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how many frames in between dithers?

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#1 joelin

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Posted 11 September 2019 - 12:46 PM

I'm wondering if anyone knows a rule of thumb for how frequently to dither? every frame? every other? every 3 frames? every 10?

 

or is it more like you shouldn't more than some X% of frames belonging to the same not dithered sequence?



#2 Midnight Dan

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Posted 11 September 2019 - 12:52 PM

For me, it depends on how long the frames are.  If I'm shooting 3 minutes or up, I'll dither every frame.  For one or two minutes, I'll go every other frame.  Below one minute I tend to go every 3rd frame.

 

-Dan


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#3 dmdouglass

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Posted 11 September 2019 - 01:05 PM

For me, on this question, i find the discussion "interesting". And there have been many threads.

One might want to ask a basic question.  Why are you dithering ? What is it doing ?

I know the answers....   and it provides the answer to this question.

 

You are trying to adjust the image, such that when "stacking", you align on the "target", and the "hot pixels" (the real "target" of dithering") are moved about. Thus, when "stacking", the astronomy "target" aligns, but the hot pixels do not. And the "hot pixels" are thus eliminated.

 

With that in mind.... i dither every frame.  Why would i want to do anything different ?


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#4 knight_parn

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Posted 11 September 2019 - 01:18 PM

It depends on the total number of frames I plan to take. Anything less than 40, I dither every frame. Up to 80, every 2 frames and Up to 140, every 3 frames. Haven't taken more than 140 subs in one session yet.


Edited by knight_parn, 11 September 2019 - 01:20 PM.

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#5 joelin

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Posted 11 September 2019 - 01:22 PM

For me dithering loses imaging time. I’ve estimated I lose 45 seconds for each dither so I want to dither as little as possible.

#6 joelin

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Posted 11 September 2019 - 01:37 PM

If pixel rejection can get rid of hot pixels if say it has at least 8 other frames where the hot pixel is somewhere else, then my % is 12.5%.

So if I take 60 frames, I can have 7 frames no dither, then dither, then 7 more no dither.
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#7 AhBok

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Posted 11 September 2019 - 01:42 PM

It also depends on your camera. I have an ASI071MC Pro and the noise and especially FPN is so low that with 2-3 minute exposures, I dither moderately every 5 frames. I used to dither more aggressive dithering every frame and the the only practical difference was lost exposure time. Some cameras require much more dithering.

My best advice is to experiment over weeks/mos.


Edited by AhBok, 11 September 2019 - 01:45 PM.


#8 kathyastro

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Posted 11 September 2019 - 02:40 PM

I dither every frame.



#9 einarin

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Posted 11 September 2019 - 02:45 PM

For me dithering loses imaging time. I’ve estimated I lose 45 seconds for each dither so I want to dither as little as possible.

 

This sounds like a long time - I would do adjustments to make it shorter.


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#10 Midnight Dan

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Posted 11 September 2019 - 02:47 PM

i dither every frame.  Why would i want to do anything different ?

 

 

For me dithering loses imaging time. I’ve estimated I lose 45 seconds for each dither so I want to dither as little as possible.

Exactly.  

 

With today's low read noise, high QE, high dynamic range cameras, you can take many short exposures rather than fewer long ones and get the same results.  Shorter exposures reduces or eliminates saturated stars, which brings out the color better and makes them look more natural.  It also reduces guiding problems and increases the number of acceptable subs.

 

I often do hundreds of exposures in the 30 second to 2 minute range these days with my ASI071 camera.  If I dithered every frame I'd lose a huge portion of imaging time.

 

-Dan


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#11 OldManSky

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Posted 11 September 2019 - 02:53 PM

As others have mentioned, it depends a lot on the hot pixels and/or fixed pattern noise of your camera.

My ZWO ASI183mm has very little of both types of artifact.  So I dither every 3 frames.

That was the result of experimentation, so I could quantify the results.  The difference in S/N ratio of stacked images with every 3rd frame and every frame was less than 0.2%.  

However, if I dithered every six frames, the S/N ratio decreased by just about 1%.  That was too much for me, so every 3 frames it is.


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#12 joelin

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Posted 11 September 2019 - 03:09 PM

How do you measure S/N of an image?

#13 joelin

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Posted 11 September 2019 - 03:11 PM

As others have mentioned, it depends a lot on the hot pixels and/or fixed pattern noise of your camera.
My ZWO ASI183mm has very little of both types of artifact. So I dither every 3 frames.
That was the result of experimentation, so I could quantify the results. The difference in S/N ratio of stacked images with every 3rd frame and every frame was less than 0.2%.
However, if I dithered every six frames, the S/N ratio decreased by just about 1%. That was too much for me, so every 3 frames it is.


Doesn’t the result matter based on the total number of frames you have?

If you dithered every 3 frames but only have 6 frames total isn’t that more worrisome than having 60 frames total?
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#14 Becomart

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Posted 11 September 2019 - 04:56 PM

I dither every frame. It doesn’t take more than seconds and the loss of time doesn’t bother me.....because I’m sleeping!



#15 dmdouglass

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Posted 11 September 2019 - 05:45 PM

I dither every frame. It doesn’t take more than seconds and the loss of time doesn’t bother me.....because I’m sleeping!

 

+1 on this (as noted above)

 

I really don't know where these quotes of large time delays are coming from.

Perhaps it is a question of "how much are you dithering"....

In my case, 2 pixels.  Mere seconds, and a few more for "settleing", and away we go !

 

Edit Add:  And keep in mind... it is the "guiding camera" pixels... not the imaging camera.


Edited by dmdouglass, 11 September 2019 - 05:46 PM.

