Calibrating CCD images involves removing noise artifacts and uneven illumination. This is done by taking dark frames, bias frames, and flat field 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.
CCD cameras are so sensitive that the heat generated by their electronics can cause noise to appear in the image. For this reason, CCD cameras are cooled, typically between 25 and 50 degrees C below ambient temperature. This minimizes the thermal noise, but in some CCDs the remaining noise is still a problem. This noise is easily eliminated by subtracting a dark frame. A dark frame is simply an image taken with the camera covered. The dark frame detects only the noise inherent in the CCD and this is then subtracted from a regular light frame. A dark must be taken at the same temperature as the light frame that it will be subtracted from, and the exposure must be of the same duration. Below is a single 600-second dark frame.
Uneven illumination of the field normally results from vignetting in the optical system. Dust on the CCD sensor or filters can also cause dark spots in an image. These artifacts are removed using a flat field image. A flat field is simply an exposure of an evenly-illuminated light source. This is often the twilight sky, but flat field panels are also made that use artificial light sources and can be used any time. A flat field image detects the uneven illumination of the field and any dust specks. Flats are filter-dependent, so if you are using a monochrome camera with red, green, and blue filters, you must take separate flats for each filter. Below is a twilight flat field image taken through a luminance filter.
To properly calibrate a flat field image, another calibration image must be taken, called a bias frame. This is effectively a zero-second exposure and it detects purely the read-noise from the camera. This is noise generated when the pixels are read out of the CCD. In a low-dark-current camera, a bias will look almost just like a dark frame. This image is used to normalize the flat field. Below is a bias frame.
Results of Applying Calibration Images
Before any calibration is applied, an image might appear speckled with hot pixels, and the background will look darkened in places by vignetting and dust. Below is a full-resolution crop from an image of the Pleiades which has not been calibrated. Notice the white specks across the image--these are hot pixels.
After subtracting a dark frame, the image appears much cleaner, as seen below.
The image below shows the full field of view of the same Pleiades exposure. The image has been stretched to show the uneven illumination resulting from vignetting of the optical system.
Below is the image after calibration with flat fields and bias frames. Note the vignetting is gone and more faint detail is visible in the nebulosity.
Applying Calibration Images in MaxIm DL
Calibrating images in MaxIm DL is easy, since the software will determine which exposures to use, and will automatically create master calibration images from multiple exposures. To begin, go to Process > Set Calibration. This will open the Set Calibration window. Click on the yellow folder icon next to the Source Folder line. Here you can navigate to the folder where you saved your calibration frames. You can put all the darks, flats, and biases into one main folder and MaxIm will generate the necessary files from there. Press the Auto_Generate button and MaxIm will figure out the parameters of all your files.
As can be seen in the image above, MaxIm has generated master frames for each calibration image type. The flat field that has been highlighted above is composed of seven individual exposures. MaxIm also lists the filter used and the temperature at which the image was taken. It will automatically match this to the light frame being calibrated and apply the proper images. Note that Median has been selected as the combine type at the bottom right of the window.
To calibrate a single image, open it in MaxIm, and simply go to Process > Calibrate. To calibrate multiple open images, select Process > Calibrate All. However, the easiest way to calibrate images, is to have MaxIm calibrate them while stacking. This way you never need to even open your files. Go to Process > Stack and in the window that appears, check the Auto Calibrate box, as shown below. Note that you must check this box before selecting your files to stack, or the calibration will not be applied.
Below is the calibrated and processed Pleiades image.
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