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Ratio between guide scope & Main telescope ?

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#1 inoxtrim

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Posted 16 May 2019 - 12:47 PM

Hello everybody  i have a question about  the ratio between  a guide escope & main telescope, in my y case 

i have a celestron 9.25 & az-eq6 skywatcher , 

Could you tell me what would be the recommended guide tube?

Thanks



#2 einarin

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Posted 16 May 2019 - 01:15 PM

Maybe 60mm guide scope but more recommended is off axis guiding.


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#3 inoxtrim

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Posted 16 May 2019 - 01:30 PM

any formula to calculate ??



#4 einarin

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Posted 16 May 2019 - 01:39 PM

https://www.cloudyni...-imaging-scale/

Dealer view:
https://agenaastro.c...hotography.html


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#5 rkinnett

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Posted 16 May 2019 - 01:54 PM

Depends on the image scales of your imaging and guide cameras, which in turn depends on the pixel widths of your guide and primary imagers, as well as the focal lengths of your guide and primary scopes.  A rule of thumb is you want your guider image scale to be less than 5x your imaging resolution, although 3x or better is advised.  Off-axis guiding gets you much better ratio of guider image scale vs primary image scale, and also benefits from co-alignment, but adds the (occasional) challenge of finding guide stars.

 

Image scale is proportional to pixel size over focal length.

 

guider-image-scale <= 5x imager-image-scale

(guide-cam-pixel-width / guide-scope-focal-length)  <=5x (imager-pixel-width / primary-scope-focal-length)

 

Solving for minimum guide scope focal length:

guide-scope-focal-length >= (guide-cam-pixel-width / imager-pixel-width) * primary-focal-length / 5


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