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BillP
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Mirror, Dielectric, & Prism Performance Comparison
      #6310450 - 01/13/14 02:57 PM Attachment (213 downloads)

I had a old thread in the EP Forum on diagonals. Will continue it here. Have collected the field finally from various kind CN souls that are letting me borrow their equipment. Below is the final field of dreams for diagonals.

Over the coming weeks I will be comparing these against each other in three main categories: CA, Planetary, Threshold Brightness. My original question was regarding planetary performance with prism vs other diagonals. Since there are a number of prisms in the field, the tests will be done in both short and fast focal ratio refractors (f/8 APO and f/6.25 APO). This is to see if prisms offer any advantages/disadvantages at short focal ratios, as they are reproted to, and for me of course, to see if they provide a planetary advantage.

Anyway, here's the glamor shot

BACK (2"): Baader Prism | Takahashi Mirror | Astro Tech Dielectric | Baader Dielectric | AP Maxbright
CENTER (1.25"): Baader T2 Prism | Vernonscope Quartz Mirror | Tahahashi Prism | Tele Vue Everbrite
FRONT (1.25"): Vixen Prism | Astro Tech Dielectric | Celestron Prism (1980s)

Edited by BillP (01/13/14 09:42 PM)


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mgwhittle
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6310506 - 01/13/14 03:31 PM

Excitedly awaiting the results. My experience is that my Zeiss prism diagonal is just slightly better than my Maxbrights on planets with my 175EDF. Will be interesting to read your conclusions.

And Happy Birthday Bill!


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Lance1234
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6310618 - 01/13/14 04:32 PM

Let no one ever say you don't like a challenge!

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Fomalhaut
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6310777 - 01/13/14 06:04 PM

Quote:

BACK (2"): Baader Prism | Takahashi Prism | Astro Tech Dielectric | Baader Dielectric | AP Maxbright
CENTER (1.25"): Baader T2 Prism | Vernonscope Quartz Mirror | Tahahashi Prism | Tele Vue Everbrite
FRONT (1.25"): Stellarvue Prism | Astro Tech Dielectric | Celestron Prism (1980s)




Bill, Are you sure the Takahashi in the back row is a prism and not their 2-inch mirror diagonal?
Chris


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Paul G
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6310857 - 01/13/14 06:48 PM

Happy Birthday, Bill!

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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Fomalhaut]
      #6310909 - 01/13/14 07:10 PM

Quote:

Quote:

BACK (2"): Baader Prism | Takahashi Prism | Astro Tech Dielectric | Baader Dielectric | AP Maxbright
CENTER (1.25"): Baader T2 Prism | Vernonscope Quartz Mirror | Tahahashi Prism | Tele Vue Everbrite
FRONT (1.25"): Stellarvue Prism | Astro Tech Dielectric | Celestron Prism (1980s)




Bill, Are you sure the Takahashi in the back row is a prism and not their 2-inch mirror diagonal?
Chris




Thanks for catching that. Made the correction. Also, it isn't Stellarviue in front row, it's a Vixen prism. So another mistake. Hope I can remember who these all go back to when I'm done

Gus - Thanks

Edited by BillP (01/13/14 09:43 PM)


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Roy McCoy
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6311367 - 01/13/14 11:13 PM

Quote:

My original question was regarding planetary performance with prism vs other diagonals




With a response like this maybe the next time you should ask a question regarding planetary performance with TEC APOs vs AP APOs vs TAK APOs!


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Daniel Mounsey
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Roy McCoy]
      #6311384 - 01/13/14 11:22 PM

Bill,
I'll be curious to see your results, granted I've already discovered answers. I'll keep checking on this thread. Once your results are posted, I'll share a few stories regarding fast scopes and slow scopes with prisms I had.
Best


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stargazer193857
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Reged: 12/01/13

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6311403 - 01/13/14 11:31 PM

Here is what I read or predict:

Dielectrics are expensive and perform differently at different frequencies, but may give the best overall performance.

Prisms give varying CA depending on how fast your light cone is and how wide your view is. The diagonal efficiency depends on the refractive index of the glass, and again on how fast your light cone is. Probably great in an SCT, not so great in a 12" dob. The entering and exiting faces can have reflections that reduce contrast or even give ghost images if they are not coated, and still reduce contrast some if they do.

A single Aluminized mirror will rob you of 12% of your brightness, but will lack these other contrast problems. I think it will be best for planetary. For galaxies, you may need the extra brightness and not notice a difference in contrast except near the edges of galaxies. I think the dielectric will do best on galaxies.


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Mariner@sg
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: stargazer193857]
      #6311591 - 01/14/14 02:31 AM

No William Optics' Dielectric?

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Jon Isaacs
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Mariner@sg]
      #6311653 - 01/14/14 05:30 AM

Quote:

No William Optics' Dielectric?




It should be the same as the Astro-Tech...

Jon


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Jon Isaacs
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6311654 - 01/14/14 05:42 AM

Quote:

Since there are a number of prisms in the field, the tests will be done in both short and fast focal ratio refractors (f/8 APO and f/6.25 APO)




Just a comment:

F/8 is still quite fast and only marginally different from F/6.25. Something in the F/12-F/15 range would seem reasonable for comparison purposes and it could be a CAT...

Jon


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Mariner@sg
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jon Isaacs]
      #6311694 - 01/14/14 06:39 AM

Quote:

Quote:

No William Optics' Dielectric?




It should be the same as the Astro-Tech...

Jon




Oh ok. Thanks for pointing it out Jon. Looking forward to the results


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dan_h
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jon Isaacs]
      #6311797 - 01/14/14 08:50 AM

I thought that too at first but after thinking on it for a bit, the experiment is to compare prisms versus mirrors in fast scopes, not to check prisms in fast versus slow scopes (although that is a logical extension of the testing). Both the scopes are relatively fast so the results should be similar if any differences are indeed due to the prisms versus the mirrors.

dan


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jon Isaacs]
      #6311830 - 01/14/14 09:11 AM

Quote:

Quote:

Since there are a number of prisms in the field, the tests will be done in both short and fast focal ratio refractors (f/8 APO and f/6.25 APO)




Just a comment:

F/8 is still quite fast and only marginally different from F/6.25. Something in the F/12-F/15 range would seem reasonable for comparison purposes and it could be a CAT...

Jon




Curious...If things look excellent in f/8, would you expect them to get worse in an f/15?

btw, f/6.25 & f/8 is all I have that can take both 1.25 and 2" diagonals...unless of course you want me to put a diagonal in the focuser of my Newt FWIW, Baader states that f/7 or longer is where their prisms work best. Most modern APOs fall in the f/6 to f/8 range, so IMO these two are good representatives for 90% of what's out there that would be using these items. Exceptions being of course CAT systems. But for those I would hazzard that the primary interest areas would be threshold brightness and planetary performance in the f/8 as it should be reliably extensible to what outcomes to expect with a CAT. I do have an older Meade 8" SCT, but it only has a 1.25" visual back. If I have time, I can spot check some of the 1.25" diagonals on that for planetary to see if they react the same after done with the APOs. But of CATs are not my cup of tea so never had what I would term as exacting planetary performance from mine due to their thermal gremlins. So not holding out much hope that I'll see any difference in that beast.

Edited by BillP (01/14/14 09:14 AM)


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Daniel Mounsey
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jon Isaacs]
      #6311944 - 01/14/14 10:09 AM

Jon,
My prism tests were conducted at F-5.9 and F-8.3. Got your PM and will post a full review here with pics. There's a lot of theory and speculation but little in the way of actual field tests until now. I respect Bill's tests because he's a visual observer calling it the way it appears in reality and that to me is what matters most.


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Jon Isaacs
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6312052 - 01/14/14 11:07 AM

Quote:

Quote:

Quote:

Since there are a number of prisms in the field, the tests will be done in both short and fast focal ratio refractors (f/8 APO and f/6.25 APO)




Just a comment:

F/8 is still quite fast and only marginally different from F/6.25. Something in the F/12-F/15 range would seem reasonable for comparison purposes and it could be a CAT...

Jon




Curious...If things look excellent in f/8, would you expect them to get worse in an f/15?

btw, f/6.25 & f/8 is all I have that can take both 1.25 and 2" diagonals...unless of course you want me to put a diagonal in the focuser of my Newt FWIW, Baader states that f/7 or longer is where their prisms work best. Most modern APOs fall in the f/6 to f/8 range, so IMO these two are good representatives for 90% of what's out there that would be using these items. Exceptions being of course CAT systems. But for those I would hazzard that the primary interest areas would be threshold brightness and planetary performance in the f/8 as it should be reliably extensible to what outcomes to expect with a CAT. I do have an older Meade 8" SCT, but it only has a 1.25" visual back. If I have time, I can spot check some of the 1.25" diagonals on that for planetary to see if they react the same after done with the APOs. But of CATs are not my cup of tea so never had what I would term as exacting planetary performance from mine due to their thermal gremlins. So not holding out much hope that I'll see any difference in that beast.




Bill:

By trade, I am an experimental materials scientist. I have learned that when making measurements, it is good to be guided by analysis, not so good to be guided by word of mouth (that's a good way to have a sample mashed beyond recognition) and it is good to make as many measurements over as wide a range of the variable(s) of interest as is possible.

I have not seen the analysis that suggests that F/7 is a cut off though I imagine that someone like Vlad could provide that if they were to happen along here. Refractors are designed by analysis, this analysis would seem to be comparatively easy.

But, in any event, my point was that I think it's important to recognize that in general terms, F/8 is not particularly slow, I would classify it a moderate focal ratio, definitely not fast but still fast enough that many simpler eyepieces will still have issues with off-axis astigmatism.

