Celestron 80GTL mod for PST? Some questions.

[poita]

G'day all,

I found a source of Celestron 80mm 900mmFL scopes brand new on ebay for AUD$190 with Nextstar computerised mount(about USD$190) and bought one to use as a donor scope for the PST.

It's been a while since I have done this, so I just want a reality check. I have a DERF for the front of the scope.

From memory, I have to place the etalon 200mm inside the focus length of the scope, so in this case it would be 700mm from the back of the objective?
Is that measurement from the back of the objective to the front of the etalon?

Do I trust the Celestron focal length, or should I focus it onto a piece of paper and measure back 200mm from that focal point?

Thanks

-Peter

[iwatkins]

Hi Peter,

I've never done a PST mod so I cannot say exactly, the measurements needed. No doubt someone will chirp in, but 200mm sounds correct.

But I never trust stated FLs, especially from the big names. Easy way is to pull the objective and bench test it. My favourite way is a pair of green laser pointers strapped together but totally parallel and mark on paper the actual FL.

I had a doublet once stated as being 2100mm FL, turned out to be 2250mm. That would have messed thins up after you cut the OTA tube.

Cheers

Ian

[poita]

Thanks for the reply!
I'm not cool enough to own two lasers, so is focusing an image of the sun or clouds going to get me close enough to measure the true Focal Length?

[colinsk]

Use a point source. If you can image a star that is the best otherwise use the farthest street lamp you can image. A laser through a spatial filter with a collimating lens or mirror is a perfect point source but spatial filters are an unusual thing to have laying around.

[Merlin66]

The safest method is to measure the focal length by imaging the sun onto a piece of card.
The front edge of the knurled ring on the PST etalon must be positioned at the "majic" distance, 200mm inside prime focus.

[poita]

Thanks, I'll do that this afternoon.
So the measurement is to the frontmost piece of glass of the PST etalon? (not looking at one at the moment, so not sure where the knurled ring is).
How accurate do we have to be, within a mm?

Thanks

-Peter

[Merlin66]

Peter,
I have a write-up available on the PST Mod. Just drop me an email address for a copy.
Within +/- 3mm or so gives good results.

[poita]

Thanks, email is xbox at mudgee dot net

[swisswalter]

please for me too

walter at nanosys dot ch

:thanx: