Snowballs are transient events observed in HgCdTe detectors with a sudden increase of charge in a few pixels. They appear between consecutive reads of the detector, after which the affected pixels return to their normal behavior. The origin of the snowballs is unknown, but it was speculated that they could be the result of alpha decay of naturally radioactive contaminants in the detectors, but a cosmic ray origin cannot be ruled out. Even though previous studies predicted a low rate of occurrence of these events, and consequently, a minimal impact on science, it is interesting to investigate the cause or causes that may generate snowballs and their impact in detectors designed for future missions. We searched for the presence of snowballs in the dark current data in Euclid and Wide Field Infrared Survey Telescope (WFIRST) detectors tested in the Detector Characterization Laboratory at Goddard Space Flight Center. Our investigation shows that for Euclid and WFIRST detectors, there are snowballs that appear only one time, and others than repeat in the same spatial localization. For Euclid detectors, there is a correlation between the snowballs that repeat and bad pixels in the operational masks (pixels that do not fulfill the requirements to pass spectroscopy, photometry noise, quantum efficiency, and/or linearity). The rate of occurrence for a snowball event is about 0.9 snowballs/hr. in Euclid detectors (for the ones that do not have associated bad pixels in the mask), and about 0.7 snowballs/hr. in PV3 Full Array Lot WFIRST detectors.
The SPEED camera is being developed to study the spectral energy distributions of high redshift galaxies, Sunyaev-Zel'dovich effect in X-ray clusters and other cold objects in the universe. Its initial runs will be done on the 10 m Heinrich Hertz Submillimeter Telescope (HHSMT), with later runs using the Large Millimeter Telescope (LMT). SPEED requires a 2x2 pixel cryogenic detector array of Frequency Selective Bolometers (FSB). Each of the pixels will have four frequency bands in the ~150-350 GHz range. Here we describe the development of the detector array of these high efficiency FSBs. The FSB design provides the multi-pixel multi-spectral band capability required for SPEED in a compact, light weight, stackable array. The SPEED FSB bolometers will use proximity effect superconducting transition edge sensors (TES) as their temperature-sensing element permitting significantly higher levels of electronic multiplexing in future applications where larger numbers of detectors may be required.
We present the performance of the IR detectors developed for the WFC3 project. These are HgCdTe 1Kx1K devices with cutoff wavelength at 1.7 μm and 150K operating temperature. The two selected flight parts, FPA#64 (prime) and FPA#59 (spare) show quantum efficiency higher than 80% at λ=1.6 μm and greater than 40% at λ>1.1μm, readout noise of ~25 e<sup>-</sup> rms with double correlated sampling, and mean dark current of ~0.04 e/s/pix at 150K. We also report the results obtained at NASA GSFC/DCL on these and other similar devices in what concerns the QE long-term stability, intra-pixel response, and dark current variation following illumination or reset.
Wide Field Camera 3 is a fourth generation instrument for the
Hubble Space Telescope (HST), to be installed during the next HST Servicing Mission 4. For its infrared channel Rockwell Scientific Company has developed a new type of HgCdTe 1Kx1K detector, called WFC3-1R, with cutoff wavelength at 1.7μm and 150K operating temperature. The WFC3-IR detectors are based on HgCdTe MBE grown on a CdZnTe substrate and use a new type of multiplexer, the Hawaii-1R
MUX. Two flight detectors, a prime and a spare, have been recently selected on the basis of the measures performed at NASA Goddard Research Center - Detector Characterization Laboratory. These parts show quantum efficiency higher than 80% at λ=1.6μm and greater than 40% at λ>1.1μm, readout noise of ~25 e- rms with double correlated sampling, and mean dark current of ~0.04 e/s/pix at 150K. We show that the IR channel of WFC3, equipped with one of these flight detectors, beats the instrument requirements in all configurations and promises to have a discovery efficiency
significantly higher than NICMOS. In particular, a two-band
wide-area, deep survey made with WFC3 exceeds the discovery
efficiency of NICMOS before and after the installation of NCS
by a factor of 15 and 10, respectively.
EDGE is a Long Duration Balloon (LDB) borne instrument designed to measure the large-scale anisotropy of the Cosmic Infrared Background (CIB). The goal is to use this signal as a new observational tool to measure the character of the spatial distribution of galaxies at the largest spatial scales. With a 6\arcmin\ beam mapping more than 400 square degrees of sky at 8 frequency bands between 250GHz and 1.5 THz the experiment can determine the variation of galaxy density on
spatial scales ranging from >200h<sup>-1</sup> Mpc, where dark matter
variations are determined directly from Cosmic Microwave Background Radiation (CMBR) anisotropy, to <5h<sup>-1</sup> Mpc where the distribution of dark matter and galaxies is determined from galaxy redshift surveys and the underlying dynamics of structure growth is non-linear. The instrument consists of a 1-meter class off-axis telescope and a Frequency Selective Bolometer (FSB) array radiometer. The FSB design provides the compact, multi-chromatic, high sensitivity focal plane needed for this measurement.
