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Hello All, I've noticed these bizarre false "star" pairs showing up in my recent M108 RGB Star image - I unscreened the stars right after the original RGB combination and did minor processing on them but didn't notice this other strangeness until I added them back much later to an L RGB "final" - in fact, it took some time before I noticed the repeat nature all over the image - I've circled some, but not all, of these seemingly identically paired false pixel sets - same size, separation, colour etc. It's not in the starless image, not caused by clone stamp and I can't seem to see it in the individual frames - I can see many single random hot pixels in individual frames, mostly fixed in PP, but nothing like this paired weirdness.  I't's probably something simple and I plan to redo all of the RGB pre-processing but any pointers would be appreciated! Thanks! Pete PS - I've been using this focal length for several months now and I haven't noticed this until now. |
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| They are hot pixels. Try a more selective rejection algorithm and run CosmeticCorrection, if using PI. |
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andrea tasselli: Hi Andrea - thanks for the info; I figured that it was that but stuck me odd as to the placement and predictability but now, upon closer examination of the RGB masters I do see it in the R and B masters and the subject framing was different by exactly that much explaining the spacing issue but what I don't get is why I haven't seen this up until now? especially in so many places - the pixel correction was working and suddenly now isn't with no changes made. Maybe just a fluke and it goes away on my next run? Pete PS - by the way, locking the mirror (manually for now until I install the secondary focuser) has definitely greatly reduced those earlier aberrations that I was getting - your analysis of that was much appreciated! |
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Maybe just a fluke and it goes away on my next run? *Maybe, or something has changed in the darks and you might have to retake them. |
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| How are you dithering? If not with a random dither, you can start to see patterns like this. Or if you are not dithering enough. |
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Chris White- Overcast Observatory: Well, I stopped dithering some time ago (without seeing this issue afterwards until now) because I'm talking either 5 or 10 minute subs and one full sub, at the dithering period, say one in three subs, would be ruined by a strange zig zag problem - I tried many different dither settings (including increasing settle time etc.) and it was always there. I got a bit frustrated at having to automatically throw out so many long subs on each occasion (especially when the occasions are so rare!) I would definitely appreciate any dithering advice; I'm at 3910 mm with a full frame ASI6200MM camera. Guide cam is an APS ASI0171 on an *on* axis Innovations Foresight splitter. I'm using Sharpcap at this time. Many thanks! Pete |
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Pete, Dithering is essential for rejection algorithms to perform properly. It's best to dither every frame if you can, although sparse dithering every 2 or 3 frames is ok as long as you collect a lot of subs. It sounds like your starting to capture a frame after dithering before your scope/mount has had enough time to settle. This can be addressed by increasing the settle time, or addressing something physically with your setup that might be unstable or prone to shaking after moving. The easiest is to just increase the settle time. Depending on your mount and other equipment it could take a little bit to settle. Maybe 15 or 20 seconds? I dont know, I have not imaged at this focal length before. You could also reduce your dither scale a little bit. You can experiment here, but my instinct would be that dithering at a smaller scale would be better than not dithering at all. I understand that there is a balance between collecting data and spending time dithering and settling, but it is worth the time investment for better data. (IMO and YMMV). |
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Chris White- Overcast Observatory: Hi Chris; It's actually a rare clear night here so I've just now set dithering up yet again with a long settle time and I'm going to watch the results. I do dither on another scope and have the same zig zag problem; but it's only a 400mm FL and the subs are shorter so it hasn't bothered me as much throwing some of those out. I guess my mystery though about the M108 data is the abruptness of it. As Andrea suggested I tried three different sets of dark frames now (one taken almost exactly on the exact date of the light frames) and it still exhibits the same issue. But other recent targets do not show it. I'll keep digging! Thanks for your advice ! |
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Pete, Those spots are small enough to be hot pixels. The problem is that they clearly repeat the same pattern in each of the groups so that's not the only problem. If they are hot pixels, something has gone wrong in the stacking process to produce this sort of pattern. I do agree with Chris that dithering is extremely important. I also agree with Chris that being careful about setting up a proper settling time for both the guider and for dithering is important to avoid issues like this. Dithering is trivial so my advice is to figure it out before working on another image. So, what is it? A number of things can cause this sort of result (e.g. wind, settling time too short, registration problems) but few that I can immediately think of would produce this pattern randomly distributed over the entire field. In order for us--or better, for you, to diagnose something like this you have to be methodical about working your way backwards to see where it came from. Help us out with a little more information: 1) Does this pattern exist exist in the individual subs? Carefully blink through each sub and examine them very carefully to see if any of these points show up in the registered subs. 2) Are all of the subs all perfectly registered? Again Blinking through the stack will quickly answer that question. Integrating a stack with just two frames mis-registered could produce this result from hot pixels alone; although if that's the case, it would also exhibit other obvious symptoms in the main object as well. John |
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John Hayes: Hi John - as of this moment, dithering seems to be working well on my current target; I've set it at 10 pixels per frame (enought I hope at 3910mm?) and 20 seconds minimum and 40 seconds maximum for settle time. Not sure what was wrong when I dithered before? I'll blink through previous data per your suggestion and henceforth keep the dithering in place! Many thanks, Pete |
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Update - Lesson learned about dithering; after getting it to function properly last night I see a *significant* improvement in the data. Not just hot pixels but almost everything looks better, smoother yet sharper. It looks as if I'll be axing my current non dithered projects! I hope it clears up! I need new data now! |
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| Great! |
