GSO Ritchey Chretien 10" Backfocus

[Lostone]

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.

[martin_h]

I have an 8" RC and didn't bother measuring back focus, I had no clue where it was for my setup I just went with the tried and tested 'try it and see' method

[inFINNity Deck]

Hi Lostone,

According to High Points Scientific the back-focus would be 233mm. On Astrograph.net it is more specifically stated as "233mm from back plate, 127mm from supplied focuser".

Nicolàs

[Xsubmariner]

My Altair Astro 250TT (GSO 10” ver2 truss ) has a back focus of 230mm from the back plate. Yours may vary slightly but it’s a good place to start.  If you are new to this type of scope, be prepared to find a lot more fiddling is needed to get it just right for your imaging train.  I have a 3” feather-touch fitted and used it with my SX36 full frame camera, so your cameras will present similar fun. Getting the guiding right with an OAG (loadstarX2) was problematic given the prism is on the edge/outside the flat field (due to sensor size), this resulted in guide stars that are elongated which compounded the usual seeing challenges. No a problem if you have an ONAG or are fortunate to have an unguided Mount, in the end I settled for using a smaller sensor with this scope.

Good luck with your scope, you will learn a lot configuring your new setup.

Martin

[Lostone]

On 27/10/2020 at 02:22, Xsubmariner said:

My Altair Astro 250TT (GSO 10” ver2 truss ) has a back focus of 230mm from the back plate. Yours may vary slightly but it’s a good place to start.  If you are new to this type of scope, be prepared to find a lot more fiddling is needed to get it just right for your imaging train.  I have a 3” feather-touch fitted and used it with my SX36 full frame camera, so your cameras will present similar fun. Getting the guiding right with an OAG (loadstarX2) was problematic given the prism is on the edge/outside the flat field (due to sensor size), this resulted in guide stars that are elongated which compounded the usual seeing challenges. No a problem if you have an ONAG or are fortunate to have an unguided Mount, in the end I settled for using a smaller sensor with this scope.

Good luck with your scope, you will learn a lot configuring your new setup.

Martin

That's sofar that's been the best help I have gotten from any of the forums.  My Moonlite 2.5 focuser just arrived today so I should be able to work on finding the focus point and if and what spacers I will need to use between the focuser and the backplane of the scope housing.  I'm hoping it won't be too much of a pain.  I know there is a learning curve with this. Especially since I've been using Refractors for so long and just now deciding to go with a reflector that has much more reach.  I'm looking forward to imaging and re-imaging some of the smaller objects out there.  I am still waiting on my Laser Collimation from starlite industries, but who knows how long that will be