upper limit to guide pixel scale ?

[mj.w...@gmail.com]

Is there an upper limit to useable guide pixel scale ?

Is that irrespective of the main imaging camera's pixel scale ?

Comes up in the GuideLogs on the astro forums.

For example:

120mm FL guidescope, 3.75um pixels, 6.45as/p

And 120mm FL guidescope, 5.86um pixels, 10.07as/p.

Michael

Wiltshire UK

[oliv...@gmail.com]

Hi Michael

This is not only a matter of pixel size but the SNR is also a key factor 

Google « PHD, subpixel guiding, and star SNR » to find the Craig Stark article which explains the sub pixel accuracy. 

On top of that we have now the multi star guiding which further improve this sub pixel accuracy.

In real life, the main limit of parallel guiding is barely the centroid calculation but differential flexures.

Clear skies

Olivier 

Le 19 mai 2023 à 10:06, mj.w...@gmail.com <mj.w...@gmail.com> a écrit :

Is there an upper limit to useable guide pixel scale ?

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[Brian Valente]

HI Michael

are you referring to a maximum image scale within PHD?

I am not aware of any max (and certainly not a maximum of any realistic pixel scale, i'm not referring to something like 968,000  pixels/arcsec), 

The values you mentioned are not unusual

However the *effectiveness* of guiding certainly considers the relative pixel scale of guide camera compared to main imaging camera's pixel scale. If it starts to get far past 3:1 (guide to imager) that can degrade guiding effectiveness

Brian

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[mj.w...@gmail.com]

Hi guys

"I am not aware of any max"

Thanks, good to know.

 "If it (pixel scale) starts to get far past 3:1 (guide to imager) that can degrade guiding effectiveness"

So the 6.45as/p guiding with a 2as/p imaging would be okay.

What might the guiding be like if that was 1as/p ?

Michael

Wiltshire UK

[Brian Valente]

>>>What might the guiding be like if that was 1as/p ?

you'd have to try it and see. There are a lot of variables other than just the relative image scale. Also what is acceptable to the user?

The general guidance is 3:1 is safe. Beyond that it can degrade guiding, but you have to try it and see 

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