The Stratospheric Observatory for Infrared Astronomy (SOFIA) is the world’s largest airborne observatory, featuring a
2.5 meter effective aperture telescope housed in the aft section of a Boeing 747SP aircraft. SOFIA’s current instrument
suite includes: FORCAST (Faint Object InfraRed CAmera for the SOFIA Telescope), a 5-40 μm dual band
imager/grism spectrometer developed at Cornell University; HIPO (High-speed Imaging Photometer for Occultations), a
0.3-1.1μm imager built by Lowell Observatory; GREAT (German Receiver for Astronomy at Terahertz Frequencies), a
multichannel heterodyne spectrometer from 60-240 μm, developed by a consortium led by the Max Planck Institute for
Radio Astronomy; FLITECAM (First Light Infrared Test Experiment CAMera), a 1-5 μm wide-field imager/grism
spectrometer developed at UCLA; FIFI-LS (Far-Infrared Field-Imaging Line Spectrometer), a 42-200 μm IFU grating
spectrograph completed by University Stuttgart; and EXES (Echelon-Cross-Echelle Spectrograph), a 5-28 μm highresolution
spectrometer designed at the University of Texas and being completed by UC Davis and NASA Ames
Research Center. HAWC+ (High-resolution Airborne Wideband Camera) is a 50-240 μm imager that was originally
developed at the University of Chicago as a first-generation instrument (HAWC), and is being upgraded at JPL to add
polarimetry and new detectors developed at Goddard Space Flight Center (GSFC). SOFIA will continually update its
instrument suite with new instrumentation, technology demonstration experiments and upgrades to the existing
instrument suite. This paper details the current instrument capabilities and status, as well as the plans for future
The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory, carrying a 2.5 m telescope onboard a heavily modified Boeing 747SP aircraft. SOFIA is optimized for operation at infrared wavelengths, much of which is obscured for ground-based observatories by atmospheric water vapor. The SOFIA science instrument complement consists of seven instruments: FORCAST (Faint Object InfraRed CAmera for the SOFIA Telescope), GREAT (German Receiver for Astronomy at Terahertz Frequencies), HIPO (High-speed Imaging Photometer for Occultations), FLITECAM (First Light Infrared Test Experiment CAMera), FIFI-LS (Far-Infrared Field-Imaging Line Spectrometer), EXES (Echelon-Cross-Echelle Spectrograph), and HAWC (High-resolution Airborne Wideband Camera). FORCAST is a 5–40 μm imager with grism spectroscopy, developed at Cornell University. GREAT is a heterodyne spectrometer providing high-resolution spectroscopy in several bands from 60–240 μm, developed at the Max Planck Institute for Radio Astronomy. HIPO is a 0.3–1.1 μm imager, developed at Lowell Observatory. FLITECAM is a 1–5 μm wide-field imager with grism spectroscopy, developed at UCLA. FIFI-LS is a 42–210 μm integral field imaging grating spectrometer, developed at the University of Stuttgart. EXES is a 5–28 μm high-resolution spectrograph, developed at UC Davis and NASA ARC. HAWC is a 50–240 μm imager, developed at the University of Chicago, and undergoing an upgrade at JPL to add polarimetry capability and substantially larger GSFC detectors. We describe the capabilities, performance, and status of each instrument, highlighting science results obtained using FORCAST, GREAT, and HIPO during SOFIA Early Science observations conducted in 2011.
FORCAST has completed 16 engineering and science flights as the “First Light” U. S. science instrument aboard SOFIA
and will be commissioned as a SOFIA facility instrument in 2013. FORCAST offers dual channel imaging (diffractionlimited
at wavelengths < 15 microns) using a 256 x 256 pixel Si:As blocked impurity band (BIB) detector at 5 - 28
microns and a 256 x 256 pixel Si:Sb BIB detector at 28 - 40 microns. FORCAST images a 3.4 arcmin × 3.2 arcmin fieldof-
view on SOFIA with a rectified plate scale of 0.768 arcsec/pixel. In addition to imaging capability, FORCAST offers
a facility mode for grism spectroscopy that will commence during SOFIA Cycle 1. The grism suite enables spectroscopy
over nearly the entire FORCAST wavelength range at low resolution (~140 - 300). Optional cross-dispersers boost the
spectroscopic resolution to ~1200 at 5 - 8 microns and ~800 at 9.8 – 13.7 microns. Here we describe the FORCAST
instrument including observing modes for SOFIA Cycle 1. We also summarize in-flight results, including detector and
optical performance, sensitivity performance, and calibration.
