The Penn Array Receiver (PAR) is a camera designed for rapid, high angular resolution imaging at 90 GHz (3.3 mm). When installed on the 100 m Green Bank Telescope it will have a 32" × 32" field of view and 8" resolution. PAR has an eight by eight planar array of superconducting Transition Edge Sensor bolometers. Currently it is in the commissioning phase and after that it will become a user instrument capable of mapping a 5' × 5' area of sky to a noise level of 40 <i>μ</i>Jy in one hour.
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.
The Fabry-Perot Interferometer Bolometer Research Experiment FIBRE, a protoype submillimeter spectrometer for astronomical observations, is based on a helium-cooled scanning Fabry-Perot and superconducting transition edge sensor bolometers (TES). The TES design takes advantage of a recently discovered method of excess noise reduction by depositing lateral normal metal bars on these devices. A SQUID multiplexer is used to read out the individual detector pixels. The spectral resolving power of the instrument is provided by a 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. This 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 is used at the Caltech Submillimeter Observatory for astronomical
Upcoming major NASA missions such as the Einstein Inflation Probe and the Single Aperture Far-Infrared Observatory require arrays of detectors with thousands of elements, operating at temperatures near 100 mK and sensitive to wavelengths from ~50 μm to ~3 mm. Such detectors represent a substantial enabling technology for these missions, and must be demonstrated soon in order for them to proceed. 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 8x32, 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 about 4 hours, 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 describe the design, construction, and predicted performance of this cryogenic detector testing facility.
The redshift (z) and Early Universe Spectrometer (ZEUS) is an echelle grating spectrometer designed to study the history of star formation in the Universe from about 2 billion years after the Big Bang to the present by observing submillimeter and far-infrared spectral lines from distant dusty galaxies. ZEUS has moderate resolving power (R~1000), and large spectral coverage so as to optimize extragalactic point source sensitivity in the telluric submillimeter (350, 450, and 610 um) windows. When completed, ZEUS will have a 4 x 64-element array of TES PUD bolometers delivering an instantaneous 64-element spectrum for each of 4 spatial positions on the sky. ZEUS is designed for use on the 15 m JCMT telescope on Mauna Kea. We also plan to use it on the 12 m APEX telescope at the Chajnantor site in northern Chile. Our scientific goals include (1) investigating star formation in the early Universe by measuring the redshifted fine-structure lines from distant (z ~1 to 4) (proto-) galaxies, (2) measuring the redshifts of optically obscured submillimeter galaxies by detecting their bright 158 um [CII] line emission, and (3) investigating the properties of starburst and ultraluminous galaxies in the local Universe by observing their [CI] and mid-J CO rotational line emission.
Far-infrared detector arrays such as the 16x32 superconducting bolometer array for the SAFIRE instrument (flying on the SOFIA airborne observatory) require systems of readout and control electronics to provide translation between a user-driven, digital PC and the cold, analog world of the cryogenic detector. In 2001, the National Institute of Standards and Technology (NIST) developed their Mark III electronics for purposes of control and readout of their 1x32 SQUID Multiplexer chips. We at NASA's Goddard Space Flight Center acquired a Mark III system and subsequently designed upgrades to suit our and our collaborators' purposes. We developed an arbitrary, programmable multiplexing system that allows the user to cycle through rows in a SQUID array in an infinite number of combinations. We provided 'hooks' in the Mark III system to allow readout of signals from outside the Mark III system, such as telescope status information. Finally, we augmented the heart of the system with a new feedback algorithm implementation, flexible diagnostic tools, and informative telemetry.
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 is an imaging Fabry-Perot spectrometer operating at wavelengths between 100μm and 700μm. SAFIRE’s key science goal is to investigate line emission in galaxies at wavelengths not visible from the ground, and to map the variation in this line emission in nearby galaxies. SOFIA will fly at an altitude where the atmosphere is mostly transparent, permitting SAFIRE to achieve a high point source sensitivity at most wavelengths. With a field of view of 160''×320'' at a spectral resolution of ~200km/s, when SAFIRE achieved first light in 2006, it will add substantial capability to the first light instrument complement of SOFIA. SAFIRE’s top priority observations will be to measure emission lines in the Galactic center, to map emission lines in nearby galaxies, and to understand the physics of the cores of ultraluminous galaxies from the local region to the high redshift universe through far-infrared fine-structure line emission.
