Focusing a PST

[iceman (Mike)]

How does a PST actually focus?

Does it move some internal elements or something? The eyepiece didn't appear to move when I turned the focus knob.

Has anyone set up any mechanical focus control with a PST focus knob? I don't really want to go back to bumping the scope when I try and focus..

[AndrewJ]

Gday Mike

I made up a motorised focusser for doing remote fine tweaking.
The large gear on the knob by itself makes a big difference
for manual tweaking.

Andrew

[sheeny (Al)]

I haven't gone to a remote focus device.

Initially I found focus to be fiddle and difficult, and while it is still fiddly I don't think its as difficult as I first experienced. You get used to it.

Al.

[casstony]

A prism is moved internally for focusing:

http://www.tavcso.hu/egyeb/meade/pst_s.php

Misalignment of the prism over time can cause astigmatism, but it can be realigned by removing the side cover.

[iceman (Mike)]

Very interesting! Thanks guys.

A bigger knob will definitely be the first option. I wonder if I can somehow adapt the Orion focus thingy I've got on my 12" to fit the PST. It just fits over the existing knob..

Cheers

[casstony]

When I owned a pst I always stored it on it's side to reduce the time gravity is pushing down on the prism, hopefully avoiding a sagging prism and consequent astigmatism.

[Paul Haese]

Mike I do all my solar imaging with manual focus. The image does not move that much if you have the PST only on the mount.

Just to confirm the focus works internally.

[Merlin66 (Ken)]

The pentaprism focus design is one of the weaknesses of the PST, it's only glued to the adjusting bar and can move out of position both in the X and Y axis. This gives astigmatism....
Also note on the later models ( blue objectives) that the extension piece below the eyepiece attachment will contain an additional UV-IR filter element. This is in addition to the BF5 blocking filter in the eyepiece section.
I've also found that the recent PST's appear to be "flooded" with Loctite on all the threaded joints ie gold tube to etalon, etalon to body and the eyepiece spacers etc. This makes it almost impossible to strip them for modification to a 80mm ERF arrangement without damage.... ( maybe Meade have caught up with me and figured using the PST etalon to build a great 80mm Ha solar telescope is to competative to their expensive models!!!)