dothead
super member
Reged: 08/13/08
Posts: 138
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: 10349
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 and three curious cats.
"Semper ubi sub ubi"
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hudson_yak
sage
Reged: 11/15/07
Posts: 480
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 + SVP
Meade 12" LightBridge
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 10349
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.
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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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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.
Galactic Stone & Ironworks - Buy/Sell/Trade Meteorites, Moon Rocks, Mars Rocks, & 35 different falls and types!
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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
member
Reged: 08/16/08
Posts: 41
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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current:
Tak FS-60c (guidescope for:)
Tak FCT-100/640 on Tak EM-10
Zeiss 7X42-T*P-Dialyt + Nikon 18x70 Binoculars
Coronado Maxscope 40
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highfnum
super member
Reged: 09/06/06
Posts: 104
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
Aunt Clootie
Reged: 04/14/07
Posts: 3613
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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.
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David E
Carpal Tunnel
Reged: 05/25/06
Posts: 1757
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
So when at times the mob is swayed
To carry praise or blame too far,
We may choose something like a star
To stay our minds on and be staid.
-Robert Frost
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Daniel Mounsey
Vendor - Woodland Hils
Reged: 06/12/02
Posts: 2882
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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
sage
Reged: 11/15/07
Posts: 480
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 + SVP
Meade 12" LightBridge
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 10349
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).
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A few telescopes of dubious value.
Understanding wife and three curious cats.
"Semper ubi sub ubi"
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Jim Rosenstock
Carpal Tunnel
Reged: 07/14/05
Posts: 2406
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: 89
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: 10349
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 and three curious cats.
"Semper ubi sub ubi"
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David E
Carpal Tunnel
Reged: 05/25/06
Posts: 1757
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
So when at times the mob is swayed
To carry praise or blame too far,
We may choose something like a star
To stay our minds on and be staid.
-Robert Frost
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proud uncle
Pooh-Bah
Reged: 01/22/07
Posts: 1336
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: 10349
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 and three curious cats.
"Semper ubi sub ubi"
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