Need help understanding the magic 200

[Allcart]

Forgive my lack of understanding, but I am way behind you guys and trying to catch up. Please correct me if I am wrong.
My understand so far is that the light cone on a f10 system is intercepted by the pst collimating lens, which is a 25mm -200mm plano concave followed by a 25mm +200mm plano convex lens to refocus the light to the EP. The -200mm lens has to be placed 200mm inside the F10 light cone.

The lenses can be changed to a different set for use on different f ratio systems, eg. 25mm -150 and a 25mm +150 and the system will still work. Assuming I changed the collimating lens to a 25mm -150 then this should be placed 150mm inside the light cone, or a -100mm would be placed 100mm inside the light cone. These figures are just examples so that I might better understand how things work.

Is the place where the lens intercepts the light cone governed by the focal length of the lens, or is it governed by the lens diameter. Visualising a simple light cone, would a 25mm dia lens be placed where the light cone is 25mm wide?

[marktownley]

It is a 25mm diameter lens, but only 20mm diameter is actually used, hence 20mm @ -200mm fl = f10.

The key thing is matching up the focal ratio of the negative focal length collimating lens with the focal ratio of the objective of the donor OTA. They need to be the same.

So, yes, you put the negative collimating lens at whatever its focal length is inside the focal point of the donor OTA.

The refocussing lens does not have to be the same focal length, or ratio, as the collimating lens / donor OTA objective, it can be any focal length. 200mm is just a convenient length. I've tried with refocusing lenses of different focal lengths and they all work just aswell.

[Allcart]

Cheers Mark. I remember your post about buying a cheap 50mm lens for testing your theories. That lens looked like it was an objective lens from a bino or small scope. Was that lens a concave lens to give the parallel beam through the etalon or was it an achromat.

[marktownley]

For my 70mm f6 mod it was a 40mm -120mm fl negative achromat from Edmunds Optics. The beauty of this is that it is the same size as glass used in 2" barlow lenses, and as such by using one of these had a ready made holder When I stopped it down from 40mm to 20mm as it still retains the -120mm fl it became an f6 lens which matched the 70mm f6 donor scope perfectly.

There are quite a few 152mm f6 scopes out there that may be a project for me later in the year :whistle:

[Allcart]

Thats the way I am leaning too Mark. I don't want to spend time and money on a mod that does not perform at its best, so I need to learn this optical stuff first.
I have most of the components and will decide on the donor when I have the info I need to make a decent job of it.

So am I right in thinking your -120mm lens was placed 120mm into the light cone.

[marktownley]

So am I right in thinking your -120mm lens was placed 120mm into the light cone.

Yup, that's right!

[swisswalter]

There are quite a few 152mm f6 scopes out there that may be a project for me later in the year :whistle:

Hi Mark

I'm very sure that that has been approved by Mrs Townley

[marktownley]

Sssshhhhh! :whistle:

[swisswalter]

well I'm like a grave :whistle:


if paid the right way

[colinsk]

The key thing is matching up the focal ratio of the negative focal length collimating lens with the focal ratio of the objective of the donor OTA. They need to be the same.

I have noticed you guys have better luck from this approach and it is how the PST was designed but I have not sorted out why it works this way. I am sure I understand why the distance between the objective and the first lens needs to be fl1+fl2 but why the focal ratios need to be the same less clear. Is it to minimize aberrations?

[marktownley]

The PST etalon has a diameter of 20mm and so this acts as a field stop in the optical train. If the focal ratio of the collimating lens is greater than the focal ratio of the donor scope objective then (assuming the collimating lens is positioned correctly) there will be vignetting in the system and will be running effectively at less aperture. If the focal ratio of the donor scope objective is greater than the focal ratio of the collimating lens then (assuming the collimating lens is positioned correctly) the full 20mm diameter of the etalon will not be utilized. Both scenarios will 'work' but don't provide as good an image as is possible if both f-ratios are matching.

[colinsk]

Ah, that makes sense. Thank you.

[Merlin66]

Colin,
Yes, In the PST etalon assembly, the front barlow and the rear positive lens are both 200mm focal length.
This is not mandatory....we found in the Lunt60 mod that the front element is a different focal length to the rear element....

[Merlin66]

IMHO it's much, much, easier to use a f10 (or greater) donor....
The issue is finding a barlow (negative lens) which will allow the -200mm positioning. Working in monochromatic light, the rest is of less improtant.

