Search

The discovery of the Trappist-1 system, which consists of an ultra cool M-dwarf star orbited by 7 planets, 3 of which are located in the habitable zone, has demonstrated that these types of planetary systems around dwarf stars are very common. Such systems are well suited for the study of exoplanets. In particular the search for bio-signatures in the atmosphere of planets in the habitable zone around M-stars will be a high-priority science goal of future space missions. The mid-infrared (mid-IR) band between 3 and 15 microns is probably the best available band for this science, because the spectral lines of methane, ozone, and nitrous oxide can be found in this spectral range. he coexistence of these constituents in a planets atmosphere would be a very strong indicator for life on the planet. Mid-IR transit spectrometers on future space missions such as Origins Space Telescope (OST) will be the instrument of choice to detect these bio-signatures in exoplanets around M-dwarfs. Current mid-IR detectors are based on impurity band conduction (IBC) devices such as Si: As detectors. Charge trapping in these device leads to a time and exposure dependent response. As a result, this detector class is not expected to provide the required 5 ppm stability needed for a reliable detection of the aforementioned spectral lines. While efforts are under way to improve IBC detectors, it is un-clear how far the performance can be improved. Here we describe the development of an ultra-stable Mid-IR Array Spectrometer demonstration for Exoplanet Transits (MI-RASET), which includes a calibration system that, as we show, is needed to achieve the required sensitivity for the detection of atmospheric bio-signatures in habitable-zone planets around M-dwarfs. The spectrometer will be demonstrated with arrays of Transition Edge Sensor detectors (TES). These devices are known to have a very linear response and are intrinsically very stable. Furthermore, the required detector parameters (sensitivity, dynamic range) for space based mid-IR transit spectroscopy can be easily met with existing devices. No new detector developments are required, only the absorbers need to be optimized for the wavelength range of the instrument. This project will include the development of a high-accuracy calibration system with a stable reference source which itself will be monitored in the visible (0.5μm) by a photo diode. At this wavelength the precision of the load temperature measurement exceeds that of an in-band calibration. This scheme will allow for real time monitoring of the detector gain and offset, which we anticipate will result in a background limited performance with the required stability of better than 5 ppm for the detection of bio-signatures in a designated spectrometer flying e.g. on the OST space telescope, and as such will help to answer one of NASA's prime questions: “Are we alone?”

The OSS on the Origins Space Telescope is designed to decode the cosmic history of nucleosynthesis, star formation, and supermassive black hole growth with wide-area spatial-spectral 3-D surveys across the full 25 to 590 micron band. Six wideband grating modules combine to cover the full band at R=300, each couples a long slit with 60-190 beams on the sky. OSS will have a total of 120,000 background-limited detector pixels in the six 2-D arrays which provide spatial and spectral coverage. The suite of grating modules can be used for pointed observations of targets of interest, and are particularly powerful for 3-D spectral spectral surveys. To chart the transition from interstellar material, particularly water, to planetary systems, two high-spectral-resolution modes are included. The first incorporates a Fourier-transform spectrometer (FTS) in front of the gratings providing resolving power of 25,000 (δv = 12 km/s) at 179 µm to resolve water emission in protoplanetary disk spectra. The second boosts the FTS capability with an additional etalon (Fabry-Perot interferometer) to provide 2 km/s resolution in this line to enable detailed structural studies of disks in the various water and HD lines. Optical, thermal, and mechanical designs are presented, and the system approach to the detector readout enabling the large formats is described.

The search for biosignatures in the atmospheres of exoplanets will be a key focus of future space telescopes that operate in the ultraviolet, visible, and near-infrared bands. Detection of biosignatures requires an instrument with moderate spectral resolving power ($R \sim 100$) and a large bandwidth ($\sim 400$ nm -- $\sim 1.8$ $\mu$m). Additionally, biosignature detection is a photon-starved science; instruments designed for these measurements would ideally combine high optical efficiency with quantum-limited photon detectors (i.e., detectors that exhibit zero dark current). In this work, we report on our efforts to develop energy resolving transition edge sensor (TES)-based detectors designed for biosignature detection. TESs operated as microcalorimeters are compelling detectors for this application. Unlike semiconductor detectors, TESs eliminate the need for dispersive optics and are truly single photon detectors -- fundamental TES noise yields uncertainty in the energies of detected photons, not in the number of detected photons. We introduce TESs designed for this application and discuss the path toward realizing a TES-based dispersionless spectrometer optimized for biosignature detection.

