NovaTJ wrote in post #9930009
DonR...I just ordered an Orion SSAG to begin autoguiding. I would like to learn more about this technique called "dithering". Can you point to any sources where I may go to learn more?
I use the EOS Utility with my 40D and can somewhat do exposures to 120 seconds with minor star trailing, but the amount of noise and color blotching bothers me. Noise Ninja helps, but doesn't eliminate it totally.
The idea of dithering is that each image produced by the DSLR contains some noise that is generated by the camera electronics (read noise), and a portion of that noise is reproducible (fixed pattern noise). The most obvious component of the fixed pattern noise manifests as vertical banding. While every frame, including very short bias frames made with the camera body capped, contains the fixed pattern noise, it isn't completely uniform from exposure to exposure. Acquiring a number of bias frames, averaging them, and subtracting the result from each exposure, gets rid of that part of the fixed pattern noise that is constant, but the noise that remains, although it is not noticeable in normal photos, can become a problem when contrast is stretched drastically, as sometimes is necessary in astrophotography images. I have attached a bias frame made with my 350D. It's a 1/4000 second exposure made with the camera body capped, and I stretched it severely in Photoshop to reveal the fixed pattern noise. There's also a little amp glow apparent in the lower right corner, even at 1/4000 second!
Dithering helps reduce the effect of fixed pattern noise by moving the subject on the sensor between exposures so that the banding doesn't accumulate in the same position relative to the subject on each frame, and therefore can be minimized instead of reinforced when the images are stacked.
I dither the old fashioned way. Between exposures I stop guiding and use the hand controller (actually a joystick since I use EQMOD to control my mount) to move the guide star a few pixels. Then I engage guiding again, let it settle down, and take the next frame. If I know I will be taking lots of frames, I will go up to three frames between dithering.
As has been mentioned in this thread, some software allows for automated dithering. Nebulosity together with PHD advertises the capability, but there are apparently still some problems with it. Tim provided some links to other automated dithering processses. The way I do it means I have to stay around and alert during the imaging session rather than going in, going out or going to sleep, but that's OK with me.
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