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Prism vs. Mirror

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#1 Daniel Mounsey

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Posted 04 July 2010 - 05:17 PM

Sadly, most consumers are focused only on reflectivity numbers rather than quality. The reason I'm aware of this fact is because I sell to the public and their never ending obsession with the numbers. This does not imply that a good prism has any less reflectivity than a mirror. All one has to do is look through an exotic pair of binoculars in order to realize, prisms are just as bright. After continual visual test through my 6" F-8 apo refractor and my 10" F-12 DK cass, I have concluded that my highest quality mirror diagonal "can not" compete with my highest quality prism on various deep sky objects and planets. Not only is the light scatter on this exotic prism noticeably superior, but it is every bit as bright and every bit as accurate as my best mirror. I myself, normally do not use dielectric coated diagonals and have always preferred a high quality, aluminized diagonal regardless, but a good prisms IMO, is unrivaled. This of course, will depend on certain optical perimeters of the telescope. The prism is the clear winner.

#2 Bill Barlow

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Posted 04 July 2010 - 07:37 PM

Thanks for the information. Do you know of any prism diagonals that come in the 2" size? Seems all the ones I see for sale are 1.25" models. Would like to try a 2" model in my larger SCT. Take care.

Bill

#3 Tamiji Homma

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Posted 04 July 2010 - 09:07 PM

Thanks for the information. Do you know of any prism diagonals that come in the 2" size? Seems all the ones I see for sale are 1.25" models. Would like to try a 2" model in my larger SCT. Take care.

Bill


Hi Bill,

Here is one Baader 2" Zeiss prism diagonal:

Posted Image

Tammy

#4 Starman1

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Posted 05 July 2010 - 12:11 AM

It should be noted that a prism diagonal, even the Baader (which is excellent) should probably not be used with scopes below f/9-f/10 or so. The more oblique the light ray entering the prism, the more likely it is for the light to be split into a spectrum.
Stars at the edge in an f/5 refractor looked like there was serious lateral chromatic aberration. It was the prism.
But for long focal lengths, a prism is great.
I compared a $50 prism diagonal (FMC) to my $150 dielectric diagonal in my f/13 Mak. and could not see any difference in brightness or image quality. I would have kept the prism diagonal but for the cheap plastic housing.

#5 Clive Gibbons

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Posted 05 July 2010 - 07:54 AM

Sadly, most consumers are focused only on reflectivity numbers rather than quality.


True, and there's a third, just as important factor, to consider--

How does the prism or mirror affect the final optical correction of the refractor. Some objective lenses deliver their best color and spherical correction when used with a prism. Since manufacturers aren't intent on informing customers about this, it's up to the user to experiment and find out for themselves.
Even a fairly "fast" refractor can benefit from the use of a 1.25" star diagonal prism. My WO Megrez 110ED (f/5.95) and 88FD (f/5.6) doublets both displayed better overall image quality with an inexpensive prism. The SV115T triplet (f/6.95) I'm using now has achieved essentially perfect correction when coupled with a Takahashi 1.25" prism diagonal.

#6 Daniel Mounsey

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Posted 05 July 2010 - 09:21 AM

Clive and Don,

On axis, color correction should be slightly improved but I also agree with Don that there are certain limitations the prism has at faster focal ratios. Perhaps you are both discussing a trade off since you and Don have slightly different thoughts here, and perhaps if you were to use a 2" prism, more of the false color may be seen around the periphery with wider angle eyepieces. I have not experimented on that level yet because I dont use short achromats, only very long ones. But Don, I do think F-9 and F-10 may be a bit extreme. I will ask Tammy to bring his 2" prism to the next NUTS party. The one I'm using is almost 2" and fully, multi coated. Sadly, A couple of high end companies do not coat theirs well enough and as a result, light was scattering even though I flocked one of them. I can only assume it was lack of coatings.

BTW Don, I just completed my observatory is the observatory forum. You sould come by soon and see what else I did not depicted in those photos.

#7 Daniel Mounsey

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Posted 05 July 2010 - 09:23 AM

Tammy,
Looks like they did not put any Baader logo on it correct?
Daniel :)

#8 Clive Gibbons

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Posted 05 July 2010 - 09:31 AM


On axis, color correction should be slightly improved but I also agree with Don that there are certain limitations the prism has at faster focal ratios. Perhaps you are both discussing a trade off since you and Don have slightly different thoughts here, and perhaps if you were to use a 2" prism, more of the false color may be seen around the periphery with wider angle eyepieces.


Hi Dan.

I have a Baader T-2 prism, but use it with 1.25" oculars.
FWIW, most of my observing is done at medium to high mags and only with 1.25" accessories. So, a big prism could introduce some lateral c.a., visible at the edge of a 2" f.o.v.-- I've not tried that. :shrug:

Thanks for bringing up the discussion of prisms vs. mirrors.
I agree.
There's much more to consider than quantum efficiency.
:waytogo:

#9 Scott99

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Posted 05 July 2010 - 10:13 AM

Before the AP Maxbright came out I was using a 2-inch "premium" prism diagonal sold by Univ. Optics. It worked very well with my f/8 refractor, better than anything I'd used previously.

