Telescope Reviews: Prism Diagonal

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dothead
scholastic sledgehammer

Reged: 08/13/08

Loc: Heidelberg, Germany

Re: Prism Diagonal new

[Re: Dylan Gladstone]
#2611735 - 08/29/08 12:29 PM

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

Loc: Oort Cloud

Re: Prism Diagonal new

[Re: dothead]
#2612011 - 08/29/08 02:39 PM

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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hudson_yak
Carpal Tunnel

Reged: 11/15/07

Loc: Hyde Park, NY, USA

Re: Prism Diagonal new

[Re: Clive Gibbons]
#2612115 - 08/29/08 03:27 PM

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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Clive Gibbons
Mostly Harmless

Reged: 05/26/05

Loc: Oort Cloud

Re: Prism Diagonal new

[Re: hudson_yak]
#2612191 - 08/29/08 04:06 PM

Thanks for mentioning that, Mike.
Yes I remember you mentioning it.

Worth noting for owners of Radian (or similar) oculars.

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Glassthrower
Vendor - Galactic Stone & Ironworks

Reged: 04/07/05

Loc: Oort Cloud 9

Re: Prism Diagonal new

[Re: Clive Gibbons]
#2612298 - 08/29/08 05:17 PM

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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Veridian
sage

Reged: 07/17/06

Re: Prism Diagonal new

[Re: Clive Gibbons]
#2612696 - 08/29/08 10:19 PM

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
scholastic sledgehammer

Reged: 08/16/08

Loc: Switzerland

Re: Prism Diagonal new

[Re: asaint]
#2612939 - 08/30/08 03:24 AM

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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highfnum
Carpal Tunnel

Reged: 09/06/06

Loc: NE USA

Re: Prism Diagonal new

[Re: Fomalhaut]
#2613893 - 08/30/08 04:54 PM

good article got me thinking of trying some test with a
bunch of diagonal I have

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Mr Onions
Two Time International Photographical Competition Winner

Reged: 04/14/07

Loc: Newcastle upon Tyne.

Re: Prism Diagonal new

[Re: highfnum]
#2613925 - 08/30/08 05:17 PM

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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David E
Post Laureate

Reged: 05/25/06

Loc: North Carolina

Re: Prism Diagonal new

[Re: helpwanted]
#2614713 - 08/31/08 08:16 AM

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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Daniel Mounsey
Vendor - Celestron

Reged: 06/12/02

Re: Prism Diagonal new

[Re: David E]
#2614847 - 08/31/08 10:34 AM

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
Carpal Tunnel

Reged: 11/15/07

Loc: Hyde Park, NY, USA

Re: Prism Diagonal new

[Re: Glassthrower]
#2616061 - 08/31/08 11:50 PM

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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Clive Gibbons
Mostly Harmless

Reged: 05/26/05

Loc: Oort Cloud

Re: Prism Diagonal new

[Re: Mr Onions]
#2618566 - 09/02/08 09:12 AM

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).

Post Extras:

Jim Rosenstock
Postmaster

Reged: 07/14/05

Loc: MD, south of the DC Nebula

Re: Prism Diagonal new

[Re: Glassthrower]
#2618644 - 09/02/08 10:06 AM

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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jumpingjackflash
newbie

Reged: 09/02/08

Re: Prism Diagonal new

[Re: Clive Gibbons]
#2619868 - 09/02/08 08:03 PM

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
super member

Reged: 07/31/06

Loc: Bruges, Belgium

Re: Prism Diagonal new

[Re: jumpingjackflash]
#2622988 - 09/04/08 08:20 AM

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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Clive Gibbons
Mostly Harmless

Reged: 05/26/05

Loc: Oort Cloud

Re: Prism Diagonal new

[Re: sdufoer]
#2623464 - 09/04/08 12:39 PM

That sounds like a large deviation.
You should contact Burgess and inquire.

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David E
Post Laureate

Reged: 05/25/06

Loc: North Carolina

Re: Prism Diagonal new

[Re: sdufoer]
#2623495 - 09/04/08 12:51 PM

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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proud uncle
Carpal Tunnel

Reged: 01/22/07

Loc: Central Texas

Re: Prism Diagonal new

[Re: Glassthrower]
#2624033 - 09/04/08 05:31 PM

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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Clive Gibbons
Mostly Harmless

Reged: 05/26/05

Loc: Oort Cloud

Re: Prism Diagonal new

[Re: proud uncle]
#2625625 - 09/05/08 01:59 PM

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.

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