We describe our efforts to build a Wide-Field Lyman alpha Geocoronal simulator (WFLaGs) for characterizing the end-to-end sensitivity of FORTIS (Far-UV Off Rowland-circle Telescope for Imaging and Spectroscopy) to scattered Lyman α emission from outside of the nominal (1/2 degree)2 field-of-view. WFLaGs is a 50 mm diameter F/1 aluminum parabolic collimator fed by a hollow cathode discharge lamp with a 80 mm clear MgF2 window housed in a vacuum skin. It creates emission over a 10 degree FOV. WFLaGS will allow us to validate and refine a recently developed scattered light model and verify our scatter light mitigation strategies, which will incorporate low scatter baffle materials, and possibly 3-d printed light traps, covering exposed scatter centers. We present measurements of scattering intensity of Lyman alpha as a function of angle with respect to the specular reflectance direction for several candidate baffle materials. Initial testing of WFLaGs will be described.

We present the main design features for the GISMO-2 bolometer camera, which we build for background-limited operation at the IRAM 30 m telescope on Pico Veleta, Spain. GISMO-2 will operate simultaneously in the 1 and 2 mm atmospherical windows. The 1 mm channel uses a 32×40 TES-based backshort under grid (BUG) bolometer array, the 2 mm channel operates with a 16×16 BUG array. The camera utilizes almost the entire full field of view provided by the telescope. The optical design of GISMO-2 was strongly influenced by our experience with the GISMO 2 mm bolometer camera, which is successfully operating at the 30 m telescope. GISMO is accessible to the astronomical community through the regular IRAM call for proposals.

Micro-Spec is a compact sub-millimeter (approximately 100 GHz--1:1 THz) spectrometer which uses low loss superconducting microstrip transmission lines and a single-crystal silicon dielectric to integrate all of the components of a diffraction grating spectrometer onto a single chip. We have already successfully evaluated the performance of a prototype Micro-Spec, with spectral resolving power, R=64. Here we present our progress towards developing a higher resolution Micro-Spec, which would enable the first science returns in a balloon flight version of this instrument. We describe modifications to the design in scaling from a R=64 to a R=256 instrument, as well as the ultimate performance limits and design concerns when scaling this instrument to higher resolutions.

著者人数: 44名, Accepted: 2006-10-03, 資料番号: SA1000630000

We describe a comparative investigation of the parameters 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 (providing differing normal state resistances and current densities), and have varieties of normal metal regions to study the effects of geometry on noise. These normal metal regions are oriented either parallel to or transverse to the direction of current flow, or both. The lowest noise detectors are found to have normal metal regions oriented transversely. For about a dozen different devices, we have measured a large set of parameters by means of a suite of tests. These include complex impedance measurements to derive time constants; IV curves to determine resistance and power; thermal conductance measurements; noise measurements as a function of device resistance; and direct resistance vs. temperature measurements.

The development of a large, far-infrared telescope in space has taken on a new urgency with breakthroughs in detector technology and recognition of the fundamental importance of the far-infrared spectral region to questions ranging from cosmology to our own Solar system. The Single Aperture Far-InfraRed (SAFIR) Observatory is 10 m-class far-infrared observatory that would begin development later in this decade to meet these needs. SAFIR's science goals are driven by the fact that youngest stages of almost all phenomena in the universe are shrouded in absorption by and emission from cool dust that emits strongly in the far-infrared, 20 μm–1 mm. Its operating temperature (4 K) and instrument complement would be optimized to reach the natural sky confusion limit in the far-infrared with diffraction-limited performance down to at least the atmospheric cutoff at 40 μm. This would provide a point source sensitivity improvement of several orders of magnitude over that of SIRTF. In order to achieve this, large arrays of detectors with NEPs ranging from a few to a hundred zeptowatts/sqrt(Hz) are needed. Very low temperature superconducting transition edge sensors and far-infrared “photon counting” detectors are critical technologies requiring development for the SAFIR mission.

