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Walking on the Moon

Light transmission, mirror vs prism diagonal?


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Thinking in terms of a Mak 90, lightweight set up, where making the most of aperture is significant, I was wondering whether there is any appreciable difference in the light transmission between a mirror and a prism diagonal. I have a Circle  T prism, is it worth forking out money for a dielectric mirror diagonal?

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8 minutes ago, Alfian said:

Thinking in terms of a Mak 90, lightweight set up, where making the most of aperture is significant, I was wondering whether there is any appreciable difference in the light transmission between a mirror and a prism diagonal. I have a Circle  T prism, is it worth forking out money for a dielectric mirror diagonal?

Like Gus, I have yet to spot the difference between the two.

I bought a prism because it is said that under excellent seeing conditions, finer planetary detail is distinguishable. Going by that, you already have the better of the two options, but when I was researching there were many MANY posts stating that people could not see a difference. Only the super experienced planetray guys claimed a small improvement with a prism.

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Thanks guys, that's good to hear, so far. I was expecting to hear that in terms of the light travel around a prism, that a mirror would  have the edge, but in real terms would there be a difference. It seems not.

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4 minutes ago, Saganite said:

I would agree with what has been said, but prism diagonals have much the shorter light path, and this may or may not be important to you.

Good point Steve. I only just have enough in focus with my filter wheel fitted using certain EP's with the prism. A diagonal wouldn't work in this configuation.

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Dielectrics were not meant to compete against prisms for light transmission but against simply aluminized diagonals. Prisms have 100% reflection on their long side but a little light loss on their other sides, it amounts to a few percents or a fraction of a percent depending on the coatings.

But prisms also scatter some light within the body of the glass, and cause some refraction that can improve refractors if they are designed for one, or deteriorate refractors that don't want one. Same for catadioptrics.

Brightness could be the same, however contrast and resolution could change. Entry-level scopes are fitted with a prism because it's much cheaper than a dielectric, less than half the price, so you would probably improve your scope with a dielectric. My prism-equipped Celestron 5 was clearly sharper and a bit brighter after switching to a GSO dielectric.

Problem is, the dielectric costs a big fraction of the whole Mak 90's price.

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10 hours ago, Ben the Ignorant said:

Dielectrics were not meant to compete against prisms for light transmission but against simply aluminized diagonals. Prisms have 100% reflection on their long side but a little light loss on their other sides, it amounts to a few percents or a fraction of a percent depending on the coatings.

But prisms also scatter some light within the body of the glass, and cause some refraction that can improve refractors if they are designed for one, or deteriorate refractors that don't want one. Same for catadioptrics.

Brightness could be the same, however contrast and resolution could change. Entry-level scopes are fitted with a prism because it's much cheaper than a dielectric, less than half the price, so you would probably improve your scope with a dielectric. My prism-equipped Celestron 5 was clearly sharper and a bit brighter after switching to a GSO dielectric.

Problem is, the dielectric costs a big fraction of the whole Mak 90's price.

That's interesting and thanks for the reply. Most of the scopes I,'ve purchased that needed a diagonal have come with a mirror item in a plastic housing which is "adequate" at best. The old-ish Japanese Circle T prism item I have has a pretty fair reputation but I've never been able to compare it in a like for like comparison. It's a fair point about cost but with almost any scope the cost of ancilliaries, eyepiece etc. wind up costing a sizeable sum, which is ok if the end is worth it. As things stand, I haven't got the scope yet, so I'll take it one step at a time, but I appreciate the comments.

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1 hour ago, nightfisher said:

Ian, if you would like to loan a 1.25 dielectric I can lend you one to see what's best for you

Hi Jules, thank you,much appreciated. I'll get back to you on that one when I get my setup together and see how it works out.

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As I understand things, the Mk1 eyeball responds logarithmically. In other words, put twice as much light in, and the image appears only a little brighter.
This is like the ear response where 3dB sound increase is heard as a 'bit louder' but is actually double the energy.

On this basis, I would think the quality of the 'light cornering' device is far more important than whether it reflects 90% or 95% or 98% of the light.

What I'm getting at is what has been raised in earlier posts. 'Not quite flat' reflecting surfaces, bubbles or impurities in glass, chromatic aberration in glass, etc are all more important than method of diverting light. On that basis I would think it is easier to make a product to a given performance if you have a mirror with one flat face, rather than a prism with several flat faces. With a prism any defects get multiplied with each reflection. Though having said this, the evidence is that good optical performance from lenses and prisms is possible - look at binos and refractors.

I reckon you would be pushed to see any difference between any of the good quality 'cornering devices'.
However, a £10 device vs a £100 device is another thing entirely.

Just my two pennorth. David.
 

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I have a Baader 1.25" prism diagonal and a 2" dielectric diagonal. I use the later 99% of the time. There's hardly any difference but where contrast is super critical such as looking for sirius b or subtle planetary detail I believe the prism does have a very slight edge. I don't bother with it much at the moment because the planets are so low atmospheric effects would totally swamp any advantage, but in the best conditions the prism would be my diagonal of choice :)

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On 02/08/2018 at 09:11, Alfian said:

Thinking in terms of a Mak 90, lightweight set up, where making the most of aperture is significant, I was wondering whether there is any appreciable difference in the light transmission between a mirror and a prism diagonal. I have a Circle  T prism, is it worth forking out money for a dielectric mirror diagonal?

I had a circle T prism diagonal for a short time about 18 months ago and it was superb. I seriously doubt you'll gain any improvement by moving to a dielectric. The circle T, though not quite as fancy, was optically equal to my Takahashi prism which has proved its worth time and again. Keep hold of your money!

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