To try and get more back focus, I attempted to modify the scope according to some advice from knowledgeable souls here (thanks Merlin66!).
DON'T MOD YOUR PST AT ALL IF YOU ARE NOT 100% SURE IT IS SAFE, YOU COULD SEND YOURSELF OR SOMEONE ELSE PERMANENTLY BLIND.
I thoroughly researched that the mod I was about to undertake was safe for my
particular PST. Many are different, and
this may not be safe to do on another PST.
In my case it involves removing the small extension tube before the eyepiece holder.
My particular model has the rear filter in the eyepiece holder and not in the extension tube, so it was perfectly safe to remove the empty extension, and replace the eyepiece holder to get more back focus.
I marked the position of the eyepiece holder in case the filter is aligned in some way (it probably isn't but I thought it couldn't hurt)
Sounded easy, I tried to unscrew it, but no go. I went out and purchased a 'Boa Constrictor' strap wrench
http://www.haron.com.au/images/strap_wrenches.png
as most guides say these will allow you to undo the extension tube.
Not. A. Chance.
It didn't even *think* about coming undone.
Eventually I held my breath, put two blocks of wood in the vice and the extension tube of the PST in between them and tightened the vice.
Still no go. Tried heating the joints. Nothing.
I ended up having to tighten the vice
way past where I thought was safe (i.e. as tight as my wimpy arms would allow) before I could get the thing to unscrew. When it
finally (2 hours of swearing later) came apart, I could see it was held in by red loctite, about a litre of it as far as I could tell.
Once the extension is removed it allows you to see inside to the prism and so forth inside the magic black box. I very carefully used my camera brush to remove any bits of loctite dust from the prism, and then carefully screwed just the eyepiece holder containing the filter back in.
It looks like the holder could potentially hit the prism, so I put a rubber o-ring in-between the PST body and the eyepiece holder to stop it screwing all the way in, just in case.
That is where tonight's story ends, I'll have to wait for the sun to come up tomorrow to check the results.
It really does feel odd to be impatient for the night to
end so that I can use my scope!
Thanks again to everyone that gave me advice, and if anyone it ever thinking of doing
any mod to a PST, remember, if you get it wrong, your's or someone else's eyesight could be lost, so if there is any doubt at all, just leave the thing alone.
Now to wait for sunrise...