Teledyne Princeton Instruments PI-MTE, System Manual

Page: 47 / 118

Chapter 5 Operation 47
To prevent condensation damage while the camera is mounted in the vacuum
chamber:
• Vacuum Chamber: Make sure that the vacuum chamber has been pumped
down to at least 1 mTorr, before cooling the camera below ambient or turning
on coolant flow.
• Camera: Before venting a chamber, turn off the camera, turn off coolant flow,
and allow the camera to warm up for at least thirty (30) minutes. Do not vent
the chamber if coolant is still flowing through the camera.
5.4 Exposure and Signal
The following topics address factors that can affect the signal acquired on the CCD
array. These factors include array architecture, exposure time, CCD temperature, dark
charge, and saturation.
5.4.1 CCD Array Architecture
Charge coupled devices (CCDs) can be roughly thought of as a two-dimensional grid of
individual photodiodes (called pixels), each connected to its own charge storage “well.”
Each pixel senses the intensity of light falling on its collection area, and stores a
proportional amount of charge in its associated “well.” Once charge accumulates for
the specified exposure time (set in the software), the pixels are read out serially.
5.4.2 Exposure Time
Exposure time (set on the Experiment Setup
►
Main tab page as illustrated in
Figure 5-3 ,) 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.