To much Dither!?

[Rustang]

My second ever guiding session last night but first time dithering, in general it went really and I'm excited for the times ahead. I have a question though and the answer may well be in the question but 'what would cause a dither to upset the quide, too greater dither!? I'm still obviously learning the ropes, so bear with me, i had the dither distance at 6 to start with and ended up down a 1 which seemed to settle everything but it was a shorth dither, say about 5 seconds from starting dither to ending dither, from what I read for a DSLR the dither needs to be fairly big!? At first the distance set to 6 didn't upset the guide but after a short while it threw the guide out of sink which threw up some errors and it took a few minutes to settle again. 

Edited September 10, 2020 by Rustang

[ollypenrice]

What is unit when 6 is the value?  DSLRs benefit from a dither of around 12 pixels to kill colour mottle in the background. (Source Tony Hallas.) The only downside of a large dither is loss of the borders which is unlikely to be a real issue.

You can test your dither scale in pixels by aligning one sub onto the other using star alignment. Look at the borders zoomed in to pixel scale. Just count the pixel offset at the borders.

Olly

[Rustang]

2 hours ago, ollypenrice said:

What is unit when 6 is the value?  DSLRs benefit from a dither of around 12 pixels to kill colour mottle in the background. (Source Tony Hallas.) The only downside of a large dither is loss of the borders which is unlikely to be a real issue.

You can test your dither scale in pixels by aligning one sub onto the other using star alignment. Look at the borders zoomed in to pixel scale. Just count the pixel offset at the borders.

Olly

Apologies, you may have to put that in a more 'I'm not sure what you mean' way!? 😊

[bottletopburly]

Apt uses a multiplication factor so 6x your  guide camera scale on my old qhy5l worked out at 5 pixels so that would be to much at 35 pixels a setting of 2-3 in Apt should be fine for most setups with a dslr 

[Rustang]

Ok cool, il try with that and see how I get on, thanks guys 👍

[ollypenrice]

5 minutes ago, Rustang said:

Apologies, you may have to put that in a more 'I'm not sure what you mean' way!? 😊

All I meant was, 'The value is 6, but 6 what?'  Bottletopburly has offered what may be the answer in your case.

Olly

[Rustang]

1 minute ago, ollypenrice said:

All I meant was, 'The value is 6, but 6 what?'  Bottletopburly has offered what may be the answer in your case.

Olly

6 was the initial 'Dither Distance' setting 🙂

[ollypenrice]

9 minutes ago, Rustang said:

6 was the initial 'Dither Distance' setting 🙂

Yes, but if I say to you that the distance from my front door to my garden gate is six you're going to ask me 'six what?' and that's what we're trying to fathom.

Olly

[bottletopburly]

Use astronomy tools to work out the scale for your guide camera , search post you should find my post on it from a few years back all explained in that 👍

[Rustang]

1 hour ago, ollypenrice said:

Yes, but if I say to you that the distance from my front door to my garden gate is six you're going to ask me 'six what?' and that's what we're trying to fathom.

Olly

I have no idea myself lol 🤣, its just a setting that you put a number in APT under 'Dither Distance' I was just going off info Ive read online and trial and error, as I say I'm still very much learning. I should probably find out what it means !😆

Edited September 10, 2020 by Rustang

[Rustang]

1 hour ago, bottletopburly said:

Use astronomy tools to work out the scale for your guide camera , search post you should find my post on it from a few years back all explained in that 👍

Thanks mate 👍

[Rustang]

So i think I've got it, my guide camera pixel scale is 3.75/225 x 206.3 = 3.4 / that by my imaging cam set up which would be 2.1 would then = 1.6 pixel so to move 12 i would need to set PHD2 to approx 6 . So then what is the 'dither distance' in APT is the question!?

 

 

Edited September 10, 2020 by Rustang

[AKB]

11 hours ago, ollypenrice said:

if I say to you that the distance from my front door to my garden gate is six you're going to ask me 'six what?

Reminds me of a professor at university who asked us if 1% accuracy was good.   He pointed out that  if he only managed that on his journey home, he wouldn’t get to his front door...

[michael8554]

Your imaging image scale is 2.1 arcsecs/pixel ?

Then a 12 pixel dither would be a 12 x 2.1 = 25.2 arcsec move.

How many pixels  would you have to move the Guidecam to move the imaging camera 25.2 arcsecs ?

That would be 25.2 / 3.4 = 7.4 pixels. 

So the remaining question is, does an APT Dither setting of 7 to 8 mean "move the mount 7 to 8 guidecam pixels ?" 

Who knows.......

Michael