Accurate measurement of the intensity of light from a distant source,
perhaps as a function of position, time, or frequency.
In principle, we have a detector response of some kind (a voltage or
count) that is a function of the input light intensity I:
Considerations for detector choice and evaluation
People actually started out measuring radiation intensity by eye. I'm impressed. But the eye is non-recording and non-integrating.
Photomultipliers: one- or few-channel systems. Very accurate and sensitive, but there can be only a limited number of channels so that a slow array beats a fast PMT. Semiconductor basics: valence and conduction bands in energy level diagram. DC/pulse counting regimes. Magnetic shielding. Fabry lenses to overcome cathode inhomogeneities. Sky subtraction, chopping. PMTs excel in time resolution; devices with nanosecond response are avaiable. These detectors currently find their largest astronomical use on the astroparticle frontier, used for measuring the very short Cerenkov flashes produced by neutrino interactions in ice or energetic particle and gamma rays producing showers in the upper atmosphere. For example, this image shows a technician preparing one of their giant PMTs for installation in one of the Ice Cube digital optical modules.
For atmospheric Cerenkov arrays, PMTs (or arrays of them) are located at the foci of large "light buckets"; the light splashes are large enough that fairly crude imaging quality is acceptable.
Photographic emulsions (hypersensitized or not) - special spectroscopic emulsions can be optimized for long exposures without reciprocity failure. Easy to use, portable, no support equipment needed, came in very large formats. On the other hand, their DQE is low (almost always <1%), they are quite nonlinear in response and painful to calibrate, and grain irregularities cannot be calibrated. They store their own results without mounds of tapes, though. Sometimes used with image intensifiers to increase DQE and evenness of spectral response (at the expense of introducing spatial structure due to the internal cathodes, limiting S/N with any detector). These may also produce temperature-dependent geometric distortions. Details are here for historical interest, in case you ever need to retrieve information from photographic data. Various artifacts can be produced in photographic images - adjacency effects can alter the appearance of closely space images, there may be reflections in the camera or off the film backing, nonlinear nature of detection can make decomposing some images very difficult.
Electronographic emulsions: record photoelectrons from a cathode rather
than photons, onto a nuclear emulsion.
Could be very sensitive, have higher dynamic range than direct
plates. Also fantastically finicky and difficult to use.
IIDS (Intensified Image-Dissector Scanner): a hybrid system with three-stage image tubes, and the final output phosphor scanned rapidly (on the decay time scale of eah photon's output flash) along spectral traces of two apertures by an image dissector, magnetically scanned into a photomultiplier. Description by Robinson and Wampler 1972, PASP 84, 161. Almost exactly linear (some applications have output = const * input1.03, for example). S/N limited to 100 or so by instabilities in exact location of output spectra, so that the flat-field correction changes. At high count rates, a coincidence correction is needed (as with true photon-counting systems). These systems (Lick, AAT, KPNO, ESO) have been widely used for surveys of stars, QSOs, galactic nuclei. The readout is visible in real time. Dual apertures serve for simultaneous sky subtraction or (for large objects) simultaneous measurement of two position and time-switched sky subtraction. Polarimetric operation is possible by scanning four spectral traces, one from each aperture as split into polarization senses by calcite blocks (Miller, Robinson, and Schmidt 1980, PASP 92, 702). Adjacent pixels are not truly independent, since each photon flash has nonzero width (clever software can improve this), so S/N statistics are not trivial to work out.
Reticons (with or without intensifiers): 1-d semiconductor arrays (as used in store checkout lines) typically having 1024 diodes, using internal connections for self-scanning readout. This introduces 2,4,8,16...-channel fixed-pattern noise (removable by bias observations). The readout noise tends to be rather high, but the electron capacity of each diode is huge. This allows observations of bright targets at S/N up to 1000. Relatives of these also exist, such as the HST-FOS Digicons.
