Detectors For Space Science

The design of detector systems for flight applications requires the consideration of a number of issues unique to space instrumentation. Flight detectors must endure hostile radiation environments and thermal extremes. Paramount importance is given to reliability since inflight replacement is at best difficult and usually impossible. Flight detectors are also significant cost and design drivers since they often determine key requirements for flight instruments such as volume, mass, power consumption, heat dissipation and communications budgets. In this paper we describe the primary concerns in developing flight detector systems, and review the challenges posed by future NASA and ESA space science missions for detector development.     

The ISO-SWS detectors: Performance trends and space radiation effects

We present a trend analysis of the ISO-SWS detector performance and a study of the space radiation effects on the SWS detectors. In particular, dark currents, dark current noise and detector responses have been checked as a function of time through the mission and as a function of time in arevolution. The results show that these parameters were stable during the mission in all bandsbut for band 3 (Si:As). Dark currents and responses were found to be higherin the first hours following the start of the science window,especially in band 2 (Si:Ga). We have studied the impacts of cosmic rays and radiation belt particles on the SWS detectors, as well as of the only large solar proton event on November 6, 1997,that occurred during the ISO mission (operated during solar minimum).The observed glitch rates in all SWS bands are found to be between 2 and4 times higher than the value predicted by the CREME96 model for the cosmic ray flux in the period considered. The bands that registered the highest glitch rates showed also a correlation with the electron fluxes measured on theGOES 9 spacecraft. From the distribution of glitchheights (voltage jumps in the detector signal), we have derived the deposited energy distributions of the particles hits. Our results lead to the conclusion that secondaryparticles produced in the shield and the detectors contributed at least as much as cosmic rays to the observed glitch rate. The effects on the detectors of the November 6, 1997 event, which caused that all observationsin a revolution were declared failed, are described in detail.     

Recognition of Space Weather Impact on the Isophot Detectors

The effects of changes in the space environment on the ISOPHOT photoconductivedetectors over the whole ISO mission were studied using the complete setof responsivity check measurements taken after the curing of the detectors.We found that the responsivity of the Ge-based, low bias voltage far-infrared detectors (P3, C100, and C200) is sensitive to the conditions of the Space Weather.We present evidence that an increased responsivity level (20% – 50%) after curing of the detectors is linked to the onset of geomagnetic storms. TheSi-based, high bias voltage detectors P1, P2 and PHT–SS show only small changesin their responsivity. An exception is the PHT–SL array which shows a similar,but less pronounced behaviour as the FIR detectors. While these relationshave been demonstrated by our study, a detailed physical understanding is still outstanding. The Space Weather dependent scatter of the responsivity,being the photometric scaling factor (conversion from measured photo currentto inband power on the detector), justifies the observing mode design to include frequent monitoring of its actual level.     

The ISOCAM/LW Detector Dark Current Behaviour

We describe the calibration, measurements and data reduction, ofthe dark current of the ISOCAM/LW detector. We point-out theexistence of two significant drifts of the LW dark-current, onethroughout the ISO mission, on a timescale of days, another within each single revolution, on a timescale of hours. We alsoshow the existence of a dependence of the dark current on thetemperature of the ISOCAM detector.By characterizing all these effects through polynomial fittings,we build a model for the LW calibration dark, that depends onthe epoch of observation (parametrized with the revolutionnumber and the time elapsed in that given revolution since theactivation) and on the temperature of the ISOCAM detector. Themodel parameters are tuned for each of ISOCAM/LW pixel.We show that the modelling is very effective in taking intoaccount the dark-current variations and allows a much cleanerdark subtraction than using a brute average of severalcalibration dark images.The residuals of the LW model-dark subtraction are, on average,similar to the pre-launch expectation.     

The transient behaviour of the long wavelength channel of ISOCAM

The detector of the long wavelength channel of the ISOCAMcamera on-board the Infrared Space Observatory is a Galliumdoped Silicon photo-conductor hybridized by Indium bump. Itpresents systematic memory effects of the response which canbias the photometry by a factor of typically 40%. The main features of the response after flux variations are discussed. A simple empirical model actually used to correct systematically the data with a photometric accuracy of 5–10% is detailed.     

