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Equipment Discussions >> Eyepieces

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Daniel Mounsey
Vendor (Woodland Hills)
*****

Reged: 06/12/02

Re: Eyepiece Performance using a Prism Diagonal new [Re: Daniel Mounsey]
      #6241780 - 12/08/13 11:01 PM Attachment (21 downloads)

BTW, Vernonscope Silver and Gold we discussed today Bill.

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urassner
sage
*****

Reged: 07/08/12

Re: Eyepiece Performance using a Prism Diagonal new [Re: BillB9430]
      #6242575 - 12/09/13 12:38 PM

Hi BillB9430, thanks for the explanation. It makes perfect sense. I just had never thought about that aspect. I am surprised though, that this is not mentioned as a marketing feature of the prism diagonal. In fact, Baader lists the optical path length for some diagonals on the is website, but not for this one. Compared to my Baader Dielectric, the optical path was 13 -4 mm shorter and that was despite the longer eyepiece holder used on the prism diagonal. I would think that for some telescopes this shorter light path could make all the difference when using binoviewers or focal reducers (it does to me with the focal reducer, but I have not tried the binoviewer).

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6242858 - 12/09/13 03:05 PM

Quote:

Quote:

Edmund will coat the hypotenuse to increase reflectivity:
http://www.edmundoptics.com/optics/prisms/right-angle-image-reflection-prisms/




=> But not for astronomical 90deg-(zenit-)prisms. Not even Zeiss ever considered doing so to be helpful.

Quote:

From MellesGriot:
a) The possibility of significant TIR failure with convergent or divergent beams should be kept in mind if polarization is important.
b) TIR can also fail if the hypotenuse face is not kept extremely clean. Even an almost invisible fingerprint can lead to TIR failure.
c) An aluminum or silver coated hypotenuse is recommended for applications where the right angle-prism is frequently handled, or where convergent or divergent beams are used.

d) There is a slight loss of reflectance at all internal angles with (!) the coating, ..."




a) => Is it to you?

b) With reasonable care during the manufacturing process and proper housing a zenit-prism will be (and stay) without any fingerprints on the hypotenuse.

c) The convergence of light before entering the eyepiece is only ~1 deg for a ~2 deg TFOV. Lightthroughput for such a case is still far above 99%.

d) Here it becomes quite clear why reflection coating the hypotenuse of astronomical zenit-prisms is not applied for standard use by any producer. Or is it? (Well, might be for infrared)

Chris



A) We do have convergent rays coming through the diagonal, so light throughput and the spectrum of transmission is important.
B) Polarization is important in planetary viewing. Use a single polarizer on Jupiter and rotate it. Interesting, eh?
C) extreme cleanliness of the 45 degree surface is guaranteed if it is coated.
D) The percentage loss with coating is negligible, but phase coherence is maintained.

At any rate, the internal reflection is not 100% except for a particular wavelength at exactly the correct angle of incidence. Prisms also add loss due to the thickness of glass and the spectrum of transmission. In an f/15 scope, though, a prism can be a reasonable alternative to a mirror system. At f/5-6, not so much.


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BillP
Postmaster
*****

Reged: 11/26/06

Loc: Vienna, VA
UPDATE: Prism Diagonals new [Re: BillP]
      #6247933 - 12/11/13 09:45 PM

UPDATE:

Just did a round of color tests with the diagonals. Used a very difficult target, the bright limb of the Moon placed dead center in the AFOV.

Scope, a fast f/6.25 APO, the Celestron Onyx 80mm. Chose this one first to see what kind of color I could tease out since it was a fast focal ratio and folks have been saying this is where the prisms do not operate well.

Eyepieces chosen were a 6mm ZAO-II and a 4mm TMB Monocentric which produce 83x and 125x respectively. Then pumped things up and used the 6mm ZAO-II with the APM 2.7x ED Barlow for 225x (.35mm exit pupil and 71x per inch aperture).

Results....

#1 -- A-T 1.25" Dielectric (no color)
#2 -- Baader T2 Prism (V. Slight)
#3 -- Celestron 1980 Prism (Slight)
#4 -- Tak Prism (Slight+)
#5 -- Baader 2" Prism (Slight++)

Not the greatest ranking words, but for the moment all I can think of. Basically it was a difficult observation to see the color once in focus with #2 and #3. Dark shadows in craters were just a little off-black, difficult to see a slight blue tinge to the black shadows. #4 it was more evident that the shadows were not completely black and definitely blue-black, and in #5 they were more obvious still having the blue tinge to the shadows. #5 is the only one also where the slight blue/purple halo off the limb was of any size. In all the others any color showing on the limb was infinitesimally thin and small when in focus.