The TopHat instrument was designed to operate on the top of a high altitude balloon. From this location, the experiment could efficiently observe using a clean beam with extremely low contamination from the far side lobes of the instrument beam. The experiment was designed to scan a large portion of the sky directly above it and to map the anisotropy of the Cosmic Microwave Background (CMB) and thermal emission from galactic dust. The instrument used a one-meter class telescope with a five-band single pixel radiometer spanning the frequency range from 150-600 GHz. The radiometer used bolometric detectors operating at ~250mK. Here, we report on the flight of the TopHat experiment over Antarctica in January, 2001 and describe the scientific goals, the operation, and in-flight performance.
The Wide Field Camera 3 (WFC3) is an instrument which is being developed for the Hubble Space Telescope. It will have a UV/VIS channel which will include two 2051 X 4096 pixel, thin, backside illuminated CCDs. These CCDs produce interference fringes in narrow band or monochromatic light images taken in the 700 nm to 1000 nm wavelength range. We have obtained 146 monochromatic images for each of the four flight candidate CCDs. These images can be used to model the physical structure of the CCD, which are described by a set of parameters deduced by solving the Fresnel equations for the absorption within the CCD as a function of wavelength. We have used the formalism developed to model the Space Telescope Imaging Spectrograph's CCD by Malumuth et. al. to determine the free parameters for a large portion of one of the WFC3 flight candidate CCDs. From these fits we are able to evaluate the ability to fit the fringing of real data by comparing a model fringe flat to an observed fringe flat. We find that we should be able to reduce the observed fringe amplitude by a factor of five or better. Finally we show that for a certain class of object (extended emission line object with a variety of radial velocities) this model is an excellent method for removing the effect of fringing.
The Frequency Selective Bolometer (FSB) is a bolometer with a patterned frequency selective absorber, coupled with a band-reflecting backshort. The resulting unit absorbs in-band radiation, and passes out-of-band radiation. Thus a series of FSBs tuned to different bands packed in series in a light pipe forms a compact multi-band photometer. The compact form factor makes it an attractive detector for a mm-wave array camera.
We have built and characterized prototypes that demonstrate this technology. We are now developing a set of FSBs for SPEED (the SPEctral Energy Distribution camera), an FSB array camera which will observe 4 pixels in 4 mm-wave spectral bands, to be used on the Heinrich Hertz Telescope and the Large Millimeter Telescope. These FSBs are fabricated on a free-standing SiN film with TES thermometers. We will discuss the design and performance of these detectors.
Advances in bolometer device and readout technologies make it possible to build photon-noise limited bolometric cameras for ground-based observations at mm-wave frequencies. However, today's bolometer cameras are limited not by photon-noise of the telescope and atmosphere but by fluctuations in the atmosphere signal. To realize the full potential of bolometer cameras on large aperture ground-based telescopes, one must find a way to defeat this foreground.
The SPEctral Energy Distribution Camera - or SPEED - is a four pixel, four frequency camera planned for eventual use on the Large Millimeter Telescope (LMT). A prototype version of this camera is currently being built for initial operation on the Heinrich Hertz Telescope (HHT). SPEED incorporates Frequency Selective Bolometers to sample the sky with a frequency-independent beam simultaneously at four frequencies (from 150 to 375 GHz) in each pixel. SPEED's ability to separate the temporally varying atmospheric signal from the true sky signal will potentially result in a per-detector sensitivity between 2 and 5 times greater than that achieved with contemporary bolometer cameras. We describe the basic design and motivation for SPEED, the expected sensitivity of the camera on the LMT, and give examples of some of the science programs we will undertake.
NASA is developing the NICMOS Cooling System (NCS) for deployment during Servicing Mission 3 of the Hubble Space Telescope (HST) in late 1999. The NCS is intended to provide mechanical cryocooling for the near IR camera and multi- object spectrometer (NICMOS) instrument that was installed during servicing mission 2 in February 1997. The NICMOS with NCS can potentially continue the near-IR capability of HST through the currently scheduled end-of-mission in 2010. The NCS hardware is currently in final integration and will soon start a series of rigorous ground and flight test that will prepare it for installation in the HST.