Quote:
Originally Posted by iceman
Doug, the purpose of this thread was specifically about "darks for your flats". Not darks, or flats, or biases on their own.
Not everyone takes darks for their flats.
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Mike , with flat framed exposure times normally held well below the 30 second mark there is absolutely no need to take darks for your flat frames. Bias files on the other hand are quite a diferent matter. The read noise must be calculated into the flat frame to ensure a good quality master flat frame. The fact that most if not all image calibration programs go through a calibration and normalisation process when creating a master Flat and sets all the pixel integer values at a scaled value to set the calibration masterfile, the need for a dark is totally negated.
Berts example image is a section of an image which could well be a read aberation or noise or could be a harmonic setup by the fact that his camera is surrounded by TEC coolers etc which it was not designed to compensate for. Astro cooled CCD cameras have any aberation or harmonics created by electronics built into the in camera processing.
It must also remember that a DSLR while manufactured to work through a wide temprature range are optimised to operate correctly at a mid range area and to change this and try to reduce the operating temprature to the bottom end of the range will affect the operation of the camera in some way or another. Even the Astro DSLR's manufactured by central ds in korea, only cool the CCD and leave the remainder of the electronics alone. The cooling system of these is designed as a specific modification and is fully filtered to reduce harmonics etc.
Any dark frame calibration is taken into account during the overall calibration process and when the final calibration is staged in the software a combined calibration is carried out by a set routine which compensates for all of these abnormalities.
Flat frame images are not just a matter of taking a short image where the peak ADU count is 1/3 of the total full well capacity of the camera but should actually be taken at an exposure level which places the exposure in the linear range of the affected camera and all CCD's/CMOS sensors have a diferent linear range. To complete these files effectively you should graph the performance of your camera output and use an exposure which puts your flat exposure in the linear range of the camera sensor.
It is all a lot more complicated than just using an ADU count to guage your exposure.
Anyone who takes an image should use Darks Flats and Bias frames to calibrate their images and I know full well that a lot choose not to use some or even all of these calibration files. I am guilty of this on numerous occasions and usually nmention this fact when I post an image and usually get a comment back to ask why not. This choise is a personal one usually made for various reasons, be it time or just pure laziness, it is just a desision made at the time. If all images were to be used for full scientific purposes almost all images produced and posted on this and other forums would not cut the mustard.
The fact that this operation has made a diference to Berts image is great for Bert but may well impart other imperfections to an image when not using a modification like Bert has made to cool his camera to a point where at times may well be operating outside it's normal operating range. Any electronic equipment operating outside the normal will create small aberations which require further processing to repair. A top of the line camera such as the 5D should not be creating the type of aberation which is present in the attached image and may require some adjustment at the factory to correct this or it could be caused by something external to the camera.
Over compensation during the calibration process can and does lead to images being washed out and devoid of depth of field and is quite evident in a lot of wide field images in particular. Images often appear to have a grey wash or appear to have been taken through fine cloud which even strips a lot of the colour depth out of the image.
as for Doug and Bert, 
excellent points to ponder on, but I might just stay out of this one. 