SIT (Silicon-Intensified Target) tubes: integrating TV cameras, used in a charge-storage mode or constantly read out in an "equilibrium" mode. The dynamic range in direct mode is quite limited by changes in the flat-field pattern, as is the S/N achievable. Use of photon-counting (event-centroiding) electronics as in the IPCS remedies some of this. The preparation time for each exposure can be long (up to 15 minutes) since each part of the camera tube must be cleared of accumulated charge. Readout can be similarly lengthy, passing an electron beam across the faceplate and recording the resulting current. This may suffer from beam-pulling effects. Vidicons of this kind were used on IUE, whose staff described their properties as being like dirty but reusable photographic plates..
IPCS (Image Photon-Counting System): uses TV or related CCD system and fast centroiding electronics to give position and time of each detected photon, accumulated in real time into a display memory. Has very low (essentially zero) ``readout noise", and is thus most effective at high dispersions and low photon rate, where CCD readout noise overwhelms the higher DQE of CCDs. Large numbers of pixels (2048 by 100) are possible. Relatives: KPCA, 2-D Frutti, 6-m IPCS, HST FOC.
MAMA (Multi-Anode Microchannel Array): microchannel plate feeding a multi-anode array that times individual electron bursts; acts as a photon counter. Like a generalized photomultiplier with many spatial channels. Can be very UV-sensitive, as used in STIS and GALEX. Refinements include curving the electron-avalanche channels to reduce "poisoning" of the detector by ions pulled backwards by the voltage. Charge depletion is an issue - FUSE and HST-COS lost sensitivity at wavelengths of goecoronal emission due to constant exposure.
CCD (Charge-Coupled Device): the current observers' darling. See C. MacKay 1986, Ann Rev 24,255 and various observatory newsletters. Solid-state array of potential wells (in fixed pixel array), in which changes in clock voltages can move charge around and eventually through an on-chip amplifier and thence to the outside world. Pixel sensitivities nonuniform but usually flatten to 1% or better. Excellent stability with time, linearity, DQE up to 90% in some spectral ranges. Readout noise usually the limiting factor. Thin/thick chips, red/blue sensitivity, blue enhancements by UV flooding or coatings, cosmic-ray sensitivity. Cooling is required to reduce thermal current enough to allow long exposures - either LN2 or thermoelectric (which is why the HST CCDs run warmer than normal). Much helpful detail in this page from Bob O'Connell at UVa, and Jorden's chapter in PSSS1.
Chip formats up to 8192 by 8192 pixels (about 120mm square) exist, with pixels typically 15-30 microns. At large format, readout time becomes an issue; sometimes only a subsection of the chip is actually read out unless the full format is needed. Larger chips often have four amplifiers which can be run simultaneously for different quadrants.
Fringing in far-red, monochromatic or spectroscopic applications; front-illuminated versus back-illuminated use.
On-chip binning for readout noise and dynamic range improvement.
Bias and charge-transfer efficiency, preflashing.
Cosmetic defects in imaging/spectroscopic applications: bad pixels, dead/hot columns, flat-field features. There are also frequently features from dust particles on the dewar widow or filters.
Can be read in analog mode (TV rates) for guiding or (with intensifier) for a pseudo-photon counter (2D-Frutti, KPCA). This amplifies individual photon events into blobs that can be seen above readout noise even for very short frame times.
Behavior at/near saturation: we distinguish saturation of the analog-digital converter, which results in loss of intensity information, and saturation of the full-well potential depth of a pixel, which leads to migration of charge into adjacent pixels along the column, migration along rows in the transfer register, and often to deferred charge that may show up on subsequent exposures. Improved circuit design and readout can reduce these (a good thing with large chips since there are stars all over the sky).
ADUs versus photons and noise calculations; the "CCD equation". If each ADU represents g photoelectrons (the gain) and the readout noise is r in ADU, the noise expected for a pixel with N detected ADU is
S/N in instrument comparisons and exposure calculations; sometimes a less sensitive detector will give better results. Some examples have been given in the NOAO Newsletter, in a continuing series on ``Which chip is right for you?".
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