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Diagonals.


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Listening to the lunchtime BBC Four Extra comedy hour yesterday, realised that my two diagonals had different weights. Intrigued, discovered slightly different shapes and slowly realised, everything comes slowly at my age if it comes at all, that the lighter ST 80 one was just a mirror and the heavier Digimax, prismatic.

Realise that some marketing boffin is about to launch a prism diagonal manufactured from sand gathered from a secret remote Pacific Island beach, melted in a special vessel using exotic woods as heating, doped with special 'natural' chemicals, matured for five years in a shared cave alongside vintage wine, vintage cheeses and horseshoe bats, not necessarily vintage, before being CNC machined and hand-finished, numbered and signed by optical genius Newt Gallileo with his Dremel and Angle Grinder. ;)

BUT apart from the lateral or vertical reversals, a surface mirror is obviously cheaper to manufacture than an accurately surfaced lump of glass but is there always less light loss using a mirror?

Thanks in advance for your learned input.

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Hi,

Neither prism nor mirror can said to be better, but here's what the different types do.

Prisms:

Scatter less light than a mirror

Introduce more false colour in fast telescopes

Prisms barely age

More expensive to make

Self collimating

Mirrors:

Scatter more light

Don't introduce false colour in fast telescopes

The coatings can wear off, and sometimes require cleaning

Cheaper to make

Clear Skies,

Luke

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I am no doubt wrong but light loss may not come into the equation, I think it will be more to do with the accuracy of the reflected image, never the less I am still puzzled as to how the BBC Four comedy hour relates to Astronomical star diagonals ;)

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A prism will lose light both as reflection at the incident face and also at the exit face. Antireflection coatings reduce the loss but do not eliminate it, and they do not put expensive antireflection coatings on the relatively simple prisms used. So go for 2% loss at each face.

Additionally there is loss through the glass itself. What goes in does not always come out.

Add in that the prism has to be descigned in as part of the optical system as at the first face although it is plane (perpedicular) to the optical axis the light is coming in as a cone. So there is refraction of different amounts owing to the different wavelengths. So a good introduction of chromatic aberations.

The mirror needs to be flat and good refelction. Flat is fairly easy these days and the reflection is usually better then 95%, good ones claim 99%.

Overall the mirror should be the better. But they can make garbage mirrors and excellent prisms.

I would opt for a mirror as the extra glass will add in more errors then a mirror, ignoring reflections. Mainly thinking of the change in path length caused by refraction of the wavelengths through the glass. One advantage of a prism is that a front silvered mirror will deteriorate over time.

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I am no doubt wrong but light loss may not come into the equation, I think it will be more to do with the accuracy of the reflected image, never the less I am still puzzled as to how the BBC Four comedy hour relates to Astronomical star diagonals ;)

It wouldnt be a comedy hour if it made sense :rolleyes:

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I am no doubt wrong but light loss may not come into the equation, I think it will be more to do with the accuracy of the reflected image, never the less I am still puzzled as to how the BBC Four comedy hour relates to Astronomical star diagonals :rolleyes:

What else does one do with one's hands during BBC Four Extra lunchtime comedy Hour? ;)

Thanks for all the replies, guys, there is a possibility, pretty slight,

that all this learning might even be retained. :hello2:

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On the subject of prism diagonals has anyone ever used one of these.

KASAI 1.25" Dielectric Coated Pentaprism

" Kasai Trading gives a "light" to this neglected optical design. Their refined pentaprism utilizes modern "dielectric coating" on both the reflecting surfaces, yielding 99% reflectivity throughout the visual wavelengths. In addition, both the transmitting surfaces are broadband multi-layer coated, to allow more than 99.8% transmission rate. As a result, even considering the light absorption in the glass material, it still yields 97% total transmission rate, which is the same or even better than enhanced-coated diagonal mirrors' (mostly 96 - 97% reflectivity)."

John

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