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Equipment Discussions >> Eyepieces

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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6233578 - 12/04/13 03:25 PM

Quote:

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.



As I would expect given the comparison between a top-notch prism and a mediocre mirror diagonal. The results might have been different with a top-notch mirror diagonal, like an AP or TeleVue.
Quote:


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 Dielectric 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.



The results would also have been different on a shorter f/ratio scope.
Quote:


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 averted 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 averted vision in the Prism and barely at all with the Dielectric. The brightness of M43 was also more obvious in the Prism vs the Dielectric. 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 Dielectric. 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.



The light path may be an indication of a slightly smaller prism used.
In general, a prism starts introducing lateral chromatic aberration in the shorter focal ratios, but at the ratio you tested it's negligible.
I did a test of a $50 multi-coated prism (plastic housing, shudder) with a high-end dielectric (note spelling) diagonal and found the prism stood up to the dielectric in every characteristic except construction quality. With Baader or Takahashi, though, the construction quality is up there, so a prism makes a reasonable alternative to a mirror. With 3 optical surfaces versus one, the prism has to be made to a high standard.
I'd really like to see a multi-coated prism with a protected silver coated diagonal surface. Light transmission would be superb and optical quality could be really high.
The spectrum of transmission for prisms is a little different than a mirror, but because most of the differences occur at longer wavelengths, where our eyes are less sensitive, they don't matter much in the field.
For a manufacturer, though, a high end prism is a lot more expensive to make: more glass, 3 surfaces to figure, and equal coating costs (or higher).


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BillP
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Reged: 11/26/06

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Re: Eyepiece Performance using a Prism Diagonal new [Re: Starman1]
      #6233619 - 12/04/13 03:53 PM

Quote:

Quote:

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.



As I would expect given the comparison between a top-notch prism and a mediocre mirror diagonal. The results might have been different with a top-notch mirror diagonal, like an AP or TeleVue.




Very valid point here. However, I do have my reservations. At planetary magnifications I work at sub 1mm exit pupils routinely. At these magnifications, as RC says, only a very small portion of the mirror is used so the actual P-V is likely to be exceedingly small. And given that the focal plane is so close to the mirror, any aberration excepting astigmatism will really be inconsequential visually. At least, this is what he says. However, if someone wants to loan me a TV or AP diagonal I would be happy to add them to the tests

btw, what about quartz diagonals? Many contend best contrast from these period! Thoughts? As far as brightness I read that the TV edges the Vernonscope Quartz, but does it also edge the AP for brightness?

ps - here's an interesting exchange - link.


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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: Eyepiece Performance using a Prism Diagonal new [Re: jrbarnett]
      #6233655 - 12/04/13 04:22 PM

Quote:

However, there is no question that prism diagonals introduce slight overcorrection and mirror diagonals do not, so assuming a refractor is not under-corrected, a prism introduces spherical aberration whereas a mirror does not.




If the intent is to use a prism diagonal to eke out that last bit of constrast in planetary observation, chances are you're already using a barlow to boost magnification. If so, a simple way to reduce the impact of the prism on color and spherical aberration when using it in faster scopes is to put the barlow ahead of the prism diagonal. It "narrows" the light cone and reduces the effect of the prism on the correction of the scope. Such a barlow also magnifies more than its nominal value so that's an added benefit. With such a setup, I can see no difference between Baaders's T2 prism and Maxbright diagonals in terms of spherical or chromatic correction.

Tanveer.


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6234167 - 12/04/13 08:41 PM

Quote:


btw, what about quartz diagonals? Many contend best contrast from these period! Thoughts? As far as brightness I read that the TV edges the Vernonscope Quartz, but does it also edge the AP for brightness?

ps - here's an interesting exchange - link.



In theory, quartz can be polished smoother than glass.
In practice? Who knows?
If the Vernonscope Quartz is a standard coating, for sure the TV dielectric version will be brighter. The AP MaxBright is also a dielectric unit, though, so I wouldn't expect any difference in brightness at all to anything short of a spectrophotometer.