High-performance, integrated spectrometers operating in the far-infrared and sub-millimeter promise to be powerful tools for the exploration of the epochs of reionization and initial galaxy formation. These devices, using high-efficiency superconducting transmission lines, can achieve the performance of a meter-scale grating spectrometer in an instrument implemented on a four-inch silicon wafer. Such a device, when combined with a cryogenic telescope in space, provides an enabling capability for studies of the early universe. Here, the optical design process for Micro-Spec (mu-Spec) is presented, with particular attention given to its two-dimensional diffractive region, where the light of different wavelengths is focused on the different detectors. The method is based on the stigmatization and minimization of the light path function in this bounded region, which results in an optimized geometrical configuration. A point design with an efficiency of approx. 90% has been developed for initial demonstration, and can serve as the basis for future instruments. Design variations on this implementation are also discussed, which can lead to lower efficiencies due to diffractive losses in the multimode region.

Upcoming major NASA missions such as the Einstein Inflation Probe and the Single Aperture Far-Infrared Observatory will require arrays of detectors with thousands of background-limited elements sensitive to wavelengths from ∼ 50 μ m to ∼ 3 mm . Because of the low power levels present in space with cryogenic telescopes, these detectors will likely operate at temperatures below 100 mK. In order to make rapid progress on detector development, the cryogenic testing cycle must be made convenient and quick. We have developed a cryogenic detector characterization system capable of testing superconducting detector arrays in formats up to 8 × 32 , read out by SQUID multiplexers. The system relies on the cooling of a two-stage adiabatic demagnetization refrigerator immersed in a liquid helium bath. This approach permits a detector to be cooled from 300 K to 50 mK in under 6 h, so that a test cycle begun in the morning will be over by the end of the day. The system is modular, with two identical immersible units, so that while one unit is cooling, the second can be reconfigured for the next battery of tests.

We have fabricated absorber-coupled microwave kinetic inductance detector (MKID) arrays for sub-millimeter and far-infrared astronomy. Each detector array is comprised of lambda/2 stepped impedance resonators, a 1.5 micrometer thick silicon membrane, and 380 micrometer thick silicon walls. The resonators consist of parallel plate aluminum transmission lines coupled to low impedance Nb microstrip traces of variable length, which set the resonant frequency of each resonator. This allows for multiplexed microwave readout and, consequently, good spatial discrimination between pixels in the array. The Al transmission lines simultaneously act to absorb optical power and are designed to have a surface impedance and filling fraction so as to match the impedance of free space. Our novel fabrication techniques demonstrate high fabrication yield of MKID arrays on large single crystal membranes and sub-micron front-to-back alignment of the microstrip circuit.

We are constructing the Primordial Inflation Polarization Explorer (PIPER) to measure the polarization of the cosmic microwave background (CMB) and search for the imprint of gravity waves produced during an inflationary epoch in the early universe. The signal is faint and lies behind confusing foregrounds, both astrophysical and cosmological, and so many detectors are required to complete the measurement in a limited time. We will use four of our matured 1,280 pixel, high-filling-factor backshort-under-grid bolometer arrays for efficient operation at the PIPER CMB wavelengths. All four arrays observe at a common wavelength set by passband filters in the optical path. PIPER will fly four times to observe at wavelengths of 1500, 1100, 850, and 500 microns in order to separate CMB from foreground emission. The arrays employ leg-isolated superconducting transition edge sensor bolometers operated at 145 mK; tuned resonant backshorts for efficient optical coupling; and a second-generation superconducting quantum interference device multiplexer readout. We describe the design, development, and performance of PIPER bolometer array technology to achieve background-limited sensitivity for a cryogenic balloon-borne telescope.

We describe our ongoing project to build a far-infrared polarimeter for the HAWC instrument on SOFIA. Far-IR polarimetry reveals unique information about magnetic fields in dusty molecular clouds and is an important tool for understanding star formation and cloud evolution. SOFIA provides flexible access to the infrared as well as good sensitivity to and angular resolution of continuum emission from molecular clouds. We are making progress toward outfitting HAWC, a first-generation SOFIA camera, with a four-band polarimeter covering 50 to 220 microns wavelength. We have chosen a conservative design which uses quartz half-wave plates continuously rotating at ~0.5 Hz, ball bearing suspensions, fixed wire-grid polarizers, and cryogenic motors. Design challenges are to fit the polarimeter into a volume that did not originally envision one, to minimize the heating of the cryogenic optics, and to produce negligible interference in the detector system. Here we describe the performance of the polarimeter measured at cryogenic temperature as well as the basic method we intend for data analysis. We are on track for delivering this instrument early in the operating lifetime of SOFIA.

