Manual dithering

Probably you won't need to, just pause between captures and the natural drift of the mount+scope will take care of that as long as the focal length/image scale is high enough (> 3"/px).

Hi Luc,

as Andrea and Carole wrote, the mounts built-in imperfections and some pauses between exposures will usually make for a sufficient dithering if you use a telescope. If you use short focal lenghts (camera lenses) though, it will probabely be necessary to alter the object's position on the sensor between exposures manually in both axes to avoid the remaining of artifacts after stacking.
I experienced that when I used a 200mm lens on a Star Adventurer. I carefully aligned the SA and used a Lacerta MGen for guiding in RA because I expected the SA to be to imprecise due to overload. Also I wanted to use dithering so I would not have to make dark frames. But it turned out the setup ran very well and the perfectly repeated offsets in only one axis were insufficient for stacking algorithms to separate object signal from sensor characteristics. When I only stacked the subframes from one run, I got very disturbing stripe artifacts. The effect disappeared when I mixed subframes from 3 imaging sessions (=3 times different offset in RA and DEC), each contributing approximately one third to the final result.
So if you use very short focal lenghts, be shure to manually alter the object's position on your sensor in RA and DEC for each ~quarter of the number of frames you inted to stack. Otherwise image processing can be quite frustrating, especially if you have spent lots of time and money to go to a dark place to find out later that you can't get rid of those stripes.

CS

Basti

I would recommend the Lacerta MGEN.

Begging your pardon, it does. I never ever dithered the way suggested by you (possibly because my mounts aren't that good) and have never had an issue during post-processing hot-pixels. Besides, whether the pixel line up or not isn't an issue if the rejection criteria is based on position and you have enough instances to create a robust statistics. Certainly the way you suggest is even better, no doubt about it, but is tedious is you take a lot frames.

andrea tasselli:
Begging your pardon, it does. I never ever dithered the way suggested by you (possibly because my mounts aren't that good) and have never had an issue during post-processing hot-pixels. Besides, whether the pixel line up or not isn't an issue if the rejection criteria is based on position and you have enough instances to create a robust statistics. Certainly the way you suggest is even better, no doubt about it, but is tedious is you take a lot frames.

Actually John Kroon is correct, dithering serves more purpose than just to remove hot pixels; more important is fixed-pattern camera noise/artifacts. If you're using your processing software correctly, hot pixels shouldn't even survive to the stacking phase, given cosmetic correction options which can occur beforehand.

Therefore, a randomized dither is "technically" required, in order to properly handle fixed pattern camera noise. I say technically, because plenty of setups don't have enough integration time, or have adequate drift, to function "well enough" this way. Most images don't have enough integration time to really show this fixed-pattern noise, and most cameras have noise low enough to prevent this problem.

However, on my setup, I've had fixed-pattern noise persist in my final stacks (DSLR, 10+ hours integration time), even with computerized dithering after every frame, if I don't dither by great enough amounts. Pixel rejection cannot function properly without adequately-sized, truly randomized dithering, mathematically speaking. Hot pixels will definitely be handled fine by any type of dithering, but fixed-pattern noise will not. In fact, I read a mathematical analysis that showed dithering by drift leads to even worse fixed-pattern noise than not dithering at all! It's likely you don't have such severe fixed-pattern noise, which is fine. I'm not saying one absolutely must dither in this way, however it is objectively better, when possible.

To Luc Aubut, you can absolutely dither manually! Many setups don't require it, however, so I recommend comparing a stack from a dithered night, to one without, and see if it matters. If it doesn't, you could save some valuable effort and time!