HET LRS Status Update
Gary J. Hill
May 27, 2002

This memo describes two new operating modes for the HET LRS, along with their availability for the August - November 2002 trimester.

1. MOS unit astrometric setups

We have now successfully tested out astrometric setups of the LRS Multi-Object Spectroscopy (MOS) unit on astronomical targets. We have achieved accuracies at least as good as those obtained using pre-images from the LRS. As a result we can now release this setup mode to astronomers in the HET consortium for use in the upcoming phase II proposals. Astrometric setups use accurate coordinates from other images, along with a field center and position angle to generate a setup. The field center and PA are chosen and the slitlets can be placed interactively. Full details of the script to create MOS configuration files from astrometry will be released shortly by Joe Tufts and myself, in time for Phase II proposals. At this time, for phase I proposals to your TAC, you should decide whether to use this mode, which requires no telescope time overhead, or the previous mode involving pre-images (which cost 10 minutes plus the integration time for the image). If you do not have accurate coordinates (better than 0.25 arcsec rms) a pre image is still recommended.

The output sent to the RAs for MOS observations will remain the same whether using astrometry or pre-images. The following are required:

2. Limited availablility of Volume Holographic Grism G3

The first volume holographic grism for LRS has now had a very preliminary test in the instrument and all indications are that it is very efficient. After consulting with Larry Ramsey, we have decided to make the grism available on a "shared risk" basis for one dark time in the upcoming trimester. Shared risk means that you can propose for the grism, but technical issues may mean that your observations cannot be completed. We also have no information on the sensitivity of observations with this grism, we only know its efficiency relative to grism G1. The LRS only has room to carry 2 grisms at any given time, and G3 would be used in place of G2, displacing it from the queue. If there is not sufficient demand for G3, then it may not be scheduled until the following trimester.

The preliminary properties of G3 are as follows:

These efficiency values are for the LRS and HET with a 8 m aperture and no obstruction. The HET+LRS with Grism G1 currently peaks at 10% efficiency, so G3 is significantly faster overall.

So G3 covers beyond the CaII IR triplet and gives sufficient resolution for galaxy dynamics work. Eventually, with the right blocking filter, it will be possible to use G3 in second order in the blue with approximate coverage from 3600 to 4500 A at the same resolving power. This mode is not yet available.

G3 should be used with the OG515 blocking filter. A typical instrument configuration would look like: lrs_g3_20_OG515. Note that the data has the wavelength _decreasing_ with increasing column on the CCD, the opposite of the other two grisms.

3. Overall efficiency of the HET+LRS and image quality

A recent measurement of the throughput of the HET+LRS with grism G1 shows the peak throughput to be 10%, compared to about 15% 18 months ago. This degradation is likley to be due to the continued degradation of the primary mirror and corrector reflective coatings and the current dirty state of the mirrors. It would be prudent to consider the sensitivity numbers given on the website for LRS as very optimistic given this degradation.

The HET seeing has been improving and will counteract this loss of throughput, but it is necessary to check the focus during a track as we appear to be drifting in focus much more rapidly than before. This may be due to SAMS locking up the mirror. As a result we recommend breaking all long exposures into 20 minute pieces with a refocus in between. The guider drift that had hampered long exposures before has been cured.