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GSO RC 10" Backfocus Question

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#1 Lostone

Lostone

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Posted 26 October 2020 - 11:06 PM

I just picked up a GSO Ritchey Chretien 10".  While i'm waiting on a focuser and a few other things.  I tried doing some research on the back focus for one of these.  I have always been using Refractors and the Ritchey Chretien are a different beast altogether.  Anyway It came with 3 spacers, 2" & 2ea 1"  Anyway the 2 camera set's that I will be using are the QHY600 Mono with filter wheel, and a ZWO Asi6200MC with filter tray.  Normally I will be using a 55mm backfocus with these camera on my Esprit 120.  However from my understanding the back focus on an RC is much further away and I am unable to find any kind of documentation that tells me what it should be and where it is measured from on the back of the scope.

 

So on that note, is there anyone using a GSO RC 10" (Truss) than can help me out.  How many and which spacers are you using and what should be back focus be along with where you are measuring it from.

 

Any help would be greatly appreciated and would probably save me a couple of nights of trial and error.  I would rather have 1 night instead of many to get it dialed in.

 

Also, I've done a search here and have come up short.

 

Thanks



#2 luxo II

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Posted 27 October 2020 - 01:42 AM

However from my understanding the back focus on an RC is much further away and I am unable to find any kind of documentation that tells me what it should be and where it is measured from on the back of the scope.

Adjusting the mirror spacing will shift the backfocus. To reduce the backfocus you need to move the mirrors apart a few mm. This can be accomplished with either or both the secondary mirror or primary mirror cells and the push-pull bolts that adjust them. And yes, you have to recollimate after.

 

For a change of 1mm, the backfocus will shift roughly in the range 9... 20mm depending on the geometry of these scopes, so be careful.

 
When re-collimating I put blueback over one bolt and only touch the other two, that way I don't change the spacing.
 
You'll need a Glatter or Hotech collimator if you do this, to get it close to correct afterwards.

Edited by luxo II, 27 October 2020 - 01:51 AM.


#3 Terry White

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Posted 27 October 2020 - 06:08 AM

A few minutes of searching provides your answer. From here the design focal length of a 10" truss RC is 2032 mm, which results in a corresponding back focus distance of 233 mm measured from the top of the focuser attachment port to the image plane. It is well-known that RCs like the GSOs are optimally designed have their focal length (and corresponding back focus) set to one, and only one, optical design spec. This guarantees good optical performance. You can measure your focal length by either plate solving or by using a Ronchi eyepiece or camera adapter here. If you arbitrarily go changing the mirror spacing from the factory specification, you will add spherical aberration and other decollimation artifacts to your images. Short of using an expensive laser interferometer, the best collimation method is to read, understand and, most importantly, follow the DSI guide here. It goes into great detail about how lasers and collimation scopes can make your collimation worse.


Edited by Terry White, 27 October 2020 - 06:30 AM.



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