Small (50-60mm) PST Mod

[nightfisher]

Just a thought, how would an 80mm f7.5 work in this set up, only saying as i bought one of these TS 80mm and have found it a rather good scope

[Moonshane]

It would work fine Jules assuming enough in focus (most scopes need to be shortened unless you change the etalon lenses). It would though clip the aperture at 60mm again unless you change the etalon lenses as the pst is set up for f10 or slower systems where the etalon Is placed 200mm Inside the focal point of the objective.

[Scooot]

I've just bought the 1.7 extender for the 60mm tak which takes it to F10 when I want to use it. It wouldn't be a cheap option but you now know it exists  it just screws onto the objective lens and is incredibly light. So the scope is now either f5.9 or f10 which makes it quite a versatile little thing.

[Peter Drew]

I think it still needs to be a native F10, something to do with field angles I seem to recall.

[Scooot]

18 minutes ago, Peter Drew said:

I think it still needs to be a native F10, something to do with field angles I seem to recall.

I don't understand enough about their workings but for information, looking at the extender, there is one lens at the narrow end and nothing elsewhere. In other words it's a big spacer adding length to the scope with a lens at one end. The lens screws into the main scope and the blank end screws onto the focusser end. Ie it's inserted between the focusser and tube.

[bomberbaz]

All sounds very complicated, I need to study this subject much more before I start opening Mr wallet up

 

[Merlin66]

Richard,

The standard lenses in the PST etalon assembly require two things to allow a mod to function properly:

1. An optical system providing a beam of f10 (or greater). If a mod is attempted with a faster system, the etalon will act as a aperture stop and maintain an f10 outcome i.e. an 80/560 telescope (f7) would become a 56/560 (f10).

2. The front barlow lens in the PST assembly must be positioned at -200mm before the prime focus. It's almost impossible to achieve this position when using an other barlow in the OTA to extend the donor focal length/ ratio.

The optimum results seen to be with an f10 donor telescope.

Hope this helps.