IRS: Instrument Description: Optics

The IRS optical system covers the wavelength range from 5.2 to 38.0 microns. There is no active focus control for any of the modules. The correct focus of the slit image on the focal plane was achieved by placing an appropriately machined spacer plate between the Focal Plane Mount Array (FPMA) and the module housing. The spacer plate also corrects for displacements and rotations of the actual detector array relative to its housing. The IRS instrument is capable of self-test for proper functional operation of the focal planes. Through the use of an internal flood illuminating stimulator source, located near the arrays within each of the four Cold Instrument Modules, the detector responsivity can be monitored. All mirrors and gratings are constructed of diamond machined aluminum. The filters are evaporated multi-layer interference filters.

The IRS cold assembly.

Since there are no moving parts in the IRS, multiple images/orders are projected onto the detector arrays simultaneously, and telescope motion is used to move a source along the slit. Each aperture in the low-resolution modules is divided into two sub-slits. These sub-slits provide spectroscopy in either the first or second order. Each high-resolution (echelle) module has a single slit. The tolerances of the slit widths are +/-5%, with the slit constant in width over its length to +/-1%. The Table below summarizes the properties of the IRS slits. The Figure illustrates the dimensions of the slits and peak-up arrays.

The optical design of each of the IRS spectrograph modules was organized along a set of common design rules. The widths of the entrance slits were set at flambda max , where lambda max is the maximum wavelength for that module and f is the telescope focal ratio (f = 12). Internally, each module maps this slit width geometrically across two pixels on its detector array. Slit lengths were set to be as long as possible, limited by the sizes of the detector arrays, the available space in the Cryogenic Telescope Assembly (CTA) focal plane, and the requirement to keep the different orders separated in the cross-dispersed echelle modules. All of the IRS optics are over-sized to minimize internal diffraction effects, and have their optical surfaces coated with gold to enhance their infrared reflectivity. With the exception of the imaging train of the Short-Low module, none of the modules has an internal Lyot stop.

Table: Sizes of the IRS slits

Schematic representation of the IRS slits and peak-up apertures. Note that the actual relative orientations and positions in the Spitzer focal plane are shown elsewhere.

Note that the IRS slits are not parallel in the Spitzer focal plane. The relative angle between the short-high and the long-high slit is 84.8 deg, and the angle between the short-low and the long-low slit is 85.9 deg.

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