IRS News

The CTIO IR Spectrometer has recently returned to service after one of its trienniel facelifts. It was removed from service for the months of March-May to be modified to work with the new 4-m tip-tilt secondary at f/14. In more detail:

• The entrance optics were modified to accept the f/14.5 beam from the telescope and convert it back to the f/24.5 beam, which the IRS uses internally. The scale at the slit and the detector are unchanged.

• In addition to re-imaging the focal plane, the foreoptics now provide an image of the telescope exit pupil on a cold Lyot stop ahead of the slit. This represents a substantial improvement in baffling the input beam. In the previous incarnation of the IRS the Lyot stop was after the slit, which left opportunities for out-of-beam radiation to scatter inside the spectrometer.

• The instrument was converted to an up-looker by eliminating a 45° entrance mirror, which also served to select light from on-board comparison lamps. These were therefore sacrificed. Comparison lamps in the 4-m RC guider are adequate for setting up the instrument. Sky lines are also abundant, of course, and more useful because they are observed with same conditions as the object (flexure, illumination).

• A new mounting bracket, dubbed "Popeye," grasps the instrument between well-muscled arms and holds it rigidly below the tip-tilt guider box. Increasing the rigidity of support was another primary goal of the upgrade.

• The fine alignment of the instrument beam with the telescope beam is now done via the folding flat mirror in the tip-tilt guider box.

• Several baffles were rebuilt to make better seals against leakage of ambient temperature radiation from the dewar walls. We also scanned the dewar interior and found one major leak, which we will plug. We also implemented a glass blocking filter immediately in front of the detector that cuts out 3-5 µm light that might have leaked in. This has the disadvantages that: 1) it must be removed for programs requiring observations in this range and 2) there are reflection losses from the glass surfaces. But until the stray light level is acceptably low without it, we believe this is the preferable configuration for observers in the 0.9-2.5 µm region.

Making the change to the f/14 secondary with tip-tilt capability has important implications for observing procedures both because of tip-tilt per se but also because of some more subtle changes in the guiding/acquisition environment.

• The on-axis dichroic of the old system is replaced by a dichroic in the tip-tilt guider box. A modified CTIO CCDTV views the on-axis field in visible light via the dichroic (in transmission). The CCDTV is no longer fixed with respect to the field. It can be scanned to cover a field about 5' square. The camera can be used for both acquisition (in a panoramic mode) and for fast-guiding (tip-tilt) anywhere within the field. The tip-tilt capability has been shown to give large gains in throughput (factor of 3-5 under conditions of moderately good seeing). The system has been used only for a very limited period and we do not have reliable data on performance as a function of magnitude.

• In addition, the off-axis guide probe is useable as well. This provides an alternative for fields lacking a tip-tilt guide star, or for bright-object programs in which sensitivity is not an issue (the off-axis guider is somewhat faster to set up).

The manual on the Web has been modified to reflect the changes in the IRS. Those wishing an archival copy are advised to wait before downloading, however. The present version is subject to frequent revision.

Brooke Gregory