Christine A. Allen

Characterization of a submillimeter high-angular-resolution camera with a monolithic silicon bolometer array for the Caltech Submillimeter Observatory

We constructed a 24-pixel bolometer camera operating in the 350- and 450-μm atmospheric windows for the Caltech Submillimeter Observatory (CSO). This instrument uses a monolithic silicon bolometer array that is cooled to approximately 300 mK by a single-shot (3)He refrigerator. First-stage amplification is provided by field-effect transistors at approximately 130 K. The sky is imaged onto the bolometer array by means of several mirrors outside the Dewar and a cold off-axis elliptical mirror inside the cryostat. The beam is defined by cold aperture and field stops, which eliminates the need for any condensing horns. We describe the instrument, present measurements of the physical properties of the bolometer array, describe the performance of the electronics and the data-acquisition system, and demonstrate the sensitivity of the instrument operating at the observatory. Approximate detector noise at 350 μm is 5 × 10(-15) W/√Hz, referenced to the entrance of the Dewar, and the CSO system noise-equivalent flux density is approximately 4 Jy/√Hz. These values are within a factor of 2.5 of the background limit.

HAWC: a far-infrared camera for SOFIA

When SOFIA enters operation, it will be the largest far- infrared telescope available, so it will have the best intrinsic angular resolution. HAWC (High-resolution Airborne Wideband Camera) is a far-infrared camera designed to cover the 40 - 300 micron spectral range at the highest possible angular resolution. Its purpose is to provide a sensitive, versatile, and reliable facility-imaging capability for SOFIAs user community during its first operational use.

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.

Multiplexed Readout of Superconducting Bolometers

Studies of emission in the far-infrared and submillimeter from astrophysical sources require large arrays of detectors containing hundreds to thousands of elements. A multiplexed readout is necessary for practical implementation of such arrays, and can be developed using SQUIDS, such that, e.g., a 32 × 32 array of bolometers can be read out using ≈100 wires rather than the >2000 needed with a brute force expansion of existing arrays. These bolometer arrays are made by micromachining techniques, using superconducting transition edge sensors as the thermistors. We describe the development of this multiplexed superconducting bolometer array architecture as a step toward bringing about the first astronomically useful arrays of this design. This technology will be used in the SAFIRE instrument on SOFIA, and is a candidate for a wide variety of other spectroscopic and photometric instruments.

Characterization of transition edge sensors for the Millimeter Bolometer Array Camera on the Atacama Cosmology Telescope.

The Atacama Cosmology Telescope (ACT) aims to measure the Cosmic Microwave Background (CMB) temperature anisotropies on arcminute scales. The primary receiver for ACT is the Millimeter Bolometer Array Camera (MBAC). The MBAC is comprised of three 32×32 transition edge sensor (TES) bolometer arrays, each observing the sky with an independent set of band-defining filters. The MBAC arrays will be the largest pop-up detector arrays fielded, and among the largest TES arrays built. Prior to its assembly into an array and installation into the MBAC, a column of 32 bolometers is tested at ~ 0.4 K in a quick-turn-around dip probe. In this paper we describe the properties of the ACT bolometers as revealed by data from those tests, emphasizing a characterization that accounts for both the complex impedance and the noise as a function of frequency.

Instrument design and characterization of the Millimeter Bolometer Array Camera on the Atacama Cosmology Telescope

The Millimeter Bolometer Array Camera (MBAC) was commissioned in the fall of 2007 on the new 6-meter Atacama Cosmology Telescope (ACT). The MBAC on the ACT will map the temperature anisotropies of the Cosmic Microwave Background (CMB) with arc-minute resolution. For this first observing season, the MBAC contained a diffraction-limited, 32 by 32 element, focal plane array of Transition Edge Sensor (TES) bolometers for observations at 145 GHz. This array was coupled to the telescope with a series of cold, refractive, reimaging optics. To meet the performance specifications, the MBAC employs four stages of cooling using closed-cycle 3He/4He sorption fridge systems in combination with pulse tube coolers. In this paper we present the design of the instrument and discuss its performance during the first observing season. Finally, we report on the status of the MBAC for the 2008 observing season, when the instrument will be upgraded to a total of three separate 1024-element arrays at 145 GHz, 220 GHz and 280 GHz.

Next generation of silicon-based x-ray microcalorimeters

After the design of the calorimeter array for the high-resolution x-ray spectrometer (XRS) on the original Astro-E was frozen, new fabrication techniques became available and our understanding of these devices continually increased. We are now able to complete the optimization of this technology and, potentially, to increase the capability of new XRS instrument for Astro-E2, our on-going sounding recket experiments, and possible further applications. The most significant improvement comes from greatly reducing the excess noise of the ion-implanted thermistors by increasing the thickness of the implanted region.

Microshutter array system for James Webb Space Telescope

We have developed microshutter array systems at NASA Goddard Space Flight Center for use as multi-object aperture arrays for a Near-Infrared Spectrometer (NIRSpec) instrument. The instrument will be carried on the James Webb Space Telescope (JWST), the next generation of space telescope, after the Hubble Space Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light from objected galaxies in space with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size close to 100x200 μm. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with minimized stress concentration. In order to enhance optical contrast, light shields are made on each shutter to prevent light leak. Shutters are actuated magnetically, latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining and packaged utilizing a novel single-sided indium flip-chip bonding technology. The MSA flight system consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays. The system will be placed in the JWST optical path at the focal plane of NIRSpec detectors. MSAs that we fabricated passed a series of qualification tests for flight capabilities. We are in the process of making final flight-qualified MSA systems for the JWST mission.

Design techniques for improved noise performance of superconducting Transition Edge Sensor bolometers

We have investigated the noise performance of MoAu-bilayer TES bolometers designed for infrared detectors. A set of devices with variations in geometry were fabricated at the NASA/GSFC detector development facility. These detectors have different bilayer aspect ratios and have varieties of normal metal regions deposited on top of the bilayer to study the effects of geometry on noise. These normal metal regions are oriented either parallel or transverse to the direction of current flow, or both. The lowest noise detectors are found to have normal metal regions oriented transversely. Our detectors with the most favorable design feature negligible excess noise in the in-band region, only slight excess noise in the out-of-band region, and low 1/f noise. The detectors are successfully used in the Submillimeter Broadband Spectrometer FIBRE which is used for astronomical observations at the Caltech Submillimeter Observatory.

Performance of multiplexed SQUID readout for Cryogenic Sensor Arrays

Abstract We report on the implementation of a multiplexer that uses superconducting quantum interference devices (SQUIDs) to read out low-impedance cryogenic detectors. Using prototype chips, a circuit was built which interfaces eight input SQUID channels with a close-packed array of eight transition-edge sensor (TES) infrared bolometers. Circuit elements were measured and crosstalk specifications are reported. Digital feedback is employed to flux-lock a single element in the array of SQUIDs.