asaint
Carpal Tunnel
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Prism Diagonal
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Jim Rosenstock
Post Laureate
Reged: 07/14/05
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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
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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, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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Glassthrower
Vendor - Galactic Stone & Ironworks
Reged: 04/07/05
Posts: 16328
Loc: Kuiper Belt
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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.
Meteorites, Moon Rocks, Mars Rocks, Tektites, Fossils, Minerals, Crystals, & Trinitite.
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
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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, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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Glassthrower
Vendor - Galactic Stone & Ironworks
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Thanks Clive! I just emailed Don Rothman to inquire about it. 
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slyke
super member
Reged: 02/21/08
Posts: 115
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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
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Posts: 16328
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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.
Meteorites, Moon Rocks, Mars Rocks, Tektites, Fossils, Minerals, Crystals, & Trinitite.
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Scott Beith
SRF
Reged: 11/26/03
Posts: 37004
Loc: Frederick, MD
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I just finished reading the review. Nice work Clive. I enjoyed it.
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   SLAP Observer (TMB130SS, SV102V, SV80ED)
"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: 13320
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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, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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Scott Beith
SRF
Reged: 11/26/03
Posts: 37004
Loc: Frederick, MD
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Good try... 
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SLAP Observer (TMB130SS, SV102V, SV80ED)
"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: 13320
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, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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proud uncle
Carpal Tunnel
Reged: 01/22/07
Posts: 1572
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: 16328
Loc: Kuiper Belt
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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.
Meteorites, Moon Rocks, Mars Rocks, Tektites, Fossils, Minerals, Crystals, & Trinitite.
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helpwanted
Carpal Tunnel
Reged: 07/04/07
Posts: 2585
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
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Posts: 16328
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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.
Meteorites, Moon Rocks, Mars Rocks, Tektites, Fossils, Minerals, Crystals, & Trinitite.
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 13320
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.
--------------------
A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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Dylan Gladstone
scholastic sledgehammer
Reged: 08/05/07
Posts: 988
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: 13320
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, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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Dylan Gladstone
scholastic sledgehammer
Reged: 08/05/07
Posts: 988
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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dothead
super member
Reged: 08/13/08
Posts: 163
Loc: Heidelberg, Germany
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If I recall correctly, Zeiss recommended the use of prism diagonals when their APQ Apos were introduced. Interesting, I think!
Ralph
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 13320
Loc: Oort Cloud
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Hi Ralph.
I recall back in the '90s when the APQ's were being offered, that a Zeiss prism diagonal was recommended for best image correction. It was that factoid which first got me thinking about prism vs. mirror diagonals.
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A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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hudson_yak
Pooh-Bah
Reged: 11/15/07
Posts: 1092
Loc: Hyde Park, NY, USA
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I mentioned this in one of the many threads about this, but not sure if you saw it. I tried using my old Meade prism on Jupiter several nights ago, and while seeing wasn't good enough to draw any conclusions about color or detail, it did make me remember one of the reasons I changed to mirror diagonals years ago, there was an extra reflection dancing around, which I assume is bouncing back from the top surface of the prism.
I haven't seen anyone else comment on this, so perhaps it's unique to my prism or high-power eyepiece choice (TV Radians).
Mike
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Meade 8" LX10
Orion ED100 + Losmandy GM-8
Meade 12" LightBridge
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 13320
Loc: Oort Cloud
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Thanks for mentioning that, Mike.
Yes I remember you mentioning it.
Worth noting for owners of Radian (or similar) oculars.
--------------------
A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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Glassthrower
Vendor - Galactic Stone & Ironworks
Reged: 04/07/05
Posts: 16328
Loc: Kuiper Belt
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Mike,
Maybe a simpler design, like an orthoscopic, would solve those ghosting issues......or maybe it was the prism. Do you have a simpler eyepiece with less elements in it to try?
Regards and clear skies,
MikeG
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Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.
Meteorites, Moon Rocks, Mars Rocks, Tektites, Fossils, Minerals, Crystals, & Trinitite.
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Veridian
sage
Reged: 07/17/06
Posts: 478
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Clive,
Good article you wrote. More good follow-up information here.
Ahhh,, now, something else to buy!! 
Well,, it's what we do, isn't it? All for those incrementals. 
Thanks again,
Mark J.D.
