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Old 07-09-2018, 09:05 AM
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graham.hobart (Graham stevens)
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dithering question

I intend to dither via maxim using my SX lodestar via an OAG on my main imaging scope FS106N-530mm plus FLI.
But I will also be imaging using a cooled full frame ZWO via my 85mm FSQ AND a DSLR via a 135mm f2.8 lens.
My question...
what amount of dithering can I get away with which will not affect the other imaging set ups?
I image mostly 300sec on each set up but they all are timed differently so one will be imaging and then dithering when the second one will be still imaging etc
I tried 3 pixels I think last time and I am sure it affected my other images.
thanks for the advice
Graham

Last edited by graham.hobart; 07-09-2018 at 09:06 AM. Reason: correction
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Old 07-09-2018, 09:58 AM
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billdan (Bill)
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I don't think you will be able to do it without having problems with the other two cameras. Even if you timed each camera to start at the same time, the download times will vary, so they will eventually go out of sync.

As Jon Rista on CN says, you need to dither at least 10 pixels to take advantage of dithering. So 3 pixel dithering is a waste of time anyway.
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Old 07-09-2018, 10:05 AM
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graham.hobart (Graham stevens)
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dithering

ok thanks for that- does the 10 pixel mean for any focal length?
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Old 07-09-2018, 10:32 AM
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Good question, I'm not sure. Jon Rista's setup is at 600mm focal length, so whether his 10 pixel rule applies across the board I'm not sure.
My scope is at 1650mm focal length and I use 6 pixel dithering and it works out OK.
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Old 08-09-2018, 10:55 AM
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Bill,


Do you have a link to John Rista's comments? My experience is quite different. With my TEC180 at ca 2.11 arc-sec resolution (OAG guider/ultrastar) I can get a huge benefit from dithering 2 pix (with my imaging camera at .88 arc-sec.). This is enough to remove hot pix. Perhaps not enough to remove large artifacts such as aircraft trails. It's an interesting question to consider.



Graham,


There is software that will allow for dithering one camera while synchronized to a different camera (but not 2 other cameras). It only requires that the subs are of equal length, or that the master camera is some multiple in length of the slave camera. It will introduce a delay and even change filters in the slave camera. It works with TheSkyX or TheSkyX + CCDSoft:


http://astro.neutral.org/software/as...-software.html


Peter

Last edited by PRejto; 08-09-2018 at 07:24 PM.
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Old 08-09-2018, 07:30 PM
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Peter

I had look on CN but I couldn't find Jon Ristas' post, but it was in reference to eliminating FPN rather than just hot pixels.
I remember him saying that for some cameras a 20 pixel dither may necessary to randomise the FPN so it got eliminated in stacking.
These numbers are for the max dither setting so the dither can move 2,3,5 or 10 pixels depending on its max setting.
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Old 09-09-2018, 07:47 AM
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multiweb (Marc)
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I dither all the time. Never checked the offset from sub to sub but I'm pretty sure I'm under 10px most of the time. My image scale is between 2 and 3 asp. How do you get rid of hot pixels by dithering? Only dark subtraction and/or a defect map works for me. Otherwise you pepper your stack with hot pixels.
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Old 09-09-2018, 08:29 AM
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Marc,


Seems to me that dithering will naturally eliminate any pix not eliminated by dark frame subtraction (which I never find to be perfect). During registration the stars are aligned which now means that the remaining hot pix are now not aligned. The sigma reject process thus easily picks out those pixels. If you have not dithered then aligning the stars also aligns the remaining hot pix and the sigma reject will not see those pix as statistical outliers. Anyway, that is how I think it works.


Peter
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Old 09-09-2018, 09:00 AM
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Quote:
Originally Posted by PRejto View Post
Marc,


Seems to me that dithering will naturally eliminate any pix not eliminated by dark frame subtraction (which I never find to be perfect). During registration the stars are aligned which now means that the remaining hot pix are now not aligned. The sigma reject process thus easily picks out those pixels. If you have not dithered then aligning the stars also aligns the remaining hot pix and the sigma reject will not see those pix as statistical outliers. Anyway, that is how I think it works.


Peter
Do you do your data rejection in PixInsight or CCDStack?
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Old 09-09-2018, 12:46 PM
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Both, but when in a hurry, CCDStack.


Peter
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