For Information: PST mod on Skywatcher 120/900 ED works well

[colmic]

Hi all,

last monday i visited a friend in atlantic coast of France. We tested my PST stage 2 mod on his Skywatcher Equinox 120/900 ED, with my 135mm D-ERF.

As i use on my TOA130, i puted the 1,25x glasspath in front of the PST etalon to reach F/10. The backfocus is enough to work with the stage 2 mod, no need to cut the tube. Results was fine

No idea if the 120/900 achromat as the same backfocus, if yes this could be a very good candidate to make a 120mm PST mod.

[peter drew]

Hello Michel, I'm still intrigued by the possibility of using a glasspath Barlow in front of a PST etalon using a shorter than F10 donor telescope. What diameter glasspath are you using and how far in front of the PST etalon do you place it. I'm still trying to convince myself that there are no vignetting issues.

[colmic]

Peter, 2 monthes ago i created a specific topic about the glasspath in this forum.

Here is an image of my PST etalon dissassembled:





This setup is used with my FS60 60mm/355mm refractor, with the TOA130 I replace the 1,7x glasspath by the 1,25x one. It is placed just in front of the PST etalon front lens (about 15mm). Glasspath measures about 30mm diameter.



It works very well with my TOA130 PST mod, you can see lot of images i made with.

[swisswalter]

Hi Michel

congratulations on the mod and the results

[DSobserver]

interesting mich'

It looks that f/d10 is definitely not a strict rule....


vincent

[peter drew]

Surely it is unless the pre etalon light path is modified in some way?. If not, with the 20mm PST etalon placed at its optimum position 200mm inside the objective focal plane will vignet any objective larger than 40mm and faster than F10.

[DSobserver]

it's worst than this : if the etalon is not placed 200mm before focal, the lens just in front of the etalon (a kind of focal reducer) will not place the light parallel in the etalon and bandwidth will not be around Halpha.

But apparently it's not so critical...until which f/d???

[Merlin66]

The front barlow element on the PST etalon assembly is a 20mm -200mm lens.
Any entrance focal ratio f10 and above will work very well, focal ratio's below f10 will have the etalon act as an aperture stop and effectively give an f10 system.
The use of the Baader Glasspath barlow seems to bring the input focal ratio to around f10.

[marktownley]

In fact, you can replace both lenses in the etalon module and so adapt focal length of your refractor. And with this option you don't need to cut of the tubus either. I find it less expensive rather than buying a barlow or a corrector lens to bring the input focal ratio to around f10.

Yup, i've done exactly the same aswell so my PST works at f6.