I want to make a Nasmyth Cassegrain. Maybe later maybe never :)

[kbrown]

Greetings,

Ever since I was about ten (several decades ago) I have been fascinated by the prospect of making my own telescope from scratch including the optics after seeing a book about it in the local library. I've read and watched numerous videos on the subject and have a fairly good understanding on what is required to complete such a project. Most of the articles I've seen are concentrating on making a Newtonian telescope but since I already have one I'm not sure if I'd want to make another one. Recently I've been reading about Cassegrain telescopes and especially the Nasmyth variation that doesn't involve a hole in the primary mirror.

Here's the somewhat crazy idea I have in my mind at the moment which I'm not sure if it's worth pursuing. Feel free to shoot it down if it doesn't make sense since I lack the experience... To get my feet wet I was thinking cutting some corners and start by making the hyperbolic secondary and borrow the parabolic primary and maybe even the secondary from my existing newt to get it all working. Once I'm happy with the secondary I could then have a go at making a duplicate of the newt primary. Does this make any sense?

The primary I have is from a SkyWatcher 250PX i.e. 10" / 1200mm. My aim is to have a long focal length cassegrain for mainly planetary imaging. So maybe an effective focal length of 6000mm (5x) or so. I do realise this would still be a fairly sizeable scope but most likely nothing my NEQ6 Pro could not handle. I entered the numbers into the CassDesign.exe program I found by Mike I. Jones. I'm not sure what the units for the Unvignetted Field Width is so I left it at its default. Likewise I'm not sure how much of Back Working Distance I'd need. Other than that do these numbers make any sense?

 

Edited October 5, 2020 by kbrown

[Merlin66]

I was lucky enough to spent a few years with Jimmy Barker while working in South Africa, he was a great guy.

The scope he used was a Cassegrain with a third mirror to make a  Nasmyth arrangement. He used a “hollow” Dec shaft to push the focused beam and the focuser was mounted on the end of the fork mount.

I have some images...

I did some observing with it, it worked very well. Not sure how it would perform for photography.

Well worth considering.

[Peter Drew]

An interesting project.  I built a 18" Cassegrain/Newtonian with a Nasmyth tertiary arrangement. I was a telescope designer and fabricator but not an optical worker.  The optics were purchased and were of the relatively simpler Dall-Kirkham configuration.  If you have the optical skill to make the Cassegrain secondary, then using the other optics that you already have is a good plan.     🙂 

[Rusted]

I suppose it makes sense to work on a small surface first rather than a very large one.
No doubt there is far more information online now.
Than there ever was in the local reference library of 50+ years ago.
Do I vaguely remember mention of figuring the Cass convex secondary in ATM Vols 1-3?
Ring star, pitch polishing tools seem to pop into my false memory module noddle.
That was after first achieving a nice convex sphere of course. 
They'd use a concave spherical test plate [perfectly matching glass surface] to monitor progress.
Such a small surface really demands a simple polishing machine.
The strokes are so short it would feel like you were polishing a spectacle lens.
Have a look for YT videos on optical fabrication in these small sizes for more ideas.

 

[Astrobits]

I think you will have to have a longer "back working distance" than that in your Cass calculation as it has to accommodate the radius of the tube plus the working distance outside the tube for focuser and camera.

Nigel

[Peter Drew]

Yes, the tube radius is a consideration but I think this "extra" will be somewhat reduced by the amount that the third mirror is placed before the primary mirror.          🙂

[skybadger]

I built one using my existing cassegrain. It was really handy to put the pole withe the tertiary mirror through the primary mirror perforation. The focuser was mounted on a plate suspended across the frame poles. I think the reason I did it was to cater for an extended focal length as I moved the mirror spacing around to find the optimal spacing. 

[Merlin66]

Many years ago I made a secondary for a Cassegrain conversion of an 12" f5 scope.

I was using the Gaviola test method (very straight forward) for testing.

https://www.osapublishing.org/josa/abstract.cfm?uri=josa-29-11-480

(It's also re-printed in Allan Mackintosh's "Advanced Telescope Making Techniques", Vol 1, p63)

 

[skybadger]

Ghats good, I never understood that method , I needed to measure my secondary, which is why I built a bath interferometer but I havent got round to using it in anger yet. 

[Chriske]

On 05/10/2020 at 14:14, Astrobits said:

I think you will have to have a longer "back working distance" than that in your Cass calculation as it has to accommodate the radius of the tube plus the working distance outside the tube for focuser and camera.

Nigel

Indeed it as bit short.
But there is an easy fix.
I did a simulation and ended up with exactly the same optical(raytrace) result.
 

Lower the ROC  of the SecMir. to 595mm
Correct the CC of that mirror to -2.14
You'll end up with a distance of about 330mm instead of 250mm back focal length.
But : The telescope has a different end value now, it has grown now to f/25.5, so   F=6395mm.
This is just an example, you can play with these two figures as needed.

[kbrown]

On 07/10/2020 at 11:43, Chriske said:

Indeed it as bit short.
But there is an easy fix.
I did a simulation and ended up with exactly the same optical(raytrace) result.
 

Lower the ROC  of the SecMir. to 595mm
Correct the CC of that mirror to -2.14
You'll end up with a distance of about 330mm instead of 250mm back focal length.
But : The telescope has a different end value now, it has grown now to f/25.5, so   F=6395mm.
This is just an example, you can play with these two figures as needed.

I entered your new focal length and back focus values to CassDesign and it gave me pretty much the same ROC and CC as you mentioned. Curious what did you use to do the simulations with?

Also I'd appreciate if anyone is able to explain how to read the raytracing graph shown below? I don't know whether this is a good result or not? All I know is that having a longer focal length (higher f-ratio) is beneficial due to the inherent coma and field curvature issues in these scopes. Surely F/25+ should be alright for planetary work?

Here's a magnified view of the above raytrace:

[Chriske]

To do my raytrace 'stuff' I always use 'Spotplotter'. A very small but powerful thing.
I could use 'Oslo' or other PRO-software, but it is over the top for just two surfaces, and you end up with the same result.

A few screenshots.

                       0.5° field - The circle in the centre is the Airydisk                                                                                                         0.12543°(your value)

 

 

[Gina]

Goodness me, you telescope builders are very clever!!  I would have liked to build my own but never got further than looking at it and joining the M-O-M group and learning a bit about grinding mirrors.  I realised though that it's a very long-winded job and wondered if I'd have the patience let alone the skill.  I very much admire you all.

Edited October 13, 2020 by Gina

[kbrown]

17 minutes ago, Gina said:

Goodness me, you telescope builders are very clever!!  I would have liked to build my own but never got further than looking at it and joining the M-O-M group and learning a bit about grinding mirrors.  I realised though that it's a very long-winded job and wondered if I'd have the patience let alone the skill.  I very much admire you all.

This is all just a day dream for me at the moment. I first have to arrange a place to do anything like this in i.e. reinstate a garage in our back garden which in itself will be a bit of a project and probably not going to happen until spring/summer 2021 earliest. Anyway. I'm not in any sort of rush with this as I can happily continue using what I already have and do a bit of learning and planning on this as a slow back burner... Got a lot of useful feedback from you guys already so thank you for that