Space Operation and Performance of Doped Germanium Photo-Conducting Detectors in the Far Infrared: Experience from the ISO LWS

The performance of the ten doped germaniumphotoconducting detectors on the Infrared SpaceObservatory Long Wavelength Spectrometer is discussed. This instrument was designed to make spectroscopicmeasurements of astronomical sources in the wavelengthrange from 43 to 198 m. It employed^* a combination of stressed and unstressed Ge:Ga detectorsand one Ge:Be detector coupled to Infrared Labs JF-4integrating amplifiers. The performance of thedetectors was affected by the ionising radiation inthe space environment. The Noise Equivalent Power ofthe detectors increased by a factor of 4 compared tothat measured during ground testing. We show thatthis was the result of reducing the operating bias,and therefore the responsivity, of the detectors andusing shorter integration ramps to alleviate theeffects of the ionising radiation impacts. ^ast ISO exhausted its helium supply on 8 April 1998.     

The first large format Si:Sb BIB arrays

The first 128×128 Si:Sb blocked impurity band (BIB) detectors, manufactured by Rockwell International, are sensitive detectors from 10 to at least 40 m. While further work is required to make these arrays suitable for the low backgrounds of space infrared telescopes, they can be used now for observations from the ground and aircraft.     

Compact Two-Dimensional Array of Stressed GE:GA Detectors

We have developed a 4 × 8 array of stressed Ge:Ga detectors. This array detector has a high density format ofentrance pupils so that we can minimize the size of the cameraoptics. The cutoff wavelength of the detector is about 170 m, and the detector's NEP is better than 10¹⁶ WHz^-1/2. We are going apply this array detector toballoon-borne astronomical observations. Furthermore, we aredeveloping this detector into a 5 × 15 array detector that will be placed onboard the IRIS satellite to be launched in 2003.     

Science, Technology and Detectors for Extremely Large Telescopes

The second decade of the third millennium will hopefully see a new generation of extremely large telescopes. These will have diameters from 30 to 100 meters and use advanced adaptive optics to operate at the diffraction limit in order to detect astronomical objects that are impossible to observe today, such as earth-like planets around nearby stars and the earliest objects in the Universe. Even for small fields of view, the requirements for detectors are daunting, with sizes of several gigapixels, very fast readout times and extremely low readout noise. In this paper I briefly review the science case for ELTs and the requirements they set on telescopes and instruments, and report on the status of the OWL 100 m telescope project and the challenges it poses.     

Solar-Blind Diamond Detectors for Lyra, the Solar VUV Radiometer on Board Proba II

Fabrication, packaging and experimental results on the calibration of metal-semiconductor-metal (MSM) photodetectors made on diamond are reported. LYRA (Lyman- RAdiometer onboard PROBA-2) will use diamond detectors for the first time in space for a solar physics instrument. A set of measurement campaigns was designed to obtain the XUV-to-VIS responsivity of the devices and other characterizations. The measurements of responsivity in EUV and VUV spectral ranges (40–240 nm) have been carried out by the Physkalisch-Technische Bundesanstalt (PTB) in Germany at the electron storage ring BESSY II. The longer wavelength range from 210 to 1127 nm was measured with monochromatic light by using a Xe-lamp at IMO-IMOMEC. The diamond detectors exhibit a photoresponse which lie in the 35–65 mA/W range at 200 nm (corresponding to an external quantum efficiency of 20–40%) and indicate a visible rejection ratio (200–500 nm) higher than four orders of magnitude.     

世界引力波探测网的现状

引力波是广义相对论的重要推论之一,迄今为止尚未被直接探测到。上世纪末共振棒引力波探测网曾联合运行.历经十余年的努力,世界激光干涉引力波探测网已经初步形成。今后一、两年将会同步运行。最新的关于中子双星的天文观测和分析给引力波探测带来了希望。     

The Isocam Responsivity in Orbit. Standard star photometry

An overview is given of the absolute flux calibration of the ISOCAMdetectors. The flux calibration is based on observations of standard stars selected from the Ground Based Preparatory Programme, for which Kurucz stellar models are available. No dependencies of the responsivity on different configurations of thecamera were found. No trend of changing responsivity is found throughout the mission for the SW and LW detectors. There exists a decreasing responsivity of about 5% for LW during the orbit.     

