Scientific/Technical Abstract:

FORCAST is a facility-class, mid/far-infrared camera for SOFIA. The instrument has a short wavelength camera (SWC) that operates from ~5 - 25 µm and a long wavelength camera (LWC) that operates from 25 - 40 µm, with several filters available in both cameras. The cameras can be used individually over the whole wavelength range, or together for simultaneous imaging of the same field of view (FOV) but with restricted wavelength coverage in the SWC of 10 - 25 µm. Using 256x256 Si:As and Si:Sb blocked-impurity-band detector arrays to provide high-sensitivity wide-field imaging, FORCAST samples at 0.75 arcsec/pixel giving a 3.2 arcmin x 3.2 arcmin instantaneous field-of-view. Diffraction limited imaging for λ > 15 µm is expected during full operations. The current filter set includes filters centered at 5.4, 6.4, 6.6, 7.7, 8.6, 11.1, 11.3, 19.7, 24.2, 31.5, 33.6, 34.8, and 37.1 µm. For sufficiently small objects, chopping can be performed on the array. The FORCAST instrument uses mature, low-risk technology ensuring high reliability.

FORCAST Performance Summary:

The instrument sensitivity and resolution summaries are provided to permit estimating feasibility of scientific investigations. The FORCAST performance for Full Operational Capability will be better than that during Early Science and commissioning.

FORCAST Angular Resolution

Shown below is a plot of the expected FORCAST angular resolution (FWHM in arcseconds) versus wavelength. The solid line is the diffraction limit of the 2.5-m primary with a 14% central obscuration. The dashed line is the image quality (IQ) expected by calculating the root sum square of the FWHM diffraction combined with the FWHM telescope jitter, here assumed to be 2.9'' (1.25'' rms). The circles are the IQ values measured from data obtained on a star during one leg of the Observatory Characterization Flight #3. Although these data points represent some of the best IQ measured during Early Science, we expect these values to be typical of the IQ obtained after the installation of the active mass dampers scheduled for implementation prior to the Cycle 1 observing period.

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FORCAST Spectral Passbands and Filter Characteristics

Wavelength range: 5 - 40 µm. FORCAST has two 256 x 256 blocked- impurity-band detector arrays (channels). The Short Wavelength Channel (SWC) is a Si:As array sensitive from ~5 - 25 µm and the Long Wavelength Channel (LWC) is a Si:Sb array with coverage from ~25 - 40 µm. Each detector has a plate scale of 0.75'' per pixel, yielding a 3.2' x 3.2' instantaneous field of view (FOV). By inserting a dichroic, both channels can be used to simultaneously observe the same FOV between 10 - 25 µm with the SWC and 25 - 40 µm with the LWC.

The central wavelengths (bandpasses) of the filters in μm are provided below along with features of interest within the bandpasses. The PostScript files contain plots of the full system throughput (excluding atmosphere) for the filters. Any one of the available filters can be used in Single Channel mode. FORCAST may be used in Dual Channel mode in which any combination of SWC and LWC filters can be used simultaneously.

In order to accommodate the FORCAST grisms, some filters have been removed from the filter wheels and will be unavailable during Cycle 1. The full filter complement with notes on which filters will not be available during Cycle 1 is provided below.

Below is an ATRAN model of the atmospheric absorption as a function of wavelength in the FORCAST band (assuming zenith angle 45 degrees and 7 µm of precipitable H2O). Overplotted are the transmission profiles for all of the SWC and LWC filters, even those unavailable during Cycle 1. The profiles have been scaled to the peak transmission. For a complete and up-to-date list of the filters available during Cycle 1, please see the Call for Proposals on the Cycle 1 page.

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FORCAST Grism Spectroscopy

FORCAST will be able to perform grism spectroscopy in both long-slit mode at a resolution of R= λ/Δλ=90-300 with coverage from 5-50 µm and in cross-dispersed mode with instantaneous coverage from 5-8 µm at R=1200 and 8-14 µm at R=800. Three slits will be available, two long slits (2.4'' x 191'' and 4.7'' x 191'') and a short-slit (2.4'' x 11.2'') for the high-resolution cross-dispersed mode, yielding a total of 6 grating/slit combinations as detailed in the table below. Spectroscopy will be performed by chopping and nodding either on-slit or off.

The grisms are designed to fit in the existing FORCAST filter wheels along with the existing suite of bandpass filters. The grisms include four micro-machined silicon gratings for use at 5-8 µm and 17-37 µm, and two ruled KRS-5 gratings for use from 8-14 µm. The slits are housed in the aperture wheel positioned at the cold field stop.