From my point of view, it seems primary question concerning the use a prism diagonal in a faster scope is the dispersion and the resulting chromatic aberration. This probably depends to some extent on the type of glass but little else. So if one limited their observations to chromatic aberration, rather than testing a number of diagonals, testing a number of focal ratios with few diagonals could provide better understanding of the relationship between chromatic aberration of the prism and the focal ratio of the telescope.

This approach need not only include refractors, in fact, because of their complete lack of chromatic aberration, a reflector might be a better test platform because of the availability in faster focal ratios as well as complete lack of chromatic aberration, any chromatic aberration observed could be ascribed to the prism. If one is using a refractor, one would have to keep in mind that individual refractors have differing residual chromatic aberrations and so the chromatic aberration one is seeing is the result of both the prism and that particular refractor's residual color correction.

With all this in mind, my approach would to begin by looking for chromatic aberration using a single prism diagonal in as many focal ratios as possible... If I could get it to focus, my 12.5 inch F/4.06 Newonian would provide an interesting data point.

As far as the performance in a slow (>F/12) refractor, I think one might find that a prism diagonal has less scatter than a mirror diagonal and so the actual performance of the diagonal as a function of focal ratio is a combination of scatter versus chromatic aberration. Since in terms of raw numbers, I have to think most prism diagonals are used in CATs, the relative performance would be of interest...

Anyway, I have outlined a much different way to approach this question.. Isolate the variable(s) of interest and investigate it over a wide range. For me, now that Venus is gone from the evening sky, the starting point would be to just put a prism diagonal in my NP-101 and look at Rigel or Sirius for signs of chromatic aberration. That would provide one point that is, in practical terms, about as fastest focal ratio that as is possible with a refractor that is well corrected for color.

Jon Isaacs


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Starman1
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Reged: 06/24/03

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jon Isaacs]
      #6312123 - 01/14/14 11:38 AM

Jon,
When you are talking chromatic aberration, you have to stipulate that you are looking for it "on axis", since many, if not most, eyepieces exhibit some lateral chromatic aberration when stars near the edge of the field, even in reflectors.


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jon Isaacs]
      #6312187 - 01/14/14 12:10 PM

I see your point, and your approach. What is often contended however (in various forum spaces, the popular wisdom as it were), and also in marketing from some is that:
1. Prisms help achros
2. Prisms add color to corrected scopes when focal ratio is short (f/6 often stated)
3. Planetary views are better in prisms
4. Dielectrics are the cats meow (marketing by everyone)
5. Mirrors & prisms are too delicate to clean (again marketing to steer us to dielectrics)

Your suggestion isolates a single parameter, which is often the best method of course, then seeks to discover a relationship. I am all on board with that. But the obvious weakness of that approach is that it does little to account for a full system test. It just gets us a single step closer in what is a loooong journey. Isolating and testing all the variables in a system test, then arriving at a working theory on how they all discretely interact would be a huge undertaking. Not something anyone can do here of course.

I prefer to cut to the chase and get an answer to a system question…is color induced and becomes a factor in the f/6-f/8 range (popular market range for APOs), does planetary performance improve, is threshold brightness impacted for DSO? My process gets me an answer. The more scientifically controlled process you describe gets us a little more knowledge on one parameter and no good way to relate that very accurately to a practical setup.

If folks want to wonder about how these discrete aspects, like just color, get impacted with scopes from f/4 to f/15, only way I can help them is for them to pony up the scopes and drop them off at my house with their observatories! Or volunteer to do that spectrum of the testing!

Don - absolutely correct. All my tests relative to color and planetary performance will be on-axis only.


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Sarkikos
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6312202 - 01/14/14 12:18 PM

For my finder scope refractors, I always use an Amici prism. I don't care if they have a line across the field (never seen it for deep sky) and don't take high power well (don't need it for a finder or rich-field scope).

I love that right-side-up non-reversed image for star hopping!

I take it for granted that none of the prisms in this comparo are Amicis.


Mike


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Jon Isaacs
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6312383 - 01/14/14 02:02 PM

Quote:


Your suggestion isolates a single parameter, which is often the best method of course, then seeks to discover a relationship. I am all on board with that. But the obvious weakness of that approach is that it does little to account for a full system test. It just gets us a single step closer in what is a loooong journey. Isolating and testing all the variables in a system test, then arriving at a working theory on how they all discretely interact would be a huge undertaking. Not something anyone can do here of course.

I prefer to cut to the chase and get an answer to a system question…is color induced and becomes a factor in the f/6-f/8 range (popular market range for APOs), does planetary performance improve, is threshold brightness impacted for DSO? My process gets me an answer. The more scientifically controlled process you describe gets us a little more knowledge on one parameter and no good way to relate that very accurately to a practical setup.




Bill:

We do what we can do they way we can do it.

That said: Isolating a relationship of interest, making a hypothesis and then investigating the relationship allows one to develop an understanding and quantify the relationship so that it can be applied to generalized, real world situations. There is good reason to carefully investigate the relationship between the chromatic aberration of a prism diagonal and the focal ratio of the telescope.

In this case, a simple analysis says that there should chromatic aberration caused by a prism diagonal, the diagonal is glass, glass exhibits dispersion so light entering the glass from air at an angle will show dispersion, the colors will separate. Add the fact that the light coming from the objective enters the diagonal over a range of angles whose maximum is approximately F/2, there is good reason to expect that a prism diagonal will exhibit chromatic aberration and that it will be a function of the focal ratio of the telescope.

Indeed, generally, there seems to be an awareness that there is a real effect here. A more involved analysis, one I am incapable of without a great deal of further study, could put real numbers on this because the parameters necessary for the analysis are well established.

Making some measurements, qualitative or quantitative, can provide the sufficient data necessary to generalize the information.

We do what we can do the way we can do it...

When you "cut to the chase" you can answer the question for a particular system or a few systems but because of the many uncontrolled variables, one cannot generalize the conclusions unless a great number of comparisons.

Starting simple, keeping things simple, isolate the variable, understand what is going on, move on...

We do what we can do the way we can do it.

Jon


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Chucke
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6312573 - 01/14/14 03:31 PM

Back in the 1960's there was a test published in S&T regarding the use of prism vs mirror diagonals for Newtonians. Perhaps someone could look it up. As I recall, there seemed to be a sort of cutoff point in terms of f-ratio where the prism began to introduce noticeable chromatic aberration.

Chuck


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schang
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Chucke]
      #6312678 - 01/14/14 04:15 PM

I do not know if this is what you mentioned, but I found it on the old CN forum:

http://www.cloudynights.com/item.php?item_id=82

http://www.cloudynights.com/item.php?item_id=1854


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schang
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Reged: 04/24/13

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6312707 - 01/14/14 04:30 PM

Bill:

I admire you for taking up this experiment. Although I'd like to comment that the task would be easier if you can narrow your goals to either

1) compare the different types of diagonals using a couple of scopes of different f-ratios, preferably low and fast ratios to be able to provide you a discernible visual differences (Hey, we got to trust your eyes to do this, and we do not know how good yours are ).

2) compare different brands with only one scope of your choice.

And, do you trust that each of these diagonals are the best they can be? Can there be a small lemon in there? So I'd suggest that you take two to three diagonals of each brand for a more statistically sound analysis.

Now you see why I admire you in doing this...

Best regards.


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Daniel Mounsey
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6312774 - 01/14/14 05:03 PM

Quote:

Jon,
When you are talking chromatic aberration, you have to stipulate that you are looking for it "on axis", since many, if not most, eyepieces exhibit some lateral chromatic aberration when stars near the edge of the field, even in reflectors.




True Don, but it's still relative between the prism and the mirror.


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Jon Isaacs
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: schang]
      #6312818 - 01/14/14 05:21 PM

Quote:

Bill:

I admire you for taking up this experiment.




I second that...

Jon


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Starman1
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6312970 - 01/14/14 06:51 PM

Quote:

Quote:

Jon,
When you are talking chromatic aberration, you have to stipulate that you are looking for it "on axis", since many, if not most, eyepieces exhibit some lateral chromatic aberration when stars near the edge of the field, even in reflectors.




True Don, but it's still relative between the prism and the mirror.



Meaning what? That if it's more visible in a prism than a mirror the prism is adding some lateral chromatic aberration?


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: schang]
      #6313876 - 01/15/14 08:45 AM

Quote:

And, do you trust that each of these diagonals are the best they can be? Can there be a small lemon in there? So I'd suggest that you take two to three diagonals of each brand for a more statistically sound analysis.




Shien,

Hi. This point comes up every now and then regarding field test comparisons. In truth though, getting a few of each unit vs just one does little statistically. As example, presuming the manufacturer make 1000 diagonals in their production run, how many samples would I need to get results with an accuracy of say +/- 10% (presume at the 95% confidence level)? My sample size would have to be 88 diagonals of each brand!! If I have 4 samples of each diagonal, then the accuracy of the results, statistically, comes to +/- 50%...so a coin toss!

Field tests, IMO, cannot be made into quantitative research. It is at its heart a qualitative process and IMO needs to be conducted and evaluated in that context. To come to any more firm conclusion then really requires that any one field test be evaluated in the context of all the others available to try to draw any more firm conclusions. Single field tests therefore become individual data points really. Field tests IMO become most valuable when teamed with a comprehensive quantitative bench test because the field test more or less informs us of the complexities of the richer operational environment and the need to add a multitude of variables into the mix for subsequent quantitative research to help develop a more comprehensive and accurate model to what is going on. The interplay of things is where it really happens for the operational realm. So in this test, while I am looking at CA on-axis with prisms at 2 focal ratios, one has to realize that what is driving the visual CA can be much more than simply the prism, as the optical chain and the target used all have their nuances that differ (e.g., predominent spectra of the target being observed, level of correction of the main objective, amount of atmospheric induced color which can not be known easily, thermal stability/acclimation of the diagonal itself, varying iris pupil dialations during observeration readings, etc. ,etc.). There are really quite a number of variables that can influence outcomes, but just can't be adequately measured and quantified in anything we amateurs do.


