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asaint
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Prism Diagonal
      #2609017 - 08/28/08 08:00 AM

Prism Diagonal

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Jim Rosenstock
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Re: Prism Diagonal new [Re: asaint]
      #2609150 - 08/28/08 09:51 AM

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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Clive Gibbons
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Re: Prism Diagonal new [Re: Jim Rosenstock]
      #2609197 - 08/28/08 10:14 AM

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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Glassthrower
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #2609307 - 08/28/08 11:07 AM

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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Clive Gibbons
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Re: Prism Diagonal new [Re: Glassthrower]
      #2609330 - 08/28/08 11:15 AM

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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Glassthrower
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #2609440 - 08/28/08 12:02 PM

Thanks Clive! I just emailed Don Rothman to inquire about it.

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slyke
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Re: Prism Diagonal new [Re: asaint]
      #2609719 - 08/28/08 01:52 PM

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
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Re: Prism Diagonal new [Re: slyke]
      #2609786 - 08/28/08 02:16 PM

I agree Stephen, perhaps this prism method is the "poor man's chromacorr" for ED scopes.

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Scott BeithAdministrator
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Re: Prism Diagonal new [Re: Glassthrower]
      #2609864 - 08/28/08 02:53 PM

I just finished reading the review. Nice work Clive. I enjoyed it.

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Clive Gibbons
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Re: Prism Diagonal new [Re: Glassthrower]
      #2609894 - 08/28/08 03:07 PM

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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Scott BeithAdministrator
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #2609936 - 08/28/08 03:23 PM

Good try...

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Clive Gibbons
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Re: Prism Diagonal new [Re: Scott Beith]
      #2609962 - 08/28/08 03:37 PM

Thanks, Scott.

Probably a good thing I'm outta wedgie range.


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proud uncle
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #2610096 - 08/28/08 04:50 PM

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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Glassthrower
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Re: Prism Diagonal new [Re: proud uncle]
      #2610112 - 08/28/08 05:01 PM

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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helpwanted
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Re: Prism Diagonal new [Re: Glassthrower]
      #2610943 - 08/28/08 11:17 PM

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
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Re: Prism Diagonal new [Re: helpwanted]
      #2611036 - 08/29/08 12:09 AM

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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Clive Gibbons
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Re: Prism Diagonal new [Re: proud uncle]
      #2611389 - 08/29/08 08:53 AM

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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Dylan Gladstone
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #2611586 - 08/29/08 10:56 AM

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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Clive Gibbons
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Re: Prism Diagonal new [Re: Dylan Gladstone]
      #2611634 - 08/29/08 11:26 AM

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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Dylan Gladstone
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #2611695 - 08/29/08 12:06 PM

Thanks for the info Clive! I'll stick to the dielectric.

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dothead
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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).


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Jim Rosenstock
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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
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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
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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
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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
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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
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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
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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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rustynailz911
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #2626362 - 09/05/08 09:33 PM

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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Clive Gibbons
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Re: Prism Diagonal new [Re: rustynailz911]
      #2630375 - 09/08/08 08:49 AM

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.


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Bradley B
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #2630547 - 09/08/08 10:57 AM

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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Clive Gibbons
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Re: Prism Diagonal new [Re: asaint]
      #2669604 - 09/29/08 09:34 AM

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.


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Clive Gibbons
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #3990179 - 08/18/10 09:09 AM

Addendum--

Further research and testing has shown that prism diagonals can also improve the image correction of some ED triplet lenses.
ED triplets typically suffer from significantly less c.a. and spherochromatism than ED doublets. Many ED triplets are optimally corrected to use a mirror-type star diagonal, but I've seen cases of triplets benefiting from a prism diagonal.
Most recent observations with a Stellarvue 115T (latest version) triplet have revealed that for high magnification views of bright objects, a Takahashi 1.25" diagonal prism results in perfectly apochromatic performance. A dielectric mirror diagonal used in the scope revealed very slight red defocus.
So, whether you have an ED doublet or a triplet, it's useful to experiment to see what diagonal performs best for high power work.