We have implemented and tested a suite of grisms that will enable a moderate-resolution mid-infrared spectroscopic
mode in FORCAST, the facility mid-infrared camera on SOFIA. We have tested the hardware for the spectral modes
extensively in the laboratory with grisms installed in the FORCAST filter wheels. The grisms perform as designed,
consistently producing spectra at resolving powers in the 200-1200 range at wavelengths from 5 to 38 microns. In
anticipation of offering this capability as a SOFIA general observer mode, we are developing software for reduction and
analysis of FORCAST spectra, a spectrophotometric calibration plan, and detailed plans for in-flight tests prior to
commissioning the modes. We present a brief summary of the FORCAST grism spectroscopic system and a status report.
We have designed and fabricated a suite of grisms for use in FORCAST, a mid-infrared camera scheduled as a
first-light instrument on SOFIA. The grism suite gives SOFIA a new capability: low resolution spectroscopy from
5 to 38 microns at resolving powers from R=200 to R=1200, without the addition of a new instrument. We have
developed an IDL based spectral data reduction and quick-look software package, in anticipation of FORCAST
grism spectroscopy becoming a facility observing mode on the SOFIA telescope. The package allows users to
quickly view their data by extracting single-order and cross-dispersed spectra immediately after acquiring them
in flight. We have optimized the quick-look software to reduce the number of steps required to turn a set of
observations into a fully reduced extracted spectrum. We present a description of the philosophy of the data
reduction software, supplemented with screen shots and examples in hopes of garnering feedback and critiques
from potential end users, software developers, and instrument builders.
FORCAST is the "first light" U. S. science instrument to fly aboard SOFIA. FORCAST offers dual channel imaging in
discrete filters at 5 - 25 microns and 30 - 40 microns, with diffraction-limited imaging at wavelengths > 15 microns.
FORCAST has a plate scale of 0.75 arcsec per pixel, giving it a 3.2 arcmin x 3.2 arcmin FOV on SOFIA. We give a
status update on FORCAST development, including the performance of new far-IR filters; design and performance of
the calibration box; laboratory operations and performance; and results from ground-based and first flight operations.
FORCAST has been selected to be the "first light" U.S. science instrument aboard SOFIA. FORCAST will offer dual
channel imaging in discrete filters at 5 - 25 microns and 30 - 40 microns, with diffraction-limited imaging at wavelengths
> 15 microns. FORCAST will have a plate scale of 0.75 arcsec per pixel, giving it a 3.2 arcmin x 3.2 arcmin FOV on
SOFIA. We give a status update on FORCAST, including filter configuration for SOFIA's early science phase;
anticipated in-flight performance; SOFIA facility testing with FORCAST; ground-based testing performance at Palomar
Observatory; performance of its new dichroic beamsplitter; and a preliminary design of the in-flight calibration box.
We have designed and fabricated a suite of grisms for use in FORCAST, a mid-infrared camera scheduled as a
first-light instrument on SOFIA. The grism suite gives SOFIA a new capability: low and moderate resolution
spectroscopy from 5μm to 37μm, without the addition of a new instrument. One feature of the optical design
is that it includes a mode using pairs of cross-dispersed grisms, providing continuous wavelength coverage over
a broad range at higher resolving power. We fabricated four silicon (n = 3.44) grisms using photolithographic
techniques and purchased two additional mechanically ruled KRS-5 (n = 2.3) grisms. One pair of silicon grisms
permits observations of the 5 - 8μm band with a long slit at R~ 200 or, in a cross-dispersed mode, at resolving
powers up to 1500. In the 8 - 14μm region, where silicon absorbs heavily, the KRS-5 grisms produce resolving
powers of 300 and 800 in long-slit and cross-dispersed mode, respectively. The remaining two silicon grisms cover
17 - 37μm at resolving powers of 140 and 250. We have thoroughly tested the silicon grisms in the laboratory,
measuring efficiencies in transmission at 1.4 - 1.8μm. We report on these measurements as well as on cryogenic
performance tests of the silicon and KRS-5 devices after installation in FORCAST.