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Fomalhaut
sage
Reged: 08/16/08
Posts: 230
Loc: Switzerland
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A diagonal prism introduces slight overcorrection and thus can diminish the slight undercorrection which is present in many refractors, such as e.g. in most Taks, resulting in slightly better color correction at the end of the focal path.
Fomalhaut
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Currently:
Tak FS-60c (guidescope for:)
Tak FCT-100/640 on Tak EM-10
Tak Mewlon-180 on same
IntesMicro-M500
Zeiss 7X42-T*P-Dialyt + Nikon 18x70 Binoculars
Coronado Maxscope 40
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highfnum
super member
Reged: 09/06/06
Posts: 191
Loc: NY
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good article got me thinking of trying some test with a
bunch of diagonal I have
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Mr Onions
Tynee Man
Reged: 04/14/07
Posts: 4695
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I remember your original thread in the refractor forum,Clive, it certainly made a big difference when you were hunting out those tight doubles.
A very interesting article that.
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Barchester Onions.
http://www.flickr.com/photos/httpwwwflickrcomphotos_caladanbrood/
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David E
Post Laureate
Reged: 05/25/06
Posts: 3003
Loc: North Carolina
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Quote:
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
In my experience, no, a prism diagonal will not reduce the color fringing in a fast achromat. I have one achromat, 80/F7 where it seemed to do the reverse, it put a blue color hue across the lunar disc as if I had put a light blue filter on the eyepiece. But I think Clive has hit on something here. If you have a long achromat like the Stellarvue 80/9D or Burgess Planet Hunter, a prism diagonal may be a "forgotten" accessory for these scopes.
David E
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David E
Confucius say: "Those who leave leftovers are condemned to re-heat it."
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Daniel Mounsey
Vendor - Woodland Hils
Reged: 06/12/02
Posts: 3200
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Clive,
This is a good reminder. In truth after having tested certain prisms, I actually favor them for planets.
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hudson_yak
Pooh-Bah
Reged: 11/15/07
Posts: 1092
Loc: Hyde Park, NY, USA
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Quote:
Maybe a simpler design, like an orthoscopic, would solve those ghosting issues......or maybe it was the prism. Do you have a simpler eyepiece with less elements in it to try?
I don't have many different eyepieces but did try a Panoptic and old 26mm Meade Plossl tonight. The same reflection happens with all of them. One thing I found though is it's pretty subtle and easy to ignore with higher magnification eyepieces, just the ones the prism would most obviously benefit, I guess.
Mike
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Meade 8" LX10
Orion ED100 + Losmandy GM-8
Meade 12" LightBridge
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 13320
Loc: Oort Cloud
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Thanks again to everybody who's either enjoyed or dismissed the article. 
A little more testing with a friend's newly acquired William Optics Megrez 88FD has tended to confirm many of the points previously mentioned.
The more data points, the better we'll be able to realize how much star diagonal prisms can help (or not).
--------------------
A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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Jim Rosenstock
Post Laureate
Reged: 07/14/05
Posts: 3657
Loc: MD, south of the DC Nebula
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Quote:
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.
Ahhhhh, another one of those half-fast ED scopes, huh?
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QUESTION AUTHORITY!
"errr....sez who??"
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jumpingjackflash
newbie
Reged: 09/02/08
Posts: 1
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worked great on my 90mm f11 acro. jupiter had a brillant blue ring on satruday. last night with the prism it was very faint. thanks for the info, the prism diagonal was just sitting in the bottom of a extra parts box. so this cost $0.00.
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sdufoer
member
Reged: 07/31/06
Posts: 94
Loc: Bruges, Belgium
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I collimated my new refractor yesterday with the chesire. So far, so good... But when I put my new 2" burgess prism in it and did another check I saw that the instrument was now heavyly out of collimation. When I put a lasercollimator in the prism only I saw that the reflection on the other side of the prism diagonal was 8mm out of the center... :-s.
I don't think it's possible to collimate this prism diagonal. Is this normal? Should I ask a new one from the dealer?
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Sjoerd Dufoer (website)
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Losmandy G11 mount
Vixen VC200L VISAC Cassegrain
Vixen ED80sf Refractor
SBIG ST10XME camera
Vixen 114mm F8 newton on GP-E
13" F5.6 ATM trusstube dobson
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 13320
Loc: Oort Cloud
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That sounds like a large deviation.
