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Old 05-11-2020, 09:58 AM
Startrek (Martin)
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Dithering with APT - Dither Distance

I’ve been using APT for 4 months now and in regard to Dithering have left the default setting of 1 in Dither Distance
Looking further into the dithering process on a few sites , a general rule of thumb or recommended Dither for the main imaging camera should be a move of around 8 to 10 pixels
Below are some calculations I put together for my two imaging scopes using APT and PHD2 which came up with vastly different Dither settings than the default setting of 1. Also when I joggle through my fits frames zoomed in I noticed the frames only shift very slightly and sometimes not at all which lead me to post this query

Dithering in APT

APT Dither settings
Assuming PHD2 Dither scale is 1
APT Guide Camera Pixels
1 0.5
2 1
3 2
4 3
5 5

6” f6 Bintel GSO Newt
ZWOASI2600MC imaging camera
Orion 60mm Guide Scope
ZWOASI120MM-S guide camera

Pixel scale for 6” f6 Bintel GSO Newt
3.76 x 206.3 / 900 = 0.86 arc sec / pixel
Pixel scale for Guide scope
3.75 x 206.3 / 240 = 3.22 arc sec / pixel

So ratio or multiplier is 3.8 x

General rule of thumb for dithering is to move the main imaging camera about 10 pixels on each frame

6” f6 Bintel GSO newt
So therefore an APT Dither Distance of 3 should move the imaging camera about 8 pixels which is satisfactory


8”f5 Bintel GSO Newt
ZWOASI2600MC imaging camera
Orion 60mm Guide Scope
ZWOASI120MM-S guide camera

Pixel scale for 8” f5 Bintel GSO Newt
3.76 x 206.3 / 1000 = 0.77 arc sec / pixel
Pixel scale for Guide scope
3.75 x 206.3 / 240 = 3.22 arc sec / pixel

So ratio or multiplier is 4.2 x

General rule of thumb for dithering is to move the main imaging camera about 10 pixels on each frame

8” f5 Bintel GSO newt
So therefore an APT Dither Distance of 2 should move the imaging camera about 8 pixels which is satisfactory

I would appreciate some feedback to see if the above is correct and if so I will change the APT Dither Distances accordingly

Thanks in advance
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  #2  
Old 06-11-2020, 10:02 AM
Startrek (Martin)
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Hello out there
Does anyone use APT ?
Does anyone Dither ?

Appreciate any advice on my calcs and suggestions
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Old 06-11-2020, 10:13 AM
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Peter Ward
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While I don't use APT (I prefer to image with MaxIm) I've found a dither of 2.5 pixels smooths the background extremely well. Any more would be overkill IMHO.
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Old 06-11-2020, 10:23 AM
Startrek (Martin)
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Peter
Thanks for taking the time to reply
I’m using the new ZWO 2600MC which has produced good results in relation to SNR. Ive always taken darks but with this camera there isn’t a noticeable difference in using them, the camera is so good
I suppose I will just bump up the dither distance one notch which will result in a 3 to 4 pixel shift and see how that compares to previous data
Thanks again
Martin
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Old 06-11-2020, 04:29 PM
RyanJones
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At the risk of sounding silly can I ask some questions regarding dithering. My understanding is that you dither to offset the fixed pattern noise. Now looking at the registration data in DSS when stacking, my X and Y axis differentiate by several pixels over the term of the session presumably due to flexure. Does this not already introduce a form of dithering ?

Sorry if that’s a silly question

Ryan
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Old 06-11-2020, 05:45 PM
Startrek (Martin)
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Quote:
Originally Posted by RyanJones View Post
At the risk of sounding silly can I ask some questions regarding dithering. My understanding is that you dither to offset the fixed pattern noise. Now looking at the registration data in DSS when stacking, my X and Y axis differentiate by several pixels over the term of the session presumably due to flexure. Does this not already introduce a form of dithering ?

Sorry if that’s a silly question

Ryan
Ryan
A few pixels over a session IMHO doesn’t constitute dithering
Dithering moves each frame in a “random pattern” by anywhere from a few pixels to 20 or 30 pixels each frame, a session could be 20 to 100 frames
Each dither is random too
Hope that puts it into perspective
Maybe others can offer more advice and explain in more detail too
Martin
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Old 06-11-2020, 06:15 PM
RyanJones
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Quote:
Originally Posted by Startrek View Post
Ryan
A few pixels over a session IMHO doesn’t constitute dithering
Dithering moves each frame in a “random pattern” by anywhere from a few pixels to 20 or 30 pixels each frame, a session could be 20 to 100 frames
Each dither is random too
Hope that puts it into perspective
Maybe others can offer more advice and explain in more detail too
Martin
Understood. To clarify I’m talking about a flexure movement of normally around 15 - 20 pixels over a single session of maybe 80 - 100 frames and then often combined over several sessions where the total can be 30 - 40 pixels. Like I said I might be missing something but i’m assuming a fixed pattern noise is fixed to the chip and as long as the signal moves but the noise doesn’t for enough frames over a given total then it will be recognised as noise and be removed. I would assume that dithering is only benificial if there is almost no differential flexure movement thoughout the entire session(s) ?

Not discounting what you’ve said Martin. I’m just trying to establish a greater understanding.

Cheers

Ryan
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Old 06-11-2020, 06:50 PM
Startrek (Martin)
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Ryan
Are you sure it’s differential flexure causing your alignment anomaly in DSS or a combination of tracking error and atmospheric dispersion ???
Just a thought
Cheers
Martin
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Old 06-11-2020, 07:06 PM
RyanJones
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Quote:
Originally Posted by Startrek View Post
Ryan
Are you sure it’s differential flexure causing your alignment anomaly in DSS or a combination of tracking error and atmospheric dispersion ???
Just a thought
Cheers
Martin
It could be either or all of the above to be honest. I haven’t run any tests to confirm what causes it. My assumption that it’s differential flexure is based on its consistency through each session. When imaging the same object over several nights, the movement seems pretty consistent from one session to the next. As I said though it’s just an assumption.

Cheers
Ryan
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