Mirror or Prism?

[GCoyote]

I have one crappy right angle diagonal. Before I start looking for a new one, what exactly should I be looking for?

[kt4hx]

Almost all of my observing is with newtonians, so I will not suggest either for you, though admittedly all of my refractors do have mirror diagonals. You may want to read the linked comparison between them, which may be helpful (or not).

https://www.cloudynights.com/articles/c ... ison-r2877

[sdbodin]

Frankly, I can't tell the difference for my casual viewing. Just go with a quality dealer, don't think an E-bay special is what I would buy. Guess that it make a bigger difference on a fast light cone, but f10 or more, shouldn't matter much.

Just 2 cents,
Steve

[helicon]

I am not an expert on refractors, but my AR152 has a mirror in the diagonal.

[Bigzmey]

Most important it should be of good quality. If you are looking for 2" I would go with mirror, if 1.25" I would go with prism for slower scope (F7 and above) and mirror for a faster scope.

[Ruud]

I prefer a mirror diagonal. With a prism you have three surfaces causing wavefront errors, with a mirror just one.

Prisms have 100% reflection internally, but there is some loss on the two glass to air surfaces. It's not much though. You have to be a bit careful cleaning them. Prisms have the slight disadvantage of causing chromatic aberration. The slower the telescope, the less CA a prism will cause.

Modern dielectric mirrors reflect 99% of the light and they are very resistant to scratches and chemicals. They are easy to clean and will last a lifetime.

I have three dielectric diagonals (William Optics) and one enhanced aluminium (Televue). When it comes to image quality, all four are equally good. Cleaning the TV requires a bit of care.

Somehow, my diagonals need more cleaning than the rest of my optics, so I'm quite happy with the dielectric ones.

[konza]

Hello GCyote, I just picked up a new diagonal. Check out re: Upgrading Baader T2.... and original post Upgrading Baader Maxbrite vs BBHS diagonal, reccomendations. Lady Fractors tip about doing a star test made the choice for me. Little tidbits like that are priceless.

[Lady Fraktor]

From a conversation with Thomas Baader, owner of Baader Planetarium:

Q:  "Do the Baader prism diagonals with BBHS coating work well with refractors faster than f/7 or f/8? I am wondering if chromatic aberration (or spherochromatism) would be a problem as the light propagates through the glass prism."
 
A: "We do not recommend a prism for refracting telescopes unless the optical calculation of the objective lens calls for using a prism instead of a mirror.
For 100 years - all Carl Zeiss Refractor telescopes were designed to compensate for the added glasspath of a prism. 
And throughout our 25 years of offering this prism we repeatedly found refractor telescopes - especially of very short focal length - where the airy disc went colorless only when adding a prism into the beam of light - as was the case with the "long gone" Astro Physics Traveler.
So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement. 
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error."

[Bigzmey]

I prefer a mirror diagonal. With a prism you have three surfaces causing wavefront errors, with a mirror just one.

Prisms have 100% reflection internally, but there is some loss on the two glass to air surfaces. It's not much though. You have to be a bit careful cleaning them. Prisms have the slight disadvantage of causing chromatic aberration. The slower the telescope, the less CA a prism will cause.

Modern dielectric mirrors reflect 99% of the light and they are very resistant to scratches and chemicals. They are easy to clean and will last a lifetime.

I have three dielectric diagonals (William Optics) and one enhanced aluminium (Televue). When it comes to image quality, all four are equally good. Cleaning the TV requires a bit of care.

Somehow, my diagonals need more cleaning than the rest of my optics, so I'm quite happy with the dielectric ones.

It is not that hard to polish one or two glass surfaces to specifications. This is why diagonal vendors are happy to report 1/10, 1/12 wave, etc. The problem is that this is BEFORE the reflecting layers are applied. Each layer degrades wavefront accuracy. Dielectric mirrors in particular have many layers and notoriously known for the light scatter. Before you compare good prism with dielectric side by side you won't even notice how much light scatter they add. It maybe not as critical for DSOs, but for splitting tight doubles or fine planetary and Lunar details the difference is quite obvious.

Now about chromatic aberration. Gabby and Baader are right, by law of physics they do add some. But how much? I have compared dielectric, hard silver and prism diagonals in my scopes with F ratio from F5 to F12 and BY EYE I could not see any added color with prism compared to others. In fact in F5 achro I see a bit less CA.

So, in my experience at EP I gained some resolution on doubles and planets, lost some light scatter and don't see (with my eyes) any added color with prisms. After this testing I use prisms exclusively in my APO/ED refractors, no matter what Baader says.

Your mileage may very, but there are quite few experienced observers (some are members here ) reporting success with prisms in fracs, even fast achros.

[GCoyote]

Actually the scope I'm using most is the 90mm Mak. It's just too handy to leave in the closet all week. Per the data plate, it's F13.9. So at that speed, I guess I should not see much difference with any good quality diagonal. If it works well in my small refractors too, so much the better.

[Jones]

If you like looking at the planets you must get a prism, any prism is better than any mirror, even that $35 Celestron 1.25". The best is the Baader Zeiss t2 size diagonal. If you don't look at planets then any old thing will work. My anything came from Hi Point Scientific 2" quartz mirror 1/12 wave dielectric. Little over $100.

[Lady Fraktor]

Not necessarily, if the refractor does not require a prism you will be increasing the spherical and chromatic aberrations of the telescope which of course degrades the views.
For planetary I use a mix of mirrors and prisms depending on which telescope I am using.