4 replies to this topic

[hypergolic]

Good morning all:

 

One of the things that should be included on new RCs (GSOs, etc.) are collimating flanges for the focuser/mirror assembly. Of course, this is just wishful thinking on my part. That being said, try finding one new in CONUS. However, I did find some available at Telescope Service in Germany and placed an order. It would seem that Telescope Service is highly regarded and this is my first transaction with them.

 

Also, one of my questions is the extreme backfocus required. Then I got to thinking....to get shorter back focus, the secondary would have to curved more strongly or be larger, increasing the already large central obstruction. Am I on the right track for understanding how this optical design works?

 

Fleep

Edited by hypergolic, 27 January 2021 - 09:48 AM.

[Terry White]

No, you aren't on the right track. See the threads here and here, and here. If you dig into all the references in those threads you will see that:

1) The optical axes of the primary and secondary control the optical performance of your RC, and RC's have very strong optical axes compared to the other types of cassegrain telescopes.

2) Don't expect geometric-center-based collimation solutions like collimating lasers, collimation telescopes, centering fixtures, etc., to be able to 100% collimate your RC. Geometric axes are not optical axes. Geometry-based collimation solutions have the potential to make your RC collimation worse!

3) Don't arbitrarily shorten your backfocus, as this will add spherical aberration and other decollimation artifacts to your RC's optical performance.

4) A long backfocus for AP is actually an advantage, not a problem, especially if you ever need to add other things to your imaging train, such as focal reducers, field flatteners, filter wheels, OAGs, camera tilt adjusters, etc.

5) Deepening the mirror curvature to shorten backfocus with introduce new unwanted aberrations, since these aberrations are already perfectly corrected, to first order, in the basic RC design.

I hope this helps.

Edited by Terry White, 28 January 2021 - 04:23 PM.

[hypergolic]

Thank you for replying. My biggest problem right now is the weather!

 

PAN

[Terry White]

Thank you for replying. My biggest problem right now is the weather!

 

PAN

Clear skies!

[RDBeck]

I am early on my journey with my RC8. 

 

If I have done the math right, the 250mm (254mm?) backfocus is a positive.  My planned layout is the 50mm extension, the focuser (I upgraded to a Moonlite), a short M48 extension (~20mm sticking out of the 2" receiver), a Celestron OAG, my filter wheel and my camera.  Everything in the train will be of greater diameter than what I have on my refractor.

 

My present equipment issue is waiting on the M48 extension tubes.