P.S. I side with Roland Christen in that discussion.

Edited by Starman1 (12/04/13 08:44 PM)


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BillP
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Starman1]
      #6234954 - 12/05/13 09:28 AM

Quote:

P.S. I side with Roland Christen in that discussion.




I only half side with him. Reason being is that what he states as fact is not so. Clever marketing (he has a product in the mix, GH did not). Silver coatings can easily achieve more than 99% reflectivity. Edmund graphs show them for their non-specialized production as being >98%. As example, the 8m Gemini telescope in the 1990s using magnetron sputtering to deposit the silver achieved 99.4% reflectivity. That was a decade before the conversation linked. Also, a very critical point that was made by someone but the everyone remained silent on was scatter. It's fine for a diagonal to have highest reflectivity and best flatness, but scatter is more important for planetary observing. Today there are lab techniques to measure and quantify this. AP diagonal is claiming 1/20-1/40 wave in the observing region and >99% transmission with no warping and perfect collimation. But if its scatter is twice the level of a 1/10 wave and 98% reflective diagonal as example, this diagonal will win on planetary details. They had a nice although brief debate...lot of posturing...no resolution though

PS - the Vernonscope diagonal that GH refers to is Quartz, Silver, 1/20th wave with individual report proving, and 99% reflective. Also way expensive.

Edited by BillP (12/05/13 09:32 AM)


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dcoyle
professor emeritus


Reged: 10/11/05

Loc: Turbulent but dark skies, N.M.
Re: Eyepiece Performance using a Prism Diagonal new [Re: John Harold]
      #6235233 - 12/05/13 11:48 AM

If anyone has experience using a prism or Tak prism on a Sky Watcher 100 ED, I sure would like learn what you thought of it compared to a mirror.

Hey, John Harold, do you know if the Orion 100 ED is the same lens as the Sky Watcher 100 ED? Have you tried the prism in your Orion 100? Wad ya think?

Thanks,

Dan

Edited by dcoyle (12/05/13 12:02 PM)


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John Harold
member


Reged: 11/29/06

Loc: Seattle
Re: Eyepiece Performance using a Prism Diagonal new [Re: dcoyle]
      #6236282 - 12/05/13 09:47 PM

Other posts agree the Sky Watcher and the Orion are the same lens (Synta?). I did use the same old Japanese Celestron prism when I used the 100ED. Now the 100 collects dust in the closet since I got the 120. I recall the 100 was really sharp, I spent many evenings investigating Mars. I'm going to set the 100 up for short run star parties. It's much easier to deal with than the 127, and it's feather light.

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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: Eyepiece Performance using a Prism Diagonal new [Re: Starman1]
      #6236619 - 12/06/13 04:13 AM

Quote:

I'd really like to see a multi-coated prism with a protected silver coated diagonal surface. Light transmission would be superb and...




Don,
I don't understand this sentence. A 90 deg (zenit-) prism made of BaK4 works entirely with total reflection (=> 100% reflection) and certainly does not need any additional reflective coating.
Do I misunderstand anything in any respect?

Chris


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6236972 - 12/06/13 10:05 AM

Quote:

Quote:

I'd really like to see a multi-coated prism with a protected silver coated diagonal surface. Light transmission would be superb and...




Don,
I don't understand this sentence. A 90 deg (zenit-) prism made of BaK4 works entirely with total reflection (=> 100% reflection) and certainly does not need any additional reflective coating.
Do I misunderstand anything in any respect?

Chris



It has been measured that aluminizing or silvering the 45 degree surface improves light transmission through the prism. Apparently, the reflection is not 100% from that surface--both because it is not perfectly flat and because not all light has exactly a 45 degree angle of incidence.


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leonard
scholastic sledgehammer
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Reged: 10/19/07

Loc: West Virginia
Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6236978 - 12/06/13 10:09 AM

Hello ,


>>> I don't understand this sentence. A 90 deg (zenit-) prism made of BaK4 works entirely with total reflection (=> 100% reflection) and certainly does not need any additional reflective coating. <<<<<

He may mean using one surface like a Mangin mirror .

