Dither Distances Simplified

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STEVE333 United States of America
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Dither Distances Simplified

#1

Post by STEVE333 »


Part way through a previous post on another subject, I started discussing this subject of determining the magnitude of dithering produced using APT with PHD2. The discussion was not easy reading and/or understanding. I thought a much simpler discussion simply explaining how to determine what settings in APT would provide the desired dithering was in order.

This post shows how to determine the amount of dithering (using APT and PHD2) you will have in your images in three easy steps.

By way of introduction, dithering is accomplished by shifting the aiming point of the telescope in random directions between exposures. The maximum distance the dithering is allowed to move away from the original aiming point is the Maximum Dither Distance, or, MDD. With that in mind let us now determine the amount of dithering to expect for your system.

First calculate the constant K for your system using the equation below.

K = 2 x (FLi / FLg) x (Pg / Pi) [Equation 1]

where:
FLi = imaging telescope focal length (mm)
FLg = guide telescope focal length (mm)
Pg = size of guide camera pixel (microns)
Pi = size of imaging camera pixel (microns)

Second, set Scale in the PHD2 Dither Settings (Global tab in Advanced settings) to 1.

Third, the Maximum Dither Distance (MDD) in image pixels is calculated from:

MDD = K x [Dithering Distance]. [Equation 2]

Dithering Distance is set in APT in the Gear/Guide/Guide Settings menu.

EXAMPLE:
For my new system (about to order ASI1600MM camera package with filter wheel and filters):
FLi = 348 mm
FLg = 160 mm
Pg = 5.2 microns
Pi = 3.78 microns

Using equation [1]: K = 2 x (348 / 160) x (5.2 /3.78) = 5.98 (let's call this 6).

Using equation [2]:
If Dithering Distance = 1 the MDD = 6 x 1 = 6 image pixels

If Dithering Distance = 2 the MDD = 6 x 2 = 12 image pixels

If Dithering Distance = 3 the MDD 6 x 3 = 18 image pixels

etc.

I hope you find this useful.

Comments/questions welcome.

Steve
Steve King: Light Pollution (Bortle 5)
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Re: Dither Distances Simplified

#2

Post by JayTee »


Nicely done Steve. I'm developing a list of posts for a subforum folder of AP "stickies", and this post will definitely go in there.

Cheers,
JT
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
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STEVE333 United States of America
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Re: Dither Distances Simplified

#3

Post by STEVE333 »


JayTee wrote: Sun Feb 02, 2020 4:00 am Nicely done Steve. I'm developing a list of posts for a subforum folder of AP "stickies", and this post will definitely go in there.

Cheers,
JT


Thanks JT. It has certainly made my choice of Dithering Distance in APT a lot simpler. I think I'll actually be able to reduce, my dithering a bit more which should reduce the settling time after dithering saving much wasted time.

Cheers,
Steve
Steve King: Light Pollution (Bortle 5)
Telescope + Mount + Guiding: W.O. Star71-ii + iOptron CEM40 EC + Orion Magnificent Mini AutoGuider
Camera: ASI 1600MM Pro + EFW Filter Wheel + Chroma 3nm Siii, Ha, Oiii + ZWO LRGB Filters
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Re: Dither Distances Simplified

#4

Post by Juno16 »


Thanks Steve,

Your last post started me calculating (based on your formulas) and I also have lowered my dithering distance in APT from 3 down to 2. Plus dithering every other sub will greatly reduce time spent between images.

By your calculations, my MDD is about 24 pixels at a APT dithering distance of 2 = 2 x (714/160) x (5.2/3.89) x 2 (APT dithering distance) = 23.9

I might even be able to drop the dithering distance down to 1.

