54 PI-MAX/PI-MAX2 System Manual Version 5.F
USB 2.0 and System On/Off Sequences
The following on/off sequences are specific to the USB 2.0 interface:
1. The ST-133 must be turned on before the WinX application software (WinView/32
or WinSpec/32) is opened to ensure communication between the controller and the
computer. If the WinX application is opened and the ST-133 is off, many of the
functions will be disabled and you will only be able to retrieve and examine
previously acquired and stored data. You must close the WinX application software,
turn on the ST-133, and reopen the WinX application software before you can set
up experiments and acquire new data.
2. The WinX application software must be closed before turning off the ST-133. If
you turn off the ST-133 before closing the WinX application software, the
communication link with the controller will be broken. You can operate the
program in a playback mode (i.e., examine previously acquired data) but will be
unable to acquire new data until you have closed the WinX application software,
turned on the ST-133, and then re-opened the WinX application software.
Pre-Exposure Removal of Accumulated Charge
Introduction
The purpose of the CCD array is to acquire a signal of interest that can then be digitized
and transmitted to the host computer for storage, display, and post-processing. It will
acquire signal whenever the camera is connected to an ST-133 that is turned on, whether
or not the intensifier is being gated to take an exposure. If the intensifier is off, dark
charge will be the main source of signal accumulation on the array. To counteract this,
clean cycles, and continuous clean cycles are used to remove the dark charge while the
camera is waiting to acquire the signal of interest. Dark charge, clean cycles, and
continuous clean cycles are described in the paragraphs that follow.
Dark Charge
As soon as the ST-133 is powered on with a connected PI-MAX camera, thermally-
induced charge will begin integrating on the CCD array even if the photocathode is
biased off. Because dark charge is thermally-induced, reducing the array temperature
significantly reduces the rate at which this charge accumulates in the pixel wells. Even
so, enough dark charge could accumulate in the pixels between data acquisitions to affect
dynamic range at the beginning of an exposure. To prevent this from happening, clean
cycles repeatedly shift and discard any signal that has integrated on the array while the
ST-133 is waiting for a Start Acquisition command from the host computer.
After the Start Acquisition command is received, a final clean cycle occurs and exposure
begins. During the exposure time, both the signal of interest and dark charge integrate on the
array. The longer the exposure time and the warmer the camera, the larger and less uniform
the dark charge will appear. To minimize the dark charge contribution to the acquired signal,
you should operate with the lowest temperature possible for your camera. Reducing the
exposure time may also be helpful.
To further reduce the dark charge contribution to an acquired signal, you may be able to
perform background subtraction, which subtracts a dark charge background from raw data
acquired using the same experiment conditions (see "Background Subtraction", page 59).
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