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misterp

PST Mod - Etalon lenses

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Hi,

As I understand it, jest before the etalon there is a lens with a -200mm focal length and just after one with a +200mm focal length. PSTs and their mods work at f10. Is this because the available aperture of the -200mm lens is 20mm, hence f10 (or even -f10)?

From what I have read if we change that first lens to a -120mm focal length then the pst mod would work with a f6 refractor and that lens/the etalon system should be placed 120mm in from focus (instead of the usual 200mm). Correct?

Furthermore if we change the last lens (the +200mm one) then the distance between the etalon and the eyepiece should be altered?

Thanks for your time

Ian

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I did the "stage 1" PST modification with an F5 4" refractor a few years ago.

I am not sure what you are trying to achieve here, but you don't need to be tampering with the ethalon to increase the F ratio. I made the mod possible by shortening the tube length.

There is a difference between F10 being the optimum, against the misconception that it HAS to be F10. The one i modded gave me some of the best views i have had from a solar scope.

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Thanks for your replies.

I have a 80mm f10 pst mod and it is great. However there is sweet spot where everything is fab but the rest of the fov is not so good. I think I read somewhere that with a shorter focal lenght the sweet spot effect reduces (I may be wrong).

Also I have a 10" f6.3sct which if I could get an erf filter for then that would be some scope!

Ian

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a 10" ERF would cost you somewhere in the region of £2500- £3000....

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Yes, it is possible to remove the front and rear lenses from the PST etalon assembly....the real issue is finding suitable replacements...

When you use a standard PST etalon assembly on other focal ratios (other than f10 and above) the etalon acts as an aperture stop and limits the effective ratio to f10...this means if you used a 150mm f4 (600mm focal length) donor it would only result in an effective (600/10)=60mm f10 system. A hellava waste of a 150mm objective!

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You said Jamie, that you made your mod possible by shortening the tube length. I don't understand this. What tube? I did a stage 1 mod on a 70mm/f.10 refractor. It works well, but if I could utilise the CELESTRON 100mm/f.5 at full aperture, so much the better.

How the heck did you do the mod ( assuming full aperture ) without substituting the front and rear etalon lenses, as Ken ( Merlin66 ) has mentioned?

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There's no problem modifying any short focus refractor for a PST mod, it's just as Merlin 66 says, if the donor is shorter than f10 then the 20mm etalon of the PST will stop the system down to f10. There is a commercially available kit for a 80mm f5 mod which results in a 40mm effective aperture, not much of an upgrade from the 40mm PST! :eek: . There is a Celestron 102mm f10 on AB&S at the moment which would make a good PST mod donor. :smiley:

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I have been testing out my pst etalon with different lenses today. I have a Tal 100mm f10 which I have shortened. Visually it works well with the standard etalon. I also have a 102mm Celestron GT f9.8 that has not been chopped yet. I had a theory that by changing the -200mm pcv lens to a -125mm pcv on the etalon, I could avoid chopping the tubes. It does work to a degree, but I do have issues with fov illumination and sweet spotting. I will no doubt shorten this tube as well and revert to the -200 lens again..

My other problem was that the helical focuser I chose, combined with a Lunt B1200 blocker from my LS60T was too long for imaging. So I replaced the rear +200 pcx lens with a +250 pcx lens, thus lengthening the light path and making it possible to image with a DMK41. Again this does work to a degree, but has even more issues. I am replacing the Borg Helical focuser with a shorter one so that I can use the original etalon lens as I think this will be better. It was only an experiment that was only half successful.

I am coming to the conclusion that the etalon works best with f10 ratio scopes and that lens changing may only be viable with faster scopes. I have yet to experiment with my 120mm f8.3 so the jury is still out.

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Very interesting trials.....

""I am coming to the conclusion that the etalon works best with f10 ratio scopes""

Yes, I think that is now well proven ;-)

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Yes the advice has always been that f10 ratio's work best, however I had to find out for myself if it was possible to alter the lenses to overcome my problems and get decent results. I can get my system to work, but so far the views are not perfect. Maybe the jump from -125 to +250 is too much. I may be flogging a dead horse but I will continue the tests until my new helical arrives from Hutech.

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The Etalon is very sensitive to the angle of light passing through it. The weak negative and positive lenses before and after it are designed to work with an f/10 incoming beam so that the light through the Etalon is essentially parallel. If you want to change the incoming beam to a different f/ratio then the first ( negative ) lens should also be changed so that the light remains parallel through the Etalon. But the exit will remain at f/10 due to the second lens. Changing that to achieve a faster f ratio will probably lead to poor performance as the edges of the enlarged field will have a different optical path length than the centre and the Etalon will be performing in tilted mode at the edges while the centre will be in straight through mode.

Nigel

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I'm not very knowledgable where optics are concerned nigel, but my tests with changing the negative lens to a faster f/ratio (shorter facal length) have shown a quite small sweetspot when used with a f10 scope, so its of no real benefit. Its better to shorten the tube and keep the -200 f10 lens. It may work better with a faster scope, but that will have to wait until i get one to test.

Changing the positive rear lens to a +250 focal length enabled me to image with my focuser, but again it hasn't produced an even field of illumination, and the sweetspot is still smaller than I expected. Until I change my focuser I cannot make a comparison to the original unchanged etalon.

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Hello Again,

So I have changed front collimating lens. I have replaced it with a 25mm -160mm fl lens (from Knight Optical). After a bit of faffing about SUCCESS. Using a 80mm f7 Apo, a 75mm ERF and the rest of my pst it works brilliantly. Using a 40mm Meade 4000 plossl I can get the full disk all in focus and see promenances all the way round as well as surface detail.

For anyone (like me) who could not aquire a nosepiece, I removed the front collimating lens cell, installed a m48 male to t-thread femail adaptor ring, a t-thread extension tube, a t-thread to 2" nosepiece and finally screwed the collimating lens call into that nosepiece.

So, another question. Would it be possible to use the entire pst with a refractor (with erf) and a 40mm collimating lens (so the light rays are parallel when they hit the pst objective). Why? Maybe you could get a double stacked pst (or other solar scope) and make it bigger.

Regards

Ian

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Ian,

I'm glad you've had some success!

I'd love to see a photo of your nosepiece arrangement...

Yes, you can use a collimating lens in front of the PST etc. Marcon has a similar arrangement using a SM40 ( I think now upgraded to an SM60) and a 150mm refractor!!! He uses a postive collimator lens, so the overall size of the instrument is quite large.....

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There has been a few folded scope designs which may have done this, but they were complicated designs and it was harder to align everything in the optical path.

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post-1261-0-67185300-1367507949_thumb.jp

Here is the nosepiece with the correcting lens installed. The m48 to t-thread ring in in the etalon assembly as I couldn't get it out. Upside down, sorry still learning the Ipad.

ian

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Congrats on this success. It iencourages me to plod on with my own modding project. Yours is similar to what I am planning with my LS35. By using a suitable negative lens in front of the etalon, you can collimate the beam from and speed scope. This front set-up is essentially a Galilean scope. The key issue is to keep the maximum field angle small. If you keep the magnification below about 4x, the field angle is below 1 deg and you should be OK (yours is 3.5x, in my design it is about 2.67x).

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