Thanks a bunch for this Steve!
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
Mount: Skywatcher HEQ5 Pro with Rowan Belt Mod
Stuff: ASI EAF Focus Motor (x2), Orion 50mm Guide Scope, ZWO 30 mm Guide Scope, ASI 220mm min, ASI 120mm mini, Stellarview 0.8 FR/FF, Sharpstar 0.8 FR/FF, Mele Overloock 3C.
Camera/Filters/Software: ASI 533 mc pro, ASI 120mm mini, Orion SSAG, IDAS LPS D-1, Optolong L-Enhance, ZWO UV/IR Cut, N.I.N.A., Green Swamp Server, PHD2, Adobe Photoshop CC, Pixinsight.
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Re: Dither Distances Simplified

#5

Post by STEVE333 »


Juno16 wrote: Sun Feb 02, 2020 4:43 am Thanks Steve,

Your last post started me calculating (based on your formulas) and I also have lowered my dithering distance in APT from 3 down to 2. Plus dithering every other sub will greatly reduce time spent between images.

By your calculations, my MDD is about 24 pixels at a APT dithering distance of 2 = 2 x (714/160) x (5.2/3.89) x 2 (APT dithering distance) = 23.9

I might even be able to drop the dithering distance down to 1.

Thanks a bunch for this Steve!


Hi Jim - You are most welcome.

Since dithering is in random directions it means you dithering will be +/- 24 pixels in all directions. that is a lot of dithering. I would think dropping the Dithering Distance to 1 would be very safe. The MDD for my last run was about 8, and, all airplane/satellite trails were completely removed. That should also help your settling time too.

Glad to see someone benefiting from the analysis.

Cheers,
Steve
Steve King: Light Pollution (Bortle 5)
Telescope + Mount + Guiding: W.O. Star71-ii + iOptron CEM40 EC + Orion Magnificent Mini AutoGuider
Camera: ASI 1600MM Pro + EFW Filter Wheel + Chroma 3nm Siii, Ha, Oiii + ZWO LRGB Filters
Software: PHD2; APT; PixInsight ***** My AP website: www.steveking.pictures
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Re: Dither Distances Simplified

#6

Post by Graeme1858 »


Thanks for this Steve. Dithering was the next aspect of the process I was looking to get my head round. I feel a spreadsheet coming on!

Regards

Graeme
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Re: Dither Distances Simplified

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Post by Graeme1858 »


Steve

Would the optimum Dithering Distance then be determined as a function of pixel size, sensor size or some other factor?

Capture.PNG

Regards

Graeme
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Re: Dither Distances Simplified

#8

Post by bobharmony »


Thanks for the calculations, Steve. They certainly do work to figure out what is going on. The next question in my mind is "how many pixels in the imaging camera do we want between images". I suspect a number in the 8 to 10 range is more than enough.

I ran through the calculation for my setup and came up with a K=4.6. I have APT dither distance set to 4, and PHD2 scale set at 3.0, which yields a maximum dither of 27.65. Interestingly, when I switched from BYEOS I copied the setting I had there, but noticed that my dithers seemed to be larger in APT and settle time increased accordingly. I will tweak PHD scale down to 1, and see if that helps. If needed I will drop the APT setting to 2 to get down to 9 pixels or so as an average.

If I read this correctly, the factor of 2 in equation 1 is meant to make the equation yield the average dither distance instead of the maximum - is that correct?

Bob
Hardware: Celestron C6-N w/ Advanced GTmount, Baader MK iii CC, Orion ST-80, Canon 60D (unmodded), Nikon D5300 (modded), Orion SSAG
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Re: Dither Distances Simplified

#9

Post by STEVE333 »


Graeme1858 wrote: Sun Feb 02, 2020 9:15 am Steve

Would the optimum Dithering Distance then be determined as a function of pixel size, sensor size or some other factor?

Regards

Graeme
Very nice Graeme - A man after my own heart. :techie-typing:

My experience has been that a MDD of 8 or more has successfully removed all artifacts from hot/cold pixels and airplane/satellite trails which is the intended purpose. Making MDD larger than that doesn't hurt the image, but, it may increase the settling time for the guiding to recover from the dither.