This paper addresses the performance of a suite of grisms as part of an Astrobiology Science and Instrument Development (ASTID) Program to implement a moderate resolution spectroscopic capability in the mid/far-IR facility instrument FORCAST for the Stratospheric Observatory For Infrared Astronomy (SOFIA). A moderate resolution mid-IR spectrometer on SOFIA will offer advantages not available to either ground or space-based instruments after the Spitzer Space Telescope ceases operation in ~2008. SOFIA will begin operations in 2008 and will have an operational lifetime of ~20 years. From aircraft altitudes, it will be possible to cover a wide range of wavelengths, particularly in the critical 5-9 micron band, where detection of astrobiologically interesting molecules have key spectral signatures that are not accessible from the ground The FORCAST grism suite consists of six grisms: four monolithic Si grisms and two KRS-5 grisms. These devices will allow long-slit low-resolution (R = 100-300) and short-slit, cross-dispersed high-resolution spectroscopic modes (R = 800-1200) over select wavelengths in the 5-40 μm spectral range and enable observing programs to gather both images and spectra in a single SOFIA flight. The silicon grisms demonstrate a new family of dispersive elements with good optical performance for spectroscopy from 1.2-8 μm and beyond 18 μm. After SOFIA flies, the grism modes in FORCAST will complement other first generation instruments on SOFIA and provide follow-up capability of bright sources observed with Infrared Spectrograph (IRS) on Spitzer. This paper highlights the design of the grism suite for FORCAST and the current laboratory cryogenic performance of the silicon grisms.
We report on new development and testing of FORCAST, the Faint Object infraRed Camera for the SOFIA Telescope. FORCAST will offer dual channel imaging in discrete filters at 5 - 25 microns and 30 - 40 microns, with diffraction-limited imaging at wavelengths > 15 microns. FORCAST will have a plate scale of 0.75 arcsec per pixel, giving it a 3.2 arcmin x 3.2 arcmin FOV on SOFIA. In addition, a set of grisms will enable FORCAST to perform long slit and cross-dispersed spectroscopic observations at low to moderate resolution (R ~ 140 - 1200) in the bandpasses 4.9 - 8.1 microns, 8.0 - 13.3 microns, 17.1 - 28.1 microns, and 28.6 - 37.4 microns. FORCAST has seen first light at the Palomar 200 inch telescope. It will be available for astronomical observations and facility testing at SOFIA first flight.
FORCAST is a mid/far-IR camera for use on NASA's SOFIA airborne observatory. We are fabricating monolithic silicon grisms to retrofit a spectroscopic capability for this facility-class instrument without affecting the imaging optics. The grisms will operate in the 5-8, 17-28, and 28-37 μm wavelength ranges. We will cover the 5-8 μm range in one exposure at a resolving power R=1200 with a 2 arcsecond slit using two grisms with one serving as a cross-disperser. For the 17-28 and 28-37 μm ranges, the resolving powers are R~140, 250 when used in low order with a slit of 3 arcseconds. We illustrate aspects of fabrication and testing during the grism development, and summarize the performance of the gratings at near- and mid-IR wavelengths. These gratings rely on procedures that can be used for modest sized (~10 cm) silicon pieces, thereby providing dispersive elements with good optical performance and large slit width-resolving power products from 1.2-8.1 μm and beyond 17 μm.
We have developed a high speed, flexible, data acquisition system and targeted it to astronomical imaging. The system is based on Field Programmable Gate Arrays (FPGAs) and provides a gigabit/sec fiber optic link between the electronics located on the instrument and the host computer. The FPGAs are reconfigurable over the fiber optic link for maximum flexibility. The system has initially been targeted at DRS Technologies' 256x256 Si:As and Si:Sb detectors used in FORCAST1, a mid-IR camera/spectrograph built by Cornell University for SOFIA. The initial configuration provides sixteen parallel channels of six Msamples/second 14-bit analog to digital converters. The system can coadd 256x256 images at over 1000 frames per second in up to 64 different memory positions. Array clocking and sampling is generated from uploaded clocking patterns in two independent memories. This configuration allows the user to quickly
create, on the fly, any form of array clocking and sampling (destructive, non-destructive, sample up the ramp, additional reset frames, Fowler, single frames, co-added frames, multi-position chop, throw away frames, etc.) The electronics were designed in a modular fashion so that any number of analog channels from arrays or mosaics of arrays can be accommodated by using the appropriate number of FPGA boards and preamps. The preamp/analog to digital converter boards can be replaced as needed to operate any focal plane array or other sensor. The system also provides analog drive capability for controlling an X-Y chopping secondary mirror, nominal two position chopping, and can also synchronize to an externally driven chop source. Multiple array controllers can be synchronized together, allowing multi-channel systems to share a single chopping secondary, yet clock the focal planes differently from each other. Due to the flexibility of the FPGAs, it is possible to develop highly customized operating modes to maximize system performance or to enable novel observations and applications.