[Allcart]

I thought the purpose of replacing the front negative lens was to change the magic -200mm to an arbitrary measurement. By choosing a negative lens with a shorter focal length we might avoid cutting the tubes of expensive scopes.

I may be off base here, but it seems to me that the purpose of the negative lens is to diverge the light cone through the etalon. Then the positive lens refocuses the light cone to the EP.

I understand that the original PST was designed for a F10 light cone, but by changing the lenses at the etalon, we should be able to match the light cone for any scope, providing the proper ratio lenses can be found.

Please correct me if I'm wrong, because I am very new to all this optics stuff and am only using my own simple logic.

[Merlin66]

Allan,
No, you're 100% correct. As long as the replacement front barlow provides a collimated beam to the etalon it will work. A f6 objective would need an -f6 barlow etc

[DavidG]

Your missing some very important points, those being the diameter of the light cone and the diameter of Etalon. The Etalon needs to be placed were it's diameter matches that of the light cone. If not your are stopping down the aperture of the telescope.
In a PST the negative lens as a focal length of -200mm and the 40mm objective has a focal length of 400mm so the negative lens is placed 200mm from the objective. Simple ratios shows that the diameter of the light cone is 20mm in diameter. The lens is 25mm in diameter and Etalon is also larger then this so the neither the negative lens or the Etalon is stopping down the system and one has the full resolution of the 40mm objective.
Now lets look at a 70mm f/6 system which has a 420mm focal length. If one uses a 40mm negative lens with -120mm focal length it will be 300mm from the objective. At this point the diameter of the light cone is 50mm in diameter and so both it and the Etalon are stopping down the system and you no longer have the resolution of 70mm system.
You also have to look at the size of the solar image formed and the field angle of that image. The longer the focal length the larger the solar image and larger the off axis field angle can be.
The PST works well because of the size of the solar image is small, just under 4mm so the field angle at the edge of the solar image is also small.
It is simply not as easy has making the f-ratio of the negative lens match that of the objective.

- Dave

[Allcart]

Can this be worked out by drawing the light cone at full size on paper and just measuring it, or is it not that simple but needs to be worked out with a formula.

[DavidG]

You can raytrace some of design on a piece of paper but be sure to give the focal plane a diameter and not draw the light coming to a point. One also needs to understand that the negative and positive lenses used in the relay system have aberations like spherical and coma. The Sun is 1/2 degree in diameter which is big so you want a well corrected field of view that as least as large as the Sun's diameter. Why a PST works so well is the fact that the diameter of the solar image it produces is small and the lenses and Etalon are large compared to it so they are not stopping down teh system and not adding any addition aberration to the image. For example, I believe the negative lenses used in a PST is a menicus in shape vs a plano concave. The reason for the meniscus is that it was most likely designed to aplanatic ie no coma and no spherical at 656 nm.
The best way to do this is to setup the optical design up in a raytracing program like OSLO EDU and check everything out. There was a fair amount of optical engineering that went into the PST design and other solar telescopes and one needs to really understand why things were done the way they were.

- Dave

[DavidG]

Opps made a mistake on the 70mm f/6 example, 40mm lens with -120 mm focal length will work since the diameter at of the light cone at 300mm is 20mm in diameter and the Etalon is 25mm is diameter. But you still need to check the off-axis aberrations of the lenses you use and also that the diameter of light cones is same size or smaller then the Etalon.

- Dave

[marktownley]

Opps made a mistake on the 70mm f/6 example, 40mm lens with -120 mm focal length will work since the diameter at of the light cone at 300mm is 20mm in diameter and the Etalon is 25mm is diameter. But you still need to check the off-axis aberrations of the lenses you use and also that the diameter of light cones is same size or smaller then the Etalon.

- Dave

Indeed it works, I have done this The etalon is 25mm in diameter from edge to edge, but in its housing presents a 20mm aperture. F6 is a nice f-ratio to work at with the PST mods as there are plenty of easily affordable and workbale alternatives to swap the front collimating lens to...

[Allcart]

Thanks Dave, I'll check out the raytracing programme.
Some more food for thought.

[Allcart]

I could almost believe you about the meniscus lens Dave, except that when I removed mine it deffinately looks like a plano concave. yet nowhere online can I find a -200mm plano concave with a 25mm dia. I suppose Meade could have had them made especially for the pst. Even anything between -100 and -200 is impossible to find at 25mm dia.
I have found a perfect meniscus lens, but not sure its right for the job. More research needed.

[Allcart]

Thanks for the link Langleif, that may be very useful.