At highest magnification, 225x out of the 80mm is where the crater shadows showed their blue-black appearance. Working at 83x and 125x pretty much everything looked black in the shadows across the Lunar surface. Image was also completely sharp and etched at 225x out of this little 80mm (I really love this scope...it is an excellent specimen and had no issues keeping up with the LOMO 80mm I used to have). I was beginning to do an assessment of how sharply the image appeared through each diagonal at the 225x magnification but only got one round in before cloud deck came through. So I have no ranking yet for who was the sharpest of the bunch but the bottom dweller was very evident....the A-T 1.25" Dielectric was no where in the same league as the Prisms! While the prisms showed a sharply etched view, the Dielectric was soft in comparison. However, I have some premium mirrors on the way to add to the comparison so will redo all these tests once they arrive in this same scope, then move to the TSA-102 to see how things change at f/8. Bottom line though, residual color was really slight and unobtrusive at f/6 IMO.


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leonard
scholastic sledgehammer
*****

Reged: 10/19/07

Loc: West Virginia
Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6248137 - 12/11/13 11:49 PM



Quite interesting Bill , keep up this great review.

Leonard


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george tatsis
professor emeritus
*****

Reged: 11/20/08

Loc: Flushing, NY - Europe
Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6248228 - 12/12/13 01:13 AM

Keep up the good work Bill, we're counting on you

God knows how many eyepieces over the years have been reported to be insufficient because of light scattering diagonals used with them.

I think we, planetary observers, should first pick our diagonals and then our eyepieces

I've placed an order for the Vixen 1.25" prism diagonal to compare it with the current WO dielectric that I use in my 120ED, and I'll report back when some solid conclusions are drawn.

An order has also been placed for the GSO 1.25" non dielectric mirror diagonal to compare the views as well.

For some reason I can see myself in the future ditching the dielectric one for planetary observing because it DOES scatter light.

I should have thought of this issue years ago when my buddy's SW 80 ED consistently outperformed my TV 85 by exhibiting darker background, less scatter, and darker Jovian belts.

At that time it didn't cross my mind that his $35 diagonal would be better for Jupiter than my Everbrite, but it was.

George

Edited by george tatsis (12/12/13 03:51 AM)


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: UPDATE: Prism Diagonals new [Re: george tatsis]
      #6248389 - 12/12/13 06:55 AM

Here is a good way to avoid all the controversy about diagonals: Use a Newt!


Mike


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george tatsis
professor emeritus
*****

Reged: 11/20/08

Loc: Flushing, NY - Europe
Re: UPDATE: Prism Diagonals new [Re: Sarkikos]
      #6248400 - 12/12/13 07:10 AM

Quote:

Here is a good way to avoid all the controversy about diagonals: Use a Newt!


Mike






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BillP
Postmaster
*****

Reged: 11/26/06

Loc: Vienna, VA
Re: UPDATE: Prism Diagonals new [Re: george tatsis]
      #6248536 - 12/12/13 08:58 AM

Quote:

Quote:

Here is a good way to avoid all the controversy about diagonals: Use a Newt!


Mike











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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: UPDATE: Prism Diagonals new [Re: george tatsis]
      #6249215 - 12/12/13 02:28 PM

It is possible that there is a little more variation in the optical surface quality in mirror diagonals than there is in prisms. I would think a planetary observer would be concerned with optical quality of the mirror.

In my experience, there is less variation in the higher-end star diagonals (AP, TeleVue, Lumicon, et al.) than there is in the economy ones, though there is some. I had occasion to compare a dozen Lumicons, and I kept one that was obviously better than the others. It was an "enhanced" coated mirror.

If you look at interferogram reports on mirrors used in diagonals, it probably would be a good thing to use a 2" star diagonal, even if using 1.25" eyepieces, because the center 1/3 of every star diagonal's mirror has the least deviation from a perfect surface.

It might even be worthwhile to invest in a really high-accuracy secondary mirror from someone like Antares optics or Protostar to use that mirror as a star diagonal's mirror to get an even better surface.

As for coatings, it does depend a lot on the quality of the coatings, but if you're concerned, a simple aluminizing with SiO overcoat might be best. You certainly won't miss the light, and the surface is likely to be less affected by the coating.

However, Roland Christen seems to think a dielectric-coated mirror is just fine for planetary viewing in an AP scope. He does say this, though:
"For critical planetary viewing it is important to have the eyepiece and diagonal surfaces as clean as possible. You will find that even after one observing session the diagonal surface will accumulate crud. The inherently high contrast and cleanability of the Maxbright insures that it is never a limiting factor when viewing subtle planetary detail."[this would be true of all dielectric-coated diagonals]

Like many labs and professional observatories, he has found that cleanliness means more than coating type when it comes to planetary details viewing. How many of you have cleaned your star diagonals regularly (like every 6 months or more)?


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Roy McCoy
Pooh-Bah
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Reged: 10/13/05

Loc: Glendale, AZ
Re: UPDATE: Prism Diagonals new [Re: george tatsis]
      #6249216 - 12/12/13 02:28 PM

Quote:

I should have thought of this issue years ago when my buddy's SW 80 ED consistently outperformed my TV 85 by exhibiting darker background, less scatter, and darker Jovian belts.




I've seen the same thing with the Orion 80ed and a T2 prism.


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BillP
Postmaster
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Reged: 11/26/06

Loc: Vienna, VA
Re: UPDATE: Prism Diagonals new [Re: Roy McCoy]
      #6249330 - 12/12/13 03:18 PM

Yup...the more I am using that T2 Prism, the more it seems to be distinguishing itself from the field.

