Dithering vs. Bad p...
 
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Dithering vs. Bad pixel map.  

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 Heno
(@heno)
Main Sequence Star Customer
Joined: 2 years ago
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November 28, 2019 16:02  

Questions as follows:
1. If you are dithering during imaging, should you still use a BPM?
(I'm thinking that if you have already smeared your bad pixels over many pixels. What would the point be?)
2. With respect to bad pixels, will a BPM do just as good a job as dithering?
(BPM will not remove any discrepancies from the sky, only bad pixels, I know.)
3. There are other benefits of dithering like the possibility of drizzle if you are  under sampled, but if you are not under sampled, are there then any other benefits?

Helge

This topic was modified 4 months ago by Vincent Groenewold - Moderator

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(@vincent-mod)
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November 28, 2019 18:59  

1. It will still benefit it yes, not all and every bit of noise is removed with dithering as it's still done using outlier rejection algorithms that aren't 100% removing everything for instance

2. Like you said, probably yes for the bad pixels, but I have to say I never tested this myself. I still use bias and darks for instance. More with the idea that having them all together give APP the best chance of removing as much noise as possible without introducing artefacts. Not sure though about this one.

3. I don't think so no, drizzling is specifically designed for undersampled data indeed.


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 Heno
(@heno)
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November 29, 2019 15:29  

@vincent-mod

OK and thanks.
Reason for asking is that I will try to run a dual setup with two telescopes. So far I have not been able to sync the dithereing (that dithering wait until the other scope has finished its ongoing frame). So that frame will be ruined. Or I can just drop the dithereing and rely om the BPM. Not sure what is the best to be honest.

Helge


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(@vincent-mod)
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November 29, 2019 15:52  

I would first just try a test session without, to see if you don't get any "walking noise" or something like that. Dithering does help a lot in a lot of cases, so I would recommend it more than avoiding it. I would then spend my time in getting the sync done, hope you can figure that one out.

This post was modified 4 months ago by Vincent Groenewold - Moderator

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 Heno
(@heno)
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November 30, 2019 11:21  

Yes, I will try to get the two scopes in sync using exposure durations that fits together, but I don't think it actually will work. Too many unknown factors like downloading time, guider settling, etc, etc.

On the other hand, even if I loose a frame every now and then, I will still have all the good ones. Alternatively I will only image with one scope and have much less data. Or drop dithering and have all. 😉
NINA capturing software is supposed to sync a dual set up, but so far I have not been able to make that work. It also force dithering after every frame which I find excessive and waste of time.

Helge


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(@vincent-mod)
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November 30, 2019 12:29  

Yes that is not necessary, I always dither every 3 frames. But if you have software that does sync 2 scopes, it should wait for one or the other to finish and then continue. I guess that must be a setting somewhere then. Would Sequence Generator Pro maybe work better?


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 Heno
(@heno)
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November 30, 2019 13:15  

@vincent-mod

Sorry. SGP don't have this possibility yet. SGP is my standard imaging software.
I have heard some rumors that this may appear in ver. 4.0, but that remains to see.

Helge


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(@carlesa25)
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November 30, 2019 13:17  
Posted by: @heno

Questions as follows:
1. If you are dithering during imaging, should you still use a BPM?
(I'm thinking that if you have already smeared your bad pixels over many pixels. What would the point be?)

Hello: If you allow me the comment; BPM have nothing to do with "Dithering" Bad Pixels are that, bad pixels, dead, failed, fixed and physically located on the sensor and that with a good BPM Master are compensated, although it is advisable to renew this Master already that Bad Pixels have a tendency to increase over time on any CCD or CMOS sensor.

The dithering is trying to compensate and thereby reduce the random, thermal and background noise of the image during the acquisition of the image making each shot or in the sequence that is designed, the image moves and thus trust that noise that is random (not fixed) is automatically canceled.

 


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 Heno
(@heno)
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December 1, 2019 11:12  

I had to sit down and read a bit to see if I had got this all wrong. I believe I have not.
But regarding my first question it should have been phrased the other way around: For bad pixels (exclusively) is there any point of dithering? If a BPM is in use the answer is no. The BPM, if in use, will neutralize the bad pixels.
If no BPM the answer is yes.  Dithering will spread bad pixels to various pixels relative to the imaged object.
Dithering will also help reduce or even remove any other artifact of type "fix pattern noise", and even random noise lik an air plane in one frame using Sigma clipping. 
(I'm writing this post mostly to have it clear in my own head. 😆 )
https://www.skyandtelescope.com/astronomy-blogs/astrophotography-jerry-lodriguss/why-how-dither-astro-images/


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(@wvreeven)
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December 1, 2019 21:15  

That only is true if the BPM is accurate and actually covers all bad pixels. But since the BPM was created the sensor may have degraded and new bad pixels may have come to existence. In that situation dithering will help to identify the new bad pixels. Since you don’t know if there are new bad pixels I think it always makes sense to dither.

 

Wouter


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(@itarchitectkev)
Red Giant Customer
Joined: 2 years ago
Posts: 68
December 2, 2019 15:28  
Posted by: @heno

Yes, I will try to get the two scopes in sync using exposure durations that fits together, but I don't think it actually will work. Too many unknown factors like downloading time, guider settling, etc, etc.

On the other hand, even if I loose a frame every now and then, I will still have all the good ones. Alternatively I will only image with one scope and have much less data. Or drop dithering and have all. 😉
NINA capturing software is supposed to sync a dual set up, but so far I have not been able to make that work. It also force dithering after every frame which I find excessive and waste of time.

Helge

@heno Slightly OT but related as you've mentioned it above. I use dual scope and synchronised dithering. It works when you set up your profile and under the Guiding for *both* profiles - select the Synchronised Dithering (experimental) option. I use NINA 1.9 for this. Set up an imaging session (number of subs, duration, target, filters, etc) and on one of the sessions choose "Start guiding". The sync option will set a dither every frame (it won't really do this) - you'll get waits as one or the other capture sessions waits for a bit longer for the timings to continue again. So be savvy on your sub duration. e.g. if you have a scope/cam that is doing 4 min exposures, maybe your other will be 2 mins - in which case it - after the 4 min sub, and the 2 min subs - then the dither will happen.


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 Heno
(@heno)
Main Sequence Star Customer
Joined: 2 years ago
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December 3, 2019 15:31  

@itarchitectkev

Hi and thanks for your info. It's been some weeks since I tried NINA, also 1.9, but I think I did exactly what you explain. (Or probably not, but I don't understand what the difference could be.)
I got error messages saying something like "Cannot start dithering while dithering is ongoing" so it seemed like both instances of NINA tried to dither.
I gave up. It was freezing cold and I did not want to waste time on this if I could not make it work. I will certainly try again at some point. 


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