asaint
Carpal Tunnel
Reged: 04/25/03
Posts: 2021
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Prism Diagonal
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Jim Rosenstock
Carpal Tunnel
Reged: 07/14/05
Posts: 2405
Loc: MD, south of the DC Nebula
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Interesting article.
Not really being a 'fractor guy, this is new-ish territory for me. Coupla questions:
1. So Clive, you're saying that a lot of the more affordable ED refractors have intentionally shifted the CA toward the red, which in your opinion is not such a good thing....and that a prism diagonal can help shift the CA back to the "normal" range. Is that more or less correct?
2. "Another optical characteristic of a diagonal prism is that it's naturally overcorrected for spherical aberration." I don't understand this. Please explain in a bit more detail. Thanks!
Clear skies,
Jim
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QUESTION AUTHORITY!
"errr....sez who??"
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 10348
Loc: Oort Cloud
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Hi Jim.
Re. #1, yes that's my take.
I've also heard from another CN member with a TV-76, who reports it has similar color correction, which has been improved by using a diagonal prism. So, it's worth checking the other TV doublets for that, too.
Re. #2, It's my understanding that when converging light passes through a diagonal prism, a small amount of spherical aberration is introduced. I've read about this in other fora. It's a fairly small effect, especially if the objective lens is of long f/ratio.
I'm not an optician, so please don't ask me for the technical details... 
P.s.,
Thanks to Allister for posting the article!  
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A few telescopes of dubious value.
Understanding wife and three curious cats.
"Semper ubi sub ubi"
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Glassthrower
Vendor - Galactic Stone & Ironworks
Reged: 04/07/05
Posts: 14693
Loc: Hurricane Alley
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I asked this previously, but I don't recall the answer, so if someone could help me again, it would be greatly appreciated.
I have an Astro-Tech 66ED doublet that I am quite pleased with overall. The color correction is very good for doublet, but it's not perfect. I currently use a 2" SCT-style di-electric mirror diagonal, because I enjoy low-power wide-field views of the Milky Way, clusters, occultations, etc. (the strength of this scope, IMO). To this end, I have a 2" widefield eyepiece that gives me 5+ degrees of sky at 15x. I also have some RKE and UO orthoscopic eyepieces that I use for general purpose observing. Since the 2" diagonal threads-on, it's a minor pain to install and remove it just for the sake of using a smaller diagonal. Ideally, I'd like to have a 2" 90-degree prism diagonal - so I can enjoy the benefits of the increased correction for higher power planetary/lunar viewing, and still be able to use my 2" eyepiece for low power views without swapping out diagonals.
So, where can I find a 2" 90-degree prism diagonal? (preferably one that doesn't cost several hundred dollars)
Regards and clear skies,
MikeG
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Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.
Galactic Stone & Ironworks - Buy/Sell/Trade Meteorites, Moon Rocks, Mars Rocks, & 35 different falls and types!
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 10348
Loc: Oort Cloud
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Hi Mike.
Check out this posting.
$150 for an older-style UO 2" star diagonal prism is probably the best deal available.
There's very few options these days, in the 2" right-angle type (NON-correct image)
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A few telescopes of dubious value.
Understanding wife and three curious cats.
"Semper ubi sub ubi"
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Glassthrower
Vendor - Galactic Stone & Ironworks
Reged: 04/07/05
Posts: 14693
Loc: Hurricane Alley
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Thanks Clive! I just emailed Don Rothman to inquire about it. 
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slyke
member
Reged: 02/21/08
Posts: 10
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Interesting discussion. The idea that an additional element in the optical train can improve color correction in a telescope is totally reasonable - all that is required is the element compensates for errors introduced by the objective lens.
Most of us think of the compensating element being part of the objective lens, like the flint glass in an achromatic lens, but there are many other ways to reduce chromatic aberration along the optical train. Schupmann and petzval designs are examples that use medial elements to improve "color correction" in telescopes.
