Santa Barbara Instrument Group ST-i Operating Manual page 15

Hot pixels are simply the pixels where the dark current is higher than
the average and thus they show up as white specs against the salt and
pepper background. Again, this is completely normal for CCD images.
The second effect you'll notice is a brightening on the left of the image,
as if something bright was just outside the field of view. This is called the
readout glow and is caused by the electronics in the CCD, in particular by a
very small but ever present glow emanating from the on-chip amplifier that
conditions the weak signals in the CCD so that they may be digitized.
The readout (or digitization) of images from CCDs is a sequential
process whereby every pixel is digitized one-after-another until the whole
image has been digitized. The readout starts at pixel (1,1) in the top-left
corner and proceeds to pixel (648 x 486) in the lower-right hand corner. As
each pixel in a row is readout the pixels to the right of it within the same
row are shifted to the left one position in preparation for the next pixel's
readout. The next pixel is then readout and the process repeated until every
pixel in the first row has been digitized.
At this point the whole CCD is shifted up one row and digitization
starts with the left-most pixel of the second row. The readout glow on the
left of the images is a buildup of light from a glow from the preamplifier
structures in the upper-left hand corner of the CCD while rows are queuing
up for readout.
The final effect you'll notice is the salt-and-pepper look of the
background. What you're seeing here is the ultimate noise floor of the
CCD whereby adjacent pixels have slightly different values due to noise in
the CCD and readout electronics. The noise in dark frames that have zero
exposure time is referred to as the read noise of the camera.
Fortunately for us there are very simple image processing techniques
we can use to eliminate the effects of dark current and readout glow.
Let's open another image. Close the first image by clicking the
the upper-right corner then use the Open command in the File menu again
but, this time double-click on Image 2. Note in the Image Parameters
dialog that this image had an Exposure Time of 10 seconds. Click in the
dialog to close it.
Now this image has a whole lot more hot pixels! That's because this
exposure was 10 times as long and the pixels built up 10 times the dark
current. It's hard to even see the background through all the hot pixels.
About this time you're probably asking yourself "How can I ever take
images with all these hot pixels?" The answer is simple. Because the build
up of dark current is a repeatable effect you can remove the effects of dark
current by taking two images, one with the shutter open (light frame) and
another of equal exposure with the shutter closed (dark frame). You then
subtract the dark frame from the light frame, and because the hot pixels and
the readout glow repeat from one image to another they are removed by the
subtraction.
Let's see how this works. Close Image 2 and Open Image 3, which is
a 10-second light frame where you can get a hint of the object but the hot
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