Thanks for the response.

Cheers,
Steve


bobharmony wrote: Sun Feb 02, 2020 2:21 pm Thanks for the calculations, Steve. They certainly do work to figure out what is going on. The next question in my mind is "how many pixels in the imaging camera do we want between images". I suspect a number in the 8 to 10 range is more than enough.

I ran through the calculation for my setup and came up with a K=4.6. I have APT dither distance set to 4, and PHD2 scale set at 3.0, which yields a maximum dither of 27.65. Interestingly, when I switched from BYEOS I copied the setting I had there, but noticed that my dithers seemed to be larger in APT and settle time increased accordingly. I will tweak PHD scale down to 1, and see if that helps. If needed I will drop the APT setting to 2 to get down to 9 pixels or so as an average.

If I read this correctly, the factor of 2 in equation 1 is meant to make the equation yield the average dither distance instead of the maximum - is that correct?

Bob
You'we welcome Bob - With Scale = 3 you must be getting huge dither excursions!

Your guess of 8 to 10 fits with my experience as I mentioned to Graeme. I would expect (as you suggested) if you change PHD2 Scale to 1 and APT Dithering Distance to 2 you would end up with MDD = 9 image pixels which is right in the sweet spot. Kind of like Little Red Riding Hood, "Not to large, not to small, but just right!". :twocents:

The factor of 2 in equation [1] was found empirically. I performed the dithering tests and determined the maximum dithering distance (image pixels) from the centroid of the individual dithers. From the analysis it turned out that, after including the effects of telescope focal lengths and pixel sizes and including the APT Dithering Distance setting, that an additional factor of almost exactly 2 was required to make the analysis agree with the measured results. Whether this factor of two is somewhere in APT or PHD2 I don't know, but, it is definitely there.

Cheers,
Steve
Steve King: Light Pollution (Bortle 5)
Telescope + Mount + Guiding: W.O. Star71-ii + iOptron CEM40 EC + Orion Magnificent Mini AutoGuider
Camera: ASI 1600MM Pro + EFW Filter Wheel + Chroma 3nm Siii, Ha, Oiii + ZWO LRGB Filters
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Re: Dither Distances Simplified

#10

Post by Juno16 »


STEVE333 wrote: Sun Feb 02, 2020 5:30 am
Hi Jim - You are most welcome.

Since dithering is in random directions it means you dithering will be +/- 24 pixels in all directions. that is a lot of dithering. I would think dropping the Dithering Distance to 1 would be very safe. The MDD for my last run was about 8, and, all airplane/satellite trails were completely removed. That should also help your settling time too.

Glad to see someone benefiting from the analysis.

Cheers,
Steve
After processing the image from last night, like you, I feel like a APT dithering distance of 1 might work well. I dropped from a setting of 3 to 2 last night and dithering every other sub worked well.

Thanks again Steve!
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
Mount: Skywatcher HEQ5 Pro with Rowan Belt Mod
Stuff: ASI EAF Focus Motor (x2), Orion 50mm Guide Scope, ZWO 30 mm Guide Scope, ASI 220mm min, ASI 120mm mini, Stellarview 0.8 FR/FF, Sharpstar 0.8 FR/FF, Mele Overloock 3C.
Camera/Filters/Software: ASI 533 mc pro, ASI 120mm mini, Orion SSAG, IDAS LPS D-1, Optolong L-Enhance, ZWO UV/IR Cut, N.I.N.A., Green Swamp Server, PHD2, Adobe Photoshop CC, Pixinsight.
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Re: Dither Distances Simplified

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Post by STEVE333 »


Juno16 wrote: Sun Feb 02, 2020 5:25 pm
After processing the image from last night, like you, I feel like a APT dithering distance of 1 might work well. I dropped from a setting of 3 to 2 last night and dithering every other sub worked well.

