We describe the Short Wavelength Camera (SWCam) for the CCAT observatory including the primary science drivers, the coupling of the science drivers to the instrument requirements, the resulting implementation of the design, and its performance expectations at first light. CCAT is a 25 m submillimeter telescope planned to operate at 5600 meters, near the summit of Cerro Chajnantor in the Atacama Desert in northern Chile. CCAT is designed to give a total wave front error of 12.5 μm rms, so that combined with its high and exceptionally dry site, the facility will provide unsurpassed point source sensitivity deep into the short submillimeter bands to wavelengths as short as the 200 μm telluric window. The SWCam system consists of 7 sub-cameras that address 4 different telluric windows: 4 subcameras at 350 μm, 1 at 450 μm, 1 at 850 μm, and 1 at 2 mm wavelength. Each sub-camera has a 6’ diameter field of view, so that the total instantaneous field of view for SWCam is equivalent to a 16’ diameter circle. Each focal plane is populated with near unit filling factor arrays of Lumped Element Kinetic Inductance Detectors (LEKIDs) with pixels scaled to subtend an solid angle of (λ/D)2 on the sky. The total pixel count is 57,160. We expect background limited performance at each wavelength, and to be able to map < 35(°)2 of sky to 5 σ on the confusion noise at each wavelength per year with this first light instrument. Our primary science goal is to resolve the Cosmic Far-IR Background (CIRB) in our four colors so that we may explore the star and galaxy formation history of the Universe extending to within 500 million years of the Big Bang. CCAT's large and high-accuracy aperture, its fast slewing speed, use of instruments with large format arrays, and being located at a superb site enables mapping speeds of up to three orders of magnitude larger than contemporary or near future facilities and makes it uniquely sensitive, especially in the short submm bands.
TripleSpec 4 (TS4) is a near-infrared (0.8um to 2.45um) moderate resolution (R ~ 3200) cross-dispersed spectrograph
for the 4m Blanco Telescope that simultaneously measures the Y, J, H and K bands for objects reimaged
within its slit. TS4 is being built by Cornell University and NOAO with scheduled commissioning in 2015.
TS4 is a near replica of the previous TripleSpec designs for Apache Point Observatory's ARC 3.5m, Palomar
5m and Keck 10m telescopes, but includes adjustments and improvements to the slit, fore-optics, coatings and
the detector. We discuss the changes to the TripleSpec design as well as the fabrication status and expected
sensitivity of TS4.
The CCAT observatory is a 25-m class Gregorian telescope designed for submillimeter observations that will be deployed at Cerro Chajnantor (~5600 m) in the high Atacama Desert region of Chile. The Short Wavelength Camera (SWCam) for CCAT is an integral part of the observatory, enabling the study of star formation at high and low redshifts. SWCam will be a facility instrument, available at first light and operating in the telluric windows at wavelengths of 350, 450, and 850 μm. In order to trace the large curvature of the CCAT focal plane, and to suit the available instrument space, SWCam is divided into seven sub-cameras, each configured to a particular telluric window. A fully refractive optical design in each sub-camera will produce diffraction-limited images. The material of choice for the optical elements is silicon, due to its excellent transmission in the submillimeter and its high index of refraction, enabling thin lenses of a given power. The cryostat’s vacuum windows double as the sub-cameras’ field lenses and are ~30 cm in diameter. The other lenses are mounted at 4 K. The sub-cameras will share a single cryostat providing thermal intercepts at 80, 15, 4, 1 and 0.1 K, with cooling provided by pulse tube cryocoolers and a dilution refrigerator. The use of the intermediate temperature stage at 15 K minimizes the load at 4 K and reduces operating costs. We discuss our design requirements, specifications, key elements and expected performance of the optical, thermal and mechanical design for the short wavelength camera for CCAT.
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 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.
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.
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.
We report the performance of Triplespec from commissioning observations on the 200-inch Hale Telescope
at Palomar Observatory. Triplespec is one of a set of three near-infrared, cross-dispersed spectrographs
covering wavelengths from 1 - 2.4 microns simultaneously at a resolution of ~2700. At Palomar, Triplespec
uses a 1×30 arcsecond slit. Triplespec will be used for a variety of scientific observations, including
moderate to high redshift galaxies, star formation, and low mass stars and brown dwarfs. When used in
conjunction with an externally dispersed interferometer, Triplespec will also detect and characterize
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 <i>Spitzer Space Telescope</i> 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.
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 FORCAST<sup>1</sup>, 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.
Four institutions are collaborating to design and build three near identical R ~2700 cross-dispersed near-infrared spectrographs for use on various 5-10 meter telescopes. The instrument design addresses the common observatory need for efficient, reliable near-infrared spectrographs through such features as broad wavelength coverage across 6 simultaneous orders (0.8 - 2.4 microns) in echelle format, real-time slit viewing through separate optics and detector, and minimal moving parts. Lastly, the collaborators are saving money and increasing the likelihood of success through economies of scale and sharing intellectual capital.
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.
MIRSI (Mid-InfraRed Spectrometer and Imager) is a mid-infrared camera system recently completed at Boston University that has both spectroscopic and imaging capabilities. MIRSI is uniquely suited for studies of young stellar objects and star formation, planetary and protoplanetary nebulae, starburst galaxies, and solar system objects such as planets, asteroids, and comets. The camera utilizes a 320 x 240 Si:As Impurity Band Conduction (IBC) array developed for ground-based astronomy by Raytheon/SBRC. For observations at the Infrared Telescope Facility (IRTF), MIRSI offers a large field of view (1.6 arcmin x 1.2 arcmin) with a pixel scale of 0.3 arcsec, diffraction-limited spatial resolution, complete spectral coverage over the 8-14 μm and 17-26 μm atmospheric windows for both imaging (discrete filters and circular variable filter) and spectroscopy (10 and 20 μm grisms), and high sensitivity (expected one-sigma point source sensitivities of 5 and 20 mJy at 10 and 20 μm, respectively, for on-source integration time of 30 seconds). MIRSI successfully achieved first light at the Mt. Lemmon Observing Facility (MLOF) in December 2001, and will have its first observing run at the IRTF in November 2002. We present details of the system hardware and software and results from first light observations.