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EastAnglian
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #5040895 - 01/27/12 12:49 PM

As I understand it the very best image is obtained from not using any form of diagonal, or zenith prism? They are supplied with a lot of (all?) lower end beginners scopes, for no reason other than to make more money when this 'beginner' decides to upgrade his equipment, and having gotten used to unecessarily using one, he buys another. I AM one of these poor unfortunate people who've been conned into using one, and am making a big effort to rid myself of the habit.
Cheers
Max


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Mark9473
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Re: Prism Diagonal new [Re: EastAnglian]
      #5041124 - 01/27/12 03:06 PM

I can think of three scenarios why you would write that: (1) you observe only things relatively close to the horizon, (2) you're rather young and an accomplished acrobat, or (3) you have a fantastic home-built mount contraption that positions your telescope just right over a reclining chair.

I hope it's 3 and you'll post a picture of the mount!


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astrogeezer41
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Re: Prism Diagonal new [Re: Clive Gibbons]
      #5041793 - 01/27/12 10:29 PM

I put some of this on the Refractor Forum a few days ago as I was referring skyjim to this thread.

I was first told about this thread on prism diagonals on another forum when someone had asked about a red halo on an ED scope. I went to this thread and read about many of the EDs being corrected towards the red end of the spectrum.
Since I believed that my 80mm,f/7 achromat was also corrected a bit to the red side I replaced my mirror diagonal with a Tak prism diagonal, and tested it on Jupiter at 150x.

The difference was amazing. Instead of doing the tweaking that I had to do with the mirror diagonal I simply had to start outside of focus with the Tak until the image was sharp. When the seeing was steady the image of Jupiter was virtually white in the zones. Since then I have always used the prism diagonal with this scope.

I also have a less expensive Orion prism diagonal which also
works well with that scope.

I am grateful for the work that Mr. Gibbons has done on this issue and to the people on the other forum who provided the link.

So thanks everyone,
Robert


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EastAnglian
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Re: Prism Diagonal new [Re: Mark9473]
      #5043797 - 01/29/12 08:40 AM

Quote:

I can think of three scenarios why you would write that: (1) you observe only things relatively close to the horizon, (2) you're rather young and an accomplished acrobat, or (3) you have a fantastic home-built mount contraption that positions your telescope just right over a reclining chair.

I hope it's 3 and you'll post a picture of the mount!



It's 3; almost! I intend to start work this week on a mount similar to that made by Grendel, here:
http://www.cloudynights.com/photopost/showphoto.php?photo=25003&size=big&password=&sort=7&thecat=3257
Then, by sitting on an astronomer's stool with the eyepiece under my eye, all that's required is to raise the stool to observe objects at a lower altitude. I'll post a pic when I'm finished.
Also see this from The Amateur Astronomer's Handbook by James Muirden:
http://f1.grp.yahoofs.com/v1/0EIlT8x_ujv05WRAvn6kzpgTPg4Rj27stXiyomWzpxSXc4SCjVUurZZkI9tpeyazzuj-GYmCiX1kFTLlJipyeC2GH2j5t4pNvcA/cut.jpg
Cheers
Max


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grendel
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Re: Prism Diagonal new [Re: EastAnglian]
      #5044353 - 01/29/12 02:47 PM

Max I have to say that with that pier and a 1250mm f/l scope near the horizon I am standing my 6'4" on the step next to the pier, and when looking at the zenith 'straight through' I am sitting on the step with the eyepiece just a few inches from the pier and quite low really. but the mount is really super steady on that 1/2" thick 6" diameter cardboard pier - that weighs in at about 20lbs.
Grendel


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grendel
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Re: Prism Diagonal new [Re: grendel]
      #5044361 - 01/29/12 02:50 PM

oh and when I use a diagonal its always a prism one - I even have a .965" / 1.25" hybrid prism diagonal.
Grendel


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EastAnglian
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Re: Prism Diagonal new [Re: grendel]
      #5044856 - 01/29/12 07:43 PM

Quote:

