7.1.5 Grisms

The suite of 6 grisms available for FORCAST provide low to medium resolution coverage throughout most of the range from 5 - 40 μm. The grisms are situated in the four filter wheels, two in each SWC wheel and one in each LWC wheel. The arrangement is chosen to minimize the impact on the imaging capabilities of the instrument. The grisms are blazed, diffraction gratings used in transmission and stacked with blocking filters to prevent order contamination. A summary of the grism properties is provided in Table 7-4. Note that during Cycle 4, the cross-dispersed XD configuration will not be available. The information on XD modes below is for informational purposes.

Grisms FOR_G063, FOR_G227, FOR_G329, and the FOR_XG063 dispersing grism provided by the University of Texas at Austin, are made of silicon to take advantage of its high index of refraction, which allows optimum spectral resolution. However, these grisms suffer from various absorption artifacts precluding their use in the 8 - 17 μm window. Coverage in this region is provided by the FOR_G111 grism, constructed of KRS-5 (thallium bromoiodide) by Carl-Zeiss (Jena, Germany). These latter two grisms have a lower spectral resolution due to the lower index of refraction of the KRS-5 material.

Three slits are available, two long slits (2.4'' x 191'', 4.7'' x 191'') and a short slit (2.4'' x 11.2''). The narrow slits yield higher resolution data. All of the slits are located in the aperture wheel of the instrument. The cross-dispersed spectra are obtained by using the short slit and passing the beam first through the low-resolution grism (either FOR_G063 or FOR_G111), followed by a disperser. Although grisms are available in both cameras, during Cycle 4 grism spectroscopy will be available only in single channel mode.

It is important to note that due to the fixed position of the slits in the aperture wheels, the lack of a field de-rotator, and the fact that SOFIA behaves in many respects as an Alt-Az telescope, the orientation of the slit on the sky will be dependent on the flight plan and will not be able to be predetermined. Furthermore, the slit orientation rotates on the sky with each telescope Line-of-Sight (LOS) rewind. These limitations may be especially important to consider when proposing observations of extended objects.

Table 7-4: FORCAST Grism Characteristics

Channel Grism Material Groove Sep. (μm) Prism Angle (°) Order Coverage (μm) R (λ/Δλ)
SWC FOR_G063 Si 25 6.16 1 4.9 - 8.0 90/180a
FOR_XG063b Si 87 32.6 15 - 23 4.9 - 8.0 1200c
FOR_G111 KRS-5 32 15.2 1 8.4 - 13.7 150/300a
LWC FOR_G227 Si 87 6.16 1 17.6 - 27.7 70/140a
FOR_G329 Si 142 11.07 2 28.7 - 37.1 110/220a

a For the 4.7'' x 191'' and the 2.4'' x 191'' slits, respectively

b Not available during Cycle 4

c Only available with the 2.4'' x 11.2'' slit

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