We report laboratory tests amd development progress for the Faint Object Infrared Camera for the SOFIA Telescope (FORCAST). FORCAST is a two-channel camera with selectable filters for continuum and line imaging in the 5 - 40 micron wavelength region. Simultaneous imaging will be possible in the two channels: 5 - 25 microns using a Si:As 256x256 blocked impurity band (BIB) detector array, and 25-40 microns using a Si:Sb BIB. FORCAST will sample 0.75 arcseconds per pixel allowing a 3.2'x3.2' instantaneous field-of view in both channels simultaneously. Imaging will be diffracted limited for lambda> 15 microns on the SOFIA telescope. Since FORCAST operates in the wavelength range where the seeing is best from SOFIA, it will provide the highest spatial resolution possible from the airborne observatory. In addition to imaging, the FORCAST optical design provides for a simple upgrade to include spectroscopic observations using grisms mounted in the filter wheels. FORCAST will be available for facility testing and astronomical observations at SOFIA first (f)light.
This paper presents results on performance testing of mid-infrared detector arrays for the Faint Object Infrared Camera for the SOFIA Telescope (FORCAST). FORCAST is a two-channel camera that utilizes a Si:As blocked impurity band (BIB) 256 x 256 detector array for imaging through discrete filters at 5 - 25 microns, and a Si:Sb BIB 256 x 256 detector array for imaging at 25 - 40 microns, over a 3.2' x 3.2' field of view, under high thermal background conditions. DRS Technologies has designed and fabricated several Si:As BIB and Si:Sb BIB engineering grade detector arrays which we test as candidate arrays for FORCAST. We present their initial laboratory test performance results.
We report final design details and development progress for the Faint Object Infrared Camera for the SOFIA Telescope (FORCAST). FORCAST is a two-channel camera with selectable filters for continuum and line imaging in the 5-40 micron wavelength region. Simultaneous imaging will be possible in the two-channels--5-25 microns using a Si:As 256×256 blocked impurity band (BIB) detector array, and 25-40 microns using a Si:Sb BIB. FORCAST will sample 0.75 arcseconds per pixel allowing a 3.2'×3.2' instantaneous field-of-view in both channels simultaneously. Imaging will be diffraction limited for lambda > 15 microns. Since FORCAST operates in the wavelength range where the seeing is best from SOFIA, it will provide the highest spatial resolution possible from the airborne observatory. In addition to imaging, the FORCAST optical design provides for a simple upgrade to include spectroscopic observations using grisms mounted in the filter wheels. We report improvements to the optical system and progress in construction of this SOFIA facility instrument and its subsystems. FORCAST will be available for facility testing and astronomical observations at SOFIA first (f)light.
We discuss plans for the construction of a 15-m class telescope located in the high Atacama desert of Northern Chile. The baseline concept is a segmented mirror telescope optimized for operation at wavelengths longer than 3.5 microns but capable of working at shorter wavelengths. An adaptive secondary will be used to achieve diffraction limited imaging while maintaining low emissivity. The facility will be designed for eventual remote/robotic operation and include a number of instruments designed to take advantage of the low precipitable water vapor and good seeing conditions.
