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Eyepiece Performance using a Prism Diagonal

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#1 BillP

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Posted 30 November 2013 - 11:14 AM

I had the opportunity last evening for a first look evaluation of a Prism Diagonal vs. a Dialectric Mirror diagonal. The Prism was the Tak 1.25" (on loan) and the Dialectric was the AstroTech 1.25". Scope used was the TSA-102 and just two targets, Jupiter and M42. Eyepieces were the 6mm ZAO-II and the 5mm XO for planetary. For M42 Sterling Plossls were used, somtimes Barlowed.

Bottom line was that the Prism won the show on both targets, producing a definite notch better results on Jupiter and being subtly better on M42.

The Tak Prism provided a clearer view of a white spot transit, showing greater perceived contrast as well as crisper definition. Structure within the main belts similarly had greater perceived contrast and crisper definition, with more detail being visible as a result. The polar region differences were even more pronounced. In the Prism there was a lot of detailed structure visible instead of just being a simple gradient of shades. The Dialectric showed none of this. Scatter around Jupiter appeared at about the same extent in both diagonals, however it was not as bright in the Prism. Finally, and oddly IMO, the focus snap was much more pronounced using the Prism. Overall I felt like these two eyepieces were performing at what I would characterize as a level better in planetary performance, similar to what I might see going from a lesser performing eyepiece to the ZAOs.

Moving to M42 I was looking for three things: faintest star transmission, nebula extent, and mottled structure performance. With transmission, both diagonals were performing the same relative to stars. In both Trap-E was just as well defined (F was not showing this evening) and adverted vision stars were showing just as well. Extent of the nebula was slightly better in the Prism, with the outer wings of M42 showing well with adverted vision in the Prism and barely at all with the Dialectric. The brightness of M43 was also more obvious in the Prism vs the Dialectric. Finally, the mottled structure presented itself close to the same with each diagonal, but my impression was that it was just a little more detailed with the Prism. This only became somewhat apparent when the magnification was increased -- not apparent using the 17mm (48x) but then seemingly better with the 2.5x Barlowed 25mm (82x). So not a firm call here.

As a side note, I do have an old Celestron 1.25" Prism from the 1980's, but wanted to see how a quality modern Prism performs against a modern Dialectric. FWIW, a surprise for me was that the light path of the Tak Prism was fairly shorter than the old Celestron Prism. Something I was not expecting.

#2 russell23

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Posted 30 November 2013 - 11:28 AM

I have a Tak 1.25" Prism and concluded much the same thing when I first got it. It definitely was an improvement upon the Williams optics diagonals (1.25" and 2" dielectric).

Currently I am using a 2" AP Maxbright diagonal, but I still have the Tak diagonal. The main thing I don't like about the Tak diagonal is that the nosepiece is not threaded for filters. Optically though it is great.

Dave

#3 PowellAstro

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Posted 30 November 2013 - 11:42 AM

This all makes good since because the mirror will transmit 99 percent of off axis light and the prism will not. It will also add a small amount of SA correction.

#4 dbowlin

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Posted 30 November 2013 - 11:46 AM

Bill, how did the 80's prism do against the TAK. Is the TAK worth the extra $$?
Dale

#5 SteveC

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Posted 30 November 2013 - 12:15 PM

Where can I get prism conversion kits.

#6 george tatsis

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Posted 30 November 2013 - 02:28 PM

I wonder if anybody has ever compared an inexpensive 1.25" prism diagonal with a premium unit. :question:

Hope Bill conducts more experiments , so that we get more info on it ;)

George

#7 mgwhittle

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Posted 30 November 2013 - 04:49 PM

Interesting observation Bill. I have a Zeiss diagonal that I use with a binoviewer but I have not used it by itself since I keep hearing that a prism will supposedly hurt the performance of a highly corrected APO. I think I have that right. Anyway, your TSA is an excellent APO so I might have to try the Zeiss out with my AP.

Thanks for posting.

