Santa Barbara Instrument Group ST-5C, Operating Manual

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Section 1 - Introduction to CCD Cameras
Page 8
CCD has a full well capacity of 50,000 electrons and a read noise of 30 electrons RMS giving a
dynamic range of roughly 1700. A 12 bit A/D offers a dynamic range of 4096 and would cover
that CCD fairly well.
One thing you do want to do with the A/D is make sure that the A/D's noise (typically
1 count) is lower than the CCD's noise so that you are truly CCD limited. Setting the A/D to
have one-eighth the noise of the CCD allows averaging several images to improve noise.
Applying this criteria to the TC255, you would want an A/D with a dynamic range of 10,000 or
so (a 14 bit A/D has 16,000 dynamic range).
Finally, the CCDs can be used in a mode where the pixels are combined in a process
called binning which is described in detail below. Binning reduces the CCD's spatial resolution
like increasing grain size in film, but increases the CCD's dynamic range. With your camera
you can bin the CCD 2 by 2 resulting in another factor of 4 in dynamic range which gets us up
to 40,000 or so, hence the need for a 16 bit A/D. While the camera only achieves 16,000 counts
unbinned, allowing it to achieve 65,000 counts binned allows taking advantage of the extra
dynamic range inherent in binning.
The final consideration regarding the A/D is how fast the data is digitized and
downloaded from the CCD to the computer. The A/D is not the only contributor to that time.
The actual transmission of the data to the computer is a significant portion of it. In your camera
an entire image can be digitized by the A/D converter and sent to the PC in 3.5 seconds. This is
done using the PC's standard Parallel port without requiring the addition of expensive (and
difficult to configure) SCSI adapter cards.
1.3.8. Binning
Binning is a process where multiple pixels in the CCD are combined to form a single larger
pixel. This reduces the CCD's resolution but increases the sensitivity. Different CCDs from
various manufacturers support different types of binning.
Some CCDs support on-chip binning, where all the pixels in the group are combined in
the CCD itself. This has the advantages of lower noise (a single read noise is generated for the
group of pixels) and higher speed digitization since fewer pixels are involved. In imaging
applications you tend to bin the images in both directions to preserve a "square" aspect ratio.
The one final advantage of binning is that it increases the overall image throughput, reducing
the digitize and download times due to the reduced number of pixels involved.
The camera software allows you to select a High resolution 10 micron square mode
where the pixels are unbinned and a Low resolution mode resulting in 20 micron square pixels.
The latter is useful for fast acquisition of faint objects.
1.3.9. Spectral Response
Like film, CCDs have a varying response to differing wavelengths. The basic fabrication
techniques used in manufacturing the CCDs greatly affect their spectral response. At the
extreme Red end of the spectrum, the CCDs loose their sensitivity because the photons simply
do not have enough energy to generate electrons in the CCD wells. At the Blue end of the
spectrum, the photons do not penetrate deep enough into the CCDs to get into the wells and are
stopped by the top layers of the CCD. Between the Red and the Blue, interference effects in the
top layers of the CCD can also cause peaks and valleys in the response.
This affects you in several ways. The most obvious is the overall effect that causes you
to take longer exposures with CCDs for various colors. For example, when taking color images,
your Blue exposure is typically several times longer than the Red exposure to give an image
with similar quality or Signal/Noise. One last interesting note about CCD's spectral response
is that they have much more response in the near infrared than film.