I wish I'd kept it because it was light in weight compared to the Maxbright. A couple years ago I checked and it was still being sold in Japan by KK.

#10 SteveG

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Posted 05 July 2010 - 04:06 PM

One problem I have with my cheap 1.25 prism is the sloppy fit in the eyepiece holder. On my Celestron, it is so bad that when the setscrew is tightened it mis-aligns the eyepiece causing distortions that are clearly seen.

Does anyone make a 1.25 prism diagonal with a compression ring?

#11 GlenM

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Posted 05 July 2010 - 05:04 PM

Yes,my Baader 11/4" prism comes with a compression ring with two screws.

#12 Starman1

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Posted 05 July 2010 - 05:41 PM

One problem I have with my cheap 1.25 prism is the sloppy fit in the eyepiece holder. On my Celestron, it is so bad that when the setscrew is tightened it mis-aligns the eyepiece causing distortions that are clearly seen.

Does anyone make a 1.25 prism diagonal with a compression ring?

Baader, Takahashi, HuTech, Carl Zeiss. Not really sure about that last one--it will depend on the attached adapter.
The last 1.25" Takahashi one i saw had a collet-style eyepiece binder (great!), but had a plastic prism housing. Someone can correct me if that's changed.

#13 Clive Gibbons

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Posted 06 July 2010 - 08:20 AM

IMO, it would be great to see more vendors offer premium quality prism star diagonals, along with their dielectric mirror models.
Even better would be if vendors tried some high magnification testing of their scopes and checked to see if a prism or a mirror resulted in a superior image.
Observers like to squeeze the best possible performance from their instruments. They can spend quite a bit of extra money hunting for the ultimate in image correction.
The mirror vs. prism diagonal choice deserves more attention than it's received in the past.

#14 Daniel Mounsey

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Posted 06 July 2010 - 09:00 AM

Clive, that's a good idea. I may just set something up like that.

#15 Clive Gibbons

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Posted 06 July 2010 - 09:46 AM

Clive, that's a good idea. I may just set something up like that.


Cheers, Dan. :bow:
If you could arrange that, it would be very interesting and informative! :waytogo:

Based on the analytical tests I've seen of Tak triplets (TOA and TSA), using a prism likely wouldn't improve their correction, since they're so darn good to begin with.
Other apos (especially Chinese doublets and triplets), which have varying degrees of undercorrection in the red, could benefit. A BK7 prism (inexpensive prisms use that) helps most with FPL51/FK61 doublets.
A higher index prism (as used in the Baader and Tak prisms) seems to work best when a lens needs a more subtle "red shift" (like with the SV115T).

#16 Alex Gastélum

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Posted 19 July 2010 - 01:51 PM

Baader ofers a "Precision Amici Star Diagonal" is that better than the "click lock 2" diagonal"(or other hi end 2" mirror) for lunar and planetary details?

#17 Alex Gastélum

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Posted 19 July 2010 - 01:54 PM

a good prisms IMO, is unrivaled. This of course, will depend on certain optical perimeters of the telescope. The prism is the clear winner.


in what kind of telescope does the prism is superior?

#18 Starman1

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Posted 19 July 2010 - 05:27 PM

Baader ofers a "Precision Amici Star Diagonal" is that better than the "click lock 2" diagonal"(or other hi end 2" mirror) for lunar and planetary details?

An Amici prism divides the light in two and the dividing line is not infinitely small. As a result, the line is always visible. This is OK for daylight viewing but not acceptable for high quality or high power viewing (though OK in a low-power finder).
An astronomical-use prism will be a 90 degree version that duplicates the orientation of a mirror.

#19 Starman1

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Posted 19 July 2010 - 05:33 PM

a good prisms IMO, is unrivaled. This of course, will depend on certain optical perimeters of the telescope. The prism is the clear winner.


in what kind of telescope does the prism is superior?

The more obliquely an image enters the prism, the more likely it is to be spread into a spectrum. Ergo, the prism's best use is in long f/ratio scopes (in my opinion f/10 and longer).
In my f/13 Maksutov, there appears to be no noticeable chromatic issues at the edge of what is a narrow field of view.
But in an f/5-f/6 refractor, I would recommend a mirror diagonal, not a prism.

As far as wavefront degradation goes, a prism will add approximately twice the wavefront error as a mirror with exactly the same specs. [The high-end prism may have a better spec, though.] However, the prism will last essentially forever and never need recoating. And the prism will have a higher transmission than a star diagonal mirror with standard aluminum coatings.

Most of the high-end star diagonals are mirrors, though, for reasons of wanting to be "universal" in use.

#20 Sarkikos

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Posted 19 July 2010 - 05:59 PM

One advantage of a 90 degree prism diagonal over a mirror diagonal, is that the prism can deliver a right-side-up correct-right-to-left image, the same orientation that you see naked-eye and on star charts. You don't have to deal with the unnatural, contra-intuitive, reversed orientation. That is a huge credit in my ledgerbook.

