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What is best, guiding at 120mm or 240mm?

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#1 marvyyk

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Posted 06 August 2022 - 09:38 AM

Hi All!

 

For guiding, I use a ZWO ASI 224mc paired with a William Optics Uniguide 32/120mm... 

 

My guiding is usually around 0,8" (with my CEM26 and Zenithstar 73)

 

I have a spared barlow 2x, do you think that I can improve my guiding performance if I used it? So my guiding FL will be 240mm instead of 120mm...

 

I will try it anyway next time just to see if it make a plus!

 

Cheers



#2 daveco2

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Posted 06 August 2022 - 10:18 AM

With the 120mm guide scope, the ratio of pixel scales (field of view) for the guide/imaging is 3.45.  For the 240mm guide scope, it would be 1.7.

 

For good guiding, the ratio of pixel scales should be less than or around 5.  Doesn't look like it'll make any difference to your guiding going to go to longer focal length, within the tolerance of such things as the seeing conditions,

ability of the guiding software to determine the guide star centroid and FWHM (variable night-to-night), and tripod and mount stability with temperature and breezes.


Edited by daveco2, 06 August 2022 - 10:26 AM.

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#3 james7ca

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Posted 06 August 2022 - 10:26 AM

It will probably depend upon how securely you can mount and attach the guide scope and the barlow to each other and then those two as a unit to your main scope. Plus, when you use a 2X barlow you're going to be cutting your guide star signal by a factor of 4X (2^2) which will affect the signal-to-noise of the guide system. And, with a 2X barlow you're only going to have one quarter of the field that you had without the barlow and that will mean fewer guide stars from which to choose.

 

That said, if you are already guiding at a scale that is no more than 5X that of your main imaging scope and camera then you are probably okay with your current setup. An ASI224MC on a 120mm focal length guide scope will produce an image scale of 6.45 arc seconds per pixel which means you can probably use that without issues when imaging at 6.45 / 5 ≈ 1.3 arc seconds per pixel and greater.

 

I use a 120mm f/4 guide scope on my 0.8X-reduced Stellarvue SV80ST2 scope but I've paired that guide scope with an ASI290MM camera which has 2.9um pixels and thus my guiding scale is 4.98 arc seconds per pixel which would allow me to guide an imaging system that produced a 1 arc second per pixel image scale. So, I'm good to go and I often guide down below 0.5 arc seconds total RMS error using PhD on a A-P Mach1GTO mount. In this case, however, part of my success is having a good mount and a really solid mounting point for the guide scope. And, if the seeing isn't very good I can't stay below 0.5 arc second RMS and in that case a longer guide scope wouldn't help anyway.


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#4 marvyyk

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Posted 06 August 2022 - 11:45 AM

Thanks Daveco2 & James7ca for your excellent explanations!

I will stay with my actual guiding system then waytogo.gif




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