Diagonals

[Arkasha (Arthur)]

Can anyone tell me which is better? A mirror or prism diagonal.

[Waxing_Gibbous (Peter)]

No idea.
All I can tell you is I can see NO visual difference between my Bintel 2" mirror diagonal and my Takahashi prism diagonal (Ooops. That was stupid - I'm trying to sell it! )
Perhaps, way back when, when mirror-making technology was a bit hit-and-miss, a prism diagonal could be counted on to offer superior views.

Or I may be missing something and prism diagonals offer superior performance with certain types of scopes - but again I've tried both in TOTL refractors, Maks, SCTs and Newts, and if anything, been more pleased with the results from the mirror diags.

FWIW, the Bintel 2" (also sold as OPt, Istar, Astrotech etc etc.) is one of the best values in astronomy.
The Meade 2" IMHO, comes a close second.
The Stellarvue Deluxe Quartz 2" is better made than either, very solid and heavy, but optically about par. I reckon iots the perfect diagonal to help balance a big triplet, but a little heavy for a Mak or SCT.

If you can get one, the diagonals from Intes-Micro in Russia are guarenteed at 1/20 wave and actually are noticably better than most others.

I don't think the uber-expensive digonals from AP, Televue, Takahashi or Baader give any noticably superior performance, so I'd stay away unless you need the brand cache.

But that's probably more info than you wanted!
Re your original question: Probably a good mirror digonal.

Peter

[dannat (Daniel)]

i think in theory these days with good quality mirror coating it should reflect light better, i.e. less light loss,
overall i agree with Peter - i had a quartz 2" -couldnt tell the diff between it & a normal dielectric.
i have a stellarvue now - its nice but i only have it cos i got a real good deal on it - save your money for other things imo

[Profiler (Profiler)]

Yep - have to agree with Peter as well - There are a few design differences with some brands like WO which vignette slightly when using large EPs (24 Pan on a 1 1/4) otherwise I couldn't see any optical difference - GSO/Astro tech are great value for money

[ausastronomer (John Bambury)]

Hi all,

Well to be honest I dont use diagonals because I like newtonains and have no need for one. However, I can tell you this. All the physics suggests that a top quality mirror diagonal "must" outperform a top quality prism diagonal. Less air to glass surfaces, less affected by temperature change and no possibility of introduced chromatic aberration through refraction. Further, it is infinitely easier to produce an optically excellent flat mirror, than it is an optically excellent refracting prism.

To the practicalities of it all.

The fact that a cheap Taiwanese mirror diagonal performs at the same level as an expensive Japanese prism diagonal has to support the theory that it's easier to get a mirror diagonal right.

The fact that several people can't pick any difference between the two tells you that while the theory suggests there should be differences, its likely they are pretty small in the field.

Cheers,
John B

[anj026]

In my experience there is no real difference in the visual views between the GSO dielectric, Astro Physics Maxbright and Baader Zeiss prism. There are other reasons why you might choose one over the other though.

The GSO is excellent value and will serve you well for visual use. The AP Maxbright is constructed to a higher level and is of higher quality though the difference probably won't be seen in the eyepiece. The Baader prism is lighter and more compact and will require less focuser in travel, this may or may not be useful for you. I have read that the prism will show better images in refractors longer than f8 and will have less scatter in the visual image though I cannot confirm that myself.

Andy

[brian nordstrom (As avatar)]

Hi Arkasha , there is one thing that has not been mentioned so far on the differances between the two , and that is the length of the light path thru the diagonal .
Some scopes can reach focus using a prisim and some wont ,as the prisim diagonal has a longer light path , eg. the light has to travel further to reach the focus of the eyepiece , .
My Zeiss has a prisim diagonal , but I use a William Optics mirror diagonal in it most times because almost every eyepiece I have wont come to focus using the Excellent Zeiss prisim Diagonal .
Its a shame , as the Zeiss one gives true'r images ( colour fidelity? ) when compared to the WO dieletric , more true to life? the Zeiss looks and the WO looks clinical , really sharp views , and if the Zeiss was not around , no body would see that differance. .
Its hard to explain , the beauty is in the eye of the beholder , I think ( my eye ) .
So what ever does it for you , get one that suits you ,its gotta be better than none at all .
Brian..

Originally Posted by Arkasha

Can anyone tell me which is better? A mirror or prism diagonal.

[Profiler (Profiler)]

Yes - very good points Brian.

From my own experience I have a supposedly 'nothing exceptional' brand prisim diagonal which does in fact produce quite good views.

I agree back in a day mass production manufacturing of mirrors probably wasnt as reliable and thus optical quality differences were more perceptible. However, what is interesting now is to see where all of these diagonals (or specifically the mirrors) are now manufactured. To my understanding the bulk of brands all come out of China or Taiwan. So, you have to wonder - does the GSO, WO, TV, Meade factories genuinely have their own glass smelting facility dedicated to making mirrors or instead is there a glass smelting facility at one factory which also supplies to many other manufacturers who then in turn may cut the glass and fit them into their own cases with their own badges etc.

Thus, a big part of mirror diagonals IMHO these days boils down to little nuances in the actual design, fit and finish of the diagonal - and then thereafter the impact of the manufacturers marketing upon your own individual psyche. For example, the WO diagonals have their slightly tapered barrel with detachable side-plates (CF now I think) whereas something like a TV is all about a one-piece housing and thus the nose piece can't unscrew. However, I am a bit unsure about the optical quality or specifically quality differences in the mirrors of various diagonals these days. Maybe its poor eyesight or lack of skill in appreciating differences but I struggled to see much difference between a GSO and a WO.

Anyhow - thats my 2 cents worth

[Arkasha (Arthur)]

Thanks for all your replies. Much clearer now.

Arkasha

[brian nordstrom (As avatar)]

cool Arkshka , go the GSO at Andrews or Bintel diagonals as these days you wont go wrong , they are both good value and offer good after sales service..
Brian.

[Profiler (Profiler)]

To my understanding Brian they are also the identical product only Bintel are a little more sophisticated in having their name painted on instead of the original 'GSO'

[brian nordstrom (As avatar)]

I think you are right there , but at this point it comes down to the money? am I right ? he he , yes both good products. I no this as I have both and the differance is non existant .
Brian.

Originally Posted by Profiler

To my understanding Brian they are also the identical product only Bintel are a little more sophisticated in having their name painted on instead of the original 'GSO'

[warpsl (Jason)]

hello.i have both baader prism diagonals.the jap version,and the zeiss version,and they both transmit much more light than a standard mirror diagonal.and produce much cleaner images,allthough they also produce a little more CA on planets.i had to use prisms instead of mirror diagonals,as i could not get 2 x 1 1/4 inch diagonals to reach focus on my big bino project.and i did not want to cut 2 inches off each OTA for the sake of reaching focus,as i also like to observe in mono mode without any diagonals.