Photometric Calibrations for the Sirtf Infrared Spectrograph

The SIRTF InfraRed Spectrograph (IRS) is faced with many of the same calibration challenges that were experienced in the ISO SWS calibration program, owing to similar wavelength coverage and overlapping spectral resolutions of the two instruments. Although the IRS is up to ∼300 times more sensitive and without moving parts, imposing unique calibration challenges on their own, an overlap in photometric sensitivities of the high-resolution modules with the SWS grating sections allows lessons, resources , and certain techniques from the SWS calibration programs to be exploited. We explain where these apply in an overview of the IRS photometric calibration planning. 1. THE IRS CALIBRATION CHALLENGE The IRS is one of three science instruments in the payload of NASA's Space Infrared Telescope Facility (SIRTF), and will provide spectroscopy over the 5.3−40 µm range at spectral resolutions λ/∆λ ∼600 between two echelle spec-trographs, and λ/∆λ ∼100 between two long-slit spectro-graphs. One of the low resolution modules allows peak up imaging at 15 and 23 µm in two subarrays, so that celestial targets with with poorly known positions may be accurately placed in the IRS slits. The four instrument modules along with the cold electronics constitute the cold assemblies located within the multiple instrument chamber, which also houses the InfraRed Array Camera (IRAC) and the Multiband Imaging Photometer for SIRTF (MIPS). The IRS and MIPS share warm electronics, with savings in mass, volume, and cost. The slit dimensions, wavelength coverage, detector materials and pixel sizes for the four modules are schematically illustrated in Figure 1. Additional details on the performance characteristics of the IRS may be found in Houck & Van Cleve 1995, Roellig et al. 1998, and the SIRTF Observer's Manual. One of the key design features of the IRS is the absence of moving parts. As a consequence, all inflight calibration activities must be performed while light from celestial sources enters through the IRS slits. This impacts Figure 1. Schematic representation of the IRS slits (not to scale). The focal plane arrays are backside-illuminated BiB detectors in 128×128 pixel format. Detector materials , pixel sizes, and wavelengths associated with each module are indicated. the measurement of dark currents and photometric responses to both internal stimulators and external sources, as light from the extended zodiacal background, potentially the cold ISM, and variously from stars or other extended sources will fall on the IRS arrays. The Short Low module will also be susceptible …