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Asiair dithering. How much ?

ZWO ASI533MC Pro Acquisition Dithering Drizzle Deep Sky
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Eric Gagne avatar
Eric Gagne avatar
I want to start dithering so I can drizzle in wbpp.  Cuiv stated in a video that drizzling requires good dithering.

I’m using a Samyang 135mm with an ASI533MC Pro.  Online calculators tell me I should dither by 20 pixels but the ASIAIR doesn’t have this option.  I have to select 10 or 30.

I don't know which one to choose.  30 seems like it would slow things down a lot but I wonder if 10 is good enough for drizzling.
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Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
Have you worked out how many pixels the imaging camera will move if your guide camera moves x amount of pixels? It depends on your guide camera and the focal length of your scope.

I've used a 50mm guide scope and dithered 5-10 pixels with my 174mini, using a 2600 as my imaging camera. If you are using a small guide scope with a narrower FOV than the lens, the lens will move a larger amount of pixels than the asiair input. If that makes sense to you. 

Also, there's this thread about it: https://www.astrobin.com/forum/c/astrophotography/deep-sky/dithering-amount/
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Brian Diaz avatar
Brian Diaz avatar
Eric Gagne:
I want to start dithering so I can drizzle in wbpp.  Cuiv stated in a video that drizzling requires good dithering.

I’m using a Samyang 135mm with an ASI533MC Pro.  Online calculators tell me I should dither by 20 pixels but the ASIAIR doesn’t have this option.  I have to select 10 or 30.

I don't know which one to choose.  30 seems like it would slow things down a lot but I wonder if 10 is good enough for drizzling.

hi

here is the link to calculate it

https://www.astrohowto.com/interactive-dithering-calculator/

CS,Brian
Tony Gondola avatar
Tony Gondola avatar
I don't think you can over-dither, I'd try 30 and see what you get…
Bob Lockwood avatar
Bob Lockwood avatar
Eric Gagne:
I have to select 10 or 30.


That's not giving you much choice. I don't use the Asiair, but I've used a 4 pixel dither (every frame) from day one regardless of that camera or scope I'm using and have never had an issue with walking noise of FPN or whatever you want to call it. I think if 10 is the least amount you can go, then do 10, I think 30 is a bit much.
Eric Gagne avatar
Eric Gagne avatar
I have the option for 2 and 5 also.  I just can’t do 20 that the calculators revommenf
Bob Lockwood avatar
Bob Lockwood avatar
2 is a bit small, I'd give 5 a shot. It will save you a lot of recovery time with 5 over 10 or 30.
Eric Gagne avatar
Eric Gagne avatar
My ratio is about 1:1.  The cameras and scopes have almost the same pixel size and FL.  Dithering 10 pixels would move it just about 10 pixels.  The question remains, is it enough for drizzling?
Bob Lockwood avatar
Bob Lockwood avatar
You did mention that from the start, I don't drizzle so I can't really help you there.smile 

This is just out there, in WBPP, I can't find anything anywhere that asks for dither info, so it may not matter what your dither pixel movement is, but I'm sure someone hear will know.
Eric Gagne avatar
Eric Gagne avatar
Finally found an answer on NINa web site.  They say it’s recommended to dither for the main camera to move 10 pixels.  So 10 it will be then
Miguel A. avatar
Miguel A. avatar
Eric Gagne:
Finally found an answer on NINa web site.  They say it’s recommended to dither for the main camera to move 10 pixels.  So 10 it will be then

I've been wondering about this lately. So 10 is distance,  so what should the interval be? By default in AsiAir it's 1.
Eric Gagne avatar
Eric Gagne avatar
I usually do 5 minutes subs if the sky and wind allow.  I will dither at every image.  If taking shorter subs I’d probably go every 5 minutes. I have to watch Cuiv’s video again, he’s got good info there
Eric Gagne avatar
Eric Gagne avatar
Here’s the video I’m talking about

https://youtu.be/ItmrAZyLeek?si=53WhMauOBHo4kPIL
Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
In short, the fewer the subs the more you should dither. So it doesn't solely depend on the exposure length, it depends on how many exposures you plan on adding for the image as a whole. I would never perform dithering for each sub-exposure as that is too often and will cost you imaging time when it adds up during a session. I usually dither each third or fifth which seem to be working rather well for me at least.
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Sam Badcock avatar
Sam Badcock avatar
Eric Gagne:
Finally found an answer on NINa web site.  They say it’s recommended to dither for the main camera to move 10 pixels.  So 10 it will be then

If you go back to Brian's comment that has the astrohowto link for an interactive dithering calculator and use that, it will give you the required dithering distance your setup needs!! Dithering works on the guiding equipment and the ultimate goal is to move the main camera 20 pixels in a random direction!! Now if your focal lengths and camera pixel sizes are pretty much the same which I'm going to assume you use the ASI120MM Mini and ZWO 30F4 guiding combo then your required dithering distance is 18pixels!! As you've noticed yeah you can't select 20 as the dither distance so stick with 10 for now but if you continue to find artifacts like hot pixels or walking noise/banding then increase the dither distance to 30 pixels!! Be careful though as some mounts don't like a big dither distance and can take some time to get back to good guiding!!

See how you go on 10 then maybe experiment with 30 for a session or 2 and see if that yields better results for ya

CS
- Sam
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Niels_L avatar
Niels_L avatar
I think that the dither distance in the ASIAIR is the max distance on the main scope. There was a discussion on AB (maybe it was somewhere else) last week or so that also wanted to figure it this out…. I cannot seem to find it anymore, so don't know 100% (I do recall they had some feedback from ZWO on this). 
So, assuming the ASIAIR calculates the dither on the main scope, this would mean that dithering 5px should normally be enough to circumvent hot pixels.
V avatar
V avatar
As for drizzling, generally speaking getting more spatial randomization is optimal, but too much can result in rough backgrounds on projects with low frame counts. Dither every frame, make sure your guider has speed set to sidereal and allow it to make longer pulses to allow for very fast dither correction.

30 pixels is enough. I use 100 pixels when doing exposures under 2 minutes, and 25 for 2 min and longer. Helps smoothen it out.
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