#8 BillP

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Posted 30 November 2013 - 05:16 PM

I wonder if anybody has ever compared an inexpensive 1.25" prism diagonal with a premium unit. :question:

Hope Bill conducts more experiments , so that we get more info on it ;)

George


I will compare it to my Celestron, but the Celestron I have never b een satisfied with...and it has some problems as a small part of the FOV on one side vignettes from something internal or maybe the prism is not entirely square. I've had it apart but have never been able to fully resolve what the issue is going on. So not sure my 1980's prism is representative for a comparison. \\With the Tak, while I was not looking for any added color, I did not notice any with last nights observing.

#9 PJ Anway

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Posted 30 November 2013 - 05:18 PM

Nice report Bill!

I find from experience that I prefer a prism over a mirror diagonal. I use an older Zeiss M44 prism diagonal in my f5.6 refractor and am very satisfied with the performance.

#10 Cyclop_si

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Posted 30 November 2013 - 05:22 PM

Once I used low cost prism diagonal, and what I really did not like was "line spike" on bright stars at high magnifications (two spikes 180° apart). I thought this is due to prism diagonal design, but reading how good Tak prism performed, it looks like this might be due to poor prism build as well. You probably have not noticed anything like that spike on Tak diagonal, right?

#11 JustaBoy

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Posted 30 November 2013 - 05:36 PM

David,

That sounds like a 'correct image' prism star diagonal to me.

I have many cheap 1.25" prism diagonals from the 60s and 70s (not correct image) and see these spikes in none of them.

Thanks,

-Chuck

#12 BillB9430

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Posted 30 November 2013 - 09:03 PM

+1 on Chuck's reply, David. Amici prisms (90 degree) and Schmidt prisms (45 degree) both have a "roof" with an edge in the center of the field. This roof portion of the prism reverses right and left to give a fully corrected image. Diffraction of light hitting the "roofline" creates a horizontal spike visible both directions from every bright object. Better quality correct image prisms have a finer edge and so a fainter spike, but all have it. Regular right angle prism astronomical diagonals show an erect image but it is reversed right and left - no roof in these, hence there should be no spike. - Bill

#13 urassner

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Posted 30 November 2013 - 10:13 PM

Bill, in some older CN threads it was mentioned that Prism diagonals are not suitable for shorter focal length telescopes (f10 and faster) due to refraction effects when light enters at an angle creating chromatic aberration. I noticed that you used a f8 refractor and it did not seem to be an issue. Might it be more of an issue with 2 inch diagonals and lower magnification? I would appreciate your insight.

Thank you, Ulrich

#14 JustaBoy

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Posted 30 November 2013 - 10:19 PM

I think the critical ratio is more like F/6, or there a bouts.

I use one on my Orion F/5 ST-80, but who could tell the difference:-)

-Chuck

#15 urassner

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Posted 30 November 2013 - 10:36 PM

Thanks, Chuck, for your input. Were you using 1.25 inch or 2 inch eyepieces? Does it make a difference?

Ulrich

#16 JustaBoy

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Posted 30 November 2013 - 10:49 PM

The ST-80 doesn't have a 2" focuser, so 1.25" Prism diagonal.

I don't really know if the size of the eyepiece field stop would make a difference or not.

Sorry,

-Chuck

#17 jrbarnett

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Posted 30 November 2013 - 11:07 PM

I think the Tak prism uses different glass than most other prism diagonals. If so, I wonder if these results are unique to the Tak prism or rather would also apply to other prisms using different glass?

I have the Tak prism as well as both vintage and recent Japanese prism diagonals. I've never checked them head to head, but I have tested every diagonal I own and many I no longer own for scatter using calipers and a GLP (measuring contact spot diameters). Prism diagonals tested in this manner show very low scatter - better than the best dielectrics and on par with the best standard coated mirrors, which tend to be smoother than the dielectrics.

Regards,

Jim

#18 george tatsis

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Posted 01 December 2013 - 12:28 AM

Bill, in some older CN threads it was mentioned that Prism diagonals are not suitable for shorter focal length telescopes (f10 and faster) due to refraction effects when light enters at an angle creating chromatic aberration. I noticed that you used a f8 refractor and it did not seem to be an issue. Might it be more of an issue with 2 inch diagonals and lower magnification? I would appreciate your insight.

