Santa Barbara Instrument Group ST-5C, Operating Manual

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Section 1 - Introduction to CCD Cameras
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whereby data from the image area, after completing the exposure, is rapidly shifted into the
storage area where it is then digitized. A fast shift from the imaging area to the storage area
insures minimal streaking. Once the image is in the storage area, it can read out by the camera
electronics without causing streaking.
The simple answer to streaking you might say is to use a mechanical shutter, and in fact
your camera does have a shutter but the accurate timing of exposures is not limited by the
speed of that shutter but by how rapidly the imaging area can be moved into the storage area.
In this way the mechanical shutter is used to cover the CCD chip for taking dark frames while
short exposure images can be achieved electronically, without the limitations of mechanical
shutters.
1.3.6. Antiblooming Protection
As described above, the individual pixels in the CCD have a limited full well capacity. When a
pixel fills up with charge, the excess charge generated has to go somewhere. Again, there are
two basic types of CCDs available.
Standard CCDs, when reaching the saturation point, will spill the charge into
neighboring pixels, typically up and down the column in a line that is called blooming. If, for
example, you had a pixel that was exposed to 10 times its full well capacity, it would bloom
until a column of ten pixels was saturated, causing streaks in the image. The second type of
CCD offers Antiblooming protection.
In an Antiblooming protected CCD, when the charge in the pixel gets above some
threshold, typically one-half the full well capacity, the majority of the excess charge gets bled off
into a drain on the CCD. For example, a CCD with a 100X Antiblooming protection will drain
off 99% of the excess charge, allowing a pixel to overexpose to 100-fold before blooming occurs.
There is a price to pay, however, with Antiblooming protection and that's why
manufacturers produce both protected and unprotected CCDs. First off, the process of
Antiblooming protection causes a nonlinearity in the response of a CCD. If you were trying to
make accurate Photometric measurements, you would want the integrated star brightness kept
below the knee where the Antiblooming kicks in. The second detriment to some Antiblooming
protected CCDs is that at the integrated circuit level, the Antiblooming structures can reduce
the sensitive area of the individual pixels, causing a slight reduction in overall sensitivity.
The Texas Instruments (TI) TC255 CCD used in your camera offers variable
Antiblooming protection and, according to TI, the structures required to implement the
Antiblooming protection do not cause any reduction in sensitivity. The benefit of the variable
Antiblooming protection in the TC255 is that you can select the amount of Antiblooming you
want, using just a small amount for fields where no bright stars would cause blooming and a
large amount for objects like the Orion Nebula where a bright star in the field of view would
otherwise bloom. With the TC255, using the minimum amount of Antiblooming protection also
has a beneficial effect in that it reduces the dark current in the CCD.
1.3.7. A/D Bits and Digitization Rate
If you browse through the literature on specifications of the various CCD cameras, you see
some of them are 8 bits, some are 12 bits and some are 16 bits. While in general an A/D
(Analog to Digital) converter with greater precision is desired, there is a point where the extra
precision doesn't get you any increased performance. In most CCD cameras it's actually the
CCD that limits the performance, not the A/D converter.
As a starting point, you can take the CCD's full well capacity and divide by the CCD's
read noise to come up with a figure for the CCD's dynamic range. In this way the dynamic
range is the ratio of the brightest object you could image without saturating to the dimmest
object you could detect. You could see that a 16 bit A/D with a dynamic range of 65,000 is
overkill for a CCD with a dynamic range of 4000, for example. Let's look at your camera. The