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#16 OldManSky

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Posted 11 September 2019 - 06:02 PM

Doesn’t the result matter based on the total number of frames you have?

If you dithered every 3 frames but only have 6 frames total isn’t that more worrisome than having 60 frames total?

Not that I can see.

If I dither every 3 frames with 6 total, I'd dither twice.  2/6 = 33% dithered

If I dither every 3 frames with 60 total, I'd dither 20 times  20/60 = 33% dithered

If I dither every 3 frames with 200 total, I'd dither 66 times.  66/200 = 33% dithered

If I dither every 3 frames with 300 total, I'd dither 100 times.  100/300 = 33% dithered

 

So why does the total # of frames matter, when the % of dithering would be the same no matter what?


Edited by OldManSky, 11 September 2019 - 06:04 PM.


#17 jdupton

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Posted 11 September 2019 - 06:03 PM

David,

 

   The amount of time is very dependent on the specific configuration of the imaging system. I am in the group of not wanting to dither too often due to the amount of time it sucks away from imaging.

 

   Keeping in mind that we are discussing Guide Camera pixels doesn't make much difference for many.

 

   I image using an OAG. My imaging scale is 0.483 "/px with 4.63 uM pixels and the guide camera isn't much different with 5.6 uM pixels. I also keep very good polar alignment. PHD2 reports my DEC drift rate as 0.02 "/minute. Because of the low drift rate and OAG guiding image scale, I sometimes guide on stars at magnitude 11.5 to 13.5 using up to 5 second guide exposures. (The mount just doesn't need more frequent guiding pulses.)

 

   Given those conditions, it can take two to four guide image iterations to settle after a dither. That translates to 10 to 20 seconds to settle and resume imaging. That can really add up in a light polluted environment where shorter exposures are often used.

 

   Another user running a separate guide scope, taking 0.5 second guide exposures, just doesn't have to contend with such down time that dithering can take away from total imaging time. It is all based on configurations and is likely very different from user to user.

 

 

John


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#18 joelin

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Posted 11 September 2019 - 06:19 PM

Not that I can see.

If I dither every 3 frames with 6 total, I'd dither twice.  2/6 = 33% dithered

If I dither every 3 frames with 60 total, I'd dither 20 times  20/60 = 33% dithered

If I dither every 3 frames with 200 total, I'd dither 66 times.  66/200 = 33% dithered

If I dither every 3 frames with 300 total, I'd dither 100 times.  100/300 = 33% dithered

 

So why does the total # of frames matter, when the % of dithering would be the same no matter what?

because in the case of dithering once in 6 frames so you have a hot pixel in the same place in 3 frames out of 6....the hot pixel frames to total frames is 50%...making it harder for rejection algorithms to get rid of it

 

when you have 60 frames, its now 3 out of 60 ...the hot pixel frames to total frames is just 5%...making it easier to reject it



#19 dmdouglass

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Posted 11 September 2019 - 07:27 PM

All very interesting numbers.

And really, there are no "Wrong Answers"...

 

In the end, its really a matter of doing ....   What works for you.

And for each person, and all the different combinations of equipment, and viewing environments.....

There will be many many different combinations of "What Works".


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#20 schmeah

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Posted 11 September 2019 - 07:42 PM

Of course if it’s LRGB you can rotate through filters and dither every 4th, effectively dithering every frame.

 

Derek


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#21 OldManSky

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Posted 11 September 2019 - 07:55 PM

because in the case of dithering once in 6 frames so you have a hot pixel in the same place in 3 frames out of 6....the hot pixel frames to total frames is 50%...making it harder for rejection algorithms to get rid of it

 

when you have 60 frames, its now 3 out of 60 ...the hot pixel frames to total frames is just 5%...making it easier to reject it

Are we talking outliers-to-make-a-point hypotheticals, or actual how-we-do-things?  'Cause I never do 6 image stacks.  Do you?

 

Darks take care of hot pixels.  My only worry is FPN.

And dithering 33% of the time takes care of that, confirmed by testing.

 

It's OK if you want to do it differently than me, I won't be offended.  Do what works for you :)



#22 AZ Maverick

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Posted 11 September 2019 - 08:00 PM

I use PHD2 and dither every two images, each random dither is about 12 pixels max on the image camera and it usually averages less than 10 seconds to do a complete dither.



#23 joelin

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Posted 11 September 2019 - 08:14 PM

Are we talking outliers-to-make-a-point hypotheticals, or actual how-we-do-things?  'Cause I never do 6 image stacks.  Do you?

 

Darks take care of hot pixels.  My only worry is FPN.

And dithering 33% of the time takes care of that, confirmed by testing.

 

It's OK if you want to do it differently than me, I won't be offended.  Do what works for you smile.gif

this isn't really about people's preference...its about what is enough dithering so that the mathematical rejection algorithms kick in and get rid of FPN ... 


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#24 joelin

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Posted 11 September 2019 - 08:15 PM

Of course if it’s LRGB you can rotate through filters and dither every 4th, effectively dithering every frame.

 

Derek

that might not work for me because i need to refocus in between filters 



#25 joelin

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Posted 11 September 2019 - 08:15 PM

Are we talking outliers-to-make-a-point hypotheticals, or actual how-we-do-things?  'Cause I never do 6 image stacks.  Do you?

 

Darks take care of hot pixels.  My only worry is FPN.

And dithering 33% of the time takes care of that, confirmed by testing.

 

It's OK if you want to do it differently than me, I won't be offended.  Do what works for you smile.gif

6 image stacks are a likely possibility if you're running low on time

 

its certainly common in narrowband when a single capture takes a looooong time 




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