Finally getting to PST Stage 2 Mod

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Oh, I get it now!
Oh, I get it now!
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Joined: Tue Oct 07, 2014 9:18 pm

Finally getting to PST Stage 2 Mod

Post by twhite » Sat Nov 10, 2018 11:04 pm

After several years and quite a bit of intervening life, I've finally assembled the parts to begin my Stage 2 PST mod.

My donor scope is a Celestron C102HD (102mm f/10). I have a Lunt 100mm ERF coming (thanks, Jack!). I have my donor PST (pre-Meade) and a BF-15 (also pre-Meade, with the Maier ITF). I have my eye on a non-rotating helical focuser to use to fine focus between the etalon and the BF.

I've been looking at quite a few articles over the years on doing this mod, and I've seen some conflicting information, so I'm going to ask for input, and patience, while I make sure I do this right. (Measure twice, cut once, right? :-) )

I am fortunate to have a friend who is a skilled amateur machinist as well as an astronomy buff -- we're working on several projects right now. He's not built telescopes before, but is excited to work with me on some of these projects (and I've probably created a monster).

I understand that the placement of the etalon needs to be between 220-225mm inside the point of focus, which should be no problem. I plan to conservatively cut my donor scope tube to ensure I don't cut too much. I plan to re-use the stock focuser on the C102HD for now.

My first question is around the need for the condensing lens in the mod. I've seen mods that use it, and other mods that don't. What's the consensus opinion on the need for it? If it is really needed, I understand that it needs to be placed in front of the etalon -- are there specific spacing considerations for it? Most things I've seen suggest that the Baader 1.25x Glaspath Corrector is the preferred solution. Still the same?

My second question is around the total spacing betweek the etalon and the BF. The majority of the references I've seen suggest that 140mm is the correct amount, and that seems to be consistent on mods with and without the condenser lens. My question is around how critical that spacing is. As I mentioned above, I have my eye on a helical focuser which I would use between the etalon and the BF15, and will make a custom extension to attach to the back of the etalon and to the front of the helical focuser (which has M48 threads) and then another adapter which will allow attachment of the BF15 to the focuser (again, M48 threads). The focuser has 50-90mm adjustment, so I'm thinking a tube length of 70mm will be perfect here and give me more than sufficient travel for critical adjustment. Anyone see a problem with this?

I'm sure this stuff has been covered before, but I hate to proceed without having a higher degree of confidence. I think the rest is pretty straightforward -- building the holder for the ERF, the adapter for the etalon to insert into the tube, a more substantial visual back for the stock focuser, perhaps -- but I'll certainly welcome additional comments.

Thanks all.

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Re: Finally getting to PST Stage 2 Mod

Post by Merlin66 » Sun Nov 11, 2018 2:07 am

I used a 102 f10 for one of my mods and didn’t need to cut the tube.
The correct distance for a standard PST etalon is exactly 200mm inside focus.
The back focus again in a standard PST etalon is 200mm.
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