We have designed, fabricated, and tested compact radiative control structures, including antireflection coatings and resonant absorbers, for millimeter through submillimeter wave astronomy. The antireflection coatings consist of micromachined single crystal silicon dielectric sub-wavelength honeycombs. The effective dielectric constant of the structures is set by the honeycomb cell geometry. The resonant absorbers consist of pieces of solid single crystal silicon substrate and thin phosphorus implanted regions whose sheet resistance is tailored to maximize absorption by the structure. We present an implantation model that can be used to predict the ion energy and dose required for obtaining a target implant layer sheet resistance. A neutral density filter, a hybrid of a silicon dielectric honeycomb with an implanted region, has also been fabricated with this basic approach. These radiative control structures are scalable and compatible for use large focal plane detector arrays.

Far-infrared bolometric detectors are used extensively in ground-based and space-borne astronomy, and thus it is important to understand their optical behavior precisely. We have studied the intensity and polarization response of free-space bolometers and shown that when the size of the absorber is reduced below a wavelength, the response changes from being that of a classical optical detector to that of a few-mode antenna. We have calculated the modal content of the reception patterns and found that for any volumetric detector having a side length of less than a wavelength, three magnetic and three electric dipoles characterize the behavior. The size of the absorber merely determines the relative strengths of the contributions. The same formalism can be applied to thin-film absorbers, where the induced current is forced to flow in a plane. In this case, one magnetic and two electric dipoles characterize the behavior. The ability to model easily the intensity, polarization, and straylight characteristics of electrically small detectors will be of great value when designing high-performance polarimetric imaging arrays.

Motivated by the need for improved resolution in Compton scattering experiments performed with high energy X-rays, we have developed a high resolution spectrometer. We have adapted calorimetric X-ray detectors, produced at Goddard Space Flight Center for X-rays around 6 keV, to Compton scattering research using synchrotron radiation near 40 keV. Such a detector is operated below 0.1 K and determines X-ray energies by measuring the temperature increase which results from the absorption of a single X-ray photon. We have investigated a number of materials as candidates for the overlayer which is affixed to a device in order to absorb the X-rays and transfer their energy to phonons. Using Sn absorbers, we have achieved a resolution of 90 eV at 32 keV. The apparatus was brought to a wiggler beam line at the Stanford Synchrotron Radiation Laboratory, where we were able to measure Compton profiles of Si for the (100) and (111) directions. The difference reproduced the previously measured anisotropy, but the measurement was limited by the low number of total counts and by a degradation in resolution experienced during operation at the synchrotron. This experiment illustrates the feasibility of using such a system for Compton scattering research. We expect that future improvements to the detectors and to the supporting apparatus will make higher resolution measurements possible.

In October 2008, for the second time we demonstrated a monolithic Backshort Under Grid (BUG) 8×16 TES array in the field using our 2 mm wavelength imager GISMO (Goddard IRAM Superconducting 2 Millimeter Observer) at the IRAM 30 m telescope in Spain. We present astronomical results from this observing run together with a discussion of the instrument performance.

In November of 2007 we demonstrated a monolithic Backshort-Under-Grid (BUG) 8x16 array in the field using our 2 mm wavelength imager GISMO (Goddard IRAM Superconducting 2 Millimeter Observer) at the IRAM 30 m telescope in Spain for astronomical observations. The 2 mm spectral range provides a unique terrestrial window enabling ground-based observations of the earliest active dusty galaxies in the universe and thereby allowing a better constraint on the star formation rate in these objects. The optical design incorporates a 100 mm diameter silicon lens cooled to 4 K, which provides the required fast beam yielding 0.9 lambda/D pixels. With this spatial sampling, GISMO will be very efficient at detecting sources serendipitously in large sky surveys, while the capability for diffraction limited imaging is preserved. The camera provides significantly greater detection sensitivity and mapping speed at this wavelength than has previously been possible. The instrument will fill in the spectral energy distribution of high redshift galaxies at the Rayleigh-Jeans part of the dust emission spectrum, even at the highest redshifts. Here1 will we present early results from our observing run with the first fielded BUG bolometer array. We have developed key technologies to enable highly versatile, kilopixel, infrared through millimeter wavelength bolometer arrays. The Backshort-Under-Grid (BUG) array consists of three components: 1) a transition-edge-sensor (TES) based bolometer array with background-limited sensitivity and high filling factor, 2) a quarter-wave reflective backshort grid providing high optical efficiency, and 3) a superconducting bump-bonded large format Superconducting Quantum Interference Device (SQUID) multiplexer readout. The array is described in more detail elsewhere (Allen et al., this conference). In November of 2007 we demonstrated a monolithic 8x 16 array with 2 mm-pitch detectors in the field using our 2 mm wavelength imager GISMO (Goddard IRAM Superconducting 2 Millimeter Observer) at the IRAM 30 m telescope in Spain for astronomical observations. The 2 mm spectral range provides a unique terrestrial window enabling ground-based observations of the earliest active dusty galaxies in the universe and thereby allowing a better constraint on the star formation rate in these objects. The optical design incorporates a 100 mm diameter silicon lens cooled to 4 K, which provides the required fast beam yielding 0.9 lambda1D pixels. With this spatial sampling, GISMO will be very efficient at detecting sources serendipitously in large sky surveys, while the capability for diffraction limited imaging is preserved. The camera provides significantly greater detection sensitivity and mapping speed at this wavelength than has previously been possible. The instrument will fill in the spectral energy distribution of high redshift galaxies at the Rayleigh-Jeans part of the dust emission spectrum, even at the highest redshifts. Here I will we present early results from our observing run with the first fielded BUG bolometer array.

