Santa Barbara Instrument Group ST-i Operating Manual page 14

This is what's referred to as a dark frame and it's what you get when you
take a picture with the nosepiece covered and the shutter closed. It's a
picture in the dark. Although there's not much interesting to look at in this
image there are some very important features that you need to understand.
The first thing you'll see is that while the image looks mostly like salt
and pepper there are hundreds of bright specks. These are called hot pixels.
They are present in every CCD image to one extent or another, even in
images from our most expensive cameras. If your digital snappy camera
didn't automatically remove them for you, they would be seen in those
images too.
What causes hot pixels? It's an attribute of CCD sensors called dark
current. A pixel in an ideal CCD, in the absence of light, would maintain a
steady value. When exposed to light the pixel's value would increase in
response to the light but then as soon as the light went away the pixel would
maintain its value again.
In the real world CCD pixels suffer from the affects of dark current
whereby the pixel's value slowly increases (brightens) over time. All the
pixels in the image shown above have some component of their signal due
to the build up of dark current.
Dark current builds up over time in a linear fashion. Cameras designed
for taking long exposures provide TE cooling of the CCD to reduced the
rate at which it accumulates. However, for the typically short exposures
used in planetary imaging and guiding, cooling is unnecessary if the camera
can otherwise take and subtract dark frames.
SBIG ST-i Manual
12