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schang
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Reged: 04/24/13

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6313975 - 01/15/14 09:32 AM

I agree what you said about the numbers of variables that can affect outcomes, that is why I admire you for taking up this experiment. The key is how you can extract the test information in a meaningful way, which is often not so trivia if the data is not treated carefully. In other words, an early computer parlance, garbage in/garbage out when a deck of punch cards fed into a card reader. No offense here.

I mentioned about using multiple samples of each diagonal for statistic variance analysis. This is not about sample size. it is about pooled standard deviation estimate to provide us some degrees of confidence level in analyzing the data. Without it, it will be difficult to conclude anything. As you know, there are quality variation within a brand of the same product. You may get one that is below par which you then conclude that it is not good compare to other brand, but it might in fact on par or better if you test enough of them. The test number of such sample needs not be very large, normally three will be sufficient in a designed experiment. Sometimes, two will be ok if you have more brands to be compared and the variance of brands is small. If it is large, you won't be able to tell the difference at all unless the test sample size is large. The question is : would you rather do a single sample test and not be able to draw a reliable conclusion, or do limited, genuine repeat tests to get what you need to know?


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6314037 - 01/15/14 10:12 AM

Update: January 14 Observations --

Had a several hour break in clouds last evening which was surprising as unexpected. And wow what a break, it was so very clear out and the Moon and Jupiter closely positioned made a perfect opportunity for some testing. Last night I chose just the prisms in the f/6.25 APO for some CA evaluations using the Moon, and some Planetary detail evaluations using Jupiter.

On the Moon, I observed the limb for on-axis CA, and then the shadows of crater walls near the limb for any coloration other than black. My first observation was straight through without diagonal to confirm no CA on the limb and black shadows. I originally used the 3.5mm XW, but the lateral color on the limb near the field stop was a distracting element for me so decided would be better to choose an eyepiece that showed no lateral color. I moved to an 8mm AP-SPL with a TV 2x, but again, a very little lateral color visible from the Barlowed combination (something I have noted before regarding Barlows), so decided against that. Eventually settled on the 5 XO and 4 Supermono as these both showed no limb color regardless of where in the FOV. So gone was my idea of having comfortable eye releif for the tests

At f/6.25, I was able to coax a very small amount of color when in focus from the prisms, all of them. Most visible in the Vixen and Celestron, the Tak then showed a notch less, and finally the two Baaders showed the least. The Vixen and the Celestron were the only two where it was enough that there was a very slight blue cast to some crater wall shadows, but it was very little really. The Tak and Baaders did not really show this and was easy to get a nice black shadow in crater walls. Took a little more coaxing with the Tak, but it was right on the heels of the Baaders. Really, IMO the amount of CA on the limb was really inconsequential it was that slight. But still, there if you looked hard for it.

Moving to Jupiter was treated to some spectacular performance using the 80mm scope. The 4 main belts were strongly visible, a rich amount of detail and structure within the 2 main equatorial belts was visible, including a nice storm, and the polar regions were also quite richly portrayed with gradations of shading and also the NNTeB peaking through, although not sharply defined and a little more ethereal.

All the prisms were showing Jupiter details exceedingly well. The Celestron and Vixen were not quite as defined as the others, lacking the more finely etched views from the others, but still quite good. The Tak and two Baaders were fairly on-par in the details and crispness being shown. I also had the impression multiple times during the evening that the Baader 2" prism was giving a brighter view than the others. I never nailed this down to confirm, I just recall my reaction each time I put it in the first look gave me that impression up front. When I get arounf the threshold brightness tests will see how this impression fares.

With the prisms putting on such a good show, and having completed my primary mission for the evening, I decided to try some the mirrors just for fun. I brought out the 1.25" mirrors first and started using them. The first impression I got was, "wow...more scatter!" So it was rather obvious that the prisms had a less distracting scatter profile. I was also surprised that the details on Jupiter were definitely softer than they were in the prisms. The NNTeB was not showing through the 1.25" mirrors, and any structure within NEB and SEB was hinted at only as a shading or brightness difference only. So no more defined structure visible within the main belts. I spent a good amount of time using the TV and Vernonscope and Astro Tech mirror and dielectrics and getting their baseline and then switching in a prism and it held consistently all evening that the prisms were showing more details and showing them more etched. The Vernonscope performed the best of the 1.25" mirror and dielectrics btw, giving a slightly better level of detail and crispness, but still not up to what the prisms were showing.

I then decided to bring out two of the 2" diagonals, the Baader dielectric and the AP Max. The AP Max went into the focuser first. To my disbelief the AP was showing astigmatism on-axis! How could this be?? I then went in and got the Baader dielectric and it too showed this! That was too much of a coincidence however, so I figured that the 2" diagonals must need some thermal acclimation time. Inside the house it's 70 degrees and outside it was about 45 degrees. I had never observed thermal acclimation on 2" diagonals before so was a bit of a shock. The 1.25" diagonals have never shown this for me. At any rate, after some time the Baader dielectric and AP MX settled themselves and the astigmatism vanished. Unfortunately, they did not in the end perform much of any better than the 1.25" mirror/dielectric diagonals as far a detail visibility and crispness on Jupiter. They did come across as being a very little bit better, but it was a difficult tell. When I move to the larger TSA-102 for testing perhaps they will pull ahead. One thing that was a bit obvious, was that the AP Max definitely came across as having less of a scatter profile than the others. But even so, the prisms were the better performers for planetary. If I had to characterize it, I would say that if I went out yesterday observing with just the mirror/dielectrics I would have come in at the end of the night thinking that it was a very good evening for observing Jupiter, much better than average. With the prisms, my impression of the evening would have been that it was simply a spectacular night...what I would call a rarer evening. So all performed well, but the prisms made it seem like it was more special evening for planetary observing.

Anyway, a productive and revealing evening with diagonals as the testing begins.


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Eddgie
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jon Isaacs]
      #6314044 - 01/15/14 10:17 AM

Somewhat related.

I used my binoviewer in my new Stellarvue 110ED and there was a lot of chromatic aberration (using a Baader T2 Maxbright mirror and T2 Prism).

When I installed the Baader 1.25x GPC into the light path, a device that was designed by Roland Cristen specifically to offset the chromoatic an spherical chromatism induced by the binoviewers themselves, the CA improved very dramatically.

There is no question in my mind that the prisms in the binovoiewr were inducing considerable color error at f/7.

Ah, but when I went back and forth between the T2 Max Mirror and the T2 Prism, the result was much harder to see, though once again, I feel as if there was less CA in the Mirror, but the scope is so well corrected that there is not much CA left with GPC in place.

So, I do believe that the prism is indeed not the right choice for a fast refractor.

And I have used mirrors with my 6" APOs and never had any issues with CA, even in the Televue 101.

Personally, I think the test will be a fun read, but I doubt very seriously it will influence me one way or the other.

I have read dozens of tests over the years that tended to overtstate the performacne benefits of this or that product. Dielectric coatings, XLT coatings, this or that eyepeice, this or that filter, and on and on.

As for dielectrics though, when they first came out, there was all this rave about how much better they were (and I found no difference between them and enhanced aluminum), but I have dielectrics that have been cleaned many, many times with absolutly no sign if damage.

I have intentiallly abraded dielectric coatings.

My findings? Oh, you can scratch anything if you try hard enough.. I was able to scrach a dielectric mirror coating by pressing down in it with the tip of a steel screwdriver, but withough resorting to such extreme measures, I found these coatings to be extremely durable.

That is why I would recommend delectrics. No performace difference that I could see, but the knowledge that a clean diagonal is important for planetary observing and that taking the fear out of the cleaning process is priceless. You can clean these over and over and over again, and not worry about damaging them. They are that tough.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: schang]
      #6314060 - 01/15/14 10:24 AM

Quote:

The question is : would you rather do a single sample test and not be able to draw a reliable conclusion, or do limited, genuine repeat tests to get what you need to know?




More is always better However, I am thankful and feel oh so fortunate to even get one unit given how expensive some of this gear is. So this is not about designing a comprehensive test, but more about doing what we can given the generosity of others. I feel quite fortunate we even got one unit from the field of excellent diagonals here! So "take what we can get, and do what we can" is the mantra for field tests we do as amateurs IMO. All that can be asked really.

Now, if people want to start contributing money so we can build an amateur astronomy consumer reports of sorts, where we can do things like you suggest, then that would be way cool! Would be nice to have an independent and funded consumer group to tackle product reviews. Couldn't be done with loaners from the vendors though as that would add some level of conflict, so really all the units would need to be purchased to be fair. Would be an expensive proposition. Heck, I don't think even Consumer Reports uses multiple units for any of their tests. Expensive undertaking for consumer wares. Plus some vendors might not appreciate it, exposing their quality variability. Given how agressive some are this testing group would have to be very well insulated and independent.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Eddgie]
      #6314075 - 01/15/14 10:34 AM

Quote:

As for dielectrics though, when they first came out, there was all this rave about how much better they were (and I found no difference between them and enhanced aluminum), but I have dielectrics that have been cleaned many, many times with absolutly no sign if damage.




