Calibrating a CCD image involves removing artifacts such as dark current and
uneven illumination. This is done by subtracting calibration images (dark
frames, flat fields) from the raw CCD images. For information on taking
calibration images, see the Capturing Images
pages in the Software Instructions section and the
Basics of Capturing Images section.
Note: CCDOPS has one major
drawback: the Undo function does not actually do anything. If you
make a mistake, you must reopen the file and start again. This is not true
of any other program in the history of computers. Not to be picky or
Begin by opening a raw image. In the image below, the tiny white specks
are electronic noise from the CCD itself.
By taking a dark frame of equal exposure length to the image the CCD detects
only the noise. By subtracting this image from the raw data, you are left
with a much cleaner image. Open one of the dark frames just to get an idea
what it looks like.
Note: CCDOPS will only allow you
to open one file at a time. This is not true with any of the other image
To dark subtract an image, open the file you wish to calibrate. Select
Utility > Dark Subtract from the menu. Select the file you wish to
use for the dark frame. CCDOPS will make the following inquiry:
Typically you will use the Standard Dark Subtract. If, after performing
the dark subtraction, you still see many hot spots or dark spots, try reopening
the file and using the Adaptive Dark Subtract.
The resulting image should be free of the specks and streaks associated with
Selecting an image to use as a flat field works much the same way as dark
subtraction. First, take a look at the image below for a good example of
an image in need of flat fielding.
Above: An image showing vignetting. This image has been
stretched to show the darkening at the corners.
Notice that there is vignetting, a darkening of the corners also called
uneven field illumination. Flat fields work by capturing the same uneven
field illumination, whether it is from the optical system (as it primarily is in
the above image) or from varying CCD sensitivity across the field, or from dust
in the optical path. Flat fields are not always used by CCD imagers.
Often you will only notice vignetting if using a very fast focal reducer and if
you significantly enhance the image (the above image shows very little
vignetting when stretched normally). Below is an example of a flat field. By
calibrating the raw image with this flat field the uneven illumination is
Note: Flat fields must be dark
subtracted before being used to calibrate an image. Be sure to take a dark
frame of the same exposure length as your flat field images, then dark subtract
the flat field using the above procedure.
Open the file you wish to calibrate. This image should already be dark
subtracted. Select Utilities > Flat Field from the menu.
CCDOPS will ask which file you would like to use for the flat field.
Select the appropriate file and CCDOPS will calibrate the image.
Above: The result of flat fielding, with the image again
stretched as above. Notice the darkening in the corners has been removed.
Above: Once properly stretched (see following sections) the
image appears like this.
Next, Basic Processing....