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GSO RC 10" Backfocus Question

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#1 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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#4 HyM

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Posted 09 July 2021 - 09:59 PM

Terry,

 

That's very helpful for my collimation efforts. Thanks! I wanted to verify what you exactly mean by "measured from the top of the focuser attachment port to the image plane".  Do you mean starting from the place that I'm measuring that's about 32mm from the back plate of the OTA in this picture: https://photos.app.g...PbHThaacJpW4HX7 all the way to the surface of the CCD imager chip in the camera? Of course that distance is slightly modified by any changes made to the tilt adjustment of the focuser tube.  Do you have a source for that detail (where the origin of the 233mm back-focus spec is) or just know by experience?

 

Thanks,

Hy



#5 Terry White

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Posted 10 July 2021 - 05:20 AM

Yes, that's the correct reference location from where the back focus distance is measured to the camera sensor. The back focus specification comes from a Highpoint Scientific GSO 10" truss RC link that I sent you in post #3. You may wish to surf around a little to find others vendors specifications for the GSO 10" truss RC. Sometimes vendors do get the numbers wrong, so it's best to verify yourself to arrive at a consensus. Also, since GSO makes all the store-brand RC's, it won't really matter if it's an Orion, Astro Tech, TPO, Bintel, Telescope Service, etc., RC. The differences are mostly cosmetic with the exception of the vendor decals and the hole patterns on the back plate of the truss RC. The optics are all made to the same optical design using CNC technology. Please note: some of these value-engineered RC's may be slightly off the design focal length and this may affect your backfocus specification, so YMMV. The DSI guide was written for premium DSI RC's and the guide assumes that you're already at the correct focal length, so it's best to check your focal length first via plate-solving before you jump into the DSI guide. This should help you converge on a collimated solution faster.


Edited by Terry White, 10 July 2021 - 11:02 AM.


#6 HyM

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Posted 10 July 2021 - 11:23 AM

Sorry to push back one more time, but I don't see the reference location specified in that link.

I'm just trying to make sure I get that right when I calibrate. How did you find out that the reference location is that attachment port, and not the back plate? 

 

Thanks.



#7 Terry White

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Posted 10 July 2021 - 12:51 PM

Hy, I checked the links and they work for me. I alway check my links before and after I post them and no one has complained until you. Here's the pasted  URL:

https://www.highpoin...telescope-10rct

The last time someone had issues with not being able to open my links, they had a malware infection that inserted extra characters into their URLs. In all the RC's I've seen, the back focus is measured from the focusing mounting collar, not the back plate.


Edited by Terry White, 10 July 2021 - 12:52 PM.


#8 HyM

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Posted 10 July 2021 - 01:23 PM

Terry, sorry if I wasn't clear. The links are fine. I can see that it says 233mm back focus. What I'm asking is, how do you know what the reference position is relative to the sensor? That is, how did you find out that 233mm starts from the screw threads of the back port that the focuser screws on to. Is that info on the same link page? I couldn't find that reference point detail.

Thanks
Hy

#9 Terry White

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Posted 10 July 2021 - 01:48 PM

That is, how did you find out that 233mm starts from the screw threads of the back port that the focuser screws on to. Is that info on the same link page?

It's my understanding that you don't measure back focus length from the end of the male threads on the focuser mounting collar, but from the end of the step on the outside diameter of the focuser mounting collar, just back from the end of the male threads. This is because any female component (focuser, adapter, etc.) you screw on will have that amount of male thread, protruding beyond the outside diameter step, incorporated into it's physical length. The component will bottom out at the step, thus making the step the logical place to measure back focus from. Back focus isn't measured from the focuser because people often replace them with better focusers anyway, so using the focuser as the reference point makes little sense. But don't take my word for it. wink.gif I suggest you call someone in tech support at your vendor that is knowledgeable about RCs and post their advice here.


Edited by Terry White, 11 July 2021 - 06:02 AM.


#10 HyM

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Posted 13 July 2021 - 11:46 PM

I contacted GSO's support page, http://www.gs-telesc...com/contact.asp and got a pretty quick response.

It showed that the design backfocus for the RC10" truss is 239.7mm between the backplate of the OTA and the focal plane.

It also showed the mirror-to-mirror distance as 463.6mm (it looks like taken at the center of the mirrors).

I'll try and collimate with those distances.


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#11 TPT

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Posted 16 July 2021 - 05:49 PM

I have a GSO RC10T. Purchased new, mid 2020.

I also found the online info about the backfocus distance.

What I found is that the info from GSO does not appear to be correct.

 

With my setup, using a Canon 7D , T adapter, and spacers and using a Batinov focus mask,  I measure 248mm from the back of the flange where the focuser mounts to the rear of the scope, to the focal plane mark on the Canon 7D.

 

I suspect that GSO makes tweaks to the design and they do not always update the documentation. I.e. the secondary mount on my unit is slightly different than what you see in their docs and pictures.

 

Hope this helps, but you've probably measured your own backfocus distance by now anyway.



#12 HyM

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Posted 20 July 2021 - 08:02 PM

Thanks @TPT,  I am definitely interested in your measurements. I guess you're saying from the backplate it's 248 + ~32mm (distance from backplate to end of focuser-mount-flange) ~280mm. That's quite different than the ~240mm spec I got.

 

My friend Paolo and I have improved the scope, but I wouldn't say it's dead-on yet. We did collimate it with the specifications I gave (that is, set the focal plane at that ~240mm back-focus distance, then moved the secondary-mirror-focuser so that the system was in focus, then collimated). I can't directly measure the backfocus, as a secondary focuser was added to it before I bought it, so backfocus is totally dependent on the position of the secondary-mirror-focuser.

 

Do you have the truss RC10 (as opposed to tube)? The diagram I have says RC10A, not RC10T. In retrospect, perhaps T means truss? However, the diagram that I have that's marked RC10A is clearly the picture of a truss.

 

Hy



#13 TPT

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Posted 04 August 2021 - 06:43 AM

I don't quite understand why anyone would add a 'secondary focuser' to an RC scope. One of the main advantages of an RC is that the optics don't move. Anyway...

Yes, TC10T is the Truss.

Some pics (I hope)...

IMG 20210802 1214136
Album: GSO Backfocus
2 images
0 comments

 


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