We present first results of Hertz/VPM, the first submillimeter polarimeter employing the dual Variable-delay Polarization Modulator (dual-VPM). This device differs from previously used polarization modulators in that it operates in translation rather than mechanical rotation. We discuss the basic theory behind this device, and its potential advantages over the commonly used half wave plate (HWP). The dual-VPM was tested both at the Submillimeter Telescope Observatory (SMTO) and in the lab. In each case we present a detailed description of the setup. We discovered nonideal behavior in the system. This is at least in part due to properties of the VPM wire grids (diameter, spacing) employed in our experiment. Despite this, we found that the dual-VPM system is robust, operating with high efficiency and low instrumental polarization. This device is well suited for air and space-borne applications., Comment: 31 pages, 11 figures, 2 tables

We are developing a kilopixel, filled bolometer array for infrared astronomy. The array consists of three individual components, to be merged into a single, working unit; 1) a transition edge sensor (TES) bolometer array, operating in the milliKelvin regime, 2) quarter-wave resonance backshorts, and 3) superconducting quantum interference device (SQUID) multiplexer readout. The detector array is a filled, square-grid of suspended, silicon membrane bolometers with superconducting thermistors. The spacing of the backshort beneath the detector grid can be set from ~30-300 microns by adjusting two process parameters during fabrication. We have produced prototype, monolithic arrays having 1 mm and 2 mm pitch detectors. The key technologies required for kilopixel arrays of detectors to be hybridized to SQUID multiplexer readout circuits have been demonstrated. Mechanical models of large-format detector grids have been indium bump-bonded to dummy multiplexer readouts to study electrical continuity. A monolithic array of 1 mm pitch detectors has been mated to a backshort grid optimized for a 350 micron resonant wavelength. Through-wafer microvias, for electroplated, low-resistance electrical connection of detector elements, have been prototyped using deep reactive ion etching. The ultimate goal of this work is to develop large-format (thousands of pixels) bolometer array architecture with background-limited sensitivity, suitable for a wide range of long wavelengths and a wide range of astronomical applications such as imaging, spectroscopy, and polarimetry and applicable for ground-based, suborbital, and space-based instruments.

Filled arrays of bolometers are currently being employed for use in astronomy from the far-infrared through millimeter parts of the electromagnetic spectrum. Because of the large range of wavelengths for which such detectors are applicable, the number of modes supported by a pixel will vary according to the specific application of a given available technology. We study the dependence of image fidelity and induced polarization on the size of the pixel by employing a formalism in which diffraction due to the pixel boundary is treated by propagating the second-order statistical correlations of the radiation field through a model optical system. We construct polarized beam pattern images of square pixels for various ratios of p/\lambda where p is the pixel size and \lambda is the wavelength of the radiation under consideration. For the limit in which few modes are supported by the pixel (p/\lambda1), the geometric limit is approached as expected. This technique gives a quantitative approach to optimize the imaging properties of arrays of planar detectors in the few-mode limit., Comment: 20 pages, 8 figures