-Stephen
Edited by slyke (08/28/08 01:53 PM)
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Glassthrower
Vendor - Galactic Stone & Ironworks
Reged: 04/07/05
Posts: 14693
Loc: Hurricane Alley
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I agree Stephen, perhaps this prism method is the "poor man's chromacorr" for ED scopes.
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Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.
Galactic Stone & Ironworks - Buy/Sell/Trade Meteorites, Moon Rocks, Mars Rocks, & 35 different falls and types!
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Scott Beith
SRF
Reged: 11/26/03
Posts: 33072
Loc: Gulfport, MS
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I just finished reading the review. Nice work Clive. I enjoyed it.
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  Scott
"The only thing necessary for the triumph of evil is for good men to do nothing." -- Edmund Burke.
"People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf." -- George Orwell
"The measure of a man’s greatness is not determined by what he accomplishes for himself, but by what he accomplishes for others.” -- Some Bald Guy
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 10348
Loc: Oort Cloud
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Thanks to the very high level of correction most ED refractors have, the prism doesn't need to do much to "tweak" the image optimally. Something the glass prism can do, but a mirror diagonal can't.
It's really only been in the past few years that ED refractors with this optical formula have risen to great popularity. Just a very nice coincidence that such an inexpensive and readily available accessory can help them achieve even better performance. 
If the data points keep accumulating to confirm that diagonal prisms improve image quality in a significant number of ED scopes (and perhaps other designs), this could be an opportunity for vendors to offer more star diagonals of the prismatic type-- especially better quality units.
More like the Baader T-2 model, but perhaps a bit less expensive. If there's competition at that end of the market, pricing should come down.
Finally, it seems to me that the term "right-angle star diagonal prism" is a bit of a mouthful. 
Doesn't exactly roll off the tongue... 
In the spirit of simplified nomenclature, I'd like to humbly suggest that from now on, such an accessory be called "the Clivagonal" (pronounced Clive-agonal).
Uhhhhhh, no good, huh?   
Sorry.... never mind.
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A few telescopes of dubious value.
Understanding wife and three curious cats.
"Semper ubi sub ubi"
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Scott Beith
SRF
Reged: 11/26/03
Posts: 33072
Loc: Gulfport, MS
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Good try... 
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Scott
"The only thing necessary for the triumph of evil is for good men to do nothing." -- Edmund Burke.
"People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf." -- George Orwell
"The measure of a man’s greatness is not determined by what he accomplishes for himself, but by what he accomplishes for others.” -- Some Bald Guy
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 10348
Loc: Oort Cloud
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Thanks, Scott.
Probably a good thing I'm outta wedgie range. 
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A few telescopes of dubious value.
Understanding wife and three curious cats.
"Semper ubi sub ubi"
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proud uncle
Pooh-Bah
Reged: 01/22/07
Posts: 1336
Loc: Central Texas
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Great, informative review, Clive.
Early in the review you mention the star diagonal prisms started waning in popularity because they do not work well with fast optical systems. But then you said nothing more about the performance of these diagonals with respect to different focal ratios. What is the fastest focal ratio that prisms can be effectively used with?
Would a star diagonal prism help with CA correction in an achromat, slower optical system?
Thanks.
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Kenneth
Zhumell 10" Dobsonian (f/4.9)
2" 32mm WA eyepiece
9mm, 12.5mm, and 20mm Plossls
6mm TMB/BO Planetary
2" 2x ED Barlow
Nikon 10x50 binocular (6.5 deg FOV)
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Glassthrower
Vendor - Galactic Stone & Ironworks
Reged: 04/07/05
Posts: 14693
Loc: Hurricane Alley
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Good question Kenneth. My AT66ED is about f/6, so I don't think that qualifies as fast. It's not slow, but not really fast either.
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Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.
Galactic Stone & Ironworks - Buy/Sell/Trade Meteorites, Moon Rocks, Mars Rocks, & 35 different falls and types!