We present design examples for instruments making use of micromachined silicon grisms and immersion gratings. The capabilities of high index grisms, transmission grating-prism hybrids, open up new possibilities in compact IR spectrograph design with spectral resolving power, R~500-5000. Coarsely grooved immersion gratings will provide for unique high resolution spectrograph designs in the near and mid-infrared (resolving power, R~104-105). The high refractive index of silicon shortens the required grating depth, to produce a given resolving power, by up to a factor of 3.4. Alternatively, at a given resolving power, an immersion grating can allow a spectrograph slit to be widened by this factor relative to an instrument using a grating illuminated in air or vacuum; this increases the instrument sensitivity without degrading the spectral resolution. Our analysis here illustrates the potential of these devices to improve spectrograph throughput, spectral resolution, and wavelength coverage while reducing the required instrument volume relative to similar instruments using non-immersed diffraction gratings and low index prisms and grisms.
Infrared spectrometers using silicon immersion gratings and prisms can have substantial performance advantages over conventional instruments. The immersion gratings and grisms share a common geometry: prism-shaped pieces of silicon with blazed grooves along one side. The grooves can either be machined directly into substrates or the grooves can be machined into thin wafers which are then bonded to flat-surfaced prisms. Chemical micromachining currently is the best method of ruling grooves directly into silicon surfaces. The tolerances for near-IR diffraction gratings make direct machining of the grooves onto one surface of a bulky, prism-shaped substrate very difficult. We encountered a number of issues that we had to resolve when we tried to etch precisely positioned grooves into massive pieces of silicon: silicon substrate purity, lithography mask alignment, photoresist thickness uniformity, temperature control, wet etching vs. reactive-ion etching. We have successfully manufactured 7 line / mm gratings on 15 mm thick substrates. We performed optical tests with these gratings used as front-surface devices to determine efficiency and diffraction limited performance. Our echelle gratings have 70\% efficiency in 365th-368th order at 632.8 nm. Testing shows that the grating preserves a diffraction-limited point-spread function making them good dispersing elements for applications requiring high spectral resolving power.
We are constructing a facility-class, mid/far-infrared camera for the Stratospheric Observatory for Infrared Astronomy (SOFIA). The Faint Object infraRed CAmera for the Sofia Telescope (FORCAST) is a two-channel camera with selectable filters for continuum imaging in the 5 - 8, 17 - 25 micron, and/or 25 - 40 micron regions. The design supports simultaneous imaging in the two-channels. Using the latest 256 X 256 Si:As and Si:Sb blocked-impurity-band detector array technology to provide high-sensitivity wide- field imaging. FORCAST will sample images at 0.75 arcsec/pixel and have a 3.2' X 3.2' instantaneous field- of-view. Imaging is diffraction limited for lambda > 15 microns.
We have designed a near IR spectrograph, sensitive in the 1.5-5 micrometers range, that uses a silicon immersion echelle grating. The cross-dispersed design demonstrates that immersion echelles allow compact spectrographs which have excellent spectral coverage and very high resolving power. Our instrument will have continuous spectral coverage over a 5.7 percent passband at 2.3 micrometers or a 7.6 percent passband at 4.6 micrometers and resolving power ranging from R equals 87,000 at 4.6 micrometers to 109,000 at 2.3 micrometers . We discuss design issues that are unique to spectrographs using silicon immersion echelle gratings in the near IR such as grating parameters, geometry, and the mechanical and thermal properties of large pieces of single crystal silicon.
Micromachined silicon gratings offer two great advantages to astronomical spectroscopy in the IR: (1) Photolithographic processing techniques permit the production of gratings with much larger groove constants than are possible with conventional wavelength coverage, despite the relatively small format of IR arrays. (2) One can use anisotropic etching to form gratings on dielectric wedges. By illuminating the grating through the dielectric, we can achieve higher spectral resolution for a given grating size or a smaller grating for a given desired resolution. We discuss the technical challenges involved in micromachining large grating grooves over large areas while holding positional accuracy to very tight tolerances. Manufacturing issues include material choices, surface preparation, and chemical and physical effects during processing. We also discuss our program for evaluation of the finished products, show result of measurements we have made on front-surface and immersion devices, and use these result to assess the potential of these devices for real-world astronomical applications.
This paper describes the Multiple Mirror Telescope (MMT) facility and discusses the demands placed upon the MMT by observations with the Thermal Infrared Photometer system, with special attention given to the subsequent modifications of the MMT-Photometer facility. It is shown that the MMT-Photometer competes well in sensitivity with similar detector systems currently operating on conventional single-mirror telescopes. The instrument will reach N (10.6 microns) magnitudes of 10 or better in integrations of one hour through a 5.4 arcsec aperture.