Thanks again Steve!
Great news Jim - I love stories with a happy ending! :romance-grouphug:

Steve
Steve King: Light Pollution (Bortle 5)
Telescope + Mount + Guiding: W.O. Star71-ii + iOptron CEM40 EC + Orion Magnificent Mini AutoGuider
Camera: ASI 1600MM Pro + EFW Filter Wheel + Chroma 3nm Siii, Ha, Oiii + ZWO LRGB Filters
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Re: Dither Distances Simplified

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Post by DEnc »


That’s handy!

Lacking this approach for SGP/PHD2 dither settings, I took an empirical approach and focused on the range of distances seen with various settings. It seemed to me that’d be desirable to achieve a 1 pixel minimum displacement, for pixel rejection while processing.

Here are a plots of distances the lock star moved on the guide camera in 68 successive dithers, derived from the PHD2 guide log. On the left the dither events are ordered in their sequence, on the right they’re sorted by distance. SGP was set to "Small dither" and the PHD2 scale was 20; OAG with FL of 770 and 539mm, and pixels of 5.4 and 7.4um for imaging and guiding, respectively.

David
DitherDistances.jpg
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Re: Dither Distances Simplified

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Post by STEVE333 »


DEnc wrote: Sun Feb 02, 2020 6:32 pm That’s handy!

Lacking this approach for SGP/PHD2 dither settings, I took an empirical approach and focused on the range of distances seen with various settings. It seemed to me that’d be desirable to achieve a 1 pixel minimum displacement, for pixel rejection while processing.

Here are a plots of distances the lock star moved on the guide camera in 68 successive dithers, derived from the PHD2 guide log. On the left the dither events are ordered in their sequence, on the right they’re sorted by distance. SGP was set to "Small dither" and the PHD2 scale was 20; OAG with FL of 770 and 539mm, and pixels of 5.4 and 7.4um for imaging and guiding, respectively.

David


Hi David - Clever idea looking at the PHD2 guide log.

I believe the PHD2 log will describe dither motions in terms of guide pixels. I chose to go directly to the dither motions of the final images. I checked the dither motions of the final images with both DSS and Pixinsight and got the same results, so, I feel pretty confident with the results.

Hopefully your analysis will provide the information you need to get good dithering.

Cheers,
Steve
Steve King: Light Pollution (Bortle 5)
Telescope + Mount + Guiding: W.O. Star71-ii + iOptron CEM40 EC + Orion Magnificent Mini AutoGuider
Camera: ASI 1600MM Pro + EFW Filter Wheel + Chroma 3nm Siii, Ha, Oiii + ZWO LRGB Filters
Software: PHD2; APT; PixInsight ***** My AP website: www.steveking.pictures
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Re: Dither Distances Simplified

#14

Post by Juno16 »


Hi Steve,

Just dug up this post from 2020. I was getting some fixed pattern noise ( walking noise) with my setup and used your calculations to determine the dither distance. I was only dithering 6 px. Now I know!
Doubled it to 12 with no walking noise. Will try 9 next time.

Just wanted to thank you again!
Jim

Scopes: Explore Scientific ED102 APO, Sharpstar 61 EDPH II APO, Samyang 135 F2 (still on the Nikon).
Mount: Skywatcher HEQ5 Pro with Rowan Belt Mod
Stuff: ASI EAF Focus Motor (x2), Orion 50mm Guide Scope, ZWO 30 mm Guide Scope, ASI 220mm min, ASI 120mm mini, Stellarview 0.8 FR/FF, Sharpstar 0.8 FR/FF, Mele Overloock 3C.
Camera/Filters/Software: ASI 533 mc pro, ASI 120mm mini, Orion SSAG, IDAS LPS D-1, Optolong L-Enhance, ZWO UV/IR Cut, N.I.N.A., Green Swamp Server, PHD2, Adobe Photoshop CC, Pixinsight.
Dog and best bud: Jack
Sky: Bortle 6-7
My Astrobin: https://www.astrobin.com/users/Juno16/
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