Deglitching methods by the ISOPHOT Interactive Analysis (PIA)

The ISOPHOT detectors were continuously exposed to high energy cosmic particles hits, resulting in an average hit rate frequency of an event per every 7 s. The net effect is a disturbance of the readout voltages, which decreases the signal-to-noise ratio and hence the photometric accuracy level. Thanks to the highly redundant readout method of ISOPHOT it is relatively easy to identify cosmic particle hits which have enough energy to produce a noticeable deviation in the readouts. The most energetic particles can cause a short term change in the detector response. Different algorithms are implemented into PIA to cope with this problem. PIA provides handy interfaces to perform individual treatment of the data and optimize the parameters of the deglitching algorithms.     

Transient Effects in ISOPHOT Data: Status of Modelling and Correction Procedures

The calibration of ISOPHOT observations is hampered in many cases by the transient response of the detectors. Here we outline thebehaviour observed in flight for the different ISOPHOT detectors. We also present some of the procedures which are currently used to correct transienteffects and their implementation in the ISOPHOT Interactive Analysis.     

Cometary Rotation: an Overview

I discuss our current understanding of the spin states of cometary nuclei. Cometary spin influences the temporal and spatial patterns of outgassing from the nucleus (through diurnal and seasonal effects) and is in turn influenced by outgassing-driven torques. The current challenge to cometary astronomers is to quantify the interactions between the spin, the outgassing, and the resultant torques on the nucleus, and to understand the role of rotation in determining the basic physical properties of the nucleus. This revised version was published online in July 2006 with corrections to the Cover Date.     

The infrared colors of main sequence stars: how much circumstellar debris is normal?

We have been awarded NASA Key Project observing time on ISO, in order to establish the true frequency of far-infrared excesses in a volume-limited sample of main sequence and related stars, and address the relative success or failure of single stars in processes related to the forming of planetary systems. For a volume-limited subset of main sequence and related stars, PHT03 measurements at 3.6, 11.5, 20 and 60 micron will be obtained, using a 120 arsec aperture in all cases to eliminate possible companion confusion with differing apertures, to ascertain spectral energy distributions. For the M dwarfs, 100 micron observations will also be obtained. For some, brighter sources, more extensive wavelength coverage and improved spatial resolution will be attempted, using CAM and SWS. Spatially over-sampled PHOT observations will be made at 60 micron of the brightest and nearest Vega-like sources to measure the characteristic sizes of the emitting regions and obtain some information regarding their shapes and orientations. The goal is not a map, but scan profiles along 3 position angles which can be deconvolved to find the intrinsic size and shape of the half-maximum contour of the emitting region. Photometry of selected lines of sight through the zodiacal dust will also be carried out to look for outer solar system (Kuiper Belt) material. Observation at a range of wavelengths, ecliptic latitudes and at 2 epochs is designed to help untangle foreground Zodiacal from background Kuiper flux, not necessarily to look for individual macroscopic objects.     

Observing and Calibration Strategies for FIR Imaging with SIRTF

We present data from a 16×32 array of unstressed Ge:Gadetectors, showing the long time constant responses and hook effectcharacteristic of these devices. This array is half the size of the70 m imaging array planned for MIPS, the Multiband ImagingPhotometer for SIRTF. We present simulations of the FIR (farinfrared) sky as viewed by a MIPS array, including realistic noiseeffects and extraction of point sources with a standard photometrypackage, showing that the photometric goals of MIPS are attainablewith the current array performance and using standard analysistechniques. Previous calculations by Heim et al. (1998) haddetermined that MIPS would achieve 5- detections of 1 mJy at 70 m in 2000 s; the simulations presented hereobtain results consistent with those calculations.     

ULTRACAM: an ultrafast, triple-beam CCD camera for high-speed astrophysics

ULTRACAM is a portable, high-speed imaging photometer designed to study faint astronomical objects at high temporal resolutions. ULTRACAM employs two dichroic beamsplitters and three frame-transfer CCD cameras to provide three-colour optical imaging at frame rates of up to 500 Hz. The instrument has been mounted on both the 4.2-m William Herschel Telescope on La Palma and the 8.2-m Very Large Telescope in Chile, and has been used to study white dwarfs, brown dwarfs, pulsars, black hole/neutron star X-ray binaries, gamma-ray bursts, cataclysmic variables, eclipsing binary stars, extrasolar planets, flare stars, ultracompact binaries, active galactic nuclei, asteroseismology and occultations by Solar System objects (Titan, Pluto and Kuiper Belt objects). In this paper we describe the scientific motivation behind ULTRACAM, present an outline of its design and report on its measured performance.