Takahashi Prism Diagonal vs Mirror Dielectric Diagonal (1.25")

[F15Rules]

In the past few months since summer I've been comparing a Takahashi Prism diagonal versus some others including dielectric versions.

My interest came about due to having heard that prism diagonals can reduced the CA on some long focus scopes and also can help better bring some eyepieces to focus due to having a shorter light path than a traditional mirror.

Like many amateurs, I guess, I've always assumed that Tak equipment is incredibly expensive..certainly outside my reach, but in the summer I saw an ad for a mint Tak 1.25" diagonal for £50 so I decided to go for it.

When it arrived, complete with original blue Takahashi box and dust plugs, I was at first struck by how light in weight it was, noticeably lighter than a dielectric mirror diagonal of the same barrel size (1.25"). The next thing I noticed was the compression ring fit at the top of the diagonal, ie where the eyepiece is inserted. This is a really clever, and superbly effective feature, whereby the more you turn the knurled aluminium retaining ring, the tighter grip is exerted on the eyepiece in the diagonal barrel.. and of course it doesn't mark your eyepiece barrel like a set screw would. In fact, I found that this ring held a fully loaded WO binoviewer very well indeed, very impressive.

I have not read anywhere what the reflectivity of this Tak diagonal prism is: however, in direct comparison to a Tal mirror (great diagonal), WO dielectric (cosmetically superb, functionally very good) I have been unable to see any measureable differences. However, for me the compression retaining ring function of the Tak Prism is the real "what counts" factor and for this alone I'd go for the Tak.

I also couldn't see any noticeable reduction of CA on my refractors, although since these are mostly F10 or more, that might be why - it's not that noticeable anyway. In shorter scopes it might be different.

Finally, I think the prism does indeed reduce the length of the light path versus an ordinary mirror diagonal (on some scopes, not all, and not needed for SCT/Maks I believe)...I'd estimate by between 50mm and 100mm max, which could be useful if you are trying to use a binoviewer without the OCS barlow and consequent higher magnification.

Perhaps one of the nicest surprises about the Tak 1.25" prism is the price. Although a 2" one is £340, you can get a new 1.25" version for just £77...see here :http://www.trutek-uk.com/pricelists/takaug13pl.pdf

It's item code number TKA00541

Hope that might be of help to someone looking to try a different diagonal..

Dave

[emadmoussa]

I used several diagonals. I had/have dielectric and mirror versions (all 2") with reflectivity ranging between 91% and 99% and for my eye I could barely see any difference in views in the ED or Achro refractors.

[melsky]

I use a Tak prism with my William Optics binoviewer for the exact same reason, glass in the light path instead of a mirror diagonal pushs the focus farther back  making it easier to use eyepieces without having to resort to a barlow.

Mel

[John]

Very interesting review Dave - thanks for posting it 

I too have read about prism diagonals reducing the CA in some scopes, notably I seem to recall the older Celestron diagonals (which used to use a prism) being used successfully with the WO Megrez 110 F/5.95 ED doublet refractor which produced a little more CA than some folks bargained for.

I've always liked the clean lines of the Tak diagonals and it's good to hear that the compression ring system works well. 

[AndyH]

Great post Dave

 I've had one of these diagonals for some years now and fully agree with your thoughts on it.

They 'use up' about 20mm less of available focus than a mirror diagonal(8mm less than a Tal type), which as you say is uber beneficial to users with setups with limited focus availability. I measured the Tak as using up 63mm, the Tal as 71mm & a dielectric as 82mm.

Another thing to note, is the Clear Aperture of the Tak prism. Often prism diagonals have a small CA, sometimes 20mm or less. Whilst I don't have my Tak to hand at the moment, I do recall it as being as large as a mirrors. I've used mine with all my scopes and have seen no loss of contrast or transmission of light, at various powers. 100% agree about the compression lock. Big glove friendly knurled ring, holds accessories level and true. Best locking system on the market. I love it Far superior to the brass ring compression locking systems.

I also have a larger Tak prism, which is similar to the Baader/Zeiss diagonals that you see on Telescope-Service. It too is of high quality. I use it on my Klevtsov, which has limited focus availability. A godsend in that regard.

The 1.25" Tak Prism is a quality product and not at the usual horrific Tak prices, IMHO.

Andy.

ps: I've read a few recent reports that prisms are best suited to slower scopes, as they can add a bit of colour on faster scopes. I think around F7>.

[estwing]

well you just made my day Dave. I've been on the look-out for a top performing diagonal to go on my Telementor and was worried about where to spend wisely...so wish I'd bought the zeiss one when I first got the scope 27yrs ago

[AndyH]

well you just made my day Dave. I've been on the look-out for a top performing diagonal to go on my Telementor and was worried about where to spend wisely...so wish I'd bought the zeiss one when I first got the scope 27yrs ago

It works really well on my Carton 60/1000, so I'd say it'll be sweet on Telebrilliant,

Andy

[estwing]

arrrgghh don't do CCards....paypal or bankers draft..i'm on it...thanks AndyH...should look nice on the zeiss!

[AndyH]

Bill Paolini is currently doing a test on various sizes and types of diagonals including the wee Tak. Will make for an interesting read, when he's done.

[F15Rules]

well you just made my day Dave. I've been on the look-out for a top performing diagonal to go on my Telementor and was worried about where to spend wisely...so wish I'd bought the zeiss one when I first got the scope 27yrs ago 

Hi Estwing,

I regularly use my Tak diagonal on the following long focus scopes..

F16.5 76.2mm refractor

F15 Pentax 60mm refractor

F15 Unitron Polarex refractor

..and now a Vixen F13 102mm Pulsar refractor.

It works perfectly on all of these scopes. I also have a 2" to 1.25" adapter from Agena Astro which lets me use my 2" eyepieces in some of the above scopes..it's great because you get a much wider field than you'd think these old long tubes could deliver!

Enjoy yours on your Telementor, a marriage made in heaven I should think!

Dave

[AndyH]

...should look nice on the zeiss!

Enjoy yours on your Telementor, a marriage made in heaven I should think!

Hell yeah !!

[alan potts]

Very nice report Dave on a subject that does not come up everyday, heck of a difference in the price between the two types, I don't think I will be running out to buy the larger one in a hurry. On the subject that John brought up about the WO 110mm, I nearly bought one of them and thankfully the supplier told me it was not the full shilling, then sold me something costing much more.

Alan.

[John]

The Megrez II0 ED followed on from the excellent Megrez 90. Everyone thought it would be knockout and it was priced at just a little more than the 90 so very tempting. What they overlooked was that i) it used FPL-51 rather than the FPL-53 of the 90 and ii) the focal ratio had shrunk to under F/6 whereas the original 90 was F/6.9. The poor old 110 doublet was never going to achieve the CA correction that a smaller, slower scope using FPL-53 element could and got panned by a few with unrealistic expectations. I expect it's a nice scope really 

Sorry thats off topic 

[AndyH]

I bought one of the 110's, new, not long after they came out. I soon swapped it for another scope.

A certain Jimi Hendrix song springs to mind

Don't get me wrong, it was beautifully finished, as always from W.O., but the lens...........