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LX200 8" SCT - Suggested Guide Scope?

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#1 quid_non

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Posted 17 February 2019 - 06:37 PM

Getting into AP - been trying to determine what is a cost effective guide scope to consider.  Any help /comments welcome!

Thanks



#2 mclewis1

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Posted 17 February 2019 - 07:07 PM

None. Particularly with an older SCT I would be looking at an off axis guider (OAG) instead of a separate guide scope. The OAG will help reduce the effects of the moving mirror in the SCT.



#3 quid_non

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Posted 19 February 2019 - 08:39 AM

Thanks Mark!

Just wondering would this still be an issue assuming I use the mirror lock on my Lx200 scope?

Any comments on a "starter" camera to consider?

Thanks!



#4 cfosterstars

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Posted 19 February 2019 - 06:04 PM

Since you state that your SCT has mirror lock, I assume that this is this an ACF OTA? I have a 10" ACF OTA with mirror lock. I have had no issues with using a guide scope even at F/10. What mount are you using? Is the OTA still on the Meade fork mount? I an on a de-forked HDX110 GEM mount and can get nice stars even with 10 minute sub frames using a guide scope. On my old LX200 fork mount, it was MUCH worse.

 

I am just starting to try using an OAG for the first time after 5 years without one. I am trying it out not because I feel that I need to, but mainly to see if its significantly better. 

 

I have used many different guide scopes. What is critical is not the scope so much as the pixel scale of your guide scope/guide camera combination. Many start with 50mm guide scope with a large pixel guide camera like a starshoot auto guider at >8 micron pixels. That leads to a huge pixel scale for your guider. This is an issue since a large pixel scale will stave the algorithm with your stars only on 1 pixel. What you need is to be less than the seeing at least and that needs to be at about 1-2"/pixel for your guider.

 

For my rig, I have a SV80 finder - 80mm aperture with a 300mm focal length and I add a 2X barlow lens to increase the FL to 600mm. I use and ASI290MM-mini guide camera and that leads to a 1" per pixel guider scale. My SCT uses an ASI1600MM-pro with a Focal reducer so the pixel scale is 0.35"/pixel. That is really not that improtant since the seeing limits the resolution. I am sigificantly oversampled. It would make sense for me to do 2X2 binning, but with a CMOS camera, you cant to hardware binning. So it make no real sense to use binning on that camera. 

 

A OAG will give you a much finer guider scale but that wont really make much difference since it is now way below the seeing. The major thing you avoid with an OAG is differential flexure. That you cant eliminate with a guider scope, but you can get it low enough - as with my rig - to not make any real difference.




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