6.1.5 Grisms
A selection of three grisms are available in FLITECAM to provide medium resolution spectroscopic capabilities across the entire 1 - 5.5 μm wavelength 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
| 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 | -- | -- |
| 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.
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).