I myself am not convinced there is a cleaning advantage. I feel it was more marketing. I never had any issues cleaning my mirror or prism diagonals in the past, not any issue cleaning the mirrors in my old SLRs back in the day. If one follows proper procedure, unlikely you will scratch anything. And today, given how tough multicoatings are and that many a prisms and mirror diagonal are available that are multicoated, unlikely much issue even when not being totally careful cleaning those. In the end, the cleaning advantage marketers say for dielectrics IMO is a solution searching for an issue...like barrel undercuts FWIW, I clean my 1980's Celestron prism diagonal all the time, and surface is still quite great


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6314182 - 01/15/14 11:28 AM

Well, then, clearly you would be wasting your money to buy a dielectric (though now the do not cost any more than enhanced aluminum).

But unless you have some way to test this, then it is a moot point.

But that is why the MilSpec almost demands dielectric coatings these days. They are harder and more scratch resistent than quartz.

When does it matter?

When you are in a desert country fighting a war and bathed in silicone dust.

And you clean your optics grabbing whatever rag you have hadny and spitting on it and whiping it off.

That is what dielectrics are able to withstand.

It is hard stuff. Very hard stuff.

But you will get some result and I am sure you will give us a table with 7 or 8 tiers of performance and maybe you will even abrasion test them....

Look forward to what I am sure will be an entertaining review.

Have fun!!!


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Eddgie]
      #6314368 - 01/15/14 01:08 PM

Lots of folks talk about dielectric hardness, but I've yet to find any real data on this. If you have links to actual hardness rating or abbrasion rating of dielectrics used for mirroring, that would be useful. Similarly would be interesting for multicoatings as well. Lots of marketing, lots of anecdotal, no data

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6315080 - 01/15/14 06:21 PM

Testing costs money that is for sure. For an industry that is large, it is feasible to do multiple tests, by various non-profit institutions on the same product per the defined performance attributes, from which the consumers can compare and decide. This is the case for camera industry where you see various testing labs using testing equipment and field evaluation for lens and body performance. For us telescope users, it is not feasible, which I understand. I really think that telescope optical equipment can be tested the same way as camera lenses using similar test equipment; and in a lot of cases, the manufactures are willing to loan their new products to the testing institutions for PR relations/consumer info. I hope eventually, telescope industry will be going that way.

Edited by schang (01/15/14 08:25 PM)


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6315113 - 01/15/14 06:39 PM

Bill,

Your results mirror my observations (pun intended).

I compared a Tak 1.25" prism, Baader Zeiss 2" prism, and a AP MaxBright in a f/6 refractor on Jupiter last year. I found that the prisms did produce some noticeable chromatic aberration, though nonetheless seemed more detailed than the MaxBright. I found the Tak prism created less chromatics than the 2" Zeiss.

Keith


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6315573 - 01/15/14 10:42 PM

Quite frankly Don, I haven't had any color issues with prisms while doing tests at F-5.9.

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The Mighty Mo
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6315661 - 01/15/14 11:44 PM

Thanks for taking this on and the first preliminary report here Bill. I do have a question, what maximum magnification did you manage to achieve in the prisms, esp the 2" Baader? I ask because I seem to remember reading here once that the prism diagonals were only good up to around 200x, which if true would make them no good to me for planetary use. I'm hoping that was an incorrect statement about them being limited.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6315879 - 01/16/14 05:57 AM

Quote:

Quite frankly Don, I haven't had any color issues with prisms while doing tests at F-5.9.




Daniel, I do not doubt your findings, but I think they should not be generalized: Mr. Wolfgang Rohr of astro-foren.de (temporarily out of service due to installation of a new software) tested several Zeiss APQ-100 f/6.4, some of which were originally designed for the use with and some for the use without additional glass-path. Those designed with glass-path showed color-issues (CA) without a prism and those designed for the use without glass-path provided color-issues when used with the Zeiss prism-diagonal. Only when used the appropriate way (what they originally were designed for), they performed (also interferometrically) as perfect apochromats.

In this context, outcoming results ("Prisms or mirrors best suited for a certain refractor?") do not only depend on the refractor's f-ratio, but also on the color-correction of its objective-lens - be it intentionally designed the way it is or not.

Chris


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Fomalhaut]
      #6315929 - 01/16/14 07:40 AM

I agree it isn't just about focal ratio as is obviously shown to be the case, but at least it's one point we are discussing. I don't doubt his data reveals some legitimate arguments about optics, but it appears we are discussing two matters. Once again, paralysis by analysis. I've conducted plenty of visual tests that don't agree with data. The forums have basically become a platform for people who take a great interest in analyzing data and enjoy discussing it in forums. Most of them are not observers who actually go outside and see how these products appear in reality and secondly they would need to have the products in hand in order to compare them yes?

I've used prisms on doublets, triplets and quads with the same results and I've played with prisms correctors. Bill is taking a telescope and going out into the real world, which is what these products were intended for and saying, hey! this is what I see. That to me, is much more valuable information for amateurs seeking advice. So what is it, that's really important to us as observers? Or are some of us really just really more interested at sitting at a table, studying data? What happened to HAL in the end?

Data would suggest that we shouldn't see any differences in contrast between one multi element eyepiece design vs another, yet very reputable purists swear they still see differences due to whatever the reason. There are even situations where the same samples are not the same. Look at consumers who think 99% dielectric diagonals are the cats meow, yet I've conducted plenty of visual tests to know better. I found specific differences with prisms and mirrors and there are good and bad examples of both but when testing the best of the best, I have found prisms to be better than most people realize in todays modern apochromats of various designs and focal ratios. BTW, there are very few APQ's to be seen these days so perhaps it would be interesting if Rohr conducted more data with more common scopes.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Eddgie]
      #6315944 - 01/16/14 07:55 AM

Quote:



That is why I would recommend delectrics. No performace difference that I could see, but the knowledge that a clean diagonal is important for planetary observing and that taking the fear out of the cleaning process is priceless. You can clean these over and over and over again, and not worry about damaging them. They are that tough.





It's unfortunate you didn't find any differences to appreciate Ed. You certainly have a lot of knowledge about optics.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6315961 - 01/16/14 08:09 AM

"If I had to characterize it, I would say that if I went out yesterday observing with just the mirror/dielectrics I would have come in at the end of the night thinking that it was a very good evening for observing Jupiter, much better than average. With the prisms, my impression of the evening would have been that it was simply a spectacular night...what I would call a rarer evening. So all performed well, but the prisms made it seem like it was more special evening for planetary observing."

Thanks Bill, very interesting, you have motivated me to test my Baader 2" prism against my 2" AP Maxbright in my TMB 92SS. I always use the Maxbright, now I'm curious about comparing each on Jupiter for detail and C/A.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6315991 - 01/16/14 08:45 AM

Quote:

The forums have basically become a platform for people who take a great interest in analyzing data and enjoy discussing it in forums. Most of them are not observers who actually go outside and see how these products appear in reality and secondly they would need to have the products in hand in order to compare them yes




If you prefer first-hand observations, I can provide you some:
After almost 30 years with other instruments, in 1992 I was lucky to get a faultless (!) Tak FCT100 f/6.4 which has been my main (but not my only) visual instrument since. Without diagonal (neither prism nor mirror) its star test is no less than perfect: Using my XO-5 or Hi-Ortho-4 or -2.8, by just comparing defocused (3 to 5 wavelengths) Airy patterns of ~1st magnitude stars, I see no possibility to decide whether the eyepiece is inside or outside of focus nor can I detect any false color: Inside and outside of focus Airy patterns are identical. An instrument colorless such as this one will undoubtedly be best suited to perceive the introduction of false color by additional elements in the optical train...

Well, using my original 45mm Tak prism the same refractor clearly shows a bit of CA, but nevertheless, when targetting Jupiter -> a wealth of surface detail.
In order to reduce prism-induced CA, I replaced the big prism by a standard 1.25" Tak-prism which resulted in even a bit better Jovian detail plus more perfect startest (less CA), but still not quite as good as without any diagonal.
In order to further improve contrast, I then bought a Baader Maxbright and finally (!) got a startest almost identical to the one without diagonal, just with a seemingly tiny bit more straylight.
But with the Maxbright, Jovian detail is clearly less contrasty than when using the 1.25" and even the 45mm-prism!!!

This as mentioned, with my 100/640 Triplet-Tak...

Chris


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6316201 - 01/16/14 10:43 AM

Thanks BillP for this great testing and write up. I recently acquired the Tak 1 1/4" prism diagonal and totally agree with your conclusion of more detail in Jupiter. I tried it out last Saturday with my Tak TSA-102 and could immediately notice a difference. I originally had a Lumicon enhanced aluminum diagonal on the Tak, and when I put the Tak prism diagonal in, the view of Jupiter came alive. It was like looking at a Hubble photo from a distance. There was a massive amount of detail in the belts. Just mezmerizing.

Thanks,


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: The Mighty Mo]
      #6316302 - 01/16/14 11:32 AM

Quote:

Thanks for taking this on and the first preliminary report here Bill. I do have a question, what maximum magnification did you manage to achieve in the prisms, esp the 2" Baader? I ask because I seem to remember reading here once that the prism diagonals were only good up to around 200x, which if true would make them no good to me for planetary use. I'm hoping that was an incorrect statement about them being limited.




That preliminary test was using an 80mm and working at a .64mm exit pupil which is only 125x. I can certainly push that instrument to 200x on the Moon and see what happens. When I move to the TSA-102 it can more confortably go beyond 200x with a brighter image and have used it higher than that productively for planetary as well so will make sure to include some high power observations that may go to smaller exit pupils that I anticipated as well.

btw, when folks told you prisms were only good up to 200x, did they tell you why?


