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In-focus difference Mirror Diagonal vs Prism

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#1 Dennis53121

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Posted 08 January 2013 - 11:00 PM

I have an older refractor with a 11.4 focal ratio. It has an older prism diagonal and I just purchased a new dielectric mirror diagonal for it. When trying the new mirror diagonal some of my eyepieces (9 & 7mm Ortho's) will not focus due to a lack of sufficient inward travel on the focuser. The profiles of the two diagonals are nearly the same. I did not realize at the time I purchased the new mirror diagonal that mirror diagonals have a longer effective light path. The refractor has a Vixen 36.4mm to 31.7mm (1.25 inch) adapter which is about an inch long on the focuser. If I were to find a shorter length adapter, say 1/2 inch long with the 36.4mm to 31.7mm it would move the diagonal closer to the focuser and should allow the eyepieces to focus but will this really work? If so does anyone know where I can get a 36.4mm to 31.7mm adapter that is only about 1/2 inch long? This is the long and short of my problem :foreheadslap:

#2 tomcody

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Posted 09 January 2013 - 02:29 AM

http://www.sciencece...org/threads.htm
Check out Borg Hutech part #7314
I think this is what you are talking about?
Also the light path difference is probably about 12 mm.
Rex

#3 Jon Isaacs

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Posted 09 January 2013 - 05:44 AM

I have an older refractor with a 11.4 focal ratio. It has an older prism diagonal and I just purchased a new dielectric mirror diagonal for it. When trying the new mirror diagonal some of my eyepieces (9 & 7mm Ortho's) will not focus due to a lack of sufficient inward travel on the focuser. The profiles of the two diagonals are nearly the same. I did not realize at the time I purchased the new mirror diagonal that mirror diagonals have a longer effective light path. The refractor has a Vixen 36.4mm to 31.7mm (1.25 inch) adapter which is about an inch long on the focuser. If I were to find a shorter length adapter, say 1/2 inch long with the 36.4mm to 31.7mm it would move the diagonal closer to the focuser and should allow the eyepieces to focus but will this really work? If so does anyone know where I can get a 36.4mm to 31.7mm adapter that is only about 1/2 inch long? This is the long and short of my problem :foreheadslap:


Dennis:

First let me say hello and welcome to Cloudy Nights. :waytogo:

It looks to me like Tom found an adapter for you that should work.

I am wondering why you decided to replace the prism diagonal a dielectric diagonal? The primary advantage of a dielectric diagonal is that the reflective surface is much harder than standard aluminum coatings so that it can be cleaned without concern for damaging the surface. But prisms are also very hard, probably harder and can be cleaned with the normal caution required for a lens rather than the extreme are necessary for a mirror.

Jon

#4 Dennis53121

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Posted 09 January 2013 - 02:29 PM

Thanks Tom, I will check it out.

#5 Dennis53121

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Posted 09 January 2013 - 02:41 PM

Hi Jon,

I had just purchased the telescope (29 year old C80 made by Vixen) and thought it best to get a new diagonal for it so I ordered a WO dielectric 1.25 while I was waiting for the scope to arrive. When I got the scope it had the original prism diagonal but it was extremely filthy. When I tested the scope however I discovered some of my Ortho eypieces would not come to focus with the new diagonal. When I tried the prism diagonal they did and this is when I realized the difference in the effective focal length prism vs mirror. I have since gone ahead and cleaned the old diagonal figuring I had nothing to lose anyway. Turns out it cleaned up really well and I hope to test it out tonight. I may have to turn around and sell the new William Optics dielectric or I may keep it and use it with my 8" SCT as I never seem to use 2" eyepieces in that one anyway. Oh well, live and learn.

Thanks again,
Dennis

#6 Jon Isaacs

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Posted 09 January 2013 - 10:26 PM

Turns out it cleaned up really well and I hope to test it out tonight.



Dennis:

That has been my experience with prism diagonals. There is not really much that can go wrong with them and the surfaces are hard and easily cleaned.

Jon






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