Classical Cassegrain Conundrum

[yuklop]

Morning All,

I am hoping experts on here have some good ideas on a little challenge with a classical cassegrain telescope.

The telescope itself came out of a Skylark rocket, so its a piece of history that would be great to get into use. The story is that I tried to figure out where the focal plane was before building anything, and I thought I found it quite far out the back of the tube. Of course I was expecting to find focus somewhere behind the primary. Turns out, I was quite wrong, and now the mirrors are recoated, I managed to find the focus by taping a an ASI224 to a metal ruler and inserting it inside the tube. It comes to focus very nicely 10 inches inside the tube, way in front of the primary. A diagram is attached. This is a little inconvenient for amateur use. It also means I can't use a traditional focuser, or indeed an eyepiece.

What I need to do is bring the focus about 20inches back, so it falls outside the back of the tube, like a normal cassegrain. The only real idea I have is to move the secondary closer to the primary. Is it as simple as moving the secondary 10 inches forward, pushes the focus 20 inches back, as you cut out 10 inches on the forward and return light path? What other impact would moving the secondary closer to the primary have? I guess it would increase the focal ratio? Before I go trying to engineer this, do folks think it would work?
Any other good ideas on how to achieve moving the focal plane back?

Any help gratefully received. Many Thanks,

Dan

Edited August 11, 2022 by yuklop
spelling

[Stu]

Certainly a very interesting scope and project. Others will know far more than me, but the trouble with moving the secondary that much closer to the primary is that the light path is wider closer to the primary so will likely be larger than the secondary ie you will be losing light. Not sure what the answer is, tricky one.

[CraigT82]

Calling @Peter Drew

[SthBohemia]

51 minutes ago, yuklop said:

Any help gratefully received.

Is it a true Cass or a Dall? Maybe remove the mirror and test (if possible) if basically spherical or parabaloid, that's just for the interest sake... Then-

http://bobmay.astronomy.net/CassNotes/index.htm  and workout the curve and secondary theoretical size of the secondary etc. If the secondary is of sufficient size in relation to the primary move it forward. How big is the hole through the primary? A few dimensions might help a bit 🙂 Is there a possibility that the original design incorporated a 3rd mirror, being a flat diagonal to shoot the focus through the OTA's side? I have seen that configuration (somewhere or other)....

PS.. Like your mount, did you fabricate it?

[yuklop]

Thanks for the response.

More details on the telescope here:

But I haven't posted all the dimensions. The hole in the primary is huge. I'd estimate about 6 inches, which is a slightly odd feature. The secondary seems oversized too, at about 90mm. I think it'll all be about experimentation and trying to move the secondary forward.

Thanks for the mount comment. It is an amazing mount, and another slightly risky auction site gamble. Turns out it is a late Rob Miller prototype from end of the 80's. More details on that one here if you're interested:

 

[SthBohemia]

21 minutes ago, yuklop said:

dimensions.

Eeek, what a BIG Cass! 150mm central hole in primary! That sounds odd!

The secondary diameter is not huge in relation to a 14" primary,  I am making a 200mm true cass (will finish it one day, 99% both mirrors figured, only started it 20-25 years ago-sheer laziness) and its secondary is 60mm.. but primary central hole only 37mm...

The whole affair, both your OTA and Scope,  are magnificent pieces of engineering 🙂  Lovely secondary holder, hmmm and all else!!

Some thoughts on what I would do (if not such a lazy .........)

Grind another secondary mirror for a Gregorian focus, utilising the full length of the tube... Greg secondaries are not hard to grind/polish.

Purchase (or make) a flat for a Newtonian focus, due to the huge primary hole it could be rectangular!

Move the Cass secondary forward. If you have a foucault tester, test the cass optics via a ronci screen when it is in its correct position.

[andrew s]

One option would be to use a transfer lens to reimagine the focal plane out side of the body of the scope.  Orion Optics do this with their ODK range.

Regards Andrew 

[yuklop]

16 minutes ago, andrew s said:

One option would be to use a transfer lens to reimagine the focal plane out side of the body of the scope.  Orion Optics do this with their ODK range.

Regards Andrew 

I get the idea I think. Would it be two equal lenses placed back to back with the extension amount between them? One lens converts the converging rays to parallel, and the second reverts them to converging... if that makes sense. Can you buy such things... I imagine they'd need to be pretty customised to the cone angle of the light from the secondary?

I think, from the post above with the link to the Cassegrain notes, and similar pages, only a relatively small shift in the secondary will have a fairly substantial effect in the position of the focal plane. 

Gregorian focus is an interesting idea. I have always thought I'd never grind a mirror (I make myself enough work with odd telescopes and mounts to engineer). Perhaps I can find one already ground and silvered to buy and test. 

Newt configuration would also work fairly well although I'd have to deface that nice titanium tube to make that happen, and I usually try and keep my shonky amateur engineering to a minimum. I can move the secondary forward easily enough with a spacer though. Ideally I'd make it adjustable if possible to find the best spot. Might be able to manage that with a single threaded hole. 

