6.1.3 Filter Suite

The core of the FLITECAM filter suite is a set of standard Barr bandpass filters used for imaging at J, H, K, L, & M, which are all located in filter wheel #1. In addition, filter wheel #2 holds a selection of specialty, narrow-band imaging filters, including Paschen α and Paschen α continuum filters, an ice filter centered on the 3.07 μm H2O ice feature, a polycyclic aromatic hydrocarbon (PAH) filter centered on the prominent 3.3 μ feature, and Lnarrow and Mnarrow filters. Finally, there are a number of order sorting filters (OSFs) for use with the grisms, including J and H (both dual purpose), Kwide, Klong, and ''L+M''. Additional details of the OSFs are given in Section 6.1.5.

As the bulk of the instrument commissioning was performed in the combined FLITECAM/HIPO (FLIPO) mode, some of the FLITECAM observation modes lack detailed performance characterics. This is due to the lack of circulation of stratospheric air down the Naysmyth tube which keeps the FLIPO foreoptics at cabin temperature, greatly increasing the observed background. Some experience in these modes (generally longward of 3.5 microns) was obtained during the first FLITECAM-only flight series in September/October 2015 and are undergoing analysis at the time of writing.

In the combined FLITECAM/HIPO configuration, imaging observations past 3 microns are required to use increasingly small subarrays; spectroscopic observations past 4 microns are not possible at all. In the FLITECAM-solo configuration, however, full-frame imaging observations are possible out to 4 microns, and the full suite of FLITECAM grisms are available.

The central wavelengths and band widths of the available imaging filters are provided in Table 6-1. This table also includes the sensitivity values (Minimum Detectable Continuum Flux; MDCF) discussed in Section 6.1.4. Filters that will only be available on a shared risk basis during Cycle 4 are indicated. Figure 6-3 displays the transmission profiles (normalized to their peak transmission) for the imaging filters over-plotted on an ATRAN model of the atmospheric transmission. The filter transmission curves are available on the SOFIA 'Science Instrument Suite'' website in the section for FLITECAM.

Table 6-1: FLITECAM Filter Characteristics

Desig-
nation
Pass-
band
λeff
(μm)
Peak Trans.
(%)
5% Band-
widtha

(μm)
R
(Δλ/λ)
MDCFb
(μJy)
MDCFc
(μJy)
Profiles
Standard Filters
FLT_J J 1.239 93.7 0.293 22.3 25.7 27.4 PNG DAT
FLT_H H 1.631 95.3 0.305 17 33.5 35.7 PNG DAT
FLT_K K 2.104 95.4 0.395 16.8 38.7 57.5 PNG DAT
FLT_Lprime L′ 3.855 93.5 0.7 15.7 - - PNG DAT
FLT_L Ld 3.53 94.1 0.65 15.7 440 - PNG DAT
FLT_M Md 4.838 91.8 0.645 10.8 2720 - PNG DAT
Specialty Filters
FLT_Pa Pa α 1.874 84.2 0.033 1.2 179 195 PNG DAT
FLT_Pa_cont Pa α Cont. 1.9 88.3 0.033 1.2 178 195 PNG DAT
FLT_ICE_308 Iced 3.049 88.1 0.191 5.2 504 1130 PNG DAT
FLT_PAH_329 PAHd 3.302 91.4 0.115 2.7 1070 2260 PNG DAT
FLT_NbL Lnarrowd 3.602 90.5 0.231 5.1 1010 2110 PNG DAT
FLT_NbM Mnarrowd 4.804 89.5 0.19 3.1 5430 - PNG DAT
Order Sorting Filterse
Hwide Hwide 1.794 97.1 0.587 29.1 - - PNG DAT
Kwide Kwide 2.299 96 0.882 35.3 - - PNG DAT
Klong Klong 2.45 95.5 0.55 19.3 - - PNG DAT
L+M L+M 4.11 92.5 2.715 64.2 - - PNG DAT

a Bandwidth for transmission level is >5%

b See Section 6.1.4

c Empirically measured MDCF values when FLITECAM is co-mounted with HIPO in the FLIPO configuration.

d Will be offered as Shared Risk during Cycle 4

e Will not be available for imaging

 

FLITECAM filter transmission curves

Figure 6-3: Plotted here are the FLITECAM filter transmission curves overlayed on an ATRAN model of the atmospheric transmission across the FLITECAM bandpass assuming a zenith angle of 45 degrees and 7 μm of precipitable water vapor. The transmission profiles of the standard filters are shown in red, specialty filters are shown in blue, and OSFs in gold. The OSF transmission profiles have been scaled by 50% for clarity.