I would say that dithering every 3 frames is the upper limit...however, I would only dither that "infrequently" if I was planning to integrate well beyond 100 subs. If you are planning to integrate fewer than 100 subs, then I would dither every 2 frames, and 50 or less every frame. Given your post, it sounds like you are probably using fairly short exposures. So I'm assuming you are probably integrating around 100 subs or so.
Dithering more frequently could help, but you may also need to increase the aggression of your dithers. If you are dithering in small enough amounts that you aren't offsetting the stars in each frame enough from each other to randomize the FPN enough, then the patterns could still align once those subs are registered and stacked. Instead of being obvious streaking, it might be softer streaking, fatter streaks, or a more "wandering walk" where the pattern meanders just a little along its path.
You need to make sure that you are dithering by enough each frame, that the patterns, when the frames are registered, just become random noise again. I recommend dithering by 5-10 pixels, which should be around 2-3x the diameter of the average size star, assuming you are sampling somewhere between 2-3 pixels across the FWHM (if you are oversampling more, or undersampling severely, then you may need to adjust this, but 5-10 pixels will usually work for most cases). That is usually enough.
Now, there is a balancing act here...you want to dither aggressively enough that you can randomize the FPN in the stack, but not so much that it becomes harder or takes too long to settle after each dither. So you should experiment a bit, and find the settings that work for you. You SHOULD be able to fully settle and get the next frame started within 10 seconds after each frame. If it takes 30, 40, 60 seconds or longer to settle, then you need to adjust your dithering settings (and specifically, what you are settling at and for how long) to bring that down to around 10 seconds. Note that, there is no reason to try and settle below your guide RMS. Usually you settle to some ratio of a guider pixel. So if your guide RMS, in guider pixels, is say 1.4 pixels...then there is no reason to try and settle below 1.4. If you try to settle below that, then it becomes merely chance that you settle, because in effect, you are trying to settle within the "tracking noise". If your guide RMS was 0.7 pixels, then you could settle at 0.7, and if it was 2.5 pixels, you wouldn't be able to settle much below 2.5. You may need to fiddle with this a bit...if it takes too long to settle right at your RMS, try slightly above...1.5, 0.8, 2.6 etc. The other factor here is, how long you expect the settling factor to be maintained...1, 2, 3, seconds etc. I usually settle for 3 seconds with my setups, however you may find you need to settle a bit longer to fully settle, and this will generally be more important when oversampling than when undersampling.
Now, dithering and how you do it is a factor here, for sure. That said, you should also try and balance out the length of exposures and number of total exposures as well as the actual frequency of dithers. If you are using very, very short subs, if you can find a way to use longer subs, that will improve the efficiency of your imaging sessions. You may also be able to improve efficiency in other ways.
First off, if you are using very short exposures, consider the gain you are using. If you are using a high or even moderate gain and are stuck with short exposures (i.e. a minute or less), then reduce your gain and increase your exposures. For LRGB, you may in fact be able to use the minimum gain, even on a 12-bit camera...and this would especially be the case for light polluted skies (its a no brainer with Bortle 7 or brighter). You may be able to double or triple your exposures, which can reduce the number of dithers you have to perform each night.
Beyond this, if reducing exposure length isn't that much of an option, you can also configure an interleaved imaging session. Instead of something like this:
LL <dither> LL <dither> LL <dither> ... RR <dither> RR <dither> RR <dither> ... GG <dither> ... BB <dither> ...
Where you dither every two frames. If you can configure automatic focusing, set up filter focus offsets (to avoid having to re-focus every time you change filters), and then interleave imaging across filters...you could greatly reduce the number of dithers you have to perform each night:
LLRGB <dither> LLRGB <dither> LLRGB <dither> ...
This would be 2.5 times more efficient than imaging one filter at a time and dithering every 2. Now, this dithers every 2 L and every 1 RGB. You could potentially extend that even more, if you plan to get enough RGB data to support dithering every 2 frames:
LLRRGGBBLL <dither> LLRRGGBBLL <dither> LLRRGGBBLL <dither> ...
This would be FIVE TIMES more efficient, than dithering every 2 frames!! Further, this can also distribute differences in seeing across all channels, as well as guarantee you have a full constituent of data across channels every night you image.
Edited by Jon Rista, 26 November 2020 - 12:30 PM.