Astronomy Principles and Practice Fourth Edition.pdf

300 Detectors for optical telescopes
18.14 TV systems and other detectors
Television sensors of various kinds have also found application as detectors. The many advantages
that they have include a high quantum efficiency, broad spectral coverage, a large dynamic range, no
reciprocity failure for long exposures and the data are in a convenient form for processing by digital
computer. As data collectors, they have been superseded by CCD technology but they still remain in
use on telescopes for field identification and guidance systems.
One detector system that had great success over a couple of decades was the Image Photon
Counting System (IPCS) designed to detect single photoelectrons and assign their positions of origin
from within a two-dimensional image, so allowing a picture to be built up. The complete apparatus
used a television camera to look at the phosphor screen of a multistage image intensifier. The intensifier
gave an amplification sufficient for a single photoelectron to be detected above the readout noise of the
television camera. The frame rate of the camera was set to be sufficiently high so that the probability
of more than one photoelectron arriving from a resolved picture element during a single frame is
extremely low and the decay time of the phosphor was sufficiently short so that the same photoelectron
was not counted twice in consecutive frames. A computer controlled the system and the photoelectron
events were fed into memory locations which opened and closed in synchronization with the position
of the scanning television raster. The IPCS was developed by Boksenberg and had particular success as
a detector for the spectrograph on the Anglo-Australian Telescope recording the weak spectra of faint
quasars.
18.15 Charge coupled devices
Based on developments of solid state technology, the invention of charge-coupled devices (CCDs) in
the 1970s has revolutionized practically every aspect of observational optical astronomy. Although
photography and photoelectric photometry are still practised, nearly all current optical imaging, stellar
photometry and spectrometry is now performed using CCDs. A basic detector is small, looking like
any familiar semiconductor ‘chip’.
The CCD chip comprises a matrix of pixels typically ∼10 4 × 10 4 in number. Each pixel in the
detector comprises a light-sensitive element with an associated charge storage capacitor similar to that
found in a metal oxide semiconductor (MOS) transistor. A schematic diagram of a basic photosite
is given in figure 18.11. By giving the gate a positive bias relative to the substrate, the majority of
carriers or holes are repelled from the Si–SiO 2 junction and a depletion layer forms with increased
depth according to the value of the voltage. As the bias voltage increases to just more than a couple of
volts, the Si–SiO 2 junction becomes sufficiently positive with respect to the bulk substrate so that any
free electrons in the vicinity are attracted to the junction and form an inversion layer. Prior to exposure,
a positive potential is applied, so charging the photosite capacitor. During the exposure, any photons
impinging on the site enter the silicon crystal lattice and raise electrons from a low-energy valenceband
state to a high-conduction-band state so that they can migrate and be captured in the potential
well, effectively discharging the capacitor. The degree of the discharge at each site is proportional to
the number of photons impinging on the pixel, the detector essentially having a linear response. If the
light levels are too high or the exposure time too long, the pixels saturate and quantitative information
is lost. Typical full-well capacity is of the order of 5 × 10 5 photoelectrons with the value dependent on
the chip design. It is sensible observational practice to plan any imaging or other photometry so that
the photoelectron accumulations in the pixels are no more than ∼80% of their full-well capacity.
The spectral sensitivity covers the whole of the optical range from about 400 to 1000 nm. The
quantum efficiency is typically ∼40% but can be higher at the longer wavelengths according to the
basic photo response of silicon. The sensitivity can be further enhanced if the chip has been ‘thinned’
in the manufacturing process and is used with back illumination.