Massive improvements to the PST

[Poita]

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.

[h0ughy]

any pics?

[trent_julie]

I would love to see the final result, I am having great troubles with full disk resolution, so this solution would be ideal.

Trent

[iceman]

The focus on the PST has to be the worst thing about its design.

Would be interested in seeing pics too.

[sheeny]

+1 for pics (out of interest ;)).

Al.

[brian nordstrom]

;) Yes , a pic or 2 ?
Brian.

[trent_julie]

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.

Trent

[dannat]

Peter we are like donkeys in the desert who have just been offered a carrot - unfortunately un donkeys don't have arms so can't reach the carrot

[Poita]

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'll try and grab a photo at lunch time.

[Poita]

Oh, and I tried a nosepiece focal reducer, and that works well for getting a full disk image.

[casstony]

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.

[Poita]

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.

[Poita]

This is a picture of the GSO focuser attached to the PST etalon and tube, you can see the collar is still removed around the etalon.

Normally you would replace this before screwing the GSO onto the etalon, but I leave it off, as I get more adjustment that way.

[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.

More pics attached:

[Poita]

I think the PST thread is M50 x 1, if anyone knows where an adapter could be sourced, that would be great.

The GSO is a big improvement, but I'd love to hook up the moonlite electronic focuser.

[brian nordstrom]

:D 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 .:thumbsup: . All set up for the 6th june .
Good work Peter .
Brian.

[Poita]

On my PST, it feels like a totally new scope to look through, YMMV.

[seeker372011]

Very Interesting......

[brian nordstrom]

:) 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 , :eyepop: 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.

[Steffen]

Very cool! So what's the issue with the stock PST, the quality of the diagonal prism/mirror?

Cheers
Steffen.

[Poita]

That is very kind. Would it be better if I sent you the unused parts that have the thread on them? I don't want to mess it up!

[h0ughy]

I can see what you mean - there would have to be some talented soles that could make the adapter, man it would give a whole lease of new life to the poor old pst. I had two over the years, and gave them up because i couldnt image with them or get a clear view without ghosting - this virtually eliminates that

[Poita]

Absolutely zero ghosting, and very sharp. I'm really happy with it, now to find some rings...

[Wodnas]

This is my amateur attempt to mount my PST. I used a cheap scope that has GOTO and made an adaptor to hold the PST. Set the tracking for solar and away you go. Complete scope including mount was $135 plus freight. Not high tech like some here but works for me.
115743

115744

115746
Cheers Bob

[Poita]

I've had a few more questions, some quick answers:

Q: What pieces did you use on the camera/eyepiece end to hold the BF?

The easiest way to do this is to use a celestron 1.25" diagonal. The blocking filter unscrews from the 'black box' and is the same thread as the Celestron eyepiece holder part of the diagonal. Unscrew the celestron eyepiece holder and screw the Blocking Filter/Eyepiece holder from the PST onto the Celestron diagonal.

Q: Any quick tips on imaging with a QHY5 or similar mono camera? e.g. will I need a barlow?

With the standard PST (the model I had anyway) the eyepiece holder was in two pieces, one piece was an empty (small) extension tube the same size as the eyepiece holder that included the BF.
For images I had to unscrew the eyepiece holder and extension tube, remove the empty extension tube and screw the eyepiece holder straight into the PST black box.

For the modified PST with the GSO focuser, I had to add an extension tube to bring it to focus. I used a barlow with the lens removed from and it just got me there.

[Marke]

If you need an adapters or fittings rty these guys , they can make up just about anything and have great service.
http://www.preciseparts.com/ppmain/adapterf.html

[Merlin66]

There's a couple of good suppliers in Europe who now supply adaptors for the front and rear threads (M50 x 1) of the PST etalon.
http://www.aokswiss.ch/index_sonne.html
http://www.teleskop-austria.at/shop/index.php?lng=de&m=2&kod=adap-pst-508-hu
The pentaprism/ focus arrangement in the original PST is marginal. An out of alignment pentaprism can cause real headaches with astigmatism. Dump it as soon as you can!
As Peter mentioned: the eyepiece holder on the Black Box comes in two versions, both use two sections....
The section nearest the Black box can be empty (just a spacer) on the original "gold" objective models - this can easily be removed to increase the available back focus.
On the later "blue" objective PST's Coronado added a small "mini-erf" to the front of this tube to reduce the energy loading on the more sensitive blocking filter.
For imaging, this can be safely removed.....
The tube section, where the eyepiece goes, contains the small (5mm diameter) blocking filter. This is a CRITICAL component in the Ha filter system.
Hope this helps.

[trent_julie]

Hey all,
I started my Nervous PST Mod tonight, I should be able to post comparison photos tomorrow weather permitting.