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Herschel Wedge imaging focus issues

solar refractor imaging accessories filters
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#1 nashtok

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Posted 06 August 2020 - 01:29 AM

Greetings!

 

I have an APM branded 2" Herschel wedge (Same as Altair and Meade wedges as far as I can tell) that I've been using for visual solar observation for a few years with my AT115EDT refractor.  A few days ago I decided to try some imaging through it at prime focus with a micro 4/3 camera, however I very quickly realized that even with the focus racked in as far as possible, I couldn't quite achieve focus.  The adapter I use to connect my camera up to the 2" receiver on the wedge is pretty low profile, only adding 9.75mm to the backfocus distance so unless there is anything even thinner I don't think there's much I can do from there.

 

Has anyone run into this same issue/does anyone have any suggestions for any way I can get focus with this setup?  I'm hoping I'm just being a bit dull and am looking over something obvious, but all I can think of is modifying the telescope/focuser which I'd really rather not do just for this...



#2 FlankerOneTwo

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Posted 06 August 2020 - 02:07 AM

If you don't have enough inward focuser travel you will need either an Optical Corrector Assembly (OCA) or something like the Allan Gee Mark II. Harry Siebert makes a 1x OCA that should work, and is very helpful with questions. These are frequently used to bring out the focus point for use with a binoviewer


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#3 sg6

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Posted 06 August 2020 - 02:09 AM

Seems to be becoming "common". Focusers fitted just handle a 2" diagonal from my experience and that at times is with an eyepiece that needs less inward travel then your situation.

 

I doubt there is a great deal you can do. I gained a small extra inward by haveing the 2"-1.25" adaptor skimmed by 2.5mm. Guess that is not an option or actually not relevant. If you have one in the wedge then find a cheap or old one and ask a machine shop to take a small amount off the lower edge of the flange so it sits down a little more. Don't do it to a good one.

 

Think after my experience I will get a WO refractor. Have their ZS61 and the focuser travel one that is "scary". Kind of huge. Word of advice on that one is do not look down the front and wind the focuser in. It literally stops just before it contacts the glass. But you have the inward travel. You also have shattered nerves.

 

Personally I doubt there is a simple answer for you.



#4 nashtok

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Posted 06 August 2020 - 02:46 AM

If you don't have enough inward focuser travel you will need either an Optical Corrector Assembly (OCA) or something like the Allan Gee Mark II. Harry Siebert makes a 1x OCA that should work, and is very helpful with questions. These are frequently used to bring out the focus point for use with a binoviewer

Thank you for pointing me in that direction, an OCA looks like it would solve my issue handily - Now I just gotta decide if it's worth dropping the cash for something like that...  Maybe I'll stick with some baader film for a bit!



#5 FlankerOneTwo

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Posted 06 August 2020 - 03:41 AM

Thank you for pointing me in that direction, an OCA looks like it would solve my issue handily - Now I just gotta decide if it's worth dropping the cash for something like that...  Maybe I'll stick with some baader film for a bit!

Actually the Alan Gee Mark II is for SCTs. Discussing with Harry Siebert would be the way to go.


Edited by FlankerOneTwo, 06 August 2020 - 03:44 AM.



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