IRS News

The CTIO IR Spectrometer has benefited from detector and grating upgrades within the last few months, and a major optical modification will take place in the March-May period.

New Detector

The IRS has been the beneficiary of the detector upgrade in the COB instrument to an Aladdin detector: COB's previous detector, a 256 x 256 Santa Barbara detector of excellent cosmetic quality and low dark current, has been handed down to the IRS. The detector has lower well capacity than the previous one, but it is adequate for operating in all bands. The most important implication of this change is to require more care in setting the integration time under high flux conditions (beyond 3µm). Here is a brief summary of its characteristics:

                        IRS Detector

               Type               InSb

            Gain                8.1 e-/ADU
            Bias                0.55 volts
            Read noise          45 e- (single read-pair)
            2% non-linear       3500 ADU
            4% non-linear       5000 ADU
            Dark                0.5 e-/sec

The stray light level in the instrument, (currently ~ 16 e-/sec), is now strongly limiting the performance of the instrument under some conditions. We are about to embark on several efforts to reduce stray light so as to get the full benefits of the detector.

New Cross-dispersed grating

The original cross-dispersed grating module used an off-the-shelf grating replica from Spectronic Instruments (formerly Milton Roy). The new one has a custom ruling designed to match the free spectral ranges more nearly to the chip dimensions. Also, the original ruling was not a particularly efficient one and it was hoped that a new ruling would improve the efficiency. Unfortunately, in the latter area of performance the cross-dispersed grating module is still significantly less efficient than the 75 line/mm, 1.9µm blaze grating in the K band. In the H band the throughput is similar and at short wavelengths the cross-dispersed prism somewhat surpasses the simple grating/filter combination. The cross dispersed mode is the clear favorite if multiband coverage is a required by the science program, for reasons both of efficiency and of the consistency of relative throughput over the bands. The following table describes the new resolution data for the device:

               (µm)                             Dispersion
  Band      (central)        2-pix Res.          10-3µm/pixel

   K          2.17              558                 1.94
   H          1.63              561                 1.45
   J          1.30              559                 1.16
   I          1.08              558                 0.97

Conversion to f/14

The foreoptics conversion to accept an f/14 input beam, postponed due to a shortage of engineering resources, will take place during March and April 1997 with telescope tests in May. This will match the instrument input to our new f/14 tip-tilt secondary in order to benefit from the tighter image cores it provides. We will also implement improvements in the cold stop and internal baffling for scattered light control. The image scale at the slit will remain unchanged, with the a two pixel slit size at the 4-m being 0.6". The internal f/ratio (after the slit) remains the same. So for proposal planning purposes the basic performance specifications (slit widths, resolutions, and throughput) will show little change.

Instrument manual

The IRS Users Manual, available on the CTIO Web pages at http://www.ctio.noao.edu/manuals/irs, has been updated to reflect these changes.

Brooke Gregory