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6316321 - 01/16/14 11:45 AM

Thanks Bill. I don't remember off-hand and not sure I could find them now. I remember reading it in posts here a year or so back (before I joined) during the last round of "prism diagonals for achromat refractors" testing and discussions, which for some reason quickly morphed into ED scopes and prisms, and mostly ignored achromat use after that.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Fomalhaut]
      #6316325 - 01/16/14 11:46 AM

Chris,
Thank you for sharing your post. In my opinion, that's the kind of post that offers real value for amateurs seeking advice. Interestingly if you take a green laser and fire it at a dielectric and an aluminized diagonal, you will see more scatter on the surface of the dielectric. It's quite obvious and a fun experiment.
Regards
Daniel


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6316416 - 01/16/14 12:35 PM

Quote:

Chris,
Thank you for sharing your post. In my opinion, that's the kind of post that offers real value for amateurs seeking advice. Interestingly if you take a green laser and fire it at a dielectric and an aluminized diagonal, you will see more scatter on the surface of the dielectric. It's quite obvious and a fun experiment.
Regards
Daniel



since calibrated dielectric coatings are designed to reflect a decent spectrum only at one angle, my question is, "At what angle?"
There shouldn't be scatter of a 532nm laser at the exact angle of 45 degrees if the coatings are correctly chosen.
At other angles, the spectrum of reflection will change, and dispersion could be either worse or better, depending on the wavelength.
The implication of your comment is that dielectric-coated diagonals will all scatter light, and that isn't necessarily the case.
To actually test this, a device to hold the laser at exactly the right angle would need to be devised, and a photographic sensor on the other end that could measure the splatter across a group of pixels to decide what happens.
I would suspect that splatter, after that, could very well be due to the smoothness of the mirror surface before coating, since, as we know, contrast-robbing light scatter from a mirror is an indication of roughness.

I had a chance to compare a fair number of Lumicon diagonals in a couple scopes (TV101 and 5" Mak), and the optical quality of the mirrors, as you would expect, varied. I kept the best two--one was an enhanced aluminum coating, while the other was a dielectric. Ultimately, I couldn't decide between the two and kept both. However, in the daytime, the dielectric appeared ever-so-slightly tinted yellow when simply looking through the diagonal at a white wall. This was not a characteristic of the spectrum of reflection at 45 degrees, but the fact that off-axis light is treated differently, spectrally, than on-axis light when reflecting off a dielectric surface. Under similar multi-angle reflections, some dielectric mirrors appear blue, others green, and others somewhat yellow. It is in the nature of constructive and destructive interference in the spectrum of light hitting the diagonal at a variety of angles. What it does point out is that flat-blackening the inside of the diagonal and providing well-placed baffles is critical on a dielectric diagonal.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6316471 - 01/16/14 12:59 PM

Hi Don,

I recall the issues with those color tones you mentioned but only with the brand you mentioned. I have never seen that by any other supplier of dielectric diagonals. Overall, in my opinion, based on the visual tests I've conducted, I actually prefer not to use dielectric diagonals. Seeing premium examples of each, I prefer silver or aluminium or prisms.

Cheers!


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6316503 - 01/16/14 01:16 PM

Quote:

since calibrated dielectric coatings are designed to reflect a decent spectrum only at one angle, my question is, "At what angle?"




Hi Don,

The "angle" caught my attention. I use Matsumoto-san's EMS (Erecting Mirror System, 60 degree angle reflection) for binoscope.

I came to know this from Matsumoto-san's experiment, enhanced aluminum coated, dielectric coated, and enhanced silver coated mirror comparison for EMS use.

Multiple reflection test result is striking to me in terms of how much we loose color and contrast by mirror reflection.
There is English description on his page:
http://www.page.sannet.ne.jp/mazmoto/emsultima.htm

Tammy


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6316855 - 01/16/14 04:09 PM

Tammy:

Is EMS system using silver coated mirror? Did Matsumoto shone LED light thru this coating to see if the light also pass through the mirror, just like he demonstrated with dielectric mirror?


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6316922 - 01/16/14 04:48 PM

Quote:

Quote:

since calibrated dielectric coatings are designed to reflect a decent spectrum only at one angle, my question is, "At what angle?"




Hi Don,

The "angle" caught my attention. I use Matsumoto-san's EMS (Erecting Mirror System, 60 degree angle reflection) for binoscope.

I came to know this from Matsumoto-san's experiment, enhanced aluminum coated, dielectric coated, and enhanced silver coated mirror comparison for EMS use.

Multiple reflection test result is striking to me in terms of how much we loose color and contrast by mirror reflection.
There is English description on his page:
http://www.page.sannet.ne.jp/mazmoto/emsultima.htm

Tammy



Were it not for the corrosion factor, there is little question that an over-coated silver would be the optimum coating, applied in a vacuum chamber using IAD.
However, it is not the optimum for longevity.
If the overcoating is multi-layer (such as in enhanced mirror coatings), reflectivity could be as high or higher than pure dielectric coatings and have a lot flatter spectral response at angles slightly off-axis.

A certain percentage of light always passes through a dielectric coating. The wavelengths of the pass-through, what type of cancellation of the wave takes place (or augmentation), etc. makes it desirable to have a non-reflective back surface for the mirror so coated, and makes such coatings questionable (though possibly useful) in a partial reflection/partial pass-through situation. This has always been true, making pure dielectric coatings only appropriate for star diagonals and secondary mirrors.

But, as I said, pure dielectric coatings can be calibrated for near-perfect reflection at pretty much only one angle (not too dissimilar to the inside surface of a prism at 45 degrees). It would not surprise me to learn some of the inexpensive dielectric-coated star diagonals are not as reflective as they claim because of improper calibration of wavefront augmentation angles and coating choices.

And, as for durability, we really don't know since this technology isn't old enough to let us know on 20-50 year old coatings.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6317225 - 01/16/14 07:28 PM

Quote:

(not too dissimilar to the inside surface of a prism at 45 degrees).




Excellent example!


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6317907 - 01/17/14 05:14 AM

Interesting link Tammy,

I recall having discussions with Bryan Greer at Protostar about silver coatings years ago and he used to talk about the corrosion factor Don expressed and I agree with him, it's probably the most ideal as far as image quality. Interestingly I've had lengthy discussions about observations of Jupiter a friend of mine, Ernie Varnum has been making this past month and he has made numerous comparisons with his diagonals. He has a TEC 7" Mak and a TEC 140. He makes comparisons often and places his diagonals in the following order. All are by premium companies and if you ever wish to PM I'll tell you the names.

prism and silver tied for 1st place (says he goes back and forth and can't tell sometimes)
aluminum in 2nd place
dielectric in 3rd place (and he says a distant 3rd due to noticeable scatter)

Also, I've noticed dielectric diagonals often have inconsistent edge quality and there's an easy way to test for it visually, it's quite striking. It's interesting to note that all the premium roof prism binoculars by the best makers are all prisms and the image quality in those are sensational.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: schang]
      #6318249 - 01/17/14 10:18 AM

Hi Shien,

Quote:

Tammy:

Is EMS system using silver coated mirror? Did Matsumoto shone LED light thru this coating to see if the light also pass through the mirror, just like he demonstrated with dielectric mirror?




Yes, current EMS uses the improved silver coated mirror. The see-through test is with dielectric coated mirror.

Tammy


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: schang]
      #6318251 - 01/17/14 10:18 AM

Quote:

I do not know if this is what you mentioned, but I found it on the old CN forum:

http://www.cloudynights.com/item.php?item_id=82

http://www.cloudynights.com/item.php?item_id=1854




Great read, thanks.



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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6318320 - 01/17/14 10:51 AM

Hi Daniel,

Yes, Matsumoto-san did quite a bit of homework to choose silver coated mirror. He and his mirror vendor did extensive durability tests along with his friends, who helped him torture-test the mirror

I don't know you have noticed the URL at bottom of the page. Here is the URL to the torture test.

http://www.page.sannet.ne.jp/mazmoto/silvertest.htm

Tammy


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6318346 - 01/17/14 11:00 AM

Tammy,
Who, currently, is selling silver-coated star diagonals and/or doing silver mirror coating?


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Starman1
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6318368 - 01/17/14 11:13 AM

Here is a trade study done on coating materials:
http://www.ssl.berkeley.edu/~mlampton/WhySilver.pdf
The visible range is 0.4 to 0.7 microns (400-700nm)
Gold is totally inappropriate for visual studies, but has superb transmission in the infrared, where it is most often used.
Silver is best and it is interesting to see the curves for bare aluminum, aluminum with SiO overcoat, and MgFl2 overcoat.
The description of the difficulties with doing silver coating show why it is not commercially common:
--base layer between silver and glass
--silver
--layer to get overcoat to adhere to silver
--overcoat

It is far easier to do aluminum coating, not to mention a lot less expensive.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6318520 - 01/17/14 12:20 PM

Quote:

Tammy,
Who, currently, is selling silver-coated star diagonals and/or doing silver mirror coating?




Hi Don,

I don't know who is making EMS silver coated mirror for Matsumoto-san in Japan. I am sure it is custom made for him.

Here is one example of the mirror:
http://www.page.sannet.ne.jp/mazmoto/lfmrs.jpg

Tammy


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urassner
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6318569 - 01/17/14 12:44 PM

Vernonscope Quartz diagonal is silver coated.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6318760 - 01/17/14 02:10 PM

Quote:

Who, currently, is selling silver-coated star diagonals and/or doing silver mirror coating?