Thank you, Ulrich


Same question here!

I remember reading the previous claim in the past, and it was the reason for my not buying a prism diagonal.

However, I suspect Jim is right by pointing out that it may have to do with the Tak design / material used in this particular prism, and not all prisms in general.

It's definitely an issue worth looking into, because according to Bill's findings, for the price of a good eyepiece you can enhance your planetary viewing by simply upgrading to this particular diagonal.

I think that this is the least expensive solution to increasing planetary contrast, because all eyepieces used with this particular diagonal will be benefited by it.

It's even more than " kill two birds with one stone " kinda thing ,since on average, we have our three to four favorite planetary eyepieces taking advantage of these wonderfully unexpected diagonal properties! :jump:

George

#19 george tatsis

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Posted 01 December 2013 - 12:35 AM

Just wondering how many CNers have been Googling "1.25 prism diagonal reviews" since Bill started this subject. :lol: :lol:

I know I have. ;)

George

#20 urassner

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Posted 01 December 2013 - 12:42 AM

I do not have experience with either diagonal. Could the observed difference be just the higher optical quality of the Tak diagonal compared to the Astrotech diagonal. Would you still see an improvement if it was compared to an AP, TV or Baader dielectric diagonal? I would assume that a Zeiss prism diagonal should be of similarly high quality as a Tak Diagonal and would also show a similar improvement as a Tak.

#21 george tatsis

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Posted 01 December 2013 - 01:30 AM

I do not have experience with either diagonal. Could the observed difference be just the higher optical quality of the Tak diagonal compared to the Astrotech diagonal. Would you still see an improvement if it was compared to an AP, TV or Baader dielectric diagonal? I would assume that a Zeiss prism diagonal should be of similarly high quality as a Tak Diagonal and would also show a similar improvement as a Tak.


PJ's reference to his excellent Zeiss prism diagonal corroborates Bill's report.

It's only logical to assume that quality and execution of design are of the utmost importance, because both samples come from reputable companies (Tak,Zeiss).

George

#22 Roy McCoy

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Posted 01 December 2013 - 01:40 AM

I now use a Baader T2 with the Zeiss prism in an Orion 80 ED f7.5 and a 2” Zeiss prism in an 86’ AP 6" f8.

Incidentally, I feel the T2 Baader Zeiss prism outperforms the 2” Zeiss prism. Maybe the shorter path? But I use 2 inch ep’s in the 6” so I’ll keep it.

As Bill said “a definite notch better”. There were several of us testing the diagonals and all involved agreed the difference was not subtle. To me the difference is as pronounced as the difference between an achro and apo.

Dust lanes and mottling pop so much more with the prism in my longer focal length scopes.

I found that the Baader 2”mirror is the better performer in the AT106 f6.5.

#23 urassner

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Posted 01 December 2013 - 01:50 AM

Thank you for the information. I am using an older AP refractor as well. I should think about a prism diagonal.

#24 Ernest_SPB

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Posted 01 December 2013 - 02:36 AM

I have 2" Amichi prizm from baader.

It is nice to see objects in right orientation. But so thick piece of glass introduces noticeable chromatic aberrations into image of my 80 mm F7.5 ED-apochromat. Then roof edge makes some artifacts: two strong rays into opposite sides of bright objects (like Jupiter, Venus) and distort a bit fine features in high magnification image (like astigmatism or doubling).

I think 2" prisms should not be used if you want to get image with high resolution/contrast.

#25 ivan86

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Posted 01 December 2013 - 05:26 AM

I have a Carl Zeiss 2" prism diagonal and i have used it with a Skywatcher 100 ED and now with my new Meade ACF 12 " SC.

This diagonal when compared to standard dieletric mirror 99% diagonal such WO / TS show better definition on planets and better contrast agaist background on deepsky objects.

The difference is more noticeable when i look at low contrast structure inside nebula.

Ill use in the future only prism diagonal and ill keep my Carl Zeiss (350 euro) forever.

Ivan






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