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Which would be the most appropriate guide scope?

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#1 keverett

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Posted 07 April 2024 - 10:57 AM

I am using an Askar 103 APO with a 0.8x reducer/flattener (real FL ~560mm) for my main scope and was wondering how I should decide which guide scope to use with it for AP.  I have a Askar 32mm scope and an Astromania 70mm Guide scope.  I assume at least one of these would be appropriate for use with the Main Scope.  I've read that you should have a guide scope that is at least 1/10th the FL of your main scope.  Then I've read that 1/5th is the target.  And, these ratios may depend on the type of scope you're using as a main scope.  So, is there a set of dependable rules to follow in selecting a guide scope?  How do you select a guide scope?  Is it really an esoteric endeavor, or is it pretty general and flexible  as to how you go about deciding what guide scope to choose?  Thanks in advance for any assistance here. 

 

73

keith


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#2 idclimber

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Posted 07 April 2024 - 11:11 AM

Either guide scope will work if you have them rigidly mounted to the main scope. If you change to a OAG the ratio goes to 1:1 depending on the guide/main camera pixel sizes. 


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#3 keverett

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Posted 07 April 2024 - 01:41 PM

Is what I've heard lately correct in that now the OAG is considered to be a better guiding system than the additional scope? 


Edited by keverett, 07 April 2024 - 01:41 PM.

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#4 idclimber

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Posted 07 April 2024 - 02:13 PM

Is what I've heard lately correct in that now the OAG is considered to be a better guiding system than the additional scope? 

No, not better. They are necessary on longer focal lengths especially SCT's. 

 

I do use an OAG on all three of my scopes. The primary reason is so I can move the entire camera package from scope to scope with less work. The only scope it is needed on is my 12" LX200 and arguably my 6" refractor. 


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#5 rgsalinger

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Posted 07 April 2024 - 02:26 PM

I prefer to use an OAG on my refractors but I think that you can get the same guiding RMS with a guide scope. What I like about the OAG is its compact nature. We may differ but I keep my refractors all set up and ready to image. (I drive carefully and most of the time the scopes are in the observatory anyway.) That makes a nice compact imaging train and makes setting up trivial. I also don't have any worries about flexure. Still, I suspect that there's not overall performance advantage until you get to 1 meter of focal length or have any sort of moving mirror solution. 


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#6 Robert7980

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Posted 07 April 2024 - 02:49 PM

Is what I've heard lately correct in that now the OAG is considered to be a better guiding system than the additional scope? 

It depends on what’s meant by better. They are lighter and simpler, usually far more rigid than a separate scope and you are probably getting more premium optics at no additional cost. There’s a lot of pluses, but  they are harder to setup properly and can have limited star feilds, they also have the same f-ratio as the optics they are mounted to, so might not be as fast as a separate scope. another downside is when rotating the camera the OAG also usually rotates requiring a calibration, offset, or another setup change to communicate with the rotator… 
 

Any of them will work for your needs. A 32mm scope should do it, 560mm isn’t challenging to guide. The 70mm would be a better choice and an OAG would be nice in my opinion as long as the reducer is compatible with one, it’s overkill though. Nothing wrong with a little overkill in book… 
 

1/10th is probably a good rule of thumb for the minimum,  I’d get it closer than that, it’s not critical, but it needs to be reasonable. More importantly than just focal length is how fast the scope is. I’d go with one that’s stop or two faster than one with 100mm more focal length as long as they are both in the ballpark. That would be my thinking for going with a larger one. Once you’re well over a thousand mm things get a bit different and OAGs start to make more sense for a lot of reasons. 
 

I don’t have any hard evidence, mostly because I don’t care enough, but I consider an OAG to be superior for guiding I just have fewer hassles and more consistent results with one. It might just be imagination though. There’s not enough “performance” difference to worry about it when either is working properly. 




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