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Eddgie
Postmaster
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Reged: 02/01/06

Re: UPDATE: Prism Diagonals new [Re: Starman1]
      #6250035 - 12/12/13 10:31 PM

Quote:

How many of you have cleaned your star diagonals regularly (like every 6 months or more)?





Every time I do serious planetary observing, I clean my diagonal.

I give everyone that asks about getting a dielectric diagonal to get one, and I explain that the main reason you should use a dielectric is simply because you can clean it for every serious planetary observing session and not damage it.

And this is one of the things I dislike about prisms. While the coatings on prisms are pretty hard (not as hard as dielectric), they are much harder to clean than a mirror.


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pga7602
sage
*****

Reged: 06/11/09

Loc: Los Angeles, CA
Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6250222 - 12/13/13 01:55 AM

If I wanted to try a 1.25 diagonal in my TV85, is it just a matter of sticking a 1.25 eyepiece adapter ahead of the diagonal? Or is there a diagonal 2-1.25 adapter made for this purpose.

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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6250313 - 12/13/13 05:46 AM

Quote:

Yup...the more I am using that T2 Prism, the more it seems to be distinguishing itself from the field.




Bill,
Since thera are three versions of the T2 prism, I would like to ask you which it is that you have:
http://www.baader-planetarium.de/sektion/s28/s28.htm#t2_90_grad_ohne
Chris


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BillP
Postmaster
*****

Reged: 11/26/06

Loc: Vienna, VA
Re: UPDATE: Prism Diagonals new [Re: Fomalhaut]
      #6250417 - 12/13/13 08:13 AM

It's on loan to me so don't know off the top of my head. Will check this evening to see if something distinguishing marked on the prism that indicates which it is.

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Ava
sage


Reged: 11/30/11

Loc: Sweden
Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6250449 - 12/13/13 08:44 AM

As has been discussed numerous times before on this forum, the amount of color that shows with any given prism / refractor combination will depend of the interplay of the chromatic aberrations from both sources. Some prisms will cancel out some of the coloring, while some will add to it. Many times it has been suggested that many doublets might benefit from a non-premium prism diagonal, e.g. the Baader T2 non-Zeiss prism, because the prism may cancel out some of the chromatic aberrations of the scope, while a good triplet will have very little color and so needs a premium prisms that introduces none either, e.g. the Baader T2 Zeiss prism. Because we're talking about color, it likely also depends on the color ranges observed. This will obviously need testing for every specific diagonal/objective/target combination.

In conclusion it's likely that you probably need to try for yourself or hear from someone that has the same scope and target preferences as yourself before buying a specific brand of diagonal, at least as far as color is concerned.

Now, regarding scatter, that's another matter and a good prism will probably win over any decent mirror diagonal. Now, when you get to the excellent (and clean!) mirror diagonals, that's where it gets interesting I guess. I'm eagerly awaiting more results!


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Ava
sage


Reged: 11/30/11

Loc: Sweden
Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6250454 - 12/13/13 08:48 AM

Bill, if it has no markings you could check the optical path length, the Zeiss version is supposed to have around 42mm length while the non-Zeiss should have around 38mm, if I remember correctly.

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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Eyepiece Performance using a Prism Diagonal new [Re: junomike]
      #6250480 - 12/13/13 09:04 AM

I have always preferred A mirror diagonal for Deep Space viewing and Prism for Planetary. I have a 10th wave enhansed Lumicon I have owned for decades, and my reysty mis level Prisms. I believe the Mirror to be a bit brighter but prism knocks glare and scattered light down better producing higher detail.

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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: UPDATE: Prism Diagonals new [Re: Ava]
      #6250650 - 12/13/13 10:35 AM

Quote:

As has been discussed numerous times before on this forum, the amount of color that shows with any given prism / refractor combination will depend of the interplay of the chromatic aberrations from both sources. Some prisms will cancel out some of the coloring, while some will add to it. Many times it has been suggested that many doublets might benefit from a non-premium prism diagonal, e.g. the Baader T2 non-Zeiss prism, because the prism may cancel out some of the chromatic aberrations of the scope, while a good triplet will have very little color and so needs a premium prisms that introduces none either, e.g. the Baader T2 Zeiss prism. Because we're talking about color, it likely also depends on the color ranges observed. This will obviously need testing for every specific diagonal/objective/target combination.





According to Rutten vanVenroij 1st ed.p.312-313, "a reflecting prism behaves as a plan-parallel plate, ...causing overcorrected spherical aberration."
Well reputed (even top-notch such as Tak) apos are slightly undercorrected, presumably i.o. to work better with zenit-prisms. => The prism's overcorrection then tends to neutralize the objectives' undercorrection.
On astro-foren.de, Taks and Zeiss APQs and others have been tested with and without prisms and many of them work better with and others without prisms. Zeiss even produced some of their famous APQs to be used with prisms and others (of same type, aperture and f-ratio) to be used with mirrors.
Mr. Rohr found their measured system-correction was indeed in accordance with the goal of their design.
Chris


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