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Mirror diagonal in a slow refractor?

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#1 Blueox4

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Posted Yesterday, 07:33 AM

I have a AP Maxbright mirror diagonal and I’m going to try it in one of my slower, f/9 refractors as soon as we get a nice night but I’ve always used prism diagonals in them prior. The Baader Zeiss prism 2” diagonal and the APM prism diagonal are two that I have. Is there a reason that we use mirror diagonals mostly in faster telescopes and prism diagonals mostly in slower telescopes? 



#2 Couder

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Posted Yesterday, 08:36 AM

I have F15 and F16 refractors and use both kinds on them. 


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#3 Eddgie

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Posted Yesterday, 01:09 PM

I have a AP Maxbright mirror diagonal and I’m going to try it in one of my slower, f/9 refractors as soon as we get a nice night but I’ve always used prism diagonals in them prior. The Baader Zeiss prism 2” diagonal and the APM prism diagonal are two that I have. Is there a reason that we use mirror diagonals mostly in faster telescopes and prism diagonals mostly in slower telescopes? 

You can use any of them you like.  There is no reason why you would not want to use a diagonal you already have, mirror or prism, in an f/9 telescope.  


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#4 KBHornblower

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Posted Yesterday, 01:54 PM

A prism can introduce some chromatic aberration in "fast" (low f ratio) scope, which a mirror diagonal prevents.  In a slower scope the CA pretty much goes away, and a good prism may be a bit brighter, depending on the coatings.  With modern coatings on mirrors there may be little or no difference.


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#5 Scott99

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Posted Yesterday, 03:25 PM

The 4-inch light path through the glass of a 2" prism diagonal introduces some aberrations that are minimized at longer focal ratio - there's also 1% light loss per inch of glass, so 4% total, that's why the mirror diagonal was named "Maxbright" for 99% transmission.  

 

I like prism diagonals however and I"m currently testing out the high-end Baader T2 amici diagonal.  I like the way prisms seal the tube and keep dirt out better than mirror IME.  The smaller Baader T2 diagonals and the Tak 1.25" prism have shorter light path than the big 2-inchers.


Edited by Scott99, Yesterday, 03:45 PM.

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#6 Cotts

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Posted Yesterday, 04:07 PM

I wonder, not for the first time either, if a sort of "reverse newtonian" would be a good idea for a refractor.  Instead of sticking a 90º diagonal in a traditional refractor focuser why not have a Newtonian-style diagonal and then a newtonian-style focuser outside the tube...  

 

In use the telescope would look very much like the 'backwards' Newtonians we sometimes see in adverts and tv shows...  Might be a laugh at star parties....  "Hey, buddy, i think you have that thing backwards!"  "I dunno, have a look...."

 

Obviously this would be mostly for visual instruments....  Wonder if there would be a cost saving....

 

Dave



#7 junomike

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Posted Yesterday, 06:04 PM

Along with aberrations introduced in a fast Refractor by Prism Diagonal, I found I preferred my Maxbright over the Prism for the increase in transmission for both Planets and DSO's (which is IMO noticeable at the eyepiece).




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