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Dither Distance Formula


Bottletopburly

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Found this on APT forum may simplify working out dither distance 

https://aptforum.com/phpbb/viewtopic.php?f=19&t=3157

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

K = 2 x (FLi / FLg) x (Pg / Pi)

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 = K x [Dithering Distance]. 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

K = 2 x (348 / 160) x (5.2 /3.78) = 5.98 ~ 6

If Dithering Distance = 1 the MDD = 6 image pixels

If Dithering Distance = 2 the MDD = 12 image pixels

If Dithering Distance = 3 the MDD = 18 image pixels
 

 

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I picked up on this one a few days ago, David. It's a very useful formula from which  I created a spreadsheet to automatically calculate the numbers for my range of optics.

 

I had terrible walking noise prior to this with APTs default settings. Once I set my optics to dither 15 or so pixels, the problem disappeared.

 

Good to see they "stickied" it.

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Bottletopburly
11 hours ago, 8472 said:

I picked up on this one a few days ago, David. It's a very useful formula from which  I created a spreadsheet to automatically calculate the numbers for my range of optics.

 

I had terrible walking noise prior to this with APTs default settings. Once I set my optics to dither 15 or so pixels, the problem disappeared.

 

Good to see they "stickied" it.

Could you share the excel sheet , I thought that could be made into an excel formula but beyond me it’s one of those invaluable tools saves getting the calculator out , I haven’t noticed walking noise since dithering plus saves taking darks with a dslr which they says pretty useless unless temperature matched mega time saver . 

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8 hours ago, Bottletopburly said:

Could you share the excel sheet , I thought that could be made into an excel formula but beyond me it’s one of those invaluable tools saves getting the calculator out , I haven’t noticed walking noise since dithering plus saves taking darks with a dslr which they says pretty useless unless temperature matched mega time saver . 

 

Of course, David

 

https://drive.google.com/file/d/1LYcBtO_LhWqeD1jXeIxBeJ7A3iXPjdJp/view?usp=sharing

 

As i say, I dither around 15 imaging camera pixels, but you may not need to go to that extreme. 

 

Simply enter your numbers into cells B4 through to B8 and it will do the rest for you.

 

I had drop down cells for my dual pixel scale QHY294m and various optics, but this just complicates things further.

 

Clear skies,

 

Kev

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That's odd. 

 

I've blinked my subs in Pixinsight and the dithering looks fine to me. I did have to remove some formatting for your copy, as it had all my data write-protected and/or in drop down boxes.

 

I can't access your lad's formula, as it's restricted at my end.

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Bottletopburly
Posted (edited)

Cheers Kev @8472 i managed to find how to edit values to my scopes cameras on the last sheet you posted and check against my lads sheet all seem to work out the same 🖖

Edited by Bottletopburly
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