Actually, this dark frame comes from a 600x600 pixel crop from my Canon 10D,
which is getting a bit old and shows quite a few hot pixels for exposures
like this one -- 30 seconds at ISO 400. All the images in the tutorial are
600x600 crops, so that you can see what is happening at full resolution.

You can download images shown here with a right-click, if you want to work along with the tutorial.

The next step is to threshold this image to map all the lower intensity noise to zero brightness,
and all the hot pixels to maximum brightness. Depending on how thorough a job you want or need,
we can use one of three images as input to the thresholding step (which actually comes later).
Our choices are:

(1) We can use this image as is, or
(2) we can capture several dark frames, average them together, and use the averaged image, or
(3) we can "compute" the absolute difference between the image made in (2), and a median-blurred version of (2).

For completeness, we will outline the entire workflow (1)-(3), but recognize you do have
the option of skipping from the single dark frame shown here directly to the thresholding step.