I usually insert a 90 degree prism diagonal in my refractors , unless I want to use 2" eyepieces in my ST80 or C4-R. Then I resort to a 2" dielectric diagonal. (Great for Rich Field views of the Veil and suchlike.) If I could afford a 2" prism diagonal, I would get one.
I imagine, though, that a 2" prism diagonal and a big 2" eyepiece would be quite a heavy load for my ST80. :tonofbricks:

I almost always use the 1.25" prism diagonal in my 6" Mak. It makes Lunar observing with the Rukl atlas much, much easier.

Mike

#21 Starman1

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Posted 19 July 2010 - 06:12 PM

One advantage of a 90 degree prism diagonal over a mirror diagonal, is that the prism can deliver a right-side-up correct-right-to-left image, the same orientation that you see naked-eye and on star charts. You don't have to deal with the unnatural, contra-intuitive, reversed orientation. That is a huge credit in my ledgerbook.

I usually insert a 90 degree prism diagonal in my refractors , unless I want to use 2" eyepieces in my ST80 or C4-R. Then I resort to a 2" dielectric diagonal. (Great for Rich Field views of the Veil and suchlike.) If I could afford a 2" prism diagonal, I would get one.
I imagine, though, that a 2" prism diagonal and a big 2" eyepiece would be quite a heavy load for my ST80. :tonofbricks:

I almost always use the 1.25" prism diagonal in my 6" Mak. It makes Lunar observing with the Rukl atlas much, much easier.

Mike

Prism diagonals do not necessarily correct the image. They do so only if of the roof prism type (Amici prism). A standard prism has the same orientation as a mirror.

I do not recommend roof prisms for astronomical viewing:
1) too many optical reflections internally
2) smaller clear apertures, both on the prism faces and in the body of the prism, and more vignetting
3) the "line in the middle" problem
4) almost never available with fully multicoated surfaces

The beautiful astronomical use prisms from Takahashi, Baader, Zeiss, et al. are of the simpler, mirror-reflection, type and have better optical performance. They do not produce correct images.

#22 GlennLeDrew

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Posted 19 July 2010 - 07:13 PM

One advantage of a 90 degree prism diagonal over a mirror diagonal, is that the prism can deliver a right-side-up correct-right-to-left image, the same orientation that you see naked-eye and on star charts.


This is true if the prism is of the roof variety, e.g., Amici. But as pointed out, its downside is diffraction introduced by the roof line, which shows up as a single, horizintal spike through bright objects--sharp if a star, and fuzzy if a resolved object such as a planet.

By the way, I have a couple of 2" format Amici prism diagonals obtained from Orion. (I think they're no longer available from Orion, but perhaps still so from William Optics?) They have a clear aperture of 40mm, which can still deliver minimally-vignetted views with eyepieces having near-to-maximal 46mm diameter field stops--if the field stop does not lie too close to the rear prism aperture. I'm going to use mine in a 120mm binoscope project.

#23 PJ Anway

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Posted 19 July 2010 - 08:08 PM

I myself, normally do not use dielectric coated diagonals and have always preferred a high quality, aluminized diagonal regardless, but a good prisms IMO, is unrivaled. This of course, will depend on certain optical perimeters of the telescope. The prism is the clear winner.



I only have prism diagonals - two Zeiss and one Baader/Zeiss. I like that they don't scatter light and yield excellent contrast. It helps on those difficult double splits.

Baader, Takahashi, HuTech, Carl Zeiss. Not really sure about that last one--it will depend on the attached adapter.


Zeiss made .965", 1.25" and 2" eyepiece adapters. The 1.25" and 2" can have compression rings, but I have never seen them on the .965". Baader also makes a M44/T2 adapter so you can use any of Baader's eyepiece holders on the Zeiss diagonals.

#24 Sarkikos

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Posted 19 July 2010 - 08:48 PM

Don, Glenn,

Hmmm... correct orientation or a fuzzier image? Which is the lesser of two evils? I hate the reversed image, but I also dislike a fuzzy image. Decisions, decisions...

For one thing, I wish they'd publish the Rukl Atlas of the Moon in reversed and inverted versions, not just terrestrial.

Well, at the very least I will continue to use my not-fit-for-astronomy Correct-Image (Amici-Roof) prism diagonal in my 70mm finder so I will have a correct image when I'm star hopping. Now isn't that the type of prism diagonal they use in RACI finder scopes? I wonder why that is? I guess it's not so bad for low power, wide field use. It works great for me. For that matter, it should be good for my ST80. I'd never use that at high power, anyway.

However, in the past I have used a mirror diagonal with good results in my C4-R for planetary observation and for detail work on the Moon.

Mike

#25 Starman1

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Posted 19 July 2010 - 10:44 PM

Mike,
Yeah, that's it. Low power isn't critical and the Amici/roof prism is ok. But not for any demanding or higher power applications.
If you give up the correct image, you gain superior optics.

Just an aside: mirrors vary in quality in star diagonals almost as much as they do in primaries (ok, maybe a little less), and I've seen noticeable differences between diagonals. That may be the key to the high-end prisms--consistency. Though I would point out that now you have 3 surfaces to polish instead of only 1.
It's the primary reason I still use a mirror diagonal. But the one I use was selected from a batch.






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