The high resolution spectra are obtained by passing the incoming beam through first one grism followed by a second grism to cross-disperse the light. This is indicated in the table below.

Note: FORCAST Grism modes will be available during Cycle 1 on a shared risk basis.

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FORCAST Imaging Sensitivities

Below are shown imaging sensitivities for a continuum point source, at the effective wavelengths of the filters offered during Cycle 1. The Minimum Detectable Continuum Flux (MDCF; 80% enclosed energy) in mJy needed to get S/N = 4 in 900 seconds is plotted versus wavelength. The values reported assume an altitude of 41,000 feet and a water vapor overburden of 7 µm. In practice, the atmospheric transmission will affect sensitivity, depending on the true water vapor overburden. The red squares indicate the MDCF values in Single Channel mode, while the blue circles are for Dual Channel mode using the dichroic. The MDCF scales roughly as (S/N) / sqrt(t) where t = net integration time. The horizontal bars indicate the effective bandpass at each wavelength. At the shorter wavelengths the bandpass is sometimes narrower than the symbol size.

For background-limited observations, as will be the case with FORCAST on SOFIA, chopping and nodding off-chip will generally result in the same signal to noise (S/N) as chopping and nodding on-chip.

Calculations of S/N for various chop-nod scenarios are provided here in detail and in a simplified version here.

Plotted below are the expected FORCAST grism sensitivities for a continuum point source using each of the grism combinations that will be available during Cycle 1. The values reported are for a S/N of 4 in 900 seconds with a water vapor overburden of 7 µm, assuming an altitude of 41,000 feet and a 60 degree zenith angle. The data are overlaid on an ATRAN model shown in light blue.

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FORCAST Observation Preparation and Data Handling

A description of the Observing Modes available for imaging with FORCAST can be found in the Observing Modes Document.

Once the observatory has been fully commissioned, additional information will be provided, including a full accounting of overheads associated with particular instrument set-ups and observing strategies; information on preparing observations using the SPT; and details regarding data formatting, calibration and reduction.

Those interested in proposing for observing time during SOFIA Cycle 1 and in preparing successful observations with FORCAST should carefully read the Cycle 1 Call for Proposals and the Cycle 1 Observer's Handbook, both of which are available from the Cycle 1 page. Users are encouraged to check back regularly on the Cycle 1 page to ensure that they have the most up-to-date version of the Observer's Handbook.

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Disclaimer

All sensitivity and resolution data are preliminary, and based on anticipated performance of the observatory and the instrument.  Actual performance of the SOFIA telescope and instrument combination will be established after flight operations begin.  Telescope performance is expected to be upgraded during the first two years, and instrument performance may be upgraded, or additional modes or capabilities may be added.

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Additional References:

Adams et al., "FORCAST: a first light facility instrument for SOFIA," Ground-based and Airborne Instrumentation for Astronomy III, Ian S. McLean, Suzanne K. Ramsay, & Hideki Takami, Editors, Proc. SPIE 7735, 77351U (2010), DOI: 10.1117/12.857049 [pdf]

Keller et al., "Progress report on FORCAST grism spectroscopy as a future general observer instrument mode on SOFIA," Ground-based and Airborne Instrumentation for Astronomy III, Ian S. McLean, Suzanne K. Ramsay, & Hideki Takami, Editors, Proc. SPIE 7735, 77356N (2010), DOI: 10.1117/12.857127 [pdf]

Deen et al., "Quick-look reduction software for FORCAST grism mode on SOFIA," Ground-based and Airborne Instrumentation for Astronomy III, Ian S. McLean, Suzanne K. Ramsay, & Hideki Takami, Editors, Proc. SPIE 7735, 773570 (2010), DOI: 10.1117/12.856608

Deen et al., "A silicon and KRS-5 grism suite for FORCAST on SOFIA," Ground-based and Airborne Instrumentation for Astronomy II, Ian S. McLean & Mark M. Casali, Editors, Proc. SPIE 7014, 70142C (2008), DOI: 10.1117/12.788565 [pdf]

Ennico et al., "Grism performance for mid-IR (5-40 micron) spectroscopy," Ground-based and Airborne Instrumentation for Astronomy, Ian S. McLean & Masanori Iye, Editors, Proc. SPIE 6269, 62691Q (2006), DOI: 10.1117/12.672256 [pdf]

Adams et al., "Testing of mid-infrared detector arrays for FORCAST," Optical and Infrared Detectors for Astronomy, James D. Garnett & James W. Beletic, Editors, Proc. SPIE 5499, 442 (2004), DOI: 10.1117/12.551490