Prism or mirror?

[bosun21]

I have used both a prism diagonal as well as a mirror one. Has any definitive conclusion been reached as to which is best? Disregarding any orientation aspects of each has one been found to be superior visually to the other. I can vaguely remember reading claims for each advocating their superiority. Thoughts anyone.

[Zermelo]

Of course, you will get differences in quality (especially the flatness of the optical surface) with either, depending on how much you spend, so I presume you're just talking about the suitability of the design.

The usual answer is that, in scopes faster than about F/7 or F/6, mirrors are preferred to prisms, because the faster-converging light cone can lead to false colour in a prism.

There's also a view that mirrors can give more scatter than prisms, so the latter might be preferred in slower scopes.

I doubt that there will be a definitive view, though.

Edited June 9, 2023 by Zermelo

[Don Pensack]

I think either works very well at f/8 but that mirrors are preferred at shorter f/ratios.

Prisms also help if the scope has insufficient in-travel at the focuser for certain eyepieces.

It's been years since I compared diagonals, but I was looking for sharpness of star images and good collimation in my tests.

And the very best and very worst were mirrors, with the prisms somewhere in the middle.

I thought that even at f/12 (the scope in the test), prisms had a little extra chromatic aberration at the edges of the 82° eyepieces I used in the test.

I had a few samples of a couple of the diagonals and the sample-to-sample variation was as great as the range in the test.

I came to the conclusion that, mirror or prism, quality of the optical surfaces was what mattered.  You could adjust collimation, but not optical quality.

 

Currently, I use a mirror diagonal in my f/7 triplet apo, and the image is quite sharp.  I don't use a refractor often enough to re-do the experiment with several diagonals.

[John]

I use a Baader Zeiss T2 prism diagonal with my F/9 Tak 100 and 2 inch Tele Vue Everbright or Astro Physics Maxbright mirror diagonals with my other refractors.

I have occasionally swapped them around but in all honesty I've not seen any differences in their performance 🤔 

 

[The Lapwing]

That’s my experience too. I have used Baader prisms, but currently use a 2 inch Tele Vue Everbright. I very much doubt that I would be able to tell the difference to be honest. My scope is 8.6 if that makes any difference I couldn’t tell you. 

Edited June 9, 2023 by The Lapwing

[bosun21]

Thank you gentlemen. My 2” WO diagonal has a 1/12 lambda dielectric mirror which I am happy with so I’ll be sticking with that. It’s just that I was always thinking, would I gain anything from a prism diagonal. This has now been answered. Thanks.

[Mr Spock]

I have a Takahashi 1.25" Prism and a Baader ClickLock 2" DiElelectric. Optically I can't see any difference between them. 

I will say the twist lock on the Tak is a real pain and so it doesn't get used.

[bosun21]

Just now, Mr Spock said:

I have a Takahashi 1.25" Prism and a Baader ClickLock 2" DiElelectric. Optically I can't see any difference between them. 

I will say the twist lock on the Tak is a real pain and so it doesn't get used.

Yes I’ve owned the Tak 1.25” prism diagonal and while optically great I also didn’t get on with its twist lock eyepiece holder.

[The Lapwing]

9 minutes ago, bosun21 said:

Thank you gentlemen. My 2” WO diagonal has a 1/12 lambda dielectric mirror which I am happy with so I’ll be sticking with that. It’s just that I was always thinking, would I gain anything from a prism diagonal. This has now been answered. Thanks.

Great diagonal the WO, nothing wrong with those at all. Good value for money too

[Louis D]

I only use prism diagonals for correct image viewing.  For everything else, I use GSO dielectric mirror diagonals.  I've never felt the need to upgrade from them.

On this topic, I have a vague recollection that some folks have noticed that prism diagonals correct chromatic aberrations in certain scopes, but I can't recall the details.  Does anyone else recollect this or is my mind on <insert illegal drug name>?

[Piero]

For a while I had both 2" Baader BBHS mirror and prism diagonals. They were used with my 4" tak f7.4.

None of them showed more details than the other, but the prism showed some chromatic aberration on that telescope and so I let it go.

The prism also seemed to take a considerable longer time to cool down.

[JeremyS]

I use 1.25-inch Tak diagonal prisms. These are superb. I’ve not tried a 2-inch prism as I am concerned that the longer light path might induce more CA. Instead I use Baader BBHS T2 or 2-inch mirror diagonals or a Tak 2-inch mirror.

[Franklin]

When using traditional eyepieces like orthoscopics and plossls I use a 1.25" prism diagonal, the Baader 32mm T2, the TS 32mm T2 and even the Vixen 1.25" prism and to me the only difference between them is the quality of build and the T2 capability. Regards the little plastic Vixen prism, I have a new "made in China" version and a vintage "Saito" and they appear identical in use. When using my larger scope and 2" eyepieces, which includes the heavy LVW range I use a 2" mirror diagonal, I have the Baader clicklock dielectric and also a vintage Vixen 2" mirror diagonal but again, visually I see no difference, other than the better build quality of the Baader and the convenience of a clicklock. I did try an Altair Astro 2" prism diagonal and optically it was very good but it added a fair bit of weight to my set up. My refractors range from around f/6 up to f/11 and the usual remarks about prisms being better in slower scopes and mirrors being better in faster scopes, although maybe true, in use I see not a lot of difference whilst observing in general, maybe when the magnification is cranked up, yes. The differences noted by others must be quite subtle to say the least. I also have a couple of Vixen 1.25" flip-mirror diagonals and even though Vixen advertise them as being able to supply two magnifications which implies their use for visual, I'm not quite sure and don't use them really. I think their main use is for centering up when using a camera on the back end.

The absolute best diagonal in the whole wide world is an invisible one and to use your objective and eyepiece combo in it's purest form whilst observing straight through, Japanese style. At least try this whilst star-testing your scopes and you will notice a difference, apart from everything being upside down of course.

[Zermelo]

I know the OP has decided, but in my resources folder I find the following that discuss prism vs mirror:

https://stargazerslounge.com/topic/331772-star-diagonal
https://stargazerslounge.com/topic/375363-in-the-market-for-a-new-125-inch-diagonal
https://stargazerslounge.com/topic/328670-mirror-vs-prism-diagonal-for-fast-refractor/
https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

 

[GordonD]

This a timely topic for me as I just got hold of an Altair 2" prism that I thought would be interesting to compare against the SW Dialectric mirror one that came with the 120ED.I have not had a chance to really compare them optically to see if there is any useful difference yet, but the first target was Venus and I immediately noticed some purple false colouring that is not present with the mirror. I was a little surprised at this as I thought at F7.5 this would not be noticeable. Venus is of course a severe test for false colour so I am certainly not writing off this prism diagonal yet, for sure. Using the mirror the 120ED has always been pretty much free any any false colour.

I will continue to test them both but I think it will take a while to come to a conclusion. Ideally a comparison on Jupiter to see if there is any difference in colour intensity and contrast.

We'll see, I guess.

Edited June 11, 2023 by GordonD