Folks who market silver or enhanced silver diagonals are:

Vernonscope
APM
Antares
Denkmeier (with their powerswitch)

And of course to confuse folks GSO markets a "dielectric enhanced silver"


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6318939 - 01/17/14 03:28 PM

Quote:

Quote:

Tammy,
Who, currently, is selling silver-coated star diagonals and/or doing silver mirror coating?




Hi Don,

I don't know who is making EMS silver coated mirror for Matsumoto-san in Japan. I am sure it is custom made for him.

Here is one example of the mirror:
http://www.page.sannet.ne.jp/mazmoto/lfmrs.jpg

Tammy



Seeing those mirrors makes me think that using a high-quality secondary mirror (like a 1/30 wave Antares Optical) in a star diagonal might be a very nice thing:
--no edge problems
--no issues with surface accuracy or smoothness
http://www.antaresoptics.com/SecEM.php
http://www.antaresoptics.com/SecAdvantages.html


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6318978 - 01/17/14 03:58 PM

Based on this site, "enhanced" Silver means that dielectric layers are put on top of the Silver. So increases the transmission of the Silver some. But is this now taking something good and ruining it by adding scatter!?

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6319000 - 01/17/14 04:07 PM

Quote:

Based on this site, "enhanced" Silver means that dielectric layers are put on top of the Silver. So increases the transmission of the Silver some. But is this now taking something good and ruining it by adding scatter!?



It's more of a protection for the silver than it is to enhance the already >97% reflectivity.
Theoretically, it could enhance scatter. But we all put overcoats on top of the aluminum on our mirrors, and besides, 1 or 2 layers means nothing compared to the 20-45 layers on a dielectric-coated diagonal mirror.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6319744 - 01/18/14 12:57 AM

Bill,

Is there any benefit to owning 2" prism diagonal as opposed to a 1.25"? I'm considering buying prism a diagonal for use with the TEC140, and I'll probably be using Supermonos, XO 5mm, and ball eyepieces most of the time.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6319849 - 01/18/14 04:28 AM Attachment (27 downloads)

Tammy,
I wonder how my Vernonscope Gold would do in the torture test.

Daniel


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: SteveC]
      #6319908 - 01/18/14 05:53 AM

That's a good question, I wonder the same thing. What would be the reason owning a 2" prism diagonal as opposed to a 1.25", except for the 2" eyepieces

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: SteveC]
      #6320029 - 01/18/14 09:08 AM

Quote:

Bill,

Is there any benefit to owning 2" prism diagonal as opposed to a 1.25"? I'm considering buying prism a diagonal for use with the TEC140, and I'll probably be using Supermonos, XO 5mm, and ball eyepieces most of the time.




Other than convenience of having just one diagonal for all purposes I can't imagine. But when the Moon goes away I will try and see if a wide field low power view has any different characteristics between the mirrors and prisms. Nothing like "looking" to tell


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6320095 - 01/18/14 09:52 AM

Bill,

I have heard that a 2" diagonal tends to be a better risk because in a 1.25" diagonal there is more of a chance that the image will be degraded by edge errors in the smaller mirror. Of course, this will depend on the quality of the diagonals.

Mike


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Sarkikos]
      #6320140 - 01/18/14 10:25 AM

Great thread and kudos for all the 'experimenters' for their hard work!

I take it from reading all this that I should get a prism diagonal for my TEC 8" f/15.5 to replace the Astro-physics Maxbright which I have been using. I assume the new prism diagonal will reduce scatter and not add any false colour thanks to my loooong focal ratio of f/15.5

My primary viewing is double stars, the closer, the better and planets, in season...

Is there a recommended brand of 2" prism diagonal I should get?

Thanks

Dave


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6320157 - 01/18/14 10:38 AM Attachment (12 downloads)

Quote:

Tammy,
I wonder how my Vernonscope Gold would do in the torture test.

Daniel



Daniel,
Gold has a very flat transmission in the infrared, where it is the preferred material for high transmission (no overcoating necessary, no corrosion!),
but it chops off most of the blue/violet end of the spectrum, so it really isn't a visual coating. It could have some specialty uses, like H-alpha photography, or searching for red giants in globulars, etc.
But for a normal visual coating, it's the wrong material.

Edited by Starman1 (01/18/14 10:43 AM)


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Sarkikos]
      #6320331 - 01/18/14 11:56 AM

Quote:

Bill,

I have heard that a 2" diagonal tends to be a better risk because in a 1.25" diagonal there is more of a chance that the image will be degraded by edge errors in the smaller mirror. Of course, this will depend on the quality of the diagonals.

Mike




Hi Mike. I have heard this too, and I personally do not understand this logic. I have heard it lots of course...but I feel it is more a marketing thing rather than based in anything discrete. Seems it should matter little on the size of the mirror and find it hard to believe that manufacturers cannot produce smaller mirrors with quality to the edge. And if there is a general problem in the industry with the edge, then seems the solution is to use an oversized mirror, regardless if 1.25" or 2" format. At any rate, at present I take that popularism with a grain of salt.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6320429 - 01/18/14 12:41 PM

Well, a 2" format diagonal would by definition be oversized compared to any 1.25".

The preference for a 2" diagonal to avoid possible edge errors in a 1.25" diagonal is similar to the idea that a Newt should have a somewhat over-sized diagonal to avoid image degradation from edge errors. I don't know how valid this idea is, either. Seems that the best way to avoid both problems is to acquire an optically decent product.

Mike


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Cotts]
      #6320530 - 01/18/14 01:26 PM

I take it from reading all this that I should get a prism diagonal for my TEC 8" f/15.5 to replace the Astro-physics Maxbright which I have been using. I assume the new prism diagonal will reduce scatter and not add any false colour thanks to my loooong focal ratio of f/15.5

My primary viewing is double stars, the closer, the better and planets, in season...

Is there a recommended brand of 2" prism diagonal I should get?

Dave
------------------------------------
__----_----

Yes, get the Zeiss Baader 2in Prism.
You can try mine but then again you would have to bring your baby over to my backyard.
It is superb on my C-14 but I have yet to test it on my refractors.
(I do think the Maxbright has less scatter then some other 'el-premo'mirror diagonals that I have laser tested).
Clears
Jerry

Edited by Full Sun (01/18/14 01:38 PM)


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Full Sun]
      #6320694 - 01/18/14 02:59 PM

Quote:

Quote:

Great thread and kudos for all the 'experimenters' for their hard work!

I take it from reading all this that I should get a prism diagonal for my TEC 8" f/15.5 to replace the Astro-physics Maxbright which I have been using. I assume the new prism diagonal will reduce scatter and not add any false colour thanks to my loooong focal ratio of f/15.5

My primary viewing is double stars, the closer, the better and planets, in season...

Is there a recommended brand of 2" prism diagonal I should get?

Thanks

Dave



Yes, get the Zeiss Baader 2in Prism.
You can try mine but then again you would have to bring your baby over to my backyard.
It is superb on my C-14 but I have yet to test it on my refractors.
(I do think the Maxbright has less scatter then some other 'el-premo'mirror diagonals that I have laser tested).
Clears
Jerry



I've been thinking about getting that 2" Zeiss Baader prism as well, but I just can't wrap my head around the $425 it would cost!

I currently have a Lumicon LumiBrite 2" diagonal. It has a dielectric mirror, and appears to not have issues with scatter, though it may be that it does, and I just don't perceive it. I recently got a Kunming United Optics 102mm f/11 achro that Stellarvue was selling, and thought that it might benefit from having a good, prism diagonal. However, the views through the Lumicon diagonal aren't bad at all. Because of that, I keep thinking that might be wasting my money getting that Baader Zeiss prism diagonal; sort of like trying to make "more gooder" what's already "good enough!"

Would the 2" Baader be better than the 2" Lumicon?

BTW, I'd also like to thank everyone involved with doing this comparo, especially Bill for dong all the dirty work. Your efforts are greatly appreciated! Thanks.

Edited by mayidunk (01/18/14 03:03 PM)


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: mayidunk]
      #6320724 - 01/18/14 03:16 PM

Remember everyone that I've just scratched the surface here with the initial report. Have a lot more tests to conduct and of course will be submitting it as an article here. So much more to come. Probably not any more interim results will be posted as time for me to get serious and ferret things out and ensure repeatability of results.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6321022 - 01/18/14 06:10 PM

Quote:

Quote:

Bill,

I have heard that a 2" diagonal tends to be a better risk because in a 1.25" diagonal there is more of a chance that the image will be degraded by edge errors in the smaller mirror. Of course, this will depend on the quality of the diagonals.

Mike




Hi Mike. I have heard this too, and I personally do not understand this logic. I have heard it lots of course...but I feel it is more a marketing thing rather than based in anything discrete. Seems it should matter little on the size of the mirror and find it hard to believe that manufacturers cannot produce smaller mirrors with quality to the edge. And if there is a general problem in the industry with the edge, then seems the solution is to use an oversized mirror, regardless if 1.25" or 2" format. At any rate, at present I take that popularism with a grain of salt.



I almost hate to throw this in, but this is from Roland Christen, concerning the edges of the AP MaxBright star diagonal:
http://geogdata.csun.edu/~voltaire/roland/dielectric.html
Also, if you exclude the outer 1mm of the edge of my secondary mirror (it came with an interferogram), the peak-to-valley measurement reduces by over 50%!. The edge is always the issue with secondary mirrors, but it isn't the only problem after mounting. Too much pressure from material behind it, and it can become convex. And, improperly-glued, it can become astigmatic. Both of those issues are worse than the edge of the mirror problem.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6321131 - 01/18/14 07:19 PM

Can anyone explain how reflectivity and scatter relate?

