Subject: preparations for new DIS gratings


Submitted: Fri, 05 May 2006 11:59:06 -0600

Message number: 964 (previous: 963, next: 965 up: Index)

To: current users of DIS on the 3.5m

As most people are probably aware, we are in the process of several upgrades
to DIS to improve performance. One aspect of the upgrades is that we are
obtaining new gratings that we hope will 1) better match the current
optics/chip in the instrument in terms of wavelength coverage, and 2)
provide improvements in throughput.

Once we obtain/test/install the new gratings, which we currently hope will
be happening in June (although since we have not yet received the new
gratings, there is some schedule uncertainty), we would like these
gratings to become the "default" set of gratings; the old gratings will
be retained in their current housings and can be mounted with grating
swaps, but, as in the past, we would like to minimize the required number
of grating swaps. The purpose of this message is to provide information
about the new gratings, to propose a new naming scheme for our grating
complement, and to solicit input about whether there are likely to be
any issues with changing the "defaults", possibly as early as the beginning of
third quarter.

If all goes with with our acquisitions, our grating complement will be:

name  descrip  proposed status   dispersion^1 coverage^2 peak throughput 
                                  A/pixel                  wavelength
B300   old BL       retire         2.43        4860        5000         
      dispersion too low for match to current chip/optics, good throughput

B400    new    default low/med     1.85        3700        5000          
      full coverage from atmospheric cutoff to dichroic w/minimal vignetting

B600   old BM       retire         1.23        2460        4750        
      full coverage but with some vignetting, throughput slightly worse

B1200  old BH  default high        0.62        1240        4500
      high res, decent blaze for entire blue range

R150   old RL       retire         5.03(4.5?)  9000        ~6500
      dispersion too low for match to current chip/optics, good throughput

R300H  old RM       retire         2.26        4520        ~6500
      dispersion good, but poor throughput 
      (H stands for the manufacturer, Hyperfine)

R300    new    default low/med     2.26        4520        6500
      new grating for full red coverage with good throughput

R830   old RH   high res >8000A    0.84        1680        8000
      high res, good throughput, but blazed for longer wavelengths

R1200   new  default high (<8000A) 0.56       1120        6500
      high res, blazed for shorter wavelength end of red, higher dispersion
1. dispersions for existing gratings from APO web page (note number for RL
  may be off as indicated, since web number doesn't match expectations); 
  dispersions for new gratings are approximate
2. coverage is for 2000 pixels, but note there is some moderate vignetting
  (up to ~30%) at each side of the chip

The current defaults are B300/R300H (BL+RM) for lower dispersion and
B1200/R830 (BH+RH) for higher dispersion.

The proposed "default" gratings would be B400/R300 for lower dispersion
and B1200/R1200 for higher dispersion, with grating swap requests of R830 
for R1200 for high res applications at longer wavelengths.

If people have good reasons for wanting to keep using the old gratings
slated for "retirement" after the new ones are obtained (assuming nominal
performance), please communicate these reasons, and the timescales over
which they will be valid, to your Users' Committee representative and
to me, so we can decide on an acceptable implementation schedule.

APO APO APO APO APO  Apache Point Observatory 3.5m  APO APO APO
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