Calibration frames

Flat field frames

Flat field frames are used to address unwanted problems in the optical path, including dirt/dust, vignetting and internal reflections. Any change in the optical path, such as focal reducers, filters, camera orientation or focus will require a new flat field frame. To obtain the "best" possible flat field frames would mean taking it at the same conditions as the light, dark and bias frames. To take flat field frames, first you need to get your scope into focus and then point it to an evenly illuminated surface. Flat field frames can be taken from the twilight sky. The exposure time has to be chosen in such a way that the intensity level reached appr. 50% of the maximum. The ATIK16IC camera has a full well value of 52000 ADU which means that the average intensity should be around 26000 ADU. Flat field frames should be taken for all filters used for the light frames, take appr. 15-20 flat frames for each filter. With these flat field frames we can generate a master flat field frames.

Dark frames

The purpose of a dark frame is to remove the non-image related thermal and electronic noise that the CCD generated when capturing light frames. It also removes imperfections of the chip, such as hot pixels. Since we are capturing the build up of thermal and electronic noise, it is important to match the dark frame to the same conditions as the light frame, this should include the same duration of exposure, chip temperature, gain, brightness and contrast. Dark frames can be taken by covering the telescope and take 15-20 images with the same exposure length as the light frames. With these dark frames we can generate a master dark frame.

Bias frames

When using a CCD chip, not all the pixels start out at a value of zero. The purpose of a bias frame is to apply it to a dark and light frame to bring all the pixels on the ccd to an equal starting value. To make a bias frame, cover the aperture or corrector plate in the same fashion as the dark frame. Capture at the lowest possible time setting your camera will allow. Bias frames should be taken at the same temperature and settings as the light frames. Take approximately 15-20 bias frames. With these bias frames we can generate a master bias frame.
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