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 with MaxIm DL pages in the Software Instructions section and the Basics of Capturing Images section.

In MaxIm DL version 4, calibration files can be automatically selected by the software.  You can save all your calibration frames into one folder and MaxIm will automatically find the correct calibration images based on temperature, binning, and exposure.  This alone is worth the price of MaxIm DL!

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.  You can open one of the dark frames just to get an idea what it looks like, but it need not be open for calibration purposes.

To dark subtract an image, open the file or files you wish to calibrate.  Select Process > Set Calibration from the main menu.  The following window appears:

Version 4 of MaxIm DL can automatically select and combine calibration frames.  All you need to do is select the folder where the proper calibration files are located.  Click the "..." button at the right side of the Auto-Generation section at the bottom of the window.  This will allow you to navigate to the proper folder, in this example C:\Starizona\M51.  Once the folder has been selected, click the Auto-Generate button.  MaxIm DL will determine the proper calibration frames from the files in the selected folder.

In the case of this example image, there are three dark frames.  These are automatically combined into a master dark frame that will be subtracted from the M51 image.  The combination method can be selected at the right side of the window.  Median is the usual method, so that is selected here.  Since we are not using any flat fields or bias frames for this image, only the Calibrate Dark box is checked at the top of the window.

Once the file is selected, as above, click OK.  (The other options can be left alone for now as they are primarily for more advanced procedures, explained later.)  Select Process > Calibrate from the menu.  This will subtract the dark frame from the image.  The resulting image should be free of the specks and streaks associated with dark current.

Note:  To process multiple open images, select Process > Calibrate All.

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:  A 60-second exposure of NGC 4565 showing a fair amount of 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.  This can be done automatically in MaxIm DL 4.  Be sure to take a dark frame of the same exposure length as your flat field images.

Open the file you wish to calibrate.  Select Process > Set Calibration, as above.  With the darks for the main image, the flats, and the darks for the flats all in one folder, navigate to that folder and select Auto-Generate.  This will automatically combine the dark frames, dark subtract the main image, dark subtract the flats, then flat field the main image.

Select Process > Calibrate to flat field the image.  The result is shown below, again stretched as above:

Once properly stretched (see following sections) the image appears as below:

Bias Frames

Bias frames remove, slight pixel-to-pixel variations in zero-point.  The base value of each pixel is slightly different and a bias frame (an exposure of essentially zero length) is used to remove this variation.

However, dark frames already contain the same bias as the regular image, so subtracting a dark frame removes the bias and no bias frame is needed.  Similarly, if you dark subtract your flat field images, no bias frames are needed.  The only time bias frames are necessary is when scaling dark frames.  If you intend to use a dark frame of a different exposure length from the regular image you will need to use a bias frame.

Note:  If you plan on performing dark, flat, and bias calibrations on a single image, you must do all of them simultaneously.  In other words, you cannot dark subtract an image and then go back and flat field it.

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