Catherine T. Marx

The Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 � m. Two nearly adjacent 5A2 ; 5A2 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 � m; 4.5 and 8 � m). All four detector arrays in the camera are 256 ; 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.

In-flight performance and calibration of the Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on board the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 μm in two nearly adjacent fields of view. We summarize here the in-flight scientific, technical, and operational performance of IRAC.

GISMO: a 2-millimeter bolometer camera for the IRAM 30 m telescope

We are building a bolometer camera (the Goddard-Iram Superconducting 2-Millimeter Observer, GISMO) for operation in the 2 mm atmospheric window to be used at the IRAM 30 m telescope. The instrument uses a 8x16 planar array of multiplexed TES bolometers which incorporates our newly designed Backshort Under Grid (BUG) architecture. Due to the size and sensitivity of the detector array (the NEP of the detectors is 4×10-17 W/√Hz), this instrument will be unique in that it will be capable of providing significantly greater imaging sensitivity and mapping speed at this wavelength than has previously been possible. The major scientific driver for this instrument is to provide the IRAM 30 m telescope with the capability to rapidly observe galactic and extragalactic dust emission, in particular from high-z ULIRGs and quasars, even in the summer season. The 2 mm spectral range provides a unique window to observe the earliest active dusty galaxies in the universe and is well suited to better confine the star formation rate in these objects. The instrument will fill in the SEDs of high redshift galaxies at the Rayleigh-Jeans part of the dust emission spectrum, even at the highest redshifts. The observational efficiency of a 2 mm camera with respect to bolometer cameras operating at shorter wavelengths increases for objects at redshifts beyond z ~ 1 and is most efficient at the highest redshifts, at the time when the first stars were re-ionizing the universe. Our models predict that at this wavelength one out of four serendipitously detected galaxies will be at a redshift of z > 6.5.

Angle-of-incidence effects in the spectral performance of the infrared array camera of the Spitzer Space Telescope

The Infrared Array Camera (IRAC) on board the Spitzer Space Telescope uses two dichroic beamsplitters, four interference filters, and four detector arrays to acquire images in four different channels with nominal wavelengths of 3.6, 4.5, 5.8, and 8 μm for channels 1 through 4 respectively. A ray-tracing analysis of the IRAC optical system indicates a distribution of angles that is position-dependent at each optical element and the focal-plane arrays. For the band-pass filters in channels 1 and 2, the angle distribution relative to the filter surface normal is 0-28°, whereas for channels 3 and 4, the distribution is from 30° to 58°. Since these angle variations will cause changes in the center-band wavelengths for these interference filters that needed to be accounted for, we performed spectral performance measurements as a function of the angle of incidence on witness samples corresponding to each of the four filters and the two beamsplitters in the IRAC instrument. These measurements were done in the 2-10 μm wavelength range and at the temperature of 5 K, which is near the operating temperature of IRAC. Based on these filter measurements, we also performed an analysis of the pass-band wavelength distributions as a function of position on the instrument focal-plane array detectors. This information is necessary to attain the highest possible photometric accuracy when using IRAC for astronomical observations.

Simfit and Focus Diversity: methods for determining the focus of the SIRTF telescope in space without a focus slew

Because of concern over possible failure of the SIRTF cryogenic focus mechanism in space, the SIRTF Project Office has directed that the focus should be set before launch so that the telescope arrives in orbit as close to optimum focus as possible. Then focus evaluation and determination of any required focus change to achieve best focus must be carried out without the conventional approach of a focus slew. For these tasks we have created two methods: Simfit and Focus Diversity. Simfit is a procedure for comparing an observed stellar image with a family of simulated point-source images with a range of focus settings. With a sufficiently accurate as-built telescope model for creating the simulated images, the focus offset and direction can be accurately and unambiguously determined because of the change in image appearance with defocus. Focus diversity takes advantage of the variation of best-focus setting over the instruments focal plane due to focal plane curvature and tilt and offsets between different instrument channels. By plotting an image quality parameter, such as noise-pixels, for observed stars at several positions on the focal plane versus a defocus variable, the focus error and direction can be determined. We have developed an efficient program for carrying out these procedures. The validity of this program has been successfully confirmed using point-source images observed with three bands of the IRAC camera during a double-pass optical test of SIRTF in a Ball Aerospace cryogenic test chamber. The two procedures are described and are illustrated with these results

Design and performance of a high-throughput cryogenic detector system

The Goddard IRAM Superconducting Millimeter Observer (GISMO) is a new superconducting bolometer array camera for the IRAM 30 Meter Telescope on Pico Veleta, Spain. GISMO uses a 3He/4He cooler mounted to a liquid He/LN2 cryostat to cool the bolometer array and SQUID electronics to an operating temperature of 260mK. The bolometer array is based on the backshort-under-grid architecture and features 128 2mm square absorbing pixels. A 101mm diameter anti-reflection coated silicon lens is used to define the beam. A single cold pupil stop prevents warm radiation from reaching the array, but no other stops are used. In the beam, filters and a cold baffling and stray light suppression system were used to define the bandpass and prevent out-of-band radiation to a very high level, including out-of-band radiation leaking through the metal-mesh filters from extreme angles. We present a detailed description of this optical design and its performance. A comprehensive report of the electronics and cryogenic integration are also included.

Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies (PISCES) for WFIRST-AFTA

Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies (PISCES) is a lenslet array based integral field spectrometer (IFS) designed for high contrast imaging of extrasolar planets. PISCES will be used to advance the technology readiness of the high contrast IFS baselined on the Wide-Field InfraRed Survey Telescope/Astrophysics Focused Telescope Assets (WFIRST-AFTA) coronagraph instrument. PISCES will be integrated into the high contrast imaging testbed (HCIT) at the Jet Propulsion Laboratory (JPL) and will work with both the Hybrid Lyot Coronagraph (HLC) and the Shaped Pupil Coronagraph (SPC) configurations. We discuss why the lenslet array based IFS was selected for PISCES. We present the PISCES optical design, including the similarities and differences of lenslet based IFSs to normal spectrometers, the trade-off between a refractive design and reflective design, as well as the specific function of our pinhole mask on the back surface of the lenslet array to reduce the diffraction from the edge of the lenslets. The optical analysis, alignment plan, and mechanical design of the instrument will be discussed.

Wide-Field InfraRed Survey Telescope (WFIRST) Slitless Spectrometer: Design, Prototype, and Results

The slitless spectrometer plays an important role in the WFIRST mission for the survey of emission-line galaxies. This will be an unprecedented very wide field, HST quality 3D survey of emission line galaxies1. The concept of the compound grism as a slitless spectrometer has been presented previously. The presentation briefly discusses the challenges and solutions of the optical design, and recent specification updates, as well as a brief comparison between the prototype and the latest design. However, the emphasis of this paper is the progress of the grism prototype: the fabrication and test of the complicated diffractive optical elements and powered prism, as well as grism assembly alignment and testing. Especially how to use different tools and methods, such as IR phase shift and wavelength shift interferometry, to complete the element and assembly tests. The paper also presents very encouraging results from recent element tests to assembly tests. Finally we briefly touch the path forward plan to test the spectral characteristic, such as spectral resolution and response.

PISCES: high contrast integral field spectrograph simulations and data reduction pipeline

The PISCES (Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies) is a lenslet array based integral field spectrograph (IFS) designed to advance the technology readiness of the WFIRST-AFTA high contrast Coronagraph Instrument. We present the end to end optical simulator and plans for the data reduction pipeline (DRP). The optical simulator was created with a combination of the IDL-based PROPER library and Zemax, while the data reduction pipeline is a modified version of the Gemini Planet Imagers (GPI) IDL pipeline. The simulations of the propagation of light through the instrument are based on Fourier transform algorithms. The DRP enables transformation of the PISCES IFS data to calibrated spectral data cubes.

Optical design of rapid infrared-visible multi-object spectrometer: a NGST demonstration instrument

A wide field (6x6 arcmin2) Rapid Infrared-Visible Multi-Object Spectrometer (RIVMOS) has been designed and is being fabricated at NASAs GSFC as part of the Next Generation Space Telescope (NGST) development and new technology demonstration. The primary goal is to demonstrate that the microshutter arrays, currently being designed for the NGST Near Infrared Spectrometer (NIRSpec) as programmable 2D selection masks, can achieve the optical performance required for faint object imaging and spectroscopy. We developed an original optical design that includes both reflective and refractive optics. The primary goal of the design was to achieve high imaging quality in both imaging and spectroscopy modes over a very wide spectral range with all spherical surfaces. The required optical performance is achieved for both multi-object spectroscopy and camera imaging over the entire field-of-view. The optical design consists of six optical subsystems including (1) an image relay consisting of a three-mirror anastigmat (TMA), (2) the microshutter assembly, (3) a triplet collimating optic, (4) a grism/filter assembly, (5) a pupil imaging optic, and (6) a five element telecentric camera design. The all-spherical optical design reduces construction costs and facilitates fabrication of the optical assembly while maintaining an encircled energy of 2 pixels within the FOV for wavelengths between 0.6 and 5.0 microns. Three spectral resolution modes (R = 50, 2000, 4000) will be available for multi-object spectroscopy as well as cross-dispersed echelle spectroscopy at the highest spectral resolution. The low resolution mode will be provided by the direct view prism, whereas silicon grisms will be used for higher resolving power. This design provides an extremely wide spectral range, wide field, very compact, high resolution imager-spectrometer with multi-object capability.