According to AP, my dielectric Maxbright diagonal has 99% reflectivity, yet it has been implied in this thread that its dielectric coating may cause scatter.

According to TEC, my TEC enhanced aluminum mirrored turret has 97% reflectivity, yet it has been implied in this thread that it may have less scatter.

Edited by SteveC (01/18/14 07:22 PM)


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6321226 - 01/18/14 08:45 PM

Yea Don, he's had that article out for quite some time. BTW, last night seeing was stunning. I rated it an 8 out of 10. Don, you should bring your Teeter to my place and set up for Jupiter soon while the timing is good. Once again I conducted tests all with high quality diagonals and the prism is absolutely incredible to say the least. It's simply crisper and cleaner than anything I've ever seen. It's like any glow you see around Jupiter in a dielectric is literally cut down more than 50% by my visual estimation. Dielectric diagonals just simply can not compete with it, even Ray Charles could see it.

I set up my FS102, FS152 and Mewlon 250 and the images just wouldn't quit. The Mewlon was stunning. So much beautiful detail and color on Jupiter while using the Mark V bino with the prism. I could see three gorgeous regions of violet ammonia ice just below the troposphere under the NEB and the RS really appears rusty this year.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: SteveC]
      #6321240 - 01/18/14 08:54 PM

Quote:

Can anyone explain how reflectivity and scatter relate?

According to AP, my dielectric Maxbright diagonal has 99% reflectivity, yet it has been implied in this thread that its dielectric coating may cause scatter.

According to TEC, my TEC enhanced aluminum mirrored turret has 97% reflectivity, yet it has been implied in this thread that it may have less scatter.




Well, I'll see if I can explain. Dielectric coatings can behave just like interference filters.
Here's how it works:
The light wave hits the surface. Some of it reflects, and some of it does not, and penetrates to a deeper level, where it reflects. If the crest of the second reflection's wave corresponds to the crest of the first wave's reflection, the height of the wave is augmented (constructive interference), and the combined reflection is higher.
That means a measurement of reflection intensity would be higher than any layer's reflection on its own. If the materials are correctly chosen, the reflection from the base glass can be augmented, bringing reflectivity up to nearly 98% at 550nm (green).
But that will be only at one angle.
At other angles, reflected waves will destructively interfere and some cancellation of the wave's reflection will occur.
This, however, is unlikely to happen at every wavelength. If it did, dielectric coatings would not scatter light at all and would be simply the best there is.
Some wavelengths, however, might be dispersed in the same way that light passing through a prism at the right angle is dispersed. At whatever that angle is, the reflection would be prismatic. In fact, prismatic reflection does occur at odd angles from many dielectric-coated surfaces.
If the stack of materials is chosen correctly, the reflection should be nearly pure at one angle--ostensibly 45 degrees on a star diagonal. But what happens to the light that doesn't reflect?
Well, it can pass through the material (hence my earlier comment about making the back of the mirror non-reflective), or it can reflect off layers in the stack and end up parallel to the original ray but perhaps out of phase. Or it can be absorbed by the material, bumping electrons in the material into more excited states. Said light would eventually be re-radiated at who-knows-what angle (but most likely not at visible wavelengths). And, if the surfaces are not perfectly parallel, some may be reflected in other directions.

Picture the Himalayas--resembles the surface of a mirror at a microscopic level. Now, coat that with a material which sticks to the first points encountered (the peaks) and the valleys later. The coating will be ever-so-slightly thicker on the peaks and exaggerate the peak-to-valley error. It's no big deal for a single layer, or even two or three. But make the layers 45 layers deep, and the final layer could be substantially more "rugged" than the first layer. Because of interference, reflectivity will still be quite high, but a fair amount of light will go in a variety of directions (admittedly, a small percentage).

This is analogous to a mirror with a rough surface. Contrast just isn't what it should be because a lot of light is scattered. Fortunately, the dielectric surfaces aren't as reflective, per layer, as an aluminum coating, or the scattering would be ferocious. If the surface is incredibly smooth (like the $795 quartz diagonal from Vernonscope), and the layers on top are applied using ion-assisted depostion, you could still end up with a very smooth surface and less scatter.

And, in fact, some high-end dielectric star diagonals are quite good. Just not cheap.

But if you compare it to a simple coating, like aluminum with a single layer overcoat, and have equally flat and smooth base surfaces, the odds are that the simple coatings will have a better finished surface, with less scatter.

The way I read that is that a dielectric coating needs a flatter and smoother base and greater skill in the application of the coatings. Unfortunately, the quality of the mirrors goes all over the place in star diagonals because they are made inexpensive so they'll sell. If a good after market secondary mirror, with a terrific surface, sells by itself for more that most high-end star diagonals, how much do you think the star diagonal that used a mirror of that quality would cost? Uh huh. You get the picture--it wouldn't sell at all.

So, what are the odds of a really good surface with smoothness at a cheap price? Well, higher with a prism. Partially because of volume (prisms are used in a LOT of other applications) and partially because they have less-expensive coatings and partially because they are partially refractive (I won't go into that here).

I've compared $50 mirror diagonals with $50 prism diagonals, and at f/10-f/15, the prism was brighter and sharper. In that price range (what comes with a lot of scopes), the mirror is cheaper to make so it makes the manufacturer more profit.

Many factors involved other that pure optics.

Edited by Starman1 (01/19/14 10:22 AM)


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6321298 - 01/18/14 09:42 PM

Quote:

I almost hate to throw this in, but this is from Roland Christen, concerning the edges of the AP MaxBright star diagonal:
http://geogdata.csun.edu/~voltaire/roland/dielectric.html




Don't worry. I will try to not be too harsh as a doubting Thomas

Of course, I know this post well. And if RC did not have a diagonal of his own in the mix then one would not have to be concerned about any conflict of interests. But alas, he is marketing a feature of his diagonal. So I can't feel like it is a totally objective point of view.

It is specific to only dielectrics. So that maybe gives it some strength. But even so, the coating technology market moves rather fast...much faster than other areas used in consumer astronomy. So even if the issue is true, is it still true? Part of the problem with some of the posts continually referred to is that they are getting a little long in the tooth and relating a previous era's technology (which in this case an era can be just 2 years, depending on the astro technology).

At any rate. Yup seen it. But given the age of it and the conflict of interest potential, have to give it a grain of salt in the context of the present.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6321300 - 01/18/14 09:44 PM

Quote:

I set up my FS102, FS152 and Mewlon 250 and the images just wouldn't quit. The Mewlon was stunning. So much beautiful detail and color on Jupiter while using the Mark V bino with the prism. I could see three gorgeous regions of violet ammonia ice just below the troposphere under the NEB and the RS really appears rusty this year.




Man!!! Did you have to post this!!! It is making my mouth water for a Mewlon!!!!!! Arrrrg!


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6321372 - 01/18/14 10:32 PM

Don,

Thanks for taking the time to explain the scatter/reflectivity issue to me. For the most part, I considered all dielectric mirrors as basically generic astro products, never really considering that there might be differences between them. Silly me.

This thread has been very informative.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6321465 - 01/18/14 11:52 PM

Quote:

It's like any glow you see around Jupiter in a dielectric is literally cut down more than 50% by my visual estimation. Dielectric diagonals just simply can not compete with it, even Ray Charles could see it.






The scene in the Blues Brothers comes to mind where the kid tries to steal the guitar from Ray Charles and he fires the .357 Magnum across the room.

Invite everyone and call it the Ray Charles Invitational Observing Session.

I guess now I'll just have to retrofit my Newtonians with prisms ….


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6321493 - 01/19/14 12:22 AM

Nice post Don, thanks for sharing.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6321869 - 01/19/14 09:57 AM

Quote:

Quote:

I set up my FS102, FS152 and Mewlon 250 and the images just wouldn't quit. The Mewlon was stunning. So much beautiful detail and color on Jupiter while using the Mark V bino with the prism. I could see three gorgeous regions of violet ammonia ice just below the troposphere under the NEB and the RS really appears rusty this year.




Man!!! Did you have to post this!!! It is making my mouth water for a Mewlon!!!!!! Arrrrg!




Yea, that was quite a remarkable evening. I like it when it's like that don't you?


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6328283 - 01/22/14 12:36 PM

Quote:

Quote:

Can anyone explain how reflectivity and scatter relate?

According to AP, my dielectric Maxbright diagonal has 99% reflectivity, yet it has been implied in this thread that its dielectric coating may cause scatter.

According to TEC, my TEC enhanced aluminum mirrored turret has 97% reflectivity, yet it has been implied in this thread that it may have less scatter.




Well, I'll see if I can explain. Dielectric coatings can behave just like interference filters.
Here's how it works:
The light wave hits the surface. Some of it reflects, and some of it does not, and penetrates to a deeper level, where it reflects. If the crest of the second reflection's wave corresponds to the crest of the first wave's reflection, the height of the wave is augmented (constructive interference), and the combined reflection is higher.
That means a measurement of reflection intensity would be higher than any layer's reflection on its own. If the materials are correctly chosen, the reflection from the base glass can be augmented, bringing reflectivity up to nearly 98% at 550nm (green).
But that will be only at one angle.
At other angles, reflected waves will destructively interfere and some cancellation of the wave's reflection will occur.
This, however, is unlikely to happen at every wavelength. If it did, dielectric coatings would not scatter light at all and would be simply the best there is.
Some wavelengths, however, might be dispersed in the same way that light passing through a prism at the right angle is dispersed. At whatever that angle is, the reflection would be prismatic. In fact, prismatic reflection does occur at odd angles from many dielectric-coated surfaces.
If the stack of materials is chosen correctly, the reflection should be nearly pure at one angle--ostensibly 45 degrees on a star diagonal. But what happens to the light that doesn't reflect?
Well, it can pass through the material (hence my earlier comment about making the back of the mirror non-reflective), or it can reflect off layers in the stack and end up parallel to the original ray but perhaps out of phase. Or it can be absorbed by the material, bumping electrons in the material into more excited states. Said light would eventually be re-radiated at who-knows-what angle (but most likely not at visible wavelengths). And, if the surfaces are not perfectly parallel, some may be reflected in other directions.