[Astrobits]

I was thinking that the optics might be incomplete and should have a transfer lens as Andrew suggested. Alternatively, there might have been plans for a very large field sensor at the focus you have identified.

As regards the design of a Cass, the secondary is not just reflecting the cone of light but is also amplyfing the focal length by a certain factor (  like a barlow ). Thus moving the secondary by 10" will move the focal point 10 x the amplification factor. Most Cass designs have an amplification factor of around 4x or 5x so you might not need to move the secondary more than 2"-3" However, the design of Cass optics ties the separation of the mirrors and their curves together so it might not perform quite as well as the original specs. As you don't have the option of using it as is you will just have to try something and see what you get.

Nigel

Edited August 11, 2022 by Astrobits

[yuklop]

This thread suggests that the spacing causes an effect of the, secondary magnification squared, on the focal plane. 

https://www.cloudynights.com/topic/73676-classical-cassegrain-spacing-question/

As I think my secondary has a magnification of 4.6 based on the design docs here:

https://articles.adsabs.harvard.edu//full/1971IAUS...41..304B/0000305.000.html

I would only need to move it an inch, to get 20 inches of back focus shift. On the design the focal plane is just at the very back of the primary. I do wonder if a half inch secondary spacer went missing somewhere in its long life. The design also shows the huge hole in the primary. I will see if I can add a simple spacer and let you know how I get on.

Thanks for all the support.

[Astrobits]

I once ( nearly 50 years ago now!) purchased a |12" Cas set but used it as a Newtonian by making a large flat. As far as I was concerned and those who also used it it performed admirably. 12" was pretty big for those days so I don't know how well it would have stood up to today's critical eyes🧐

Nigel

[SthBohemia]

@yuklop I assume from your engineering you have  a 2"+ micrometer, thus grab the secondary and measure the edge and the centre of it (protect the coating with a fag paper (me) or something else). Or maybe a spherometer, bah anything... 🙂  I assume it has a flat back....

Let's get an idea of the focal length of both primary (which you know) and secondary (unknown) and begin from there. If for nothing else than for interest sake...

That enormous hole in the primary was obviously for something.. Maybe  photographic gear was installed from the rear of the scope...??? Something BIG must have been present...

""I'd have to deface that nice titanium tube "" 😞 OK, leave the 'I am a butcher' tactics to me , waves a 9.25" angle grinder around 🙂 

A gregorian secondary would be hard to find from the commercial suppliers. I have come across amateurs that have finished 6" newts in 2 days (maniacs). T'would not take you long to make a secondary.

Maybe someone within SGL would be happy to grind a custom mirror, I'm old and lazy as stated previously 😞 

[yuklop]

You know, I think the massive hole is a weight saving exercise. Since the secondary is 90mm, and with the metal around, it would shadow that area of the primary, so why not cut it out. The weight saving is important for space flight!

I'll measure the secondary when I get home this evening.

 

[SthBohemia]

4 minutes ago, yuklop said:

The weight saving is important for space flight!

🙂 ohhh, me, being a little 'thick' between the ears didn't think of that! 😞 

[yuklop]

The secondary is 90mm diameter and measures 17.5mm at the edge and 19.7mm in the middle.

The secondary holder is comprised of two parts screwed together, one bracket part that fixes the assembly to the main tube, and a second part that holds the mirror. It will be really pretty easy to put a spacer in between these. With the holes that are already present I might even be able to fit three screws with springs over, so it is collimatable.

The screws are some imperial thread that is very close but a bit smaller than M3. I may just retap these to M3 so I can use screws I have. Happy that it looks fairly achievable.

[inFINNity Deck]

5 hours ago, Astrobits said:

Most Cass designs have an amplification factor of around 4x or 5x so you might not need to move the secondary more than 2"-3" However, the design of Cass optics ties the separation of the mirrors and their curves together so it might not perform quite as well as the original specs. As you don't have the option of using it as is you will just have to try something and see what you get.

I am not sure about the Cassegrains, but have been fiddling with a few RCs (8" and 10") and found the focal length to change with about 10-11 times the change in inter-mirror distance. Best is to change the distance and then test the scope using a Ronchi-test. Gerd Neuman sells these great photographic Ronchi filters, which can easily be mounted to a camera (with lens!) to test the inter-mirror distance. I have written an article on RC-adjustments. It is in Dutch, but opening it with Chrome should result in a reasonable translation (scroll down to step 3 for the Ronchi-test): https://www.starry-night.nl/stap-voor-stap-collimatie-van-een-rc/

I have to say that the Ronchi test on a RC is not terribly sensitive, but then, if the test says it is okay, the scope should perform fine.

Nicolàs

[Mandy D]

47 minutes ago, yuklop said:

The screws are some imperial thread that is very close but a bit smaller than M3. I may just retap these to M3 so I can use screws I have. Happy that it looks fairly achievable.

