Objective/ etalon separation

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JohnM
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Objective/ etalon separation

Post by JohnM »

My play with trying to utilize my old PST DS etalon yesterday ended up a bust, so I'm back working on the 102mm f10 which is stopped down with a 82mm red #25 as an ERF.
When I measure to the forward edge of the rubber band on the tuning ring should I measure from the front, center, or rear of the OG to get the most desirable separation?


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Re: Objective/ etalon separation

Post by Allcart »

If you're trying to find the -200mm position for the etalon John, measure 200mm forward from the focal point to the front of the rubber tuning ring. If you already have the measurements then I guess from the centre of the OG is best.


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Re: Objective/ etalon separation

Post by Merlin66 »

What Allan says.....
IMHO I would find the exact focus by focusing an image of the sun on a piece of card and measuring back to the rear edge of the OTA.....


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Re: Objective/ etalon separation

Post by JohnM »

There's a close relationship between those 2 distances.
This afternoon I changed OG/etalon spacing to read off the center of the OG, that required a matching change in focuser travel.

Depending on weather I might make a change from the rear of the OG, but I expect the same final results.
Seeing has just been atrocious so it's tough to do much.
And I expect I'm going to be faced with what I consider poor focus and sharpness till I can finally get a DERF and replace that red glass I'm using now.


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Re: Objective/ etalon separation

Post by Merlin66 »

John,
When you measure (re-measure?) the focal point and position the front of the PST knurled ring at -200mm, I'm sure (based on over 100 successful mods, so far) that you'll be cooking with gas.....


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Re: Objective/ etalon separation

Post by JohnM »

Oh yeah, been there. That paper you put together years ago is still my main source of info when I'm trying to figure what gives.
This is a bad time to be doing this stuff.
Low Sun, short days, and constant storms take away all the fun.


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Re: Objective/ etalon separation

Post by swisswalter »

This is a bad time to be doing this stuff.
Low Sun, short days, and constant storms take away all the fun.

Hi John

I'm very much with you


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