You should contact Burgess and inquire.
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A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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David E
Post Laureate
Reged: 05/25/06
Posts: 3003
Loc: North Carolina
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Quote:
I collimated my new refractor yesterday with the chesire. So far, so good... But when I put my new 2" burgess prism in it and did another check I saw that the instrument was now heavyly out of collimation. When I put a lasercollimator in the prism only I saw that the reflection on the other side of the prism diagonal was 8mm out of the center... :-s.
I don't think it's possible to collimate this prism diagonal. Is this normal? Should I ask a new one from the dealer?
First I'd double check the diagonal's collimation with your Cheshire.
It took a few spoonfulls of aggravation and a cupfull of cuss words, but I managed to collimate an old Orion prism diagonal of mine. But in your case if you bought it new I'd call for a replacement, or let them collimate it and return it.
David E
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David E
Confucius say: "Those who leave leftovers are condemned to re-heat it."
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proud uncle
Carpal Tunnel
Reged: 01/22/07
Posts: 1572
Loc: Central Texas
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Quote:
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.
Interesting, Michael. The AT66ED is exactly the scope I had in mind when I asked the question. I'm seriously considering getting one for a grab and go travel scope, and looking at options for a diagonal.
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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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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 13320
Loc: Oort Cloud
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Digging back to an earlier thread I started (in Dec. '07), shows this interesting post by Doug Peterson, who tried a prism with several different refractors.
His findings seem to support the idea that individual testing and experimentation are the only way to know how well a diagonal prism will work in any given scope.
Thanks to Doug and everyone else who contributed to that discussion.
--------------------
A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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rustynailz911
professor emeritus
Reged: 01/30/08
Posts: 538
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i have budget equipment and have a 45deg Meade diagonal with a prism. i had some serious ghosting as well i was thinking it was the MA eyepieces i removed the diagonal and went just eyepiece perfect focus and no ghosting so it was the diagonal.i just ordered a 90 deg mirror diagonal.im not saying that prisms are bad they just didnt work for me it might work awsome in another scope .plus i really disliked the 45 deg angle it is uncomfortable on my scope for viewing.
Rusty
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6"F8 Newt (a work in progress)
4.5"F8 Newt
9.7mm Meade 4000 Plossl 52 Deg FOV
15mm Meade 4000 Plossl 52 Deg FOV
26mm Meade 4000 Plossl 52 Deg FOV
2x Shorty Acromatic Barlow
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 13320
Loc: Oort Cloud
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Just to reiterate...
a 45-degree erecting prism diagonal is an entirely different animal, compared to a 90-degree star diagonal prism. If you had a poor experience with an erecting prism (either 45 degree or 90 degree type), I'd strongly recommend that you try the simpler, traditional star diagonal prism. Much less chance of introducing stray reflections and other optical errors with that type.
--------------------
A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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Bradley B
sage
Reged: 05/14/07
Posts: 453
Loc: Sacramento, California
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That old Burgess 2" Diagonal was of the "correct image" variety, so no wonder there may be issues of collimation . . . however 8mm is extreme.
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Celestron C8
William Optics ZS66
* * * * * * * * *
It is error only, and not truth, that shrinks from inquiry. - Thomas Paine
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 13320
Loc: Oort Cloud
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Further testing with ED doublets has shown that star diagonal prisms using BK7 glass typically have a greater benefit with regards to improving color correction in many ED doublet refractors.
It's usually the less expensive star diagonal prisms which use BK7 glass.
More expensive prisms, such as the Baader T-2 model (Zeiss) and the Takahashi 1.25" star diagonal, use a higher index glass (BAK-4, or analogous). This results in more complete "total internal reflection" when used in a very fast optical system (f/5 or faster). However, since most refractors aren't of such a short f/ratio, it's not necessary to use a high-index prism. BK7 glass will work just fine.
But, MOST IMPORTANTLY, the high index glass in more expensive prisms doesn't correct the prevalent color error (red defocus) which so many ED doublets display.
In conclusion, it would appear that the optimal type of prism for use in many of today's ED doublet refractors, is one using BK7 glass.
Inexpensive 1.25" star diagonal prisms by Celestron, Meade and Orion all use BK7 glass.
--------------------
A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.
"Semper ubi sub ubi"
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