allow more complete spectral
coverage and slightly better S:N on the blue side.

Here are the three proposed combinations:

           (using the old grating names)
 Setup 1:  medium red, low blue
   spectral coverage: >6000 angstroms either side of the dichroic at 5350
   resolution ~3.1 A/pix

 Setup 2:  high red, high blue
   spectral coverage: 2300 to red, 1600 to blue of dichroic
   resolution: 1.2 A/pix red, 0.8 A/pix blue

 Setup 3:  high red, medium blue
   spectral coverage: 2300 to red, 3200 to blue of dichroic
   resolution: 1.2 A/pix red, 1.6 A/pix blue

My inclination is to install Setups 1 and 2 more or less permanently.
Observers who have a particular scientific need for the extra
throughput or spectral coverage of Setup 3 would be required to make a
special request with plenty of advance warning.  I anticipate that
most observers' needs would be satisfied by the first two
combinations.  However, I thought it would be best to check with the
user community first and find out if my expectations are reasonable.

So here are the questions for DIS users to consider:

1) Would your science needs be met by a permanent installation of
   Setups 1 and 2 above?

2) How often do you anticipate needing Setup 3?

3) Do any other combinations of our six gratings seem scientifically
   valuable to you?  [Note that blue gratings cannot be used on the
   red side or vice versa, since the blaze would go in the wrong

4) Since we are mixing and matching the gratings that used to be
   called low, medium, and high, do you have any suggestions for what
   we should call the new combinations (aka Setups 1, 2, and 3)?

Please let us know what you think in the next week or so.


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