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To illustrate basic terminology used for detectors, we will
consider 3 examples of detectors: photographic plate, photomultiplier, and
array detector.
Detectors work because they are made of some material which interacts with
photons. An incoming photon generates a chemical reaction (photographic) or
photoelectron. Photoelectrons are counted, either
- one at a time, usually with amplification, or
- by collection, then subsequent measurement.
In a photomultiplier, some fraction of incident photons hit a
photosensitive material and eject a photoelectron. This electron is
amplified numerous times to create a large ``swarm'' of electrons which
is detected as a pulse. Thus, photons are ``counted'' as they come in.
Generally, simple photomultipliers do not retain any information about
the location on the detector where the photon hits. There are some modern
devices, however, called microchannel plates, which are essentially arrays
of small photomultipliers where positional information can be obtained; one
of the more common of these is called a MAMA, and exists in several instruments
on the Hubble Space Telescope. Traditional photomultipliers were the
workhorse of photometry from the 50's to the late 70's.
Array detectors are in more common usage today. In these devices, incoming
photons create photoelectrons which are trapped in local potential wells.
The amount of energy needed to eject a photoelectron depends on the type
of material used. In the optical, silicon provides a good choice, but
the excitation energy for silicon is too high to be used in the infrared.
In the IR, various different substances are used, including HgCdTe, InSb,
and PtSi. After a specified amount of time, the photoelectrons are ``counted''.
The method by which this is done differs between different types of arrays.
In CCDs, the charge is physically clocked down columns of the device, a
single row at a time (a parallel transfer) then read out of serial
register; CCDs are inherently asymmetric in rows and columns.
In IR devices, each pixel is read individually, in sequence.
When discussing detectors, one should be familiar with some terminology:
- Quantum efficiency: fraction of photons detected. Of course, the
quantum efficiency is usually a function of wavelength. Some typical
values are:
- photographic : %
- photomultiplier : %
- array detector : %
- Size and resolution elements.
- photographic: large size (many inches), good resolution.
- photomultiplier: several inches, no resolution (although there are
arrays of photomultipliers, e.g. MAMA)
- array detector: individual detectors started out small but are
continually growing. The largest current CCDs are around 2.5 inches square,
whereas the largest IR arrays are roughly an inch square. Larger effective
sizes are now available (for CCDs) because some modern devices are made to
be ``buttable'', i.e. several can be placed side by side with only very
small gaps between them. Pixel sizes of array detectors are typically
15-25 microns.
- Full well: maximum number of photons possible to detect.
- photographic: limited by number of grains
- photomultiplier: unlimited
- array detector: 100,000 photons.
- Readout noise. The process of counting electrons is never totally exact,
so noise is introduced. Generally this is at a level of 1-100 electrons rms.
If photon events are counted, rather than counting the number of photoelectrons,
as is the case for a photomultiplier, there is no readout noise.
In array detectors, readout noise is often important. Readout noise can
actually come in two different forms: fixed pattern and random noise. The
latter is what people usually refer to as readout noise. Fixed pattern noise
is noise which is introduced in the readout process but which has the same
spatial pattern over the detector from exposure to exposure.
- Dark current. Electrons in a given substance will be moving around at
speeds which are correlated with the operating temperature of the
device. If there is enough thermal motion, an electron can actually be
liberated from the substance and then counted as a (spurious) photon
detection. This is called dark current. Devices with lower photoelectric
threshholds (used at longer wavelengths) are more susceptible to dark
current, thus they must be operated at a colder temperature. CCDs are
typically operated betwee -70 and -120 C, IR arrays are operated colder
(LN = 77K, Liquid He =4K).
- Linearity: relation between number of output electrons and input photons.
In a fully linear detector, this relation is linear, with the slope of
the relation given by the quantum effeciency. An alternate way to consider
nonlinearity is to consider that the quantum efficiency is a function of
the count level.
- photographic: nonlinear (characteristic curve)
- photoelectric: linear except for dead-time correction. A dead-time
correction applies for bright sources; if two photons arrive essentially
simultaneously, they will only be counted as a single photon.
- array detector: CCDs usually linear up to 50-90% of full
well. IR arrays are usually slightly nonlinear over their entire range, but
repeatably so.