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helpwanted
Carpal Tunnel
Reged: 07/04/07
Posts: 1757
Loc: Phoenix, AZ
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nice article... makes me wonder about non-ED's, like the Orion 100mm f6... good old fashioned achro.
how would the prism help one of these?
thanks,
david
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Glassthrower
Vendor - Galactic Stone & Ironworks
Reged: 04/07/05
Posts: 14693
Loc: Hurricane Alley
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Hi David,
I'm not sure about the prism diagonal, but a violet-reduction filter works wonders with the Orion 100mm f/6. I used to own the Orion 100mm f/6, and I also had a William Optics VR-1 filter - which was a must for planetary and lunar viewing. The benefit was negligible on other targets, like deep sky, but it certainly helped tame the color on brighter targets and at higher powers. FWIW, I tried 2 other violet reduction filters (Orion and Baader FK), and the WO was the best.
Regards and clear skies,
MikeG
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Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.
Galactic Stone & Ironworks - Buy/Sell/Trade Meteorites, Moon Rocks, Mars Rocks, & 35 different falls and types!
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 10348
Loc: Oort Cloud
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Hi Kenneth 'n' all.
An achromat might benefit a bit from a prism diagonal.
I believe Jim Burnett tried one in his long focus 4" achro and liked the result.
An achro (even an f/15 one) has quite a bit of c.a., compared to a good ED doublet (even a rather fast one). So, I doubt a prism will make a huge difference in the view... but for the price of trying one out, I'd say it's worth experimenting.
For a 100mm f/6 achro, well... it has much more c.a. and the subtle assistance a prism provides would get largely overwhelmed by the objective's spurious color.
I know how well a prism works in my F/5.95 110mm scope, so other ED's of smaller aperture in the F/6 range shouldn't present a problem using one.
IIRC, prisms really started to fall out of favor when folks with SCT's began using f/5 telecompressors to make them into "RFT" systems. Then, along came TeleVue with their F/5 (or thereabouts) Petzval refractors in the mid to late 1980's.
Using a prism with an F/5 probably isn't a good thing. Too much spurious color and spherical aberration gets added.
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A few telescopes of dubious value.
Understanding wife and three curious cats.
"Semper ubi sub ubi"
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Dylan Gladstone
scholastic sledgehammer
Reged: 08/05/07
Posts: 955
Loc: Connecticut, USA
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Would a Prism Diagonal do anything for a slow Mak?
Even with a dielectric I think I'm losing something. When playing with a makeshift "artificial star" I noticed that the diffraction rings were cleaner when viewed without the diagonal in place.
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Orion SkyView Pro 127mm Maksutov
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 10348
Loc: Oort Cloud
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Hi Dylan.
I think you'll see the best image with the Mak, when you don't use any diagonal. A friend of mine tried 1.25" dielectric and prism diagonals with his 6" SCT recently. He then looked through the scope without a diagonal. Target was Jupiter (thankfully quite low in the sky-- no neck straining required). The view "straight through" was contrastier. Of course, taking the diagonal out of the optical system will result in a somewhat shorter effective focal length if the scope uses a moving primary mirror to focus. The shorter e.f.l will lower the magnification a bit, which naturally results in a brighter, contrastier appearing image.
His opinion about the prism vs. mirror in the SCT:
The mirror (dielectric 99% reflectivity) looked brighter but the prism appeared a little bit contrastier. Best view was "straight thru".
FWIW, Questar Mak-Cassegrain telescopes have always used a built-in star diagonal prism, so that company seems to think they're worthwhile.
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A few telescopes of dubious value.
Understanding wife and three curious cats.
"Semper ubi sub ubi"
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Dylan Gladstone
scholastic sledgehammer
Reged: 08/05/07
Posts: 955
Loc: Connecticut, USA
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Thanks for the info Clive! I'll stick to the dielectric.
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Orion SkyView Pro 127mm Maksutov
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