Baader Amici Prism Diagonal

[markse68]

4 minutes ago, DaveL59 said:

Doesn't sound like they are using phase coatings and perhaps not even silvered? Most Binos are in the f5-f4 range I think so perhaps they've no experience of using them in faster optic assemblies, or maybe they've done this in R&D before and found it just doesn't work well enough? hard to say as they didn't seem to want to share much in that reply

Very unlikely i think Dave- this is what they said to me:

”This kind of coating is uncomon/senseless in astronomy. ”

I was just wondering if anyone knew for certain- I am on the look out for a suitable prism (not Baader!) and if it works it works, if it doesn’t i’ll have a new diagonal for low power use on the Tal 200k

Mark

[DaveL59]

The only erecting diagonal I have came with the LT70AZ and that's not all that great tbh so I got a proper star diagonal for it, svbony dielectric IIRC. Then again the celestron plastic one is likely at the bottom end of things given the cost of the outfit so not the best example to compare to 😉 

[Louis D]

Phase coatings are a necessary evil of roof prisms to realign the slight phase shift experienced by the light passing through it.  Porro prisms do not experience this phenomena and neither do right angle prisms.  I'm not sure that Amici prisms would experience the same effect as a roof prism since the light paths are vastly different:

Amici Prism:

An Amici prism is just a right angle prism with one extra 90 degree reflection on the hypotenuse.

Edited May 20, 2022 by Louis D
Corrected number and type of extra reflections.

[Don Pensack]

7 hours ago, markse68 said:

Hi Dave, I have read a few differing explanations of why phase coatings improve the image in roof prism bins but this one seems typical. Here they say this:

“One remedy is to silver the faces of the roof prism, as Buchroeder found experimentally in 1982. An even better way is to apply dielectric phase-correcting coatings. Zeiss Oberkochen started doing this in 1988, and other firms quickly followed suit.“

https://skyandtelescope.org/astronomy-resources/astronomy-questions-answers/why-do-the-best-roof-prism-binoculars-need-a-phase-correction-coating/

Still am left wondering why it’s deemed acceptable to use amici prisms (are they phase coated? unlikely- are they even silvered?) with f4 finders (Baader even sell one!) yet it won’t work at f3.3. At what focal ratio does it stop working?That may well be the case but i’d like to understand why.

Mark

 

It works in finders but that's because of the extreme low powers.

And, if you really analyzed the finder like you do a telescope, you'd find excessive astigmatism, spherical aberration, chromatic aberration, vignetting, and a darker image than should be the case.

So in an f/3.3 scope, you'd not only get a loss of brightness, you'd have excessive lateral chromatic aberration, serious vignetting, a visible line in the field, and likely a poorer quality axial image.

In general, prisms of any kind are not recommended below f/8 because the lateral rays enter at too oblique an angle, yielding significant chromatic aberration.  And, if the clear aperture is insufficient, vignetting. 

[markse68]

1 hour ago, Don Pensack said:

It works in finders but that's because of the extreme low powers.

And, if you really analyzed the finder like you do a telescope, you'd find excessive astigmatism, spherical aberration, chromatic aberration, vignetting, and a darker image than should be the case.

So in an f/3.3 scope, you'd not only get a loss of brightness, you'd have excessive lateral chromatic aberration, serious vignetting, a visible line in the field, and likely a poorer quality axial image.

In general, prisms of any kind are not recommended below f/8 because the lateral rays enter at too oblique an angle, yielding significant chromatic aberration.  And, if the clear aperture is insufficient, vignetting. 

Hi Don,

I was quite clear when talking to Baader, and have mentioned in this thread that this IS for a finder- not a main scope. Magnification will be 8x. Given that, do you see any reason that it should be acceptable in an f4 finder but not in an f3.3 finder? If the prism is big enough, why would you get vignetting?

PS the optics I want to use for the finder are from porro binoculars- so they were used with prisms and worked excellently before.

Mark

Edited May 20, 2022 by markse68

[markse68]

2 hours ago, Louis D said:

Phase coatings are a necessary evil of roof prisms to realign the slight phase shift experienced by the light passing through it.  Porro prisms do not experience this phenomena and neither do right angle prisms.  I'm not sure that Amici prisms would experience the same effect as a roof prism since the light paths are vastly different:

Amici Prism:

An Amici prism is just a right angle prism with one extra 90 degree reflection on the hypotenuse.

