6.1 FLITECAM Instrument Overview

The First Light Infrared TEst CAMera (FLITECAM) is an infrared camera operating between 1.0 - 5.5 microns. It consists of a 1024x1024 InSb detector with 0.475''x0.475'' pixels and uses refractive optics to provide a ~8' diameter field of view. The instrument has a set of broadband filters for imaging, as well as grisms for moderate resolution spectroscopy.

Additionally, FLITECAM was designed to be co-mounted on the telescope with the High-speed Imaging Photometer for Occultations (HIPO), providing simultaneous optical through near-IR (NIR) imaging capabilities. This mode, called FLIPO, is described in detail under Section 7, below.

6.1.1 Design

FLITECAM consists of a cryogenically cooled near-IR (NIR) camera that can be used for both imaging and grism spectroscopy. A schematic of the optical bench is shown in Figure 6-1 with a full ray trace diagram in Figure 6-2. The incoming beam first passes through the entrance aperture and into the collimator assembly, a stack of custom designed lenses that allow imaging of nearly the entire 8'x8' SOFIA field of view (FOV). The beam is then repositioned using three flat fold-mirrors so that it passes through the image pupil and through a pair of 12-position filter wheels. A fourth flat fold-mirror redirects the beam through the f/4.7 refractive imaging assembly, which then focusses the beam on the array.

When observing in spectroscopy mode, only minimal changes to the optical path are required. First, the slit mask is inserted into the beam immediately behind the aperture window at the telescope focus. The slit is a single 16.5 mm long slit (2' on the detector) divided in half with two different widths, one approximately 2'' and the other 1''. Second, the chosen grism and order sorting filter, located in filter wheel #2 and #1, respectively, are set in place. Details are presented in Section 6.1.5 below.

Diagram of the front end of FLITECAM

Figure 6-1: This is a block diagram of the front end of the FLITECAM instrument with labels of important components.

Ray diagram for FLITECAM

Figure 6-2: This is the ray diagram for the FLITECAM instrument. The inset at the upper left displays the additional lenses inserted into the light path for the pupil-viewing mode.

6.1.2 Camera Performance

The FLITECAM detector is a 1024x1024 pixel indium antimonide (InSb) Raytheon ALADDIN III array cryogenically cooled to 30 K. The detector has 27 μ pixels which translate to a plate scale of 0.475 per pixel, resulting in a nearly 8' square FOV. The detector is optimized for use between 1 - 5.5 microns, and has a read noise of ~40 e- and dark current of ≤1 e- sec-1.

The on-axis image quality (IQ) of the camera is excellent and images obtained with FLITECAM are expected to be diffraction limited from 3 - 5.5 μm. From 1 - 3 μm, the IQ will be limited by contributions from the telescope optics, telescope jitter, shear layer seeing, and the diffraction limit. Ground based tests under the best conditions at the Lick Observatory indicated that the IQ is stable within 1.5' of the optical axis, but that beyond this distance, coma begins to appear. The extent of the coma has not yet been parameterized.

The detector well depth is relatively shallow, ~80,000 e-, which, when combined with the detector QE, may necessitate relatively short exposure times, particularly on bright sources or in regions of high sky background. The shortest exposure time available for a full 1024x1024 detector readout is 0.2 seconds. However, faster readout times can be achieved using sub-arrays -- 0.08 seconds for a 512x512 sub-frame and 0.015 sec (~67 Hz) for a 64x64 pixel sub-frame. These subarrays are required to be symmetric and relative to the center of the detector. An additional ``movie'' mode is avaiable to sample a single 512x512 quadrant at ~0.08 seconds without any deadtime between images, or down to the 32x32 sub-frame (closest to the detector center) at < 0.015 seconds. Users interested in these modes should consult the FLITECAM Instrument Scientist via the SOFIA Help desk (sofia_help@sofia.usra.edu).

Care must be taken not to over-expose the detector since charge persistence can be a significant problem for InSb arrays. If the detector is over-exposed, it is necessary to take several long exposures (~5 minutes each) of blanks mounted in the filter wheel to allow the detector to recover. To help mitigate the problem of accidental saturation of the detector, filter changes are carefully orchestrated to ensure that the sky is never viewed through the two open filter wheel positions.

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.

6.1.4 Imaging Sensitivities

At the time of writing, the commissioning of FLITECAM has yet to be completed, and therefore the results of the analysis of in-flight data are not available. Instead, the Minimum Detectable Continuum Flux (MDCF; 80% enclosed energy) in μJy needed to get S/N = 4 in 900 seconds has been estimated for each filter for an altitude of 41,000 feet (7.3 μm of precipitable water vapor) from a model of the instrument and the atmosphere. The values are presented in Table 6-1 above and are plotted in Figure 6-4. The horizontal bars on the data in the figure indicate the effective bandpass at each wavelength. At the shorter wavelengths the bandpass is sometimes narrower than the symbol size.

Atmospheric transmission will affect sensitivity, particularly at wavelengths >4 μm, depending on water vapor overburden. In addition to the theoretical sensitivity values for FLITECAM we have included the as-measured imaging sensitivities in the FLIPO (combined HIPO+FLITECAM) mode. Until the instrument has been more accurately characterized at these longer wavelengths, the L and M band filters will be offered on a shared risk basis.

FLITECAM imaging sensitivity

Figure 6-4: Theoretical FLITECAM imaging sensitivities for a continuum point source at the effective wavelengths of the FLITECAM filters listed above (red boxes). Yellow boxes indicate the imaging sensitivities for the FLIPO configuration (i.e. FLITECAM co-mounted with HIPO). The values reported are for a S/N of 4 in 900 seconds at water vapor overburdens of 7 μm. The horizontal bars correspond to the photometric band pass.

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

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).

6.1.6 Spectroscopic Sensitivities

Figure 6-6 presents the expected continuum point source sensitivities for the FLITECAM grisms combined with an ATRAN model of the atmospheric transmission. The Minimum Detectable Continuum Flux (MDCF; 80% enclosed energy) in Jy needed for a S/N of 4 in 900 seconds at a water vapor overburden of 7.3 μum, an altitude of 41K feet, and an elevation angle of 45°: (an airmass of 1.4) is shown.

FLITECAM and FLIPO grism sensitivities

Figure 6-6: Plotted here are the FLITECAM (green line) and FLIPO (blue line) grism sensitivities for a continuum point source across the entire FLITECAM bandpass.