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CCDs require electronics to generate electrode biases, clocking waveforms, low-noise amplification of weak signals, A/D conversion, buffering, and digital storage. They also require temperature-controlled environments mountable at different telescope foci for direct imaging, and at spectrograph foci for use as spectroscopic detectors. The details of how operational systems meet these demands differ, but the major elements are similar. To illustrate these, Fig 1.4 shows the outline for RGO-built systems now in use at telescopes of the Isaac Newton Group, La Palma.
The cryostat or dewar provides the cooled environment for the CCD. A
liquid-nitrogen cryostat designed at RGO (see Figure C), can be used in either downward or
upward-looking modes, for example in direct imaging at prime or
Cassegrain foci. It has a hold time of some 12 hours so that a single
filling of N
lasts through the observing night. The CCD itself is
mounted behind an anti-reflection coated quartz window and on a copper block
with which it makes good thermal contact and which has the temperature-sensing
element attached to it. Resistive heating controlled by a feedback
loop maintains the temperature to
0.05
about the optimum operating temperature of 150.
This circuitry is mounted on the dewar, as is
circuitry to sample and pre-amplify the output signal.
A local driver-box sits within 1 metre of the cryostat, holding the remaining circuitry for which proximity to the CCD is important for optimum performance: the A/D converter and buffer, clock driver circuitry, and telemetry circuitry to monitor the status of the CCD, its environment and its electronics.
The camera controller
is a purpose-built
microcomputer which determines
the basic sequencing of all camera operations. It provides the timing for
the clock-driver circuitry, and generates commands for signal sampling.
This provides for a system of maximum flexibility - reprogramming the
microprocessor allows different CCDs to be used, for instance to
accommodate the new generation of large chips, and it also allows
different ways of reading out the chip, involving on-chip binning.
Before
considering this, first consider the standard readout sequence which the
microprocessor controls. It runs as follows:
after the observer requests a run of N seconds to be initiated, the controller
- calls for two erase cycles of the chip,
- opens the shutter,
- closes the shutter after N seconds,
- clocks out, samples, and stores the individual pixel responses
as described in Section 1.1.1
The erase cycles consist of clockingout the (unexposed) CCD to remove any charge which has built up due to dark current (section 1.1.1) or cosmic rays (section 1.3.2). This standard sequence reads the CCD out pixel-by-pixel. With the camera
controller, it is possible to change the clocking routines via software to provide on-chip binning in which the
summed charge from several adjacent
pixels is read out as a single charge signal. Spatial resolution is
correspondingly reduced, but this may not matter, for instance in direct
imaging with heavy oversampling, or in spectroscopy if spatial resolution
perpendicular to the slit can be sacrificed. On-chip binning
causes the
summed charge signal from the adjacent pixels to be measured with the
readout noise corresponding to a single sample, and if the signal is
dominated by readout noise rather than shot (photon-statistic) noise, the
improvement in signal-to-noise is a factor of n, for the binning of n
pixels. Of course post-readout binning can be done via software at the
analysis stage; but signal-to-noise only improves by 
n in this
instance. A further feature offered by the camera-controlling
microprocessor is the ability to opimise chip performance through
software changes, timing and the form of clocking signals being set by
the microprocessor. All these changes are implemented by different
versions of the microprocessor program, which is loaded or downloaded
through the interface shown in Fig 1.4.
One final feature is worth mentioning - the size of the storage medium if
on-line monitoring of images is required. Each image must be
digitised to > 12 bits to maintain the dynamic range
for which CCDs are
so valuable. Each frame from the small (400 x 500 format) CCDs thus
occupies some 0.3 megabytes in a 16-bit medium; 32-bit systems are used
at the ING.
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Next Page: Limitations, Extra Observations, and the First Stages of Analysis
