PST mod question

[vineyard]

Hello,

I'm not sure whether this is better in the Equipment forum or not, so I thought I'd start here.

I'm slightly intrigued by the idea of doing a PST mod to allow Ha through my normal refractor.  I've come across various references to it having been done (including this useful article).  But since I don't want to use a donor scope but my existing scope (which is my baby), I wanted to see if that might be possible (b/c I'm not physically altering my baby in any way ).

My current scope is 102mm aperture, 880mm focal length (ie f8.6).  I've seen references to the donor scope needing to be f/10 (I guess the PST is geared for f/10 optical lengths) and then also seen references to people cutting about 12cm off their donor scopes.  Which would take 1000mm optical path to...880mm give or take!

So, I'd be v grateful to those with experience on this mod (or better knowledge than me about light paths) if they could shed [sun]light on whether it would work if I took the black section of the back of the PST and attached it into the drawtube of my refractor?  I'm guessing I wouldn't be able to view the full disk with just a stage 1 mod, but I don't mind tbh - I'm more interested in looking more closely at features (and I suppose I could always use a reducer if needed as I know some SGLers have done to get full disks on bigger aperture with stunning results)?

If that is possible, then (just as an aside) could the front bit of the PST that's been taken off also be used to construct a guide-scope as a separate project (albeit in a v garish gold colour...)?

I'm hoping its possible (esp as that article has photos where the author has attached the PST mods to TEC140s which I'm sure they wouldn't have chopped, unless they were oligarchs!).  If so, then what are the things I should keep in mind about getting a used PST (eg: the article says how the penta prism can move out of alignment - how do I guard against that before buying, or is that correctable?).

Thank you!  (And btw, if this is path of madness & frustration that I'm intrigued by, pls feel free to say so!).

Cheers,

Vin

[newbie alert]

Peter Drew on here can help..he's done a few .. or merlin from down under..

Not so sure that it would really help for a visual scope thou..imaging yes but I'm sure that I read somewhere the difference was minimal visually..

[Merlin66]

The standard PST etalon ideally needs an f10 input beam and needs to be positioned at the Majic -200mm inside the prime focus.

you may be lucky and find by removing the focuser from the OTA that you can achieve the -200mm.

If it works you’ll end up with a 88mm f10 system.

[Peter Drew]

As Ken (Merlin66) says.  Like Ken I have modded several Ha telescopes based on a PST.  PM me if I can be of further help.   😀

[Rusted]

Baader do a 1.125 GPC [Glass Path Corrector] for those who cannot quite manage the f/10 requirement.
It acts as a weak Barlow to stretch the focal length less than the usual 2x Barlow.
It also adds to the cost and complexity of the PST modification.

I owe all my H-alpha/PST mods to expert knowledge from Peter and Ken.

[Peter Drew]

And I got most of mine from Ken.     😀

 

 

 

[vineyard]

Thanks all 3 of you!  Can PMs be like WhatsApp group messages (ie have more than 2 people on the PM)?  If not, thank you for the offer Peter - your kindness gets you the short straw :)  I'm going to call the scope maker first just to figure out a few factual things and then there'll be a message incoming 😀

This could be the beginning of a great adventure (for me anyway!).

Cheers,

Vin

[Helen]

Yes you can include multiple people in PMs 😊