If those screws are slightly smaller than M3 and given the age of this telescope, I suspect they would be 6 BA, which are 2.75 mm in diameter. You should be able to obtain some from a model engineering supplier. It seems a shame to change parts for metric equivalents on this projject.

[Peter Drew]

The change in final focal plane in a Cassegrain  system is approximately the distance changed between the primary and secondary times the amplification factor squared so the final focal plane changes rapidly in a non linear fashion.  Based on this, it should not be necessary to alter the secondary spacing towards the primary much to provide an accessible focal plane position.  The optical dimensions suggest that the original purpose involved some form of photography, possibly infra-red which might reduce its benefits for normal photographic/visual purposes.  Adjusting the inter-mirror distance would certainly be the best way forward.    🙂

[yuklop]

12 hours ago, Mandy D said:

It seems a shame to change parts for metric equivalents on this projject.

I agree with that. Actually, they are slightly smaller than M4, not M3 as I said earlier. So I think they are probably 4BA screws. There are some on ebay at 13/4 in length that should do the job.

Edited August 12, 2022 by yuklop

[yuklop]

Just to close this thread out.... over the last weekend, I modified the secondary holder to allow primary - secondary spacing adjustment, and be collimatable. I had to drill two extra small holes to mount a bracket on the back of the holder, but these would be completely hidden if the scope was returned to original condition. I also had to tap an M5 thread into 3 existing holes to allow the collimation screws to fit. The collimation screws have pointed tips and engage with existing holes in the secondary holder. The centre screw is held by a top nut and has a spring to tension. The image attached makes it look a bit scruffier than it really is!

As I completed this work, the clouds turned up, and I couldn't test it. Last night looked possible at about 19:30, so I got it all set up and the clouds came again. By 21:00 I was all but ready to strike the evening off as a failure. I went inside, came out again 15 mins later and a large hole in the cloud revealed Arcturus, a really good bright test star. And it all worked beautifully. After many hours faffing around... I had it centred and focussed inside a few minutes. I had to lower the secondary just slightly to allow focus. The gap to the primary has been shortened by 27mm, and this allows focus using a 2'' diagonal in the big focuser I installed. Very happy indeed with the results, and now I know it works, I might refine and neaten it all a bit. Next job, is to replace the cork gaskets in the mount that haven't liked the weight of the thing... over to PTFE I think. The job list doesn't end!

Thanks very much for all the support and ideas.

Dan

[inFINNity Deck]

Good to hear you achieved focus!

Just curious: the inter-mirror distance was shortened by 27mm. Roughly how much did that change the focus position?

Are you going to test the current set-up with a ronchi-test?

Nicolàs

[johnturley]

On 11/08/2022 at 13:02, yuklop said:

Thanks for the response.

More details on the telescope here:

But I haven't posted all the dimensions. The hole in the primary is huge. I'd estimate about 6 inches, which is a slightly odd feature. The secondary seems oversized too, at about 90mm. I think it'll all be about experimentation and trying to move the secondary forward.

Thanks for the mount comment. It is an amazing mount, and another slightly risky auction site gamble. Turns out it is a late Rob Miller prototype from end of the 80's. More details on that one here if you're interested:

 

The base of the mount looks a bit like a Rob Miller, Astro Systems (Luton) design, similar to that on my fork mounted 14in Newtonian.

John 

[yuklop]

Yes, indeed it is a Rob Miller mount. It doesn't have clutches as such, rather a gasket between the metal surfaces that allows the turning. When I got it had milk bottle plastic as the gasket, which probably works quite well, but I replaced that with cork. The cork has been fine until now, but doesn't seem to have enjoyed the pressures of the added weight of this scope, and has squeezed itself out one side of the gap! I think I'll swap the cork out for PTFE. Only question is... will I do a single 1mm thick PTFE sheet between the surfaces, or two 0.5mm thick sheets. Not really sure which option would work best. Probably both would work just fine.

The focus position changed a lot! I think about 20 inches. Physics has been my friend this time. For the few minutes I had at the eyepiece last night all looked well with a star. I probably won't go to the lengths of Ronchi testing... unless something looks wrong. First tests I do will probably be looking at Saturn and seeing how happy I am with it! Then I might take a photo of a starfield and see how flat (or not) the frame is. I also have some hope that a Meade 0.63x focal reducer flattener would work OK at this FL, so I'll test that too. By then I'll have a decent idea of how much the spacing change has messed with the correction. 

 

[inFINNity Deck]

Ok, so here the ratio is roughly 1:20, so 1" change in mirror-distance changes the focal length by 20". Looking forward to your first images!

Nicolàs

Edited August 17, 2022 by inFINNity Deck

[andrew s]

You might find this helpful. Regards Andrew 

PS see section 3 on the impact of moving the secondary on the optical correction of the system 

Edited August 17, 2022 by andrew s