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Help needed to resolve the appearance of the pixels and their misalignment


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The image was taken using a mono camera and RGB filters, plus flats on the red filter, all using Sequence Generator Pro. The subs for each filter were combined in DeepSkyStacker. The resulting RGB Tiffs were then aligned in AstroArt, before combining in Photoshop. When I zoom in, I can see these annoying pixels, that don't seem to disappear even if I apply darks at the Deep Sky Stacker stage. They obviously don't even align when the alignment process takes place (aligning on stars).

Could it be an issue with dithering? I can't remember what I did about dithering in PHD2. Maybe I dithered on some light or flat frames, but not others. Maybe it is something other than dithering - perhaps my workflow.

Is this is somehow recoverable without me having to take a load more subs to start from scratch? I use Deep Sky Stacker, Photoshop CS6 and AstroArt and would prefer to keep using this software for now. 

Any advice on how to overcome this problem would be appreciated, because I consistently have this problem. I know that the image has other capture / processing issues that could be addressed, but can we please just keep to the pixel issue? I'd like to take one step at a time 🙂. Thanks!2091260049_RosetteNov2022RGBandFlatsAdjFW.jpg.ba9b98b6c093486ea09f2c39a30bdad3.jpg

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Your image is perfectly normal for an image with no darks applied, mine look exactly the same without darks and no dithering…so if you did use darks them something is wrong…also you need flats and dark flats for all the separate filters….

Edited by Stuart1971
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It looks like hot pixel walking noise. You have to dither very frequently (every 5 or so if taking 60 second exposures) and around 30 odd pixels (significant enough for you to see a slight fov shift after the dither).

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1 hour ago, Stuart1971 said:

Your image is perfectly normal for an image with no darks applied, mine look exactly the same without darks and no dithering…so if you did use darks them something is wrong…also you need flats and dark flats for all the separate filters….

Ive attached a screenshot of what happens when I include darks with my lights and flats. Looking closely, you may be able to see black pixels. Do you think these would disappear if I had taken dark flats and added them in Deep Sky Stacker? If so, I'd better build myself a dark flats library too.

Do I need to dither as well as do all the darks?

787926934_blackpixels.jpg.7231daecdb4cbee21733ebff09cfccc6.jpg

Edited by OutThereSomewhere
extra question
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5 minutes ago, Elp said:

It looks like hot pixel walking noise. You have to dither very frequently (every 5 or so if taking 60 second exposures) and around 30 odd pixels (significant enough for you to see a slight fov shift after the dither).

I used 300 sec exposures for my lights and darks, plus 2.35 sec exposures for my flats. If I dithered, would that negate the need for darks altogether? I'm not sure whether I dithered or not when taking these images. Maybe I did sometimes, I can't remember.

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It's always good practice to apply all calibration frames (bias, dark, flat, dark flat) as you want the best from your images.

If you're taking 300s exposures it might be wise to dither after every image, or decrease your exposure duration and dither later. The longest duration I ever exposed for is 3 minutes, then 2, now I mostly do 60 seconds RGB and narrowband, less likely you'll capture satellite trails and if the odd sub is bad, it's no major loss. Take more images to improve the signal to noise ratio.

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If you already have AstroArt (Which version?) then why not stack and combine in AstroArt? The stacking is very fast, and the RGB combination tool is now very much better than it was when I first started using AA, in V5.

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4 minutes ago, DaveS said:

If you already have AstroArt (Which version?) then why not stack and combine in AstroArt? The stacking is very fast, and the RGB combination tool is now very much better than it was when I first started using AA, in V5.

Yes, I've got AstroArt V5 but it's stacking procedure hasn't resolved this problem, so I've tried using DSS then AA5, still without success.

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56 minutes ago, Elp said:

It's always good practice to apply all calibration frames (bias, dark, flat, dark flat) as you want the best from your images.

If you're taking 300s exposures it might be wise to dither after every image, or decrease your exposure duration and dither later. The longest duration I ever exposed for is 3 minutes, then 2, now I mostly do 60 seconds RGB and narrowband, less likely you'll capture satellite trails and if the odd sub is bad, it's no major loss. Take more images to improve the signal to noise ratio.

Thanks for the advice. I'll have to work out how to pause my imaging and guiding in SGP; dither; then resume guiding and imaging again. I think that must be my problem.

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1 hour ago, Elp said:

It's always good practice to apply all calibration frames (bias, dark, flat, dark flat) as you want the best from your images.

If you're taking 300s exposures it might be wise to dither after every image, or decrease your exposure duration and dither later. The longest duration I ever exposed for is 3 minutes, then 2, now I mostly do 60 seconds RGB and narrowband, less likely you'll capture satellite trails and if the odd sub is bad, it's no major loss. Take more images to improve the signal to noise ratio.

You should not use both Dark flats and bias, as on modern CMOS low noise cameras they are the same thing, you should only use one or the other…..this is what I have been told a few times….i use just Darks, flats and dark flats, and I don’t dither, I don’t find a need to….

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15 minutes ago, OutThereSomewhere said:

Thanks for the advice. I'll have to work out how to pause my imaging and guiding in SGP; dither; then resume guiding and imaging again. I think that must be my problem.

That should all be done by the software, you don’t actually stop the guiding and dither then restart, the software does it all and needs no input from you, if SGP does not do that, then it’s time to switch to the better NINA software…😉

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1 hour ago, Stuart1971 said:

You should not use both Dark flats and bias, as on modern CMOS low noise cameras they are the same thing, you should only use one or the other…..this is what I have been told a few times….i use just Darks, flats and dark flats, and I don’t dither, I don’t find a need to….

Bias contains the sensor readout noise, though it is also present in the darks so it isn't necessary to use both. Dark flats are related to correcting for readout noise within flats. You are however using one of the workflows contained herein:

http://deepskystacker.free.fr/english/theory.htm#CalibrationProcess

Also if you are using average mode stacking, try kappa sigma at 2 and 5.

Edited by Elp
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