I was having trouble focusing my PST for photography, and decided that the prism focus assembly in 'the black box' was the main culprit.
I unscrewed the Etalon from the black box and removed the blocking filter/eyepiece holder from the back of the black box.
I took the GSO SCT crayford focuser, and found that the threads were pretty close to the threads on the PST etalon, not exact, but close enough.
I threaded the etalon onto the SCT crayford, and used some adapters to put the blocking filter and eyepiece holder at the other end of the crayford focuser. I ended up having to add about 40mm of 1.25" tube extensions to the eyepiece end of the crayford.
The difference was astounding, razor sharp images compared to the stock PST when looking through the eyepiece, and it was *much* easier to focus for the CCD camera with the 10:1 crayford.
The only problem is you need to use tube rings or a clamshell to mount the PST onto the tripod. I solved the problem temporarily by feeding the PST tube into the finder-scope rings on my 105mm refractor to do the testing, but a more solid solution will be required!
But for a 1/2 hour's work, it was the best DIY mod I have ever done. The views are considerably better and the focusing is much more refined.
The focus on the PST has to be the worst thing about its design.
Would be interested in seeing pics too.
I agree, I have had massive troubles with mine, most of the images I am getting seem to suffer astigmatism so if such a simple solution exists, I'm all in.
Sorry guys, I squeezed that post out just before heading to oncology at Randwick POWH, it is a four hour trip each way for me, so I was in radio silence for the last 24 hours!
I'm home again now so I'll take some pictures and post them today.
A couple of caveats.
1) The threads are slightly different. The GSO focuser fits, but unfortunately none of my other SCT focusers do (I would have loved to use the electric ones). Also there is a small amount of slop in the threads, but some plumbers thread tape seems to take care of it.
2) You suddenly have no way to mount your PST, if anyone can point me to some suitable rings or some other idea, I'd really appreciate it, I can leave it mounted in the finder-scope!
3) The back filter happened to have the exact screw thread of my Celestron diagonal eyepiece holder, but nothing else seems to have this thread. This is annoying as the diagonal isn't the best. What I ended up doing was removing the BF from the eyepice holder (amazingly it pushes out) and putting it inside an old barlow with the barlow lenses removed. This works well and allows me to position the BF for maximum CCD coverage, as the little 6mm is annoyingly small.
Other than that, it really is a revelation, with the astigmatism gone, the PST is a much better performer. I've seen PSTs that have no astig with the stock focuser, so your mileage may vary. It is also worth opening up your black box (hex bolts on the the size panel) and see if your prism is dirty/crooked/loose, just adjusting it can give better results too.
I think the astigmatism in PST's comes from the focusing prism drooping down over time such that the light is not entering and exiting perpendicular to the prism surfaces. You can remedy this by removing the side cover and repositioning the prism, and avoid reccurrence by storing the PST on it's side.
That is correct, some of the astig comes from the droopy prism, and it can be improved by adjustment.
It is quite a sloppy mechanism though.
The only real problem with using the GSO focuser is getting the tube to unscrew from the black box in the first place, it is doused in threadlok, the red variety, so you need two Boa Constrictors to be able to unscrew it and it takes an obscene amount of force to do so.
It is best to remove the rubber etalon adjustment collar first (it just peels off) and remove the locking screw in the collar underneath before attempting this, otherwise you can shear off the locking screw.
Thanks Peter , that looks so cool , well done mate .
Is it that much of an improvement ? as the GSO focusers are only $99 at Andrews com at the moment , what a steal . . All set up for the 6th june .
Good work Peter .
Brian.
Quote:
Originally Posted by Poita
If someone would make a simple PST thread to SCT thread adapter, they would have a real winner on their hands.
All thanks for these experiments really have to go to Merlin and his wonderful PST help over the years.
Peter If you can get me the dimentions and drawings for both I will have a go at machining up an adaptor , it will have to be after mid june before I get a go at this , as we , up here in Darwin are full on doing a major shut down at the local DLNG plant , yours wil be the first free adaptor .
Just need measurements .
Jpeg is alright .
Brian.
).
Thanks Peter , that looks so cool , well done mate . 
. All set up for the 6th june .
Peter If you can get me the dimentions and drawings for both I will have a go at machining up an adaptor , it will have to be after mid june before I get a go at this , as we , 
up here in Darwin are full on doing a major shut down at the local DLNG plant , yours wil be the first free adaptor . 
