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Chapter 5 Operation 47
47
Exposure Time
Exposure time (set on the Experiment
Setup|Main tab) is the time between Start
Acquisition and Stop Acquisition commands sent
by the application software to the camera. In
combination with triggers, these commands
control when continuous cleaning of the CCD
stops and when the accumulated signal will be
readout. The continuous cleaning prevents
buildup of dark current and unwanted signal prior
to the X-ray pulse. At the end of the exposure
time, the CCD is readout and cleaning starts
again.
Because PI-SCX cameras do not incorporate an
internal shutter, some signal may accumulate on
the array while it is being readout. This
continuous exposure of the array during readout
may result in some smearing. However,
exposures that are significantly longer than the readout time can be performed without a
shutter, as the amount of smearing will be low.
If smearing or other factors require a shutter, the NOT SCAN or the SHUTTER signal at
the ST-133's output can be used to control a customer-supplied external X-ray
shutter. By using one of the signals to synchronize the shutter operation with exposure,
the CCD can be read out in darkness. Alternatively, the X-ray source can be interrupted
elsewhere in the system while readout is taking place.
CCD Temperature
Each PI-SCX camera contains a Peltier-effect thermoelectric cooler that cools the CCD.
This cooling, which may be assisted by an internal fan or by liquid coolant circulation,
reduces the amount of dark charge that is generated on the array. A thermal sensing diode
attached to the cooling block of the camera monitors the array temperature and the
current temperature is reported back to the operating software. The operating (or target)
temperature is settable in software. Ambient temperature, cooling type, and CCD array
size all affect the time required to reach and stabilize at the appropriate operating
temperature (see Table 8, page 87, for typical cooldown temperatures).
Setting the Temperature
The temperature of the CCD array is set
through software. With WinView/32, you
enter and set the target temperature after
selecting Detector Temperature from the
Setup menu.
Temperature Stabilization
After the system begins cooling, it takes about 60 minutes for the CCD to reach its preset
temperature. Because the control loop is designed to achieve temperature lock as quickly as
possible, overshoot may occur. If this happens, temperature lock will be briefly indicated and
then discontinue during the overshoot. However, the lock indication will be quickly restored