We present images taken with the first deployed astronomical instrument to use multiplexed superconducting bolometers. The Fabry-Perot Interferometer Bolometer Research Experiment (FIBRE), a broadband submillimeter spectrometer, took these images as a detector investigation at the Caltech Submillimeter Observatory (CSO). FIBRE's detectors are superconducting bilayer transition edge sensor (TES) bolometers read out by a SQUID multiplexer. An order-sorted Fabry-Perot provides illumination of a 16-element linear bolometer array, resulting in five orders at a spectral resolution of around 1200 covering the 350 micron atmospheric band. We present multiwavelength images of Jupiter, Venus and the high-mass star-forming region G34.3+0.2 taken with this instrument at several wavelengths in the 350 micron band, separated by approximately 8 microns. These images have validated the use of multiplexed superconducting bolometers in an astronomical application and have helped inform the design of our future instruments.

We report on methods to minimize thermally-induced deformation in a MEMS-based reconfigurable aperture. The device is an enabling component of the Near-Infrared Spectrometer, a principle instrument on NASA’s James Webb Space Telescope. The Microshutter Array consists of 384x175 individually addressable shutters which can be magnetically rotated 90° into the plane of the array and electrostatically latched open. Each shutter is a 100x200 μm rectangular membrane suspended by a small neck region and torsion flexure. The primary materials in the shutter are a 5000a Si3N4 layer for mechanical rigidity, 2000a Al for opacity and electrostatic latching, and 2200a Co90Fe10 for magnetic actuation. This multi-layer stack presents a challenge due to the operating temperatures required for the device: both room temperature (300K) and cryogenic temperature (30K). Thermal expansion of the materials causes the shutters to bow out of plane excessively, which can prevent actuation of the shutters, cause damage to portions of the array, and allow light leakage around closed shutters. Here we present our investigation of several methods to prevent microshutter bowing including deposition of additional materials on the shutters to create a symmetrical layer stack and replacing the current stack with low-coefficient of thermal expansion materials. Using shutter-size suspended cantilever beams as a rapid-development test bed, we have reduced out-of-plane bowing between 300K and 30K to 10% or better. We are currently applying these results to microshutter arrays to develop shutters that remain flat from room temperature to cryogenic temperature while retaining the required mechanical, optical, and magnetic properties.

著者人数：34名, 資料番号: SA1001798000

We describe a polarimeter that will be a testbed for a novel polarization modulator. This modulator is composed of two modified Martin-Puplett interferometers in which the input polarization state is transformed by adjusting the phase delays between linear orthogonal polarizations in each of the two stages. This type of modulator represents a potential improvement over existing technology in the following three ways. First, these modulators can fully characterize the polarization state by measuring Stokes Q, U, and V. The characterization of V is especially useful as a diagnostic for systematic errors. Second, the Martin-Puplett modulators can be easily retuned to observe at multiple frequencies. Finally, the small translations required for modulation can be accomplished with fewer moving parts than the rotational motions in wave plate modulators, thus potentially leading to modulators with longer lifetimes than those currently employed in polarimeters. The next generation of polarimeters, designed for both the study of Galactic magnetic fields in the far-infrared and submillimeter and the measurement of the polarized component of the Cosmic Microwave Background, will need to incorporate modulators with these attributes. This prototype polarimeter will be a modified version of the Hertz polarimeter that had until recently been operating on the Caltech Submillimeter Observatory. After modification, Hertz will be moved to the Heinrich Hertz Telescope in Arizona. We will utilize dynamic scheduling to efficiently observe during the best fraction of weather at this site.

