4.1 Specifications

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4.1   Specifications

4.1.1   Instrument Overview

The First Light Infrared TEst CAMera (FLITECAM) is an infrared camera operating between 1.0–5.5 μm. It consists of a 1024x1024 InSb detector with 0.475 x 0.475 arcsec pixels and uses refractive optics to provide a 8 arcmin 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 is called FLIPO. Proposers wanting to use this mode can find the details of HIPO in Chapter 9.

NOTE: FLITECAM, either stand-alone or co-mounted with HIPO, is not being offered for observations in Cycle 6.  Limited new observations may be requested through the Director's Discretionary Time process.  Please see Section 1.0 of the Call for Proposals for a statement of the policy.

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4.1.1.1   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 4-1 with a full ray trace diagram in Figure 4-2. 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 x 8 arcmin 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 arcmin on the detector) divided in half with two different widths, one approximately 2 arcsec and the other 1 arcsec. 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 4.1.2.4.

Figure 4-1.
FLITECAM schematic

Figure 4-1. This is a block diagram of the front end of the FLITECAM instrument with labels of important components.

 
Figure 4-2.
FLITECAM ray diagram

Figure 4-2. 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.

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4.1.2   Performance

4.1.2.1   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 arcmin square FOV. The detector is optimized for use between 1–5.5 microns, and has a read noise of approximately 40 e- and dark current ≤ 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 been parameterized.

The detector well depth is relatively shallow (~ 80,000 e-) which, when combined with the detector quantum efficiency (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.

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4.1.2.2   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, and 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, Paschen α continuum, an ice filter centered on the 3.07 μm H2O ice feature, a polycyclic aromatic hydrocarbon (PAH) filter centered on the prominent 3.3 μm feature, Lnarrow, and Mnarrow. 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 4.2.3.4.
 
As the bulk of the instrument commissioning was performed in the combined FLITECAM/HIPO (FLIPO) mode, some of the FLITECAM only 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.
 
In the combined FLITECAM/HIPO configuration, imaging observations past 3 microns are required to use increasingly small subarrays; spectroscopic observations past 4 μm are not possible at all. In the FLITECAM-solo configuration, however, full-frame imaging observations are possible out to 4 μm, and the full suite of FLITECAM grisms are available.
 
The central wavelengths and band widths of the available imaging filters are provided in Table 4-1. This table also includes the sensitivity values (Minimum Detectable Continuum Flux; MDCF) discussed in Section 4.1.2.3. Filters that will only be available on a shared risk basis during Cycle 6 are indicated. Figure 4-3 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 and linked in the last column of Table 4-1 below.
 
Table 4-1
FLITECAM Filter Characteristics
Name Bandpass λeff
(μm)
Peak Trans.
(%)
Band-
widtha

(μm)
Rb MDCF (μJy) Profile
FLITECAM FLIPO
Standard Filters
FLT_J J 1.239 93.7 0.293 0.36 25.7 27.4 PNG DAT
FLT_H H 1.631 95.3 0.305 0.50 33.5 35.7 PNG DAT
FLT_K K 2.104 95.4 0.395 0.83 38.7 57.5 PNG DAT
FLT_Lprime L′ 3.855 93.5 0.7 2.70 - - PNG DAT
FLT_L Lc 3.53 94.1 0.65 2.29 440 - PNG DAT
FLT_M Mc 4.838 91.8 0.645 3.12 2720 - PNG DAT
Specialty Filters
FLT_Pa Pa α 1.874 84.2 0.033 0.06 179 195 PNG DAT
FLT_Pa_cont Pa α Cont. 1.9 88.3 0.033 0.06 178 195 PNG DAT
FLT_ICE_308 Icec 3.049 88.1 0.191 0.58 504 1130 PNG DAT
FLT_PAH_329 PAHc 3.302 91.4 0.115 0.38 1070 2260 PNG DAT
FLT_NbL Lnarrowc 3.602 90.5 0.231 0.83 1010 2110 PNG DAT
FLT_NbM Mnarrowc 4.804 89.5 0.19 0.91 5430 - PNG DAT
Order Sorting Filtersd
Hwide Hwide 1.794 97.1 0.587 1.05 - - PNG DAT
Kwide Kwide 2.299 96 0.882 2.03 - - PNG DAT
Klong Klong 2.45 95.5 0.55 1.35 - - PNG DAT
L+M L+M 4.11 92.5 2.715 11.16 - - PNG DAT
a Bandwidth for transmission level is >5%
b R = λ/Δλ
cWill be offered as Shared Risk during Cycle 6
dWill not be available for imaging
 
Figure 4-3.
plot showing FLITECAM imaging filters

Figure 4-3. 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.

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4.1.2.3   Imaging Sensitivites

The Minimum Detectable Continuum Flux (MDCF; 80% enclosed energy) in μJy needed to get S/N = 4 in 900 s 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 4-1 above and are plotted in Figure 4-4. 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 affects sensitivity, particularly at wavelengths > 4 μm, depending on the 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.

Figure 4-4.
plot showing FLITECAM imaging sensitivity

Figure 4-4. 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.

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4.1.2.4   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 4-1) to prevent order contamination. A summary of the grism properties is provided in Table 4-2.

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 4-2).

Table 4-2
Grism Characteristics
Namea Grism Sep. (l/mm) Order OSFb Coverage (μm) High-Res R Low-Res R
FLT_A1_LM A 162.75 1 L+Md 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+Md 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 Ce 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 USPOT tool
R = λ/Δλ
d This combination of Grism and OSF will be available on a Shared Risk basis during Cycle 6.
e 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 4-5.
plot of band passes for each of the grism plus order sorting filter (OSF) combinations available for FLITECAM grism spectroscopy

Figure 4-5. 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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4.1.2.5   Spectroscopic Sensitivities

Figure 4-6 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 s at a water vapor overburden of 7.3 μm, an altitude of 41K feet, and an elevation angle of 45° (i.e. at an airmass of 1.4) is shown.

Figure 4-6.
FLITECAM and FLIPO grism sensitivities

Figure 4-6. Plotted here are the FLITECAM (green line) and FLIPO (blue line) grism sensitivities for a continuum point source across the entire FLITECAM bandpass.

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