- Reciprocity: sensitivity changes as a function of photon incidence rate.
Exists in photographic plates, maybe in some array detectors?
Dead-time correction is a reciprocity failure.
- Defects. Most detectors are not perfect; there are often small regions
which are unusable because of very low quantum efficiency, blocked columns,
etc. Many devices are rejected for astronomical use because of too many
defects.
- Readout speed: typically 25 microsec/pixel for array detectors.
However, the chips can in many cases be read at a variety of speeds. In
general, when chips are read faster, the readout noise increases.
- Digitization. In array detectors, after the charge is collected and
read out, it is sent through a chain of electrons which digitizes the
signal, often after amplifying it. The digitization is made by a device
known as an A/D convertor; these work by comparing an input signal with
a set of reference voltages which successively differ by factors of two.
Thus an input signal is translated into a series of bits depending on whether
the input voltage exceeds a series of reference voltages. Typical A/D
correctors in use in astronomy consider 16-bits. The digital
signal which comes out of the CCDs is variously referred to as counts,
digital numbers (DN), or analog-to-digital units (ADU). The number
of output counts is related to the number of input counts by a constant
which depends on the amplification in the electronics. The amplification
factor is known by most people as the gain, but astronomers define the
gain of a device by the number of input electrons divided by the number
of output counts (i.e., the inverse gain); the ``astronomical'' gain is
usually specified in units of e/DN. Because the number which we
receive from the electronics chain differs from the number of input
electrons, the calculation of noise must take this into account. The
photon counting noise (rms) is given by the square root of the number of
detected photons. The number of detected photons is given by , where
is the (inverse) gain and is the number of detected counts.
Consequently, the noise in electrons is , and in
units of counts is given by . This is apart from readout
noise; the latter is usually specified in units of electrons, giving
a total noise in electrons of , or, in units of counts,
by
.
A/D convertors can only measure a positive incoming signal. At low light
levels, the true input signal can be negative in the presence of readout
noise. To avoid trucation of the negative signals, a constant voltage, called
the bias, is added to the signal before it passes through the A/D. This
bias must later be removed to preserve the correct count ratios between
different sources.
A/D convertors can introduce small systematic errors in recorded count
rates if the reference voltages are not carefully controlled.
- Dynamic range. A detector system can be characterized by its dynamic
range, which is the ratio of the signals of the brightest and faintest
sources which can be detected (with some definition of ``detection'').
At the bright end, the system is limited by either the full well of the
detector (or the number of electrons at which the detector goes significantly
non-linear), or alternatively by the limitation of the A/D convertor (e.g.,
if an A/D convertor has 16 bits, you can never see counts higher than
. At the faint end, the system is limited either by
the A/D convertor (you can't detect less than one count), or by the
readout noise (source buried by readout noise cannot be detected). The
gain of a system is often set to maximize the dynamic range; if the readout
noise is electrons, one can maximize dynamic range by digitizing
the signal by several electrons/DN if the detector has sufficient full well.
- Saturation behavior. When full well is reached, the chip is said to
be saturated. In some array detectors, especially CCDs, when a given pixel
is saturated, additional charge often leaks into adjacent pixels, usually
into adjacent rows rather than columns because of the construction of the
CCD. Separately from detector saturation, one occasionally sees saturation
of the readout electrons which can have the effect of a saturated pixel
affecting the counts in subsequent pixels.
- Hysteresis generally refers to processes in which the prior exposure
history of a detector affects subsequent exposures. A common example is
residual image, in which an area of a detector which was subject
to a very bright source in a previous exposure will continue to ``glow''
in subsequent exposures. Another form of hysteresis is so-called quantum
efficiency hysteresis (QEH) in which the quantum efficiency of the chip
changes as a function of previous exposure history. One manifestation of
this effect has been used by astronomers, mostly with older CCDs; for
some reason, when these CCDs were subjected to an extended influx of
ultraviolet light, their optical quantum efficiency was found to be increased,
and to stay increased at a stable level as long as the chip was kept
cold. This led to the practive of ``UV-flooding'' CCDs as they were being
cooled.
Next: CCDs
Up: Detectors
Previous: Detectors
Rene Walterbos
2003-04-14