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<sup>-19</sup> 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.
The next generation of far-infrared and submillimeter instruments require large arrays of detectors containing thousands of elements. These arrays will necessarily be multiplexed, and superconducting bolometer arrays are the most promising present prospect for these detectors. We discuss our current research into superconducting bolometer array technologies, which has recently resulted in the first multiplexed detections of submillimeter light and the first multiplexed astronomical observations. Prototype arrays containing 512 pixels are in production using the Pop-Up Detector (PUD) architecture, which can be extended easily to 1000 pixel arrays. Planar arrays of close-packed bolometers are being developed for the GBT and for future space missions. For certain applications, such as a slewed far-infrared sky survey, feedhorn-coupling of a large sparsely-filled array of bolometers is desirable, and is being developed using photolithographic feedhorn arrays. Individual detectors have achieved a Noise Equivalent Power (NEP) of ~10<sup>-17</sup> W/√Hz at 300mK, but several orders of magnitude improvement are required and can be reached with existing technology. The testing of such ultralow-background detectors will prove difficult, as this requires optical loading of below 1fW. Antenna-coupled bolometer designs have advantages for large format array designs at low powers due to their mode selectivity. We also present a design and preliminary results for an enhanced-dynamic-range transition edge sensor suitable for broadband ultralow-background detectors.
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.
Large format, two dimensional arrays of close-packed bolometers will enable submillimeter cameras and spectrometers to obtain images and spectra orders of magnitude faster than present instruments. The South Pole Imaging Fabry-Perot Interferometer (SPIFI) for the AST/RO observatory and the Submillimeter and Far-InfraRed Experiment (SAFIRE) on the SOFIA airborne observatory will employ a large-format, two-dimensional, close-packed bolometer arrays. Both these instruments are imaging Fabry-Perot spectrometers operating at wavelengths between 100μm and 700μm. The array format is 16×32 pixels, using a 32-element multiplexer developed in part for this purpose. The low backgrounds achieved in spectroscopy require very sensitive detectors with NEPs of order (formula available in paper). Superconducting detectors can be close-packed using the Pop-Up Detector (PUD) format, and SQUID multiplexers operating at the detector bas temperature can be intimately coupled to them. We have fabricated and assembled an engineering model array of close-packed bolometers with a multiplexed readout that features a very compact, modular approach for large format arrays.
SAFIRE is a versatile imaging Fabry-Perot spectrograph covering 145 to 655 microns, with spectral resolving powers ranging over 5 - 10,000. Selected as a `PI' instrument for the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA). SAFIRE will apply 2D pop-up bolometer arrays to provide background-limited imaging spectrometry. Superconducting transition edge bolometers and SQUID multiplexers are being developed for these detectors. SAFIRE is expected to be a `First Light' instrument, usable during the initial SOFIA operations. Although a PI instrument rather than a `Facility Class' science instrument, it will be highly integrated with the standard SOFIA planning, observation, and data analysis tools.
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 SOFIA's user community during its first operational use.
The Far InfraRed Absolute Spectrophotometer (FIRAS) was built to measure the spectrum of diffuse emission from 1 to 100 cm<SUP>-1</SUP>, with particular attention to possible differences between the spectrum of the cosmic microwave background radiation (CMBR) and a blackbody spectrum as small as 0.1% of the peak of the CMBR spectrum. The FIRAS has differential inputs and outputs, a full beam external calibrator, a controllable reference blackbody, and a polarizing Michelson interferometer with bolometer detectors. It is operated at a temperature of 1.5 K inside a liquid helium cryostat to suppress instrument emission and improve detector sensitivities. It has an intrinsic frequency resolution of the order of 0.7%, maximum path lengths of 1.2 and 5.9 cm, and a beamwidth of 7 degree(s), and achieved its goals for accuracy and rms sensitivity for νI<SUB>ν</SUB>, which are better than 10<SUP>-9</SUP> W/cm<SUP>2</SUP>sr over the frequency range from 2 to 20 cm<SUP>-1</SUP>.