Leonard


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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: Eyepiece Performance using a Prism Diagonal new [Re: Starman1]
      #6237090 - 12/06/13 10:59 AM

Quote:

Quote:

Quote:

I'd really like to see a multi-coated prism with a protected silver coated diagonal surface. Light transmission would be superb and...




Don,
I don't understand this sentence. A 90 deg (zenit-) prism made of BaK4 works entirely with total reflection (=> 100% reflection) and certainly does not need any additional reflective coating.
Do I misunderstand anything in any respect?

Chris



It has been measured that aluminizing or silvering the 45 degree surface improves light transmission through the prism. Apparently, the reflection is not 100% from that surface--both because it is not perfectly flat and because not all light has exactly a 45 degree angle of incidence.




"...and because not all light has exactly a 45 degree angle of incidence." => According to physics, doesn't have to - not even with BK7-prisms, so much the less with BaK4.
Total reflection is total. The range of angles under which it occurs depends on the refractive index of the glass used.

So please give your sources.

Chris


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6237582 - 12/06/13 03:08 PM

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

From MellesGriot:
"The index of BK7 is sufficiently high to guarantee the TIR (total internal reflection) of a collimated beam at 45 degrees internal incidence over the visible and near infrared region. The possibility of significant TIR failure with
convergent or divergent beams should be kept in mind if polarization
is important. TIR can also fail if the hypotenuse face is not
kept extremely clean. Even an almost invisible fingerprint can lead
to TIR failure. 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.

There is a slight loss of reflectance at all internal angles with the coating,
and no critical angle exists."
I also found a hundred other links in a few minutes, but the note from MellesGriot should suffice.
TIR is only 100% at the surface for collimated light at exactly one angle.


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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: Eyepiece Performance using a Prism Diagonal new [Re: Starman1]
      #6239207 - 12/07/13 02:21 PM

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


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urassner
sage
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Reged: 07/08/12

Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6239860 - 12/07/13 09:13 PM Attachment (24 downloads)

I did a little shootout last night (or attempted at least). Conditions were suboptimal, with less than ideal seeing, and thin changing cloud cover and 20 degrees temperature with me standing in snow.
I used a Starfire 152 with Jupiter as the target. I used a Pentax 5 mm XW and 5 mm XO (more time with the XW). Contenders were a Baader 2 inch dielectric, a Baader 2 inch prism (with Zeiss prism) and a Vernonscope gold.

I saw no striking difference between the two Baaders under these conditions (with brief moments of reasonable seeing and cold feet). Sharpness seemed the same. I did not see color fringing with the prism. The Vernonscope seemed just as sharp to me, but with the gold diagonal it is like a planetary filter and seem to complement the appearance of Jupiter and may have made subtle cloud band more conspicuous.
Here is the clear difference that I did not expect. The Vernonscope is clearly the smaller built diagonal, yet the Baader prism had the shortest optical path, even with the longer (non Clicklock) eyepiece holder. I looked on the draw tube measurement on my focuser. The Baader dielectric came to focus at 49 mm , Vernonscope Gold at 59 mm, and Baader prism at 62 mm. This actually makes a difference to me, since I could not reach focus with the Baader dielectric when using my 5 inch telescope with the 0.75x focal reducer and my low power eyepiece. I cold reach focus when using a TV Everbrite with some, but also not all eyepieces.
I am hoping for clearer and warmer weather to spent more time, however I do not have the experience or eye of many keen observer on this forum.