HAWC (High-resolution Airborne Wideband Camera) is a facility science instrument for SOFIA (StratosphericObservatory for Infrared Astronomy). It is a far-infrared camera designed for diffraction-limited imaging in fourspectral passbands centered at wavelengths of 53, 89, 155, and 216 m. Its detector is a 12x32 array of bolometerscooled to 0.2 K by an adiabatic demagnetization refrigerator. In this paper, we report on the development and testing ofthe instrument and its subsystems.Keywords : far-infrared, sub-millimeter, bolometer, instrumentation, cryogenic, camera, SOFIA 1. INTRODUCTION The requirements and design concepts for HAWC were discusse d in [1]. In this paper, we describe changes made sincethat time and report on progress in construction and testing of the instrumental subsystems. Major subsystems includedetector, optics, cryogenics, electronics, and software. We di scuss the fabrication and tes ting of the det ector separately[2][3]. This paper will focus on the other hardware subsystems. Software will be addressed in a future paper.The optical path and spectral passbands were described in [1]. Since that time, the optical design has remainedunchanged, but the filter passbands have been slightly modified (see Section 2.2 for band definitions andcharacteristics). Briefly, light from the telescope first passes through a warm fore-optics system. It reflects from a flatmirror, comes to a focus, then reflects from a spherical mirror that forms an image of the telescope at a cold pupil insidethe cryostat. From there, the light passes through one of four interchangeable lenses that re-image the telescope focalplane onto the detector array. Each lens assembly also contains a filter that defines the instrumental spectral response.The magnifications are such that there are 2.7 pixels per FWHM of the Airy function at each wavelength.The cryostat uses liquid helium and two vapor-cooled radiation shields to cool the optics and detector assembly to atemperature of ~4 K. An adiabatic demagnetization refrigerator (ADR) cools the detector array itself to 0.2 K. Thecryogenic suspension must be both thermally efficient and extremely stiff (to minimize microphonic noise andaccurately locate the optics under varying accelerations and orientations). The instrument must also meet Federal contact: al@oddjob.uchicago.edu ; telephone: (262)-245-5555; 373 W. Geneva St., Williams Bay, WI 53191

We introduce an architecture for changing the polarization state of far-infrared through submillimeter radiation that employs two Martin-Puplett interferometers. One interferometer is oriented with its beam-splitting grid at an angle of 22.5 degrees with respect to the Stokes Q axis. The second is oriented with its beam-splitting grid at an angle of 45 degrees. By modulating one of the arms of each interferometer, it is possible to arbitrarily adjust the polarization state that a polarization-sensitive detector measures when placed at the output of the device. Because of this flexibility, one application of this device is as a calibrator for a polarimeter. In addition, it is conceivable to use such a device as a modulator for a far-infrared/submillimeter polarimeter. As such, this system has several advantages over a half-wave plate. First, the capability to measure circular polarization will provide the instrument with a novel method for checking systematic errors, as the circular polarization of most astronomical continuum sources is expected to be near zero. Second, such a device is easily adapted to work at different wavelengths, thus facilitating the construction of far-infrared and submillimeter polarimeters with multiple passbands. Finally, the small linear throws necessary for modulation eliminate the need for complicated systems of gears and low temperature bearings that are common in wave plate systems and often prone to failure. We present a Jones matrix analysis of this modulator architecture and compare the performance of this device with that of a half-wave plate.

We have developed a high resolution near-infrared temperature tunable cryogenic spectrometer with solid Fabry-Perot etalons. It is designed and built for diffuse emission of ionized hydrogen Brγ studies, although with the appropriate pre-filter it can be configured for any near infrared lines. The etalons made from silicon and germanium operate near 77K. The high refractive index of these etalons allows for the construction of a very compact spectrometer. Germanium etalon with 20mm clear aperture is equivalent to a gas spaced Fabry-Perot interferometer of about 80 mm in diameter. A strong temperature dependence of the refractive index for these two materials makes it easy to tune etalons. Combination of these factors allowed to build a compact, high resolution (R=12000) high throughput instrument.

We present the Fabry-Perot designed for FIBRE and its evolution for its use in the SAFIRE imaging spectrometer for the SOFIA airborne telescope. The Fabry-Perot Interferometer Bolometer Research Experiment (FIBRE) is a broadband submillimeter spectrometer for the Caltech Submillimeter Observatory (CSO). FIBRE's detectors are superconducting transition edge sensor (TES) bolometers read out by SQUID multiplexers. During the first light of FIBRE in June 2001, we measured a spectral resolution of about 1200. The Fabry-Perot concept has its heritage in the ISO/LWS instrument, scaled and adapted to the submillimeter range. The semi-reflecting optics consist of a metallic meshe deposited on a lens and a wedged plate made of monocrystalline quartz. We use three voice coil actuators in the Fabry-Perot design to achieve a displacement of 600 microns of the moving plate. The use of NbTi superconducting wire for the coils allows operation at 1.5 K without any Joule dissipation. Capacitive sensors in line with each actuator and their AC readout provide three independant position measurements. These measurements are fed into a triple PID amplifier controlling the actuators. Because of the high level of vibrations present on an airborne instrument platform, it it necessary to reject the vibrations in the Fabry-Perot up to the resonance frequencies. We propose an original method to obtain a frequency response of the PID system up to 60 Hz. The updated Fabry-Perot will be used for the next FIBRE run in autumn 2003, aiming to detect the Doppler-broadened line emission from external galaxies.

