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Prism star diagonals


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I'm still a refractor newbie and when hearing the term "prism diagonal", have thought about the cheap starter-kit erecting prism diagonals. Therefore, in my mind, a decent mirror diagonal was better than a prism.

Anyway, turns out my assumption is wrong - especially after seeing the prices of some higher-end prism diagonals. So - a few quick questions from me:

  • Are all prism diagonals correct image, or are the high-end ones the same as a mirror?
  • Fast scopes - mirror best, but slow scopes - prism better?
  • At what prices do decent prism diagonals start ? Say compared to reasonable mirror diagonals starting around £70?

Happy Friday, everyone 🥳

 

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Prism diagonals have the same image as mirror diagonals. Correct, Mirror fast and Prism slow. About the same price really. Baader do a 32mm prism for around £80 which is really good but it's part of the T2 system so you need nosepiece and eyepiece holder on top. The best 1.25" prism diagonal is the Takahashi for around £99. The 2" Zeiss prisms begin to get quite expensive but so are high end 2" mirror ones.

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Prism diagonals can achieve much greater light throughput. In order to get in high 90s (like 97-99%), mirrors need dielectric coatings - and many layers. These impact surface quality as there is limit to how precise thickness of layer can be.

Prisms can easily get 99% or higher (depending on glass clarity and applied coatings on air glass surfaces - two of them).

If you want to know more about how each performs - read this excellent article:

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

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The Baader T2 prism with 32mm aperture is now about 100 quid and it has much better build quality (no plastic) than the Tak and of course as it has T2 fittings you can change the nosepiece or eyepiece holder to suit yourself. The Tak and basic Baader are similar optical qualty but if you want better there is the Baader Zeiss prism.

The Tak prism can often be picked up second hand quite cheaply.

Edited by johninderby
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2 minutes ago, johninderby said:

The Baader prism with 32mm aperture is now about 100 quid and it has much better build quality than the Tak and of course as it has T2 fittings you can change the nosepiece of eyepiece holder to suit yourself. The Tak and basic Baader are similar optical quality but a bit better is the Baader Zeiss prism.

This is initially for the Vixen 80 (f11). Was hoping for something not too big - the original diagonal is a 0.96" one (very dinky). The Baader looks nice and with so many fitting, I could probably connect it direct to the visual back. But I think the Tak might be more appropriate.

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1 minute ago, johninderby said:

Ah - not worried about that.  I have a 1.25" visual back that fits the Vixen draw tube, using 1.25" EPs. So I've been using an  Astro tech 1.25" mirror diagonal (shared with the ST80). But I'm wanting a separate diagonal now, so I can have both scopes set up simultaneously. 

So really wanting a neat 1.25" prism diagonal. The Vixen ones have a single little clamp screw, but I'd prefer either a compression ring or a twist lock. The Tak seems perfect.

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3 minutes ago, Pixies said:

Ah - not worried about that.  I have a 1.25" visual back that fits the Vixen draw tube, using 1.25" EPs. So I've been using an  Astro tech 1.25" mirror diagonal (shared with the ST80). But I'm wanting a separate diagonal now, so I can have both scopes set up simultaneously. 

So really wanting a neat 1.25" prism diagonal. The Vixen ones have a single little clamp screw, but I'd prefer either a compression ring or a twist lock. The Tak seems perfect.

I love the locking mechanism on the Tak prism, it’s so simple and brilliant but it doesn’t secure Delite eyepieces due to the exaggerated under cuts. No more thumb screws though and light as a feather too. 

 

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The Tak prism does have a plastic body but from what I've read it's not some kind of soft and flexible plastic, it's more like a reinforced composite and is fairly substantial and perfectly good for the job. I've not read about any issues with its strength or durability but its optics are very good and it's decent value for money.

I've got a Baader prism which is also great but I wouldn't mind picking up the Tak one to compare at some point.

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11 hours ago, Andrew_B said:

The Tak prism does have a plastic body but from what I've read it's not some kind of soft and flexible plastic, it's more like a reinforced composite and is fairly substantial and perfectly good for the job. I've not read about any issues with its strength or durability but its optics are very good and it's decent value for money.

I've got a Baader prism which is also great but I wouldn't mind picking up the Tak one to compare at some point.

Agreed. In spite of some claims, the Tak prism diagonal is well made. I have three. I also have a Baader 1.25 inch and T2 diagonals. I don’t differentiate on construction quality between them. The Baaaders are mirrors.

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11 hours ago, JeremyS said:

Agreed. In spite of some claims, the Tak prism diagonal is well made. I have three. I also have a Baader 1.25 inch and T2 diagonals. I don’t differentiate on construction quality between them. The Baaaders are mirrors.

Plastic construction has a bad reputation in astronomy that's not entirely undeserved given that it's mostly used in very cheap products and is often poor quality. The right plastics used in the right way are every bit as good as metals and it's worth remembering that even things like missile re-entry vehicles are made of reinforced plastic so it's hardly weak or easily damaged stuff.

Have you ever had a chance to compare the Tak prism to prisms from other manufacturers?

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I use one of the Baader T2 Zeiss prism diagonals with my F/9 Tak 100mm. As John says above, it can be configured in a number of different ways by swapping T2 bits. I;ve used mine as a 2 inch as well as a 1.25 inch diagonal. I'm a big fan of this Baader click-lock 1.25 fitting just now which seems to work so well with a wide range of eyepiece barrel designs:

P1050825.JPG.69ee6ce6d6507aaed26322d45ef15162.JPG

I've also owned a couple of the Tele Vue Everbright 1.25" diagonals which I used in faster refractors. They are expensive but very high quality and excellent performers for a mirror diagonal IMHO.

 

 

 

 

Edited by John
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The Baader T2 diagonal looks interesting, and not as big as I thought initially. One thing I'm concerned about is how long the optical path might get - as the original 0.96" diagonal was obviously pretty short.

I could try and find an adapter so the T2 diagonal could screw directly to the M43 female thread at the end of the Vixen draw tube (although I'd lose the ability to put a filter on that side of the diagonal). 

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