The Long-Low (LL) module is a grating spectrograph that covers, in two orders, the nominal spectral range from 14.0-38.0 micron at 57 < R < 126. The red coverage of the LL module extends to slightly longer wavelengths (42.3 microns) than indicated here, but LL spectra could not be calibrated redward of 38.0 microns because of light leakage at the long wavelength end caused by pre-launch delamination of the order-sorting filter. The detector is a 128x128 Si:Sb BIB array with a plate scale of 5.1 arcsec/pixel. As with the SL module, the LL aperture consists of two in-line sub-slits with a small gap between them. When the source is in one sub-slit, light passes through a 14-21 micron bandpass filter and is diffracted by the grating in second order (LL2). When the source is moved to the other sub-slit, light passes through a 21-40 micron bandpass filter and is diffracted by the grating in first order (LL1). Long-Low also has a “bonus segment,” similar to that in SL, which covers 19.4-21.7 micron (R=57-63). Figure 2.5 depicts the locations of the LL module components in the optical paths.
Figure 2.5: Long-Low module optical components and paths.
The IRS LL module displayed degraded sensitivity in 1st order during ground testing prior to launch due to delamination of the order-sorting filter. The LL1 filter survived launch to return useable science data. Based upon observations of hot stars, contamination of the LL1 spectrum from filter leakage is no more than 5% (shortward of about 38 microns). That is, the flux of the obtained spectrum deviates by less than 5% from the expected spectrum. This leakage is most pronounced beyond about 38 microns, where the spectrum can be severely contamined by blue light
