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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 anything....

Dark Subtraction

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 processing programs.

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 dark current.


Flat Fields

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 removed.

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....

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