Quote:
Originally Posted by SB
if I dither 1 or 2 pixels on the 550mm FL image train, will that make a noticeable difference to the image appearance on the 180mm image train (if the 550mm dithering occurs during the 180mm exposure time)?
BTW, how many pixels do you set for your dithering?
SB
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If you dither the 550mm focal length scope 1 or 2 pixels AND IF your 180mm focal length scope is coupled to it and uses a camera of the same diagonal ( I think you do as I note a 183mc and 183mm in your gear signature), then the 180mm scope will move through 1/3 to 2/3rds of a pixel, since the 180mm scope has ~3 times (550/180 ~= 3) the field of view of the 550mm focal length scope. As a result you may see a
slight defocus/blur in the image from the 180mm focal length scope depending on how close you look or how you use it. It would probably be fine, viewed as a whole image without zooming in, especially on a typical monitor.
On the issue of how much to dither: a 1 or 2 pixel dither might be enough to deal with a hot or dead pixel, after all it's only one pixel, but it won't be enough to deal with larger scale fixed pattern noise. I have heard it recommended (Tony Hallas) that a dither of something like 2 or 3 star diameters on the imaging scope is considered appropriate. This makes sense with other recommendations I've seen to dither 10 to 20 pixels and is consistent with the size of the "colour mottle" I've noted with DSLRs, whose sensors are not dissimilar and often the same or brothers of their astro counterparts. You may also get by with much less dither depending on your camera's level of fixed pattern noise profile / mount/ etc...
As an aside: Since you are slightly undersampling the 183 sensor on the 550mm scope at around ~0.9 arcseconds/pixel, dithering may potentially open you to the benefits of drizzling should you choose to go that way to recover more detail. That's a whole other ballgame that I've not ventured in to, other than to create some giant files...
I'm also interested in dithering a multiple telescope system (triple) using the ZWO ASI Air, but as it stands at present, there's no facility for that in the ZWO hardware or software, although I do hold up hope for something I haven't tried yet: paralleling the analog signal from One ZWO ASI Air DSLR trigger output to signal 3 DSLRs to fire and of course independently store their respective images in their own individual storage cards onboard each of the 3 DSLRs. It would be great if it could work, there's nothing clear enough in any ZWO literature on the DSLR output function - possibly on their forum(?) I will eventually experiment with 3 Nikon D800 DSLRs to find out for myself-TBA
Best
JA