The InfraRed Spectrograph (IRS) is one of three focal plane instruments on the Spitzer Space Telescope. It consists of four separate modules (Short-Low, Short-High, Long-Low, Long-High; see Figure 2.1) which provide low (R~60-130) and moderate (R~600) resolution spectroscopic capabilities from 5.2 to 38 microns. The low-resolution modules employ long-slit designs that allow both spectral and one-dimensional spatial information to be acquired simultaneously on the same detector array. Each spectroscopic aperture in the low-resolution modules is divided into two in-line sub-slits. These subslits provide spectroscopy in either the first or second order. The high-resolution modules use a cross-dispersed echelle design that allow broad spectral coverage in a single exposure. Each high-resolution module has a single slit. Spectra can be obtained in either Staring (Section 3.1) or Mapping (Section 3.2) mode.
In addition, the IRS provides imaging in two filters (13-18 and 18-26 microns) and onboard software to autonomously identify point sources and accurately place them (by offsetting the telescope) in any of the IRS slits. These peak-up arrays are also used to obtain calibrated images of the sky at 16 and 22 microns over an approximately 1x1 square arcminute field of view, providing the only direct imaging capability on Spitzer between 8 and 24 microns. See Figure 2.2 for an illustration of the relative sizes of the slits and Table 2.1 for a summary of the wavelength and resolution ranges of each module.
Figure 2.1: The IRS Cold Assembly.
Table 2.1: IRS module characteristics.
*SL1 spectra may exhibit the "14 micron teardrop", which affects wavelengths beyond approximately 13.2 microns (see Chapter 7). LL1 spectra become very noisy beyond 38 microns.
**The resolving power is approximately constant as a function of wavelength within each low-resolution spectroscopic order.
***The width of each slit is constant to 1% over its length.
^ This represents the field of view for the peak-up arrays.
Figure 2.2 Schematic representation of the IRS slits and Peak-Up apertures. Note that the IRS slits are not parallel in the Spitzer focal plane.
The interaction of the light beam with the telescope surfaces resulted in the appearance of characteristic diffraction patterns at the telescope focal plane. As with other telescopes, these patterns consist primarily of Airy rings (due to the finite telescope aperture) and radial spikes (due to the secondary mirror supports). For bright sources, the power in these features may be significant. For example, the simulation software STINYTIM indicates that, for a source with a flux density of 1 Jy at 8 microns, the PSF will be detectable at the 1 mJy level up to 10 arcsec away from the source centroid. For a source that is 1 Jy at 25 microns, the 1 mJy level is reached at a distance of about 30 arcsec. As a rule of thumb, the flux will be smaller than 1/100 of the peak source flux density at distances larger than 1.5 to 2 slit widths from the source center (for the low-resolution modules). This rule of thumb is provided as a general guideline only and should not replace a detailed PSF analysis using STINYTIM. See Figure 2.3 for a simulation of the 8 micron PSF and Section 2.7 for more information about the PSF in the PUI mode.
Figure 2.3: The PSF at 8 microns, centered in the SL slit (shown as a rectangle 3.6 arcsec wide). Simulated image using STINYTIM.
2.3 Short-Low (SL) Module
The Short-Low (SL) moduleis a grating spectrograph and imager that covers, in two orders, the nominal spectral range from 5.2 to 14.5 microns at 60 < R < 128. The red coverage of the SL module extends to slightly longer wavelengths (15.4 microns) than indicated here, but SL spectra could not be calibrated redward of 14.5 microns. The plate scale for the SL 128x128 Si:As Blocked Impurity Band (BIB) array is 1.8 arcsec/pixel. When the source is on one half of the long slit, its light passes through a 7.5-15 micron bandpass filter and is diffracted by the grating in first order (SL1). When the source is moved to the other half of the same slit, its light passes through a 5-7.5 micron bandpass filter and is diffracted by the grating in second order (SL2). The module also produces a second, short spectral segment that covers the 7.3-8.7 micron spectral region in first order (R=60-72), when the source falls on the 2nd order (SL2) sub-slit. This overlap region, known as the “bonus segment,” was intended to aid the user in normalizing the first- and second-order segments of the spectrum. Figure 2.4 shows the optical layout of the SL module.
In addition to obtaining spectra, the SL module includes the IRS peak-up arrays. These arrays are used for both Peak-Up Acquisition (PUA; see Section 3.1) and for Peak-Up Imaging (PUI; see Section 3.3). Light passes through one of two bandpass filters and is imaged onto the SL detector array focal plane (in a different detector region from that used for the spectrum).