ASIAIR Dithering Pixels

[Jim Smith]

I'm soon to try out my new Asiair mini. I have a question about dithering...

When you choose the maximum number of pixels for dithering moves, are you specifying main camera pixels or guide camera pixels?

Thanks in advance, Jim

Edited September 25, 2023 by Jim Smith

[The Admiral]

4 hours ago, Jim Smith said:

I'm soon to try out my new Asiair mini. I have a question about dithering...

When you choose the maximum number of pixels for dithering moves, are you specifying main camera pixels or guide camera pixels?

Thanks in advance, Jim

I've thought it was the guide camera pixels, but I've never been sure about this. I'll be interested to know the answer to this too.

Ian

Edited September 25, 2023 by The Admiral

[geeklee]

@Jim Smith @The Admiral Agreed, I believe its guide camera pixels and it's how I've been using it.  I found a few references to this in older posts/release notes but nothing concrete.

[Ouroboros]

I feel I ‘know’ it’s guide cam pixels from knowledge lost in the mists of time. You have to multiply by the focal length of the main scope divided by the FL of the guide scope to get the dithered distance on the main camera. That assumes both camera pixels are identical in size. If not you have to further multiply by the ratio of the guide cam pixels to main cam. 

[Jim Smith]

This from ZWO when asked the exact same question...

"Simon Lewis (ZWO)

Sep 26, 2023, 4:11 PM EDT

hi Jim 
 
thanks for the question 
 
this is the number of pixels for the main camera 
 
thanks

Simon "

It seems there are two schools of thought!

[geeklee]

11 hours ago, Jim Smith said:

It seems there are two schools of thought!

Thanks Jim.  Even reading that answer I'm not convinced - I don't know why! I'll have a rethink when I'm next out with the gear.

[Clarkey]

I don't have an ASiair, but given you need to put the camera and telescope details in the system for plate solving, there is no reason why it shouldn't be this way.

[Elp]

6 hours ago, geeklee said:

Thanks Jim.  Even reading that answer I'm not convinced - I don't know why! I'll have a rethink when I'm next out with the gear.

It doesn't make sense to me either considering the guide camera and guidescope are doing the scope adjustment and dithering. The main camera must be doing something in between because whenever my dithering and guiding has gone to pot the system knows it's off target after a few subs and reslews, centres and plate solves.

Edited September 27, 2023 by Elp

[Ouroboros]

On 27/09/2023 at 08:57, Clarkey said:

I don't have an ASiair, but given you need to put the camera and telescope details in the system for plate solving, there is no reason why it shouldn't be this way.

Yes, good point. I’ve never bothered to measure it. I set it at 5 pixels.  The maximum is 10 isn’t it? Can’t remember.  Anyway as long as I’ve see it shift a bit between subs I have accepted it as working OK.

I am also uncertain just how it important it is to dither with a CMOS camera in my case given that I use a master dark. I started using dither back when I used a DSLR to avoid having to take darks after imaging. 

[Elp]

The max distance is 30 which is what I use usually within the space of 5 total imaging minute segments. Tried 10 but still got black diagonal lines in the stack.

[Ouroboros]

I decided to have a look at some of my subs to see how many pixels they shift between subsequent shots.  

The was data obtained with a 910mm FL main scope and 176mm FL guide scope. Focal lengths determined by the ASIair. 

Anyway there's a x5.2 multiplier between the two.  The pixel sizes between the main and guide cam are identical.

I had set the dither to 5.  Presumably that's +/-5.   I'm not sure whether the 5 pixel  dither referred to is measured along the x and y axes, or whether along the hypotenuse. Anyway it doesn't matter a lot because of the large difference in focal lengths.  

I do get some odd measured dither values ie more than 5 but much less that 5 times 5. I think the maximum dither was 13.5. But most were less than 5 along the hypotenuse.  The over size ones are probably down to a shift in pointing or something. Odd because my stars were (mostly) round. 

Anyway, I think this confirms what ZWO told @Jim Smith  ie dither is main camera pixels.