Info on folded old mod needed

[Allcart]

While reading all the old mod topics I came across this photo of the Macongun experiment by SwissWalter and mark Townley.



Some questions.
What lens was used in front of the Lunt60.
Can this be done with any larger scope as the primary.
Does this mod work without altering or shortening the primary scope.

It looks to be a fairly simple and effective way of increasing aperture without actually making any real modification to either scope.

I would be interested in knowing if Mark or Walter perfected the setup.

Attached files

[highfnum]

What a wild and original idea
Does it work?

[Allcart]

it appeared to have great potential. Swisswalter posted some preliminary pics as a taste of things to come, but I can't find any others. I don't know if mark had any success with his pst version.

Its not an original idea, more of a variation on a theme, but I like it a lot. It lets you have the eye piece at a more body friendly position for viewing.

I'm trying to work out what happens to the light path as it travels through the system.

[marktownley]

I think this design is impractical really unless you have an observatory grade mount and an optical test bed to mount all the components properly - like Rogerio has done.

In terms of optics you would need - the lens that collimates the beam from the main scope is a positive achromat, this has to have the same focal ratio as the main scopes objective, so say in the case of Walters scope above it is 150mm f8, you would need to have say a 50mm f8 as the collimating lens. This 50mm f8 needs to sit 400mm (50x8) behind the focal point of the 150mm scope - hence all the diagonals and tube extensions to fold the light path back. The problem I found was keeping all the angles at 90 degrees etc. Sounds simple in principle? well the problem is all those diagonals and extenions introduce sag at various points in the optical train so that the beam that enters the Lunt is not perpendicular in the optical axis. Then there's differential flexure in the optical train - huge long tubes and extensions have a tendency to be springy with respect to each other - this makes the light path with respect to the optics bounce about. This is why Rogerios is an optical test bed to keep everything square and steady - done like this it works, but the design walter and myself pursued I think is just a dead end. It all looks like it will work when it is shown in a pic like Walters above, but don't have it pointing at Polaris and point it in a normal part of the sky the sun is in and it doesn't.

A PST is by far the best mod route to follow.

[swisswalter]

Hi Allan

thank you for digging out the old pic. Yes as Mark wrote, it first seems to be quite easy but then the proper alignment with this home work version is almost impossible. I gave up, not because of that, but I got a Lunt 100 and sold the LUNT 60. Now I'm modding around with filters

[Allcart]

Wow, that sad news mark. It really looked like it would work. I understand the bits about flexture and lining everything up correctly. That does present problems.

I don't know much about the optics side of things. can you explain why the lens has to match the focal ratio of the primary objective. I would have thought it had to match the secondary objective of the Lunt60, or have I got it wrong. Has the objective from the Lunt been removed?

I could understand all this better if someone could draw the optical path to show what happens to it.

[highfnum]

but if optical train was correct what is gain?

[Allcart]

Thanks for the reply Walter, maybe its back to the drawing board for me then. Its a shame, I really liked this idea.

[marktownley]

Wow, that sad news mark. It really looked like it would work. I understand the bits about flexture and lining everything up correctly. That does present problems.

I don't know much about the optics side of things. can you explain why the lens has to match the focal ratio of the primary objective. I would have thought it had to match the secondary objective of the Lunt60, or have I got it wrong. Has the objective from the Lunt been removed?

I could understand all this better if someone could draw the optical path to show what happens to it.




The lenses have to have same focal ratio so as to produce a parallel or collimated beam to enter the front of the Lunt. Actually a f109 beam is the ideal but this is considerably harder to engineer. The objective from the Lunt remains in place to bring the collimated beam back to a focus...

[marktownley]

So, just to add, lens C has to have the same f ratio as lens A. The optical group that makes up D f-ratio is unimportant.

[Allcart]

So looking at the diagram, the light passes through the first prism and is focused at the mirror or second prism. The light cone then diverges again, so the distance to lens 'C' will be critical, and everything must be perfectly aligned.

Ok, not such a simple mod after all.

Maybe I should be thinking about modding my 100RS with the pst.

[marktownley]

Yep, Lens C has to sit at exactly its focal length behind the focal point of Lens A. Angles of dangle are everything in this type of setup.

Modding the 100RS with a PST is easy peasey in comparison...

[colinsk]

The lenses have to have same focal ratio so as to produce a parallel or collimated beam to enter the front of the Lunt. Actually a f109 beam is the ideal but this is considerably harder to engineer. The objective from the Lunt remains in place to bring the collimated beam back to a focus...

f/109 makes sense to me but I have not ever read a reference to it. Do you have one?

[marktownley]

[quote]

The lenses have to have same focal ratio so as to produce a parallel or collimated beam to enter the front of the Lunt. Actually a f109 beam is the ideal but this is considerably harder to engineer. The objective from the Lunt remains in place to bring the collimated beam back to a focus...

f/109 makes sense to me but I have not ever read a reference to it. Do you have one?

http://luntsolarsystems.com/blog/intern ... erformance

[shindog]

Ok, I have to ask -- why f109? I realize the relative size of Sun to Earth is 1/109, but that can't be it, can it?

[Spectral Joe]

Actually, they're not saying F/109 is ideal, but that an internal etalon in an F/109 beam is no worse off than a front mounted one.

[shindog]

Spectral Joe,

Understood. But what I *don't* understand is the basis premise -- that sunlight is f/109. Where does that come from?

I asked the same question at the Lunt blog site marktownley mentioned above as the source - to colinsk's request for a reference - but as that entry is a few years old, I'm not sure if it's active. I see my message is still not posted on the site, as "Your comment is awaiting moderation"

Thanks for being patient - it's all so new to me...

[marktownley]

It is the distance from the Earth to the Sun (150,000,000km) divided by the size of the Sun (1400000km). It is saying the light coming from the sun is not entirely parallel (as it has an angular diameter and is not a point source) but equiavalent to a light cone coming from a telescope that is f109.

[shindog]

Thanks Mark. That's more like it. THAT makes perfect sense. I should have looked up those figure myself. mea culpa

I was looking at the wrong ratio (which just happens to be the same!?!) and couldn't make the connection.

The more I learn, the more I realize I don't know.

Now back to lurking mode.

[swisswalter]

Hi Mark

you surely meant mea culpa, mea culpa, mea maxima culpa ;-). I'm happy you asked the question , and I'm happy Mark knew the answer

[shindog]

How apropos today

[Merlin66]

We are always learning.....
I've been in this hobby for 50 years and still practising....I'll get it right someday ;-)