The Submillimeter and Far-InfraRed Experiment (SAFIRE) on the SOFIA airborne observatory will employ a large-format, two-dimensional, close-packed bolometer array. SAFIRE is an imaging Fabry-Perot spectrometer operating at wavelengths between 100μm and 700μm. The array format is 16×32 pixels, using a 32-element multiplexer developed in part for this instrument. The low backgrounds achieved in spectroscopy require very sensitive detectors with NEPs of order 10 -19 W/√Hz. An architecture which permits 512 pixels to be placed adjacent to each other in an area the size of a postage stamp, integrate them with multiplexers, and provide all the necessary wiring interconnections is a complex proposition, but can be achieved. Superconducting detectors can be close-packed using the Pop-Up Detector (PUD) format, and SQUID multiplexers operating at the detector base temperature can be intimately coupled to them. The result is a compact array, easily scalable to kilopixel arrays. We describe the PUD architecture, superconducting transition edge sensor bolometers we have manufactured and tested using the PUD architecture, and the electronics of SQUID multiplexed readouts. We show the design and assembly of the mechanical model of a 512-element bolometer array.

We have built a prototype submillimeter spectrometer, FIBRE, which is based on a helium-cooled scanning Fabry-Perot and superconducting transition edge sensor bolometers (TES). SQUID multiplexers are used to read out the individual detector pixels. The spectral resolving power of the instrument is provided by the Fabry-Perot spectrometer. The outgoing light from the Fabry-Perot passes onto a low resolution grating for order sorting. A linear bolometer array consisting of 16 elements detects this dispersed light, capturing 5 orders simultaneously from one position on the sky. With tuning of the Fabry-Perot over one free spectral range, a spectrum covering Δλ/λ=1/7 at a resolution of ~1/1200 can be achieved. The spectral resolution is sufficient to resolve doppler broadened line emission from external galaxies. FIBRE operates in the 350 μm and 450 μm bands. These bands cover line emission from the important PDR tracers neutral carbon [CI] and carbon monoxide CO. The spectrometer was used at the Caltech Submillimeter Observatory to obtain the first ever astronomical observations using multiplexed arrays of superconducting transition edge bolometers.

We investigate the polarization modulation properties of a variable-delay polarization modulator (VPM). The VPM modulates polarization via a variable separation between a polarizing grid and a parallel mirror. We find that in the limit where the wavelength is much larger than the diameter of the metal wires that comprise the grid, the phase delay derived from the geometric separation between the mirror and the grid is sufficient to characterize the device. However, outside of this range, additional parameters describing the polarizing grid geometry must be included to fully characterize the modulator response. In this paper, we report test results of a VPM at wavelengths of 350 microns and 3 mm. Electromagnetic simulations of wire grid polarizers were performed and are summarized using a simple circuit model that incorporates the loss and polarization properties of the device., Comment: 25 pages, 10 figures, accepted by Applied Optics

Thermal detectors operating at cryogenic temperatures can be used as sensitive microcalorimeters for measuring small pulses of energy such as produced by the absorption of individual X-ray photons. This scheme has two major advantages for photon detection. First, in principle, all the X-ray energy can be degraded into phonons to which we are sensitive, thus avoiding the statistical noise associated with the partition of the energy into several channels. Second, the limiting noise of the energy measurement is set by thermodynamic fluctuations. At low temperatures, these fluctuations in practical devices will allow energy resolution on the order of a few electron volts (eV) FWHM. We present results of experiments performed to evaluate the characteristics of thermal detectors. We find that our measurements agree well with the the general theory of thermal detectors. Specifically, the magnitude of the equilibrium temperature fluctuations agrees well with that predicted by thermodynamics for the detectors tested. The completeness of energy thermalization, however, varies among the devices tested. We will discuss this as it affects detector energy resolution.

The SAFIRE, the Submillimeter and Far InfraRed Experiment, was designed for interstellar physics in the airborne Observatory SOFIA. SAFIRE is a cryogenic Echelle Grating spectrograph for covering 27 to 470 microns, with R ranging from 2-6,000. Here we will discuss the details of the optical design, the design process, and the performance of the instrument.