6.1.5 Grisms

A selection of three grisms is available in FLITECAM to provide medium resolution spectroscopic capabilities across the entire 1 - 5.5 μm range. The grisms are mounted in filter wheel #2 and are used along with Order Sorting Filters (OSFs) mounted in filter wheel #1 (see Table 6-1) to prevent order contamination. A summary of the grism properties is provided in Table 6-2.

Grisms were chosen for use in FLITECAM since they minimize the changes necessary to the optical path while still allowing moderate spectral resolution. The grisms are made of direct-ruled (i.e. etched) KRS-5 (thallium bromoiodide) material that provides a relatively high index of refraction (n ~ 2.4) without the significant absorption features that plague grisms with adhered transmission gratings. The three grisms were fabricated by Carl-Zeiss (Jena, Germany) each with an apex angle of 34.4°, but with different groove spacings to allow coverage of the entire 1 - 5.5 μm band (see Table 6-2).

Table 6-2: FLITECAM Grism Characteristics

FLITECAM Grism Characteristics
Groove Coverage (μm) High-Res (R=λ/Δλ) Low-Res (R=λ/Δλ)
Designationa Grism Sep. (l/mm) Order OSFb
FLT_A1_LM A 162.75 1 L+Mc 4.395-5.533 N/A N/A
FLT_A2_KL      2 Klong 2.270-2.722 1690 1140
FLT_A3_Hw     3 Hwide 1.550-1.828 1710 1290
FLT_B1_LM  B 217 1 L+Mc 3.303-4.074 1780 1200
FLT_B2_Hw       2 Hwide 1.675-2.053 1750 1320
FLT_B3_J       3 J 1.140-1.385 1720 1425
FLT_C2_LM Cd 130.2 2 L+M 2.779-3.399 1670 1300
FLT_C3_Kw     3 Kwide 1.910-2.276 1650 1390
FLT_C4_H      4 H 1.500-1.718 1640 1400

a Order Sorting Filter

b Identifier used in the SOFIA Proposal Tool (SPT)

c This combination of Grism and OSF will be available on a Shared Risk basis during Cycle 4

d The C Grism suffers from excess light at the blue end of each order. This issue is under investigation at the time of this writing.

The FLITECAM slit mask is mounted in a fixed slide mechanism at the telescope focus, immediately inside the entrance window to the dewar.

No calibration lamps are installed in FLITECAM. Consequently, wavelength calibration will be performed using atmospheric absorption lines.


FLITECAM OSF band passes

Figure 6-5: Plotted here are band passes for each of the grism + order sorting filter (OSF) combinations available for FLITECAM grism spectroscopy. The grism used for each OSF set is labeled on the right. In light blue is a plot of the ATRAN model of the atmospheric transmission across the FLITECAM bandpass (assuming a zenith angle of 45 degrees and 7 μm of precipitable water vapor).

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