Max I have to say that with that pier and a 1250mm f/l scope near the horizon I am standing my 6'4" on the step next to the pier, and when looking at the zenith 'straight through' I am sitting on the step with the eyepiece just a few inches from the pier and quite low really. but the mount is really super steady on that 1/2" thick 6" diameter cardboard pier - that weighs in at about 20lbs.
Grendel



Hi Grendel!
From my home position I can't observe anything near horizontal, and due to atmospheric conditions, don't do anything serious below thirty. Using that info as a starting position, actually diminishes the up/down eye movement considerably. I like to sit when I observe, preferably almost recline, so a comfortable astonomers stool with notched risers in fairly small increments, within safety limits should suit me fine. I hear what you're saying about closeness to pier, and will either make a simple alt/az balanced mount with large altitude projection, or just simply not observe at zenith. On that note, re your other post, although I'm going to give it a go in not using a diagonal, I have to say that I've never seen a hybrid .965/ 1 1/4" PRISM diagonal; where did you get it?
Cheers
Max


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grendel
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Reged: 04/12/09

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Re: Prism Diagonal new [Re: EastAnglian]
      #5045386 - 01/30/12 04:43 AM

I picked both mine up from telescope house - in their box of odds and ends, one is a 45 degree hybrid diagonal, the other a standard diagonal. not even sure what brand they are. maybe I just got lucky or they took a standard .965 diagonal and happened to have a 1.25 eyepiece holder that fit the same thread.or a standard 1.25 and had a .965 nose that fit the thread - never really thought about it much, it was just available when I went in and asked - just 10 each from the bits box too.
Grendel


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EastAnglian
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Re: Prism Diagonal new [Re: grendel]
      #5045515 - 01/30/12 08:57 AM

Very nice too! I have a couple of standard plastic bodied mirror type, but haven't seen a prism in this configuration.
Cheers
Max


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Sarkikos
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Re: Prism Diagonal new [Re: asaint]
      #5045631 - 01/30/12 10:36 AM

FWIW, a couple years ago I experimented with five different prism and mirror diagonals on my ST80 (80mm f/5 achromat). The diagonals included 1.25" and 2" mirror diagonals, dielectric and otherwise. I used several 1.25" prism diagonals, correct image and reversed image, BAK4 and BK7. I did not see any improvement in CA correction using any of the diagonals.

Recently I acquired a 70mm f/12.9 achromat. I might try the experiment again using all the 1.25" diagonals. Maybe a prism diagonal will improve the slight CA in this scope?

Mike


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EastAnglian
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Re: Prism Diagonal new [Re: Sarkikos]
      #5045677 - 01/30/12 11:01 AM

Hi Mike,
That's a very interesting experiment you did there. I for one would be very interested to hear the result, if you do something similar with the new scope. It's not often that anybody has such a comprehensive set of diagonals to do such a test.
Cheers
Max


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grendel
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Reged: 04/12/09

Loc: Canterbury, Kent, UK
Re: Prism Diagonal new [Re: EastAnglian]
      #5046290 - 01/30/12 04:33 PM Attachment (47 downloads)

I just dug my two hybrid prism diagonals out and took some pics, the 45 degree one is a meade and has what is obviously an adaptor thread for the 1.25" eyepiece holder.
the other looks like it might have the same, but I couldnt unscrew it.
Grendel


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grendel
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Re: Prism Diagonal new [Re: grendel]
      #5046291 - 01/30/12 04:34 PM Attachment (39 downloads)

and the two.
Grendel


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grendel
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Re: Prism Diagonal new [Re: grendel]
      #5046312 - 01/30/12 04:47 PM

a bit of research shows a 'Meade Diagonal Prism #917 Hybrid' with the 90 degree diagonal and
'Meade Hybrid Erecting Prism #931' with a 45 degree prism
so obviously they were made - both are on clearance at telescope house in the uk for 37
Grendel

Edited by grendel (01/30/12 04:48 PM)


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EastAnglian
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Re: Prism Diagonal new [Re: grendel]
      #5046457 - 01/30/12 06:05 PM

Hi Grendel!
Many thanks for taking the pics, and doing the research! The numbers are particularly useful.
Cheers
Max


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