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BillP

Diagonals (Prism vs Dielectric vs Aluminum vs Silver)

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Just completed this comparison (mainly planetary) of 12 diagonals -- made possible by the generous equipment loans from Cloudy Nights members!  Will get it posted on their site as soon as they stabilize their software (they are going though some changes now).  Enjoy :)

Mirror-Prism-Dielectric Diagonal Comparison v2014-03-06 Final v2.pdf

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Thanks Bill,just skimmed the comparison,I will give it the full read in the morning.I think I'm going to be surprised!

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Bill,first off I want to thank you and the people who donated the fantastic equipment for this professional comparison.I have heard in the past that the diagonal does not impact the views....I am now enlightened,and will try my frac "straight through" as a test.My diagonal was just checked for stray light,not good,we'll see if it does impact my planetary views.The equipment used,the method of presentation and the references provided are all above compare.

Thank you

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Excellent and interesting review Bill. Thanks very much for posting it  :smiley:

Gosh, it looks like I'll have to budget for a couple of excellent prism diagonals now to compliment my Tele Vue and Istar mirror ones :rolleyes2:

Like Gerry, I've hitherto not noticed much difference in the performance of the various diagonals I've owned other than the really low cost units. Maybe a re-think is needed.

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I like you BillP, check my kit list...... :)

Edited by Tinker1947

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I've just managed to find a UK supplier listing the Vernonscope 1.25" Enhance Silver Mirror diagonal that you rate highly in this review. £469 is a high cost for a 1.25" diagonal  :shocked:

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I've just managed to find a UK supplier listing the Vernonscope 1.25" Enhance Silver Mirror diagonal that you rate highly in this review. £469 is a high cost for a 1.25" diagonal  :shocked:

Yes...very expensive...plus being silver deteriorates faster than aluminum I understand so re-silvering probably needed in 10 years.  I prefer no-work solutions and prisms have been around forever, so went with that myself.  Proven technology, lasts forever, and works fantastically too!!  Given that the range of "theoretical" transmission of these diagonals was 97% to 99%, it was comforting to see that for non-planetary observing, a difference in a few percent of transmission difference had no impacts.  So IMO, takes that out as a critical factor when selecting diagonals.  I can only presume that the less scatter of the prism is what was driving the better performance on planetary...but not sure.

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I'm sure glad my VX10 dob doesn't need one! :grin: My straight thru test confirmed for me the results of BillP test(not that it needed confirming!)-I had to try.The Baader/Zeiss is very tempting, maybe someday,thing is my 90mm frac will never beat my 10" for resolution anyway-but if I had a big refractor.....The Baader wedge prism sure makes the sun sharp in the 90mm though,awesome

ps the 10" dob took 300x very well last night

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Very useful and interesting Bill, I shall have a proper read when I get a chance.

I have a Baader T2 Zeiss prism on order with TS, but the lead time has been a couple of months so far, it is supposed to be due now but no confirmation yet. Looking forward to using it with the f9 Vixen 150ED :-)

Cheers,

Stu

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Thanks alot for posting this.  From now on, I'll leave the prism diagonal in my f10 scope, which I use primaraly for lunar/planetary/double star observing; and I'll leave my dielectric prism in my f5 scope, which I use primaraly for faint fuzzy observing. 

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Great review, but just wondering why no inclusion of the William Optics Dielectric which is something of a workhorse in these parts?

Also,  as the proud owner of a Baader Zeiss T2 Prism I can attest to its superb performance - incredibly sharp and great contrast! 

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Great review, but just wondering why no inclusion of the William Optics Dielectric which is something of a workhorse in these parts?

Also,  as the proud owner of a Baader Zeiss T2 Prism I can attest to its superb performance - incredibly sharp and great contrast! 

Bill included 2" and 1.25" Astro Tech diagonals in the review which I reckon would be pretty much the same as the William Optics ones. They may well be made by the same manufacturer.

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Bill included 2" and 1.25" Astro Tech diagonals in the review which I reckon would be pretty much the same as the William Optics ones. They may well be made by the same manufacturer.

OK, thanks - I wasn't aware of that.   It didn't detract anyway from a really thorough and informative review. 

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Thanks for such a detailed report. I must admit i have never considered prism diagonals - just assumed that that they would be less bright.

 I note from a few reports now that it seems they can give sharper planetary detail over the common dialectic types.  I wonder now if the prism could be just as important to the view as the eyepiece ? 

Forgot to add would the best possible view be straight through without any diagonal ?

andrew

Edited by andrew63

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Thanks for such a detailed report. I must admit i have never considered prism diagonals - just assumed that that they would be less bright.

I note from a few reports now that it seems they can give sharper planetary detail over the common dialectic types. I wonder now if the prism could be just as important to the view as the eyepiece ?

Forgot to add would the best possible view be straight through without any diagonal ?

andrew

Straight through would certainly give best results, fewer losses or scattering. You do need a rubber neck though ;-)

I have a parcel on my desk from TS which I believe contains a 2" Zeiss prism so will be able to give it a go soon :-)

Stu

Sent from my iPhone using Tapatalk

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6a7agume.jpg

Looks nice ;-)

Stu

Sent from my iPhone using Tapatalk

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6a7agume.jpg

Looks nice ;-)

Stu

Sent from my iPhone using Tapatalk

That, attached to the vixen with an ortho should be a Mars buster Stu !

andrew

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That, attached to the vixen with an ortho should be a Mars buster Stu !

andrew

That's the plan anyway :-)

Will report back......

Stu

Sent from my iPhone using Tapatalk

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Great review! Thank you.

I bought a Tak prism diagonal on a whim ages ago.

I always thought that the views were ever so slightly superior than through a typical enhanced/dielectric diagonal.

Definitely less scatter and a smidgen better contrast.

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