Yes I find it odd that Baader said this given there are loads of finders using amici prisms that seem to work fine without phase coatings. I'm still a little unsure what phase coatings do exactly but have read that they are needed in binoculars as the light path has different lengths on each side of the split. So it is weird to me that Baader would say what they said about their Amici prism which is symmetrical.

Mark

[DaveL59]

10 minutes ago, markse68 said:

Yes I find it odd that Baader said this given there are loads of finders using amici prisms that seem to work fine without phase coatings. I'm still a little unsure what phase coatings do exactly but have read that they are needed in binoculars as the light path has different lengths on each side of the split. So it is weird to me that Baader would say what they said about their Amici prism which is symmetrical.

Mark

well in effect one half of your viewed image is bounced off one slope edge of the roof and the other from the opposite, so in Bino use the light is out of phase either side of the roof, hence the dielectric coating to correct for that.

But then in bino use it only really makes much noticeable difference above x8. There is of course the possibility you can discern a line in the image where the roof edge is, for sure on some cheaper binos looking at a bright moon you can, but what are you planning to target with the franken-finder?

If it's just regular night sky use then you ain't exactly looking at very bright targets most of the time and could make allowance on those that do show the roof edge. Maybe they're just not wanting a tetchy customer complaint when you do...? 😉 

Edited May 20, 2022 by DaveL59

[markse68]

Its just for a diy raci finder for my new scope Dave- wanted to build my own that's all. Turning into a journey of discovery frustrated by brick walls and misinfo. But it's all good fun

I found this which explains the need for phase correction coating quite well I think- its not that the paths are of different lengths but that the light reflected off the prism roof halves gets partially polarised by some polarisations reflecting and other refracting, and when recombined suffers cancellation and reinforcement dependent on phase. Or something like that. Seems odd then that Baader said these coatings were senseless for astro.

https://second.wiki/wiki/phasenkorrekturbeschichtung

https://astarmathsandphysics.com/a-level-physics-notes/waves-and-oscillations/3105-partial-polarization-of-light.html#:~:text=Partial polarization of light occurs,of certain polarizations are removed.

Anyway, I'll chance it and see.

Mark

[DaveL59]

well I guess they don't feel the need given you aren't looking at bright colourful targets like you would with binos where the phasing would be more noticeable? 

Can't see tho why their prism wouldn't work ok but an expensive experiment if not, tho I'm sure you'd find other uses for it 🙂 

Could always send you that cheapo Celestron one I have for you to test with, tho I've no idea what the light path length on it would be.

Edited May 20, 2022 by DaveL59

[markse68]

17 minutes ago, DaveL59 said:

well I guess they don't feel the need given you aren't looking at bright colourful targets like you would with binos where the phasing would be more noticeable? 

Can't see tho why their prism wouldn't work ok but an expensive experiment if not, tho I'm sure you'd find other uses for it 🙂 

Could always send you that cheapo Celestron one I have for you to test with, tho I've no idea what the light path length on it would be.

Cheers Dave, the only reason I was considering theirs was because of its size- whatever it is- they still didn't tell me but it sounds quite big. The mirror coating they apply though should correct the partial polarisation anyway- so again odd that they would mention phase coating... But I've since discovered that the Long Perng one that Flo sell has at least a 25.5mm aperture prism which as far as optical path is concerned should work just fine. It isn't silvered though but like you say- not much colour up there though Albireo might not be so spectacular though at 8x it will appear a white dot I think. That's the one I will buy- it far more affordable too.

Mark

Edited May 20, 2022 by markse68

[Don Pensack]

23 hours ago, markse68 said:

Hi Don,

I was quite clear when talking to Baader, and have mentioned in this thread that this IS for a finder- not a main scope. Magnification will be 8x. Given that, do you see any reason that it should be acceptable in an f4 finder but not in an f3.3 finder? If the prism is big enough, why would you get vignetting?

PS the optics I want to use for the finder are from porro binoculars- so they were used with prisms and worked excellently before.

Mark

It should work equally as well or poorly in an f/3.3 finder as the typical f/4 as long as the clear aperture is adequate for the fatter light cone.