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urassner
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Reged: 07/08/12

Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6239861 - 12/07/13 09:14 PM Attachment (26 downloads)

Second image

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BillP
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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6241042 - 12/08/13 03:41 PM

I just started some testings with the diagonals I have on hand, thanks to the loaners from folks

Just did some indoor tests to measure their light paths of the diagonals only, without any 2" to 1.25" converters if they needed that, Here's the results of the measures:

MM -- DIAGONAL
51.9 -- Tak 1.25" Prism
53.3 -- Celestron 1980 Prism
62.5 -- Baader T2 Prism*
81.2 -- AT 1.25 Dielectric Mirror
84.3 -- Baader 2" Prism
96.1 -- AT 2" Dielectric Mirror

These raw numbers are a little deceptive as the Baader T2 is configured with a 2" nosepiece and a 1.25" eyepiece holder so it does NOT need a 2"-to-1.25" adapter anyplace in the train when used with a scope with a 2" focuser and a 1.25" eyepiece. So for the actually operating backfocus one needs to add the length of the 2"-to-1.25" adapter to these numbers for total operational backfocus (except for the Baader T2). I have several of these converters, the shortest adds 9.6mm, but others I have are 12.4mm. So the Baader T2 and the Tak fight with each other as to which needs the least backfocus. If I use the 9.6mm length adapter with the Tak prism then it is slightly shorter overall than the Baader, if I use the 12.4mm adapter then the Baader T2 is shortest of the bunch.


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BillB9430
super member
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Reged: 12/02/06

Loc: Illinois
Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6241307 - 12/08/13 06:17 PM

urassner: Because the apparent thickness of the prism or the "air equivalent" path length is the actual path length divided by the glass refractive index, your results are just what should be expected. For example, assuming a n = 1.5 prism glass index, if a prism in-glass path length was 100 mm, the air equivalent prism path length would be 100mm/1.5 = 66.7 mm. So the light path in the prism would take up only 66.7mm of the focal distance. (Even though it measures 100 mm by a ruler alongside the prism!) Thus the location of the image plane when using the prism diagonal would be moved back by an extra (100 mm- 66.7mm) = 33.3 mm as compared to a non-refractive mirror diagonal. The bigger the prism (i.e. longer in-glass path), the farther back the image plane will be.

Folks are often confused by this because adding a prism erecting system, as in binoculars, often seems to move the eyepiece closer to the objective. But this is only because of the "folding" reflections of the light path within the prism cluster. When focusing light travels within prism glass, the effect is to move the location of focus farther away from the objective. Hence, the thicker your prism, the farther back your eyepiece will focus. This idea is why a telescope without sufficient "in" focus to accommodate a particular eyepiece when a mirror diagonal is used can often successfully focus with the same eyepiece when a prism diagonal is substituted.

BillP : How did you get your diagonal light path measurements using the diagonal only? A mirror diagonal's path length can be directly measured on the diagonal, but I do not understand how you measured the prism diagonal light path length without either calculation with refractive index or using a scope and eyepiece. Maybe I'm missing something in understanding your method. Thanks, BillB


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BillP
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillB9430]
      #6241340 - 12/08/13 06:46 PM

Quote:


BillP : How did you get your diagonal light path measurements using the diagonal only? A mirror diagonal's path length can be directly measured on the diagonal, but I do not understand how you measured the prism diagonal light path length without either calculation with refractive index or using a scope and eyepiece. Maybe I'm missing something in understanding your method. Thanks, BillB




I measured it operationally. So the scope was focused on an object with no diagonal using an eyepiece where there is no offset of the field stop from the shoulder. Length of focusers extension was then measured. Diagonal was put in place and the same eyepiece used, refocused, then focuser extension re-measured. Difference was the amount of backfocus being used by the prism and its fixed housing components...minus the amount being used by the 2-to-1.25: adapters since not all the units needed these.


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BillB9430
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Reged: 12/02/06

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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6241466 - 12/08/13 08:02 PM

Thanks Bill, that makes sense. I somehow got the incorrect notion that you measured indoors without a telescope and could not figure out how that could be done. The Baader 2" prism diagonal's position seems an anomaly, but perhaps is due to the larger prism requiring that the eyepiece be farther from the principal axis? It certainly is in proper position relative to the 2" mirror diagonal. - BillB

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Daniel Mounsey
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6241776 - 12/08/13 10:58 PM

My conclusions match yours Bill. Good work.

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