Santa Barbara Instrument Group ST-5C, Operating Manual

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Section 2 - The First Day with the Camera
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brings up more advanced crosshair commands. Hit Esc twice to view just the image. This is
what you will typically view after you capture an image.
Next, Esc back to the main menu and hit the Grab command again. Select Dark
frame:None. With the camera nosepiece uncovered, hit Enter to begin another exposure. This
time you are taking a light frame. Notice how smooth the image looks. Repeat the above steps
used in evaluating pixels in the dark frame. Notice that they all saturated with 32,767 counts.
This is the maximum amount of light entering as photons and converted to electrons that the
CCD can read.
Next, cover the nosepiece of the camera and use the Grab command again, only this time
select Dark frame:Also and hit Enter. This image has a much lower background and range, and
represents a dark subtracted image. Still, pixel intensity variations fall between 50 and 250.
Some individual hot and cold pixels may show at 30 to 900 counts, but their Average Value is
100 or so. This "Avg" value (displayed in the crosshairs data) represents the low end intensity
of your pixel image data, while 32,767 represents the maximum intensity value. All your CCD
image data is represented as shades of grey between 100 and 32,767 counts. This is a very large
range, far beyond the photographic process capability.
Let's Esc to the main menu and use the Setup command in the Camera menu. We will
cool the camera to 0 degrees C. with a new Setpoint setting of 0. Also, set Temperature
regulation to Active. Nothing happens yet until you hit Enter. Notice the lower corner of the
Camera Status that shows Temperature and that it is rapidly dropping. When it stabilizes at
around 0, you can see the percentage cooler capacity you are using shown in parentheses. Grab
another dark subtracted image(Dark frame:Also) and study the display. First notice the much
more uniform look the pixels show. The benefits of CCD cooling are very evident. Notice that
the average count remains consistently around 100 but that the pixels are much more closely
clustered around 100 counts.
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If your room temperature is about 70 degrees F., try further cooling to -8 degrees C.
Nearly 100% cooler capacity may be indicated. For good astroimages, don't run above 90%.
Grab an image once the temperature stabilizes in the -8 degree range. Notice that the image is
even more uniform appearing now. The average pixel count is still around 100, but "hot" pixels
have greatly diminished except for an errant few that may always read high. No amount of
cooling can help these. They are inherent in the manufacture of most CCDs.
Summary
Even at a 1 second exposure, the value of cooling is demonstrated to be important as well as the
dark taking and subtraction procedure. These steps are absolutely necessary to get the most
from your CCD camera. If you were at the telescope, all that would be left to do is to save this
image. Let's do that now but first we will need to create a directory for some images. Under
Windows 95 or the Mac OS you can do this from the File Save dialog when you got to save the
image. Under DOS you use the Create Directory command in the File menu to make a new
directory for our images. Execute the command and when it asks you for a directory name type
CCD.
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Hit Enter to create the directory.
Finally, get back to the main menu and use the Save command in the File menu. Type a
name for the image (that follows your OS naming convention). Then use the Open command in
the File menu and look for your name there. Highlight it and hit Enter to open it. Notice the
displayed header data has the file name you gave it, the date and time setting of your computer
as well as exposure time and temperature, etc. Some other parameters were not yet set by you
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You might think the pixel values would be clustered around 0 counts but instead they are clustered
around 100. That's because the CCDOPS software adds 100 counts to the dark subtracted image to
stop pixels from going below zero when the two images are subtracted. Just remember that 100
counts represents zero signal.
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As a convenience the software makes up directory names for you by adding the current month and
day to DATA. You can use this name or type in your own name.