CCD Camera Imaging - Image Calibration
Image calibrating means :
  • Adjusting each exposure of the target object (usually referred to as a "light") to deduct blemishes caused by "noise" generated by the camera, hot pixels generated by the camera's CCD chip and uneven illumination across the CCD chip generated by the telescope's optics.
  • Deducting a "dark" frame and dividing by a "flat field frame" corrects the hot pixels and the uneven illumination.
  • Stacking all of the dark and flat adjusted exposures ("subs") taken of the target object through the same filter eliminates most or all of the noise.
It is necessary to pre-prepare the required dark frames and flat field frames before calibration can be done. The dark frames (which are simply exposures of the same duration as the lights taken with the camera's shutter closed) are usually taken during the same imaging session(s) as the lights. The flat field frames can be taken at any time but a separate set needs be taken for each filter used. Like each light exposure, each dark frame exposure and each flat field frame exposure is a FITS file of 16.57 MB.

When the adjusted individual exposures are stacked they must be aligned so as to correct for any slight differences caused by camera aiming errors.
The calibration is easily handled by the excellent software supplied with the Atik Camera. It is called "Dawn" and has an intuitive user interface which lets you identify all of the exposures (lights, darks and flats) to be used and then assemble the necessary workflow by dragging-and-dropping the appropriate processing steps into the assembly area.
Picture
Dawn software makes it easy to calibrate all of the "lights" to get a single properly adjusted image that is ready for post-rocessing work.
The end result of each calibration run is a single 5.23 MB image in TIF format that is then saved for further processing with Photoshop.