Picture the Himalayas--resembles the surface of a mirror at a microscopic level. Now, coat that with a material which sticks to the first points encountered (the peaks) and the valleys later. The coating will be ever-so-slightly thicker on the peaks and exaggerate the peak-to-valley error. It's no big deal for a single layer, or even two or three. But make the layers 45 layers deep, and the final layer could be substantially more "rugged" than the first layer. Because of interference, reflectivity will still be quite high, but a fair amount of light will go in a variety of directions (admittedly, a small percentage).

This is analogous to a mirror with a rough surface. Contrast just isn't what it should be because a lot of light is scattered. Fortunately, the dielectric surfaces aren't as reflective, per layer, as an aluminum coating, or the scattering would be ferocious. If the surface is incredibly smooth (like the $795 quartz diagonal from Vernonscope), and the layers on top are applied using ion-assisted depostion, you could still end up with a very smooth surface and less scatter.

And, in fact, some high-end dielectric star diagonals are quite good. Just not cheap.

But if you compare it to a simple coating, like aluminum with a single layer overcoat, and have equally flat and smooth base surfaces, the odds are that the simple coatings will have a better finished surface, with less scatter.

The way I read that is that a dielectric coating needs a flatter and smoother base and greater skill in the application of the coatings. Unfortunately, the quality of the mirrors goes all over the place in star diagonals because they are made inexpensive so they'll sell. If a good after market secondary mirror, with a terrific surface, sells by itself for more that most high-end star diagonals, how much do you think the star diagonal that used a mirror of that quality would cost? Uh huh. You get the picture--it wouldn't sell at all.

So, what are the odds of a really good surface with smoothness at a cheap price? Well, higher with a prism. Partially because of volume (prisms are used in a LOT of other applications) and partially because they have less-expensive coatings and partially because they are partially refractive (I won't go into that here).

I've compared $50 mirror diagonals with $50 prism diagonals, and at f/10-f/15, the prism was brighter and sharper. In that price range (what comes with a lot of scopes), the mirror is cheaper to make so it makes the manufacturer more profit.

Many factors involved other that pure optics.




I read this once then two more times, please be patient with me as I am trying to understand it, Roland states on the AP web site Advantages of the Astro-Physics Dielectric Coatings:
"Reflectivity - Reflectivity is above 99% over the entire 4000 to 7000 Å photo-visual range. Thin film coatings have extremely low surface scatter compared to aluminum or enhanced aluminum coatings. Dielectric coatings (if done right) will result in less scattered light. Examination with a laser source shows approximately a 5 fold reduction in surface scatter, a tremendous improvement over aluminum mirrors."

I realize he is selling a product but I'm hard pressed to believe his info is misleading. Sooooo my question is when does the aluminum or enhanced aluminum coating perform better then the dielectric ?? because how could this statement be true ?? "And, in fact, some high-end dielectric star diagonals are quite good. Just not cheap.
But if you compare it to a simple coating, like aluminum with a single layer overcoat, and have equally flat and smooth base surfaces, the odds are that the simple coatings will have a better finished surface, with less scatter."

Don, if you are reading this I am in no way questioning your knowledge on the subject, I'm just a little confused because the way I read it from AP the dielectric has a advantage over the aluminum or enhanced aluminum coating, or is this not true in all cases?? and if not then why?? why is Roland wrong in some cases if he is?? I kind of got what you said and I kind of didn't, if that makes sense.

Please keep in mind that I'm full of cold medicine and my head feels like a bubble, my mind is not fully firing on all cylinders right now.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6328646 - 01/22/14 03:17 PM

After doing some more reading I guess the question for me in regards to scatterd light comes down to this, if dielectric coatings are resistant and they don't degrade like aluminium then this is the main advantage of these star diagonals.

"the mirror will have the same high 99% reflectivity 10 years from now when most aluminium coated will have dropped well below 90% and should be realumined. Markus Ludes from APM Telescopes confirms that testing reflection of good quartz protected aluminium coating showed after 10 years a reflection of about 83-85%."

"decay due to exposure and damage to the coatings in its use could significantly worsen quality of a reflective coating, resulting in unacceptable amount of scattered light."

The implication "is that dust/dirt are the dominant - and significant - factor of scatter from mirror surface in a relatively short period of time (one year, or so) in between two successive removals, with the combined scatter reaching 5%, or more. But scatter due to the coating may have high relative rate of increase with time, and could become dominant with a coating that is several years old.

And the implication would be not so with the dielectric, at least thats the way I'm understanding it.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6328777 - 01/22/14 04:12 PM

John,

Many good points you brought out. Unfortuantely, different manufacturing processes have different lifespans. I'm not sure anyone can say with assurance the rate of reflectance loss as so many variables affect it, like humidity and protective coatings used. Here's an interesting link as example of some research - A NEW METHOD TO CHARACTERIZE DEGRADATION OF FIRST SURFACE ALUMINUM REFLECTORS.

I don't know if there is any good research yet on the longevity of dielectric coatings either. So will be interesting to see what time tells with these. But like any coating process, the more coats used then the more important clenliness becomes between the coats as particulates cause scatter. There's also research out there that shows that using ion-beam-sputtering to deposit the coatings causes less coating scatter than do evaporation techniques. My fear is that there are multiple methods to deposit coatings, whether protective or dielectric. Not knowing the specifics for each item sold, means impossible to draw much of any conclusions as to scatter, lifespans, and rate of degredations. One thing we do know though I think...a simply polished glass prism with some standard AR coatings has little to go wrong with it over time and will have the same longevity as the main objective.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6328888 - 01/22/14 05:04 PM

Quote:


I read this once then two more times, please be patient with me as I am trying to understand it, Roland states on the AP web site Advantages of the Astro-Physics Dielectric Coatings:
"Reflectivity - Reflectivity is above 99% over the entire 4000 to 7000 Å photo-visual range.



Nope. No commercial dielectric-coated mirror actually has that transmission curve. the transmission at the extremes is not low (96%+), but not 99%.
Quote:


Thin film coatings have extremely low surface scatter compared to aluminum or enhanced aluminum coatings. Dielectric coatings (if done right) will result in less scattered light. Examination with a laser source shows approximately a 5 fold reduction in surface scatter, a tremendous improvement over aluminum mirrors."



Depends on at what angle. At low angles, yes. Wavefront cancellation will occur (variable depending on wavelength) but not the "nearly on axis" scatter than some planetary gurus are talking about. The issue is different at different wavelengths. Complicating that is that not all dielectric-coated mirrors utilize exactly the same materials in exactly the same layers.
Quote:


I realize he is selling a product but I'm hard pressed to believe his info is misleading.




On axis, a dielectric coating should perform magnificently. But, truly, the scatter depends on the quality of the original mirror (substrate) and the quality of the coatings applied. Even RC illustrates the downside of dielectric coatings with his interferograms.
Quote:


Sooooo my question is when does the aluminum or enhanced aluminum coating perform better then the dielectric ?? because how could this statement be true ??



If the mirror is better (smoother), and the coatings are smoother, and transmission is not the only measured characteristic.
Quote:


"And, in fact, some high-end dielectric star diagonals are quite good. Just not cheap.
But if you compare it to a simple coating, like aluminum with a single layer overcoat, and have equally flat and smooth base surfaces, the odds are that the simple coatings will have a better finished surface, with less scatter."



Simple: apply one coating layer--it's smoothness depends on the substrate and the application (IAD preferred). Add additional layers and with every additional layer the surface gets rougher and more irregular.
Since each layer doesn't reflect the entire wavelength spread, this could result in phase errors and overall wavefront errors. Some of the high-end dielectric diagonals, though, have really high-end mirrors and the most expensive processes of deposition. But they are still not the best that labs can do. They charge $thousands for a really accurate coating if ordered by NASA or the military because they know it will be tested and tested exhaustively.
Quote:


Don, if you are reading this I am in no way questioning your knowledge on the subject, I'm just a little confused because the way I read it from AP the dielectric has a advantage over the aluminum or enhanced aluminum coating, or is this not true in all cases??



It's not true in all cases. Dielectric coatings would be preferred for dimmer objects, where cleaning is important, and, possibly, longevity (jury still out). They might not be ideal for an achromatic refractor with violet haze around the stars.
Quote:


and if not then why??




slight off-axis scatter, wavefront deformation, cost/benefit ratio, etc.
Nonetheless, I would gladly accept a quartz substrate with a dielectric coating. The differences we are discussing are smaller than the differences in seeing from hour to hour.
Quote:


why is Roland wrong in some cases if he is?? I kind of got what you said and I kind of didn't, if that makes sense.



Well, read my clarifications, and wait for Bill's remarks. The best thing i can tell you is that commercial star diagonals vary a LOT in quality.
Myself, I have witnessed that variation, and I would say that price isn't the only indicator of quality in that realm, but it is one. I have not seen any $100 dielectric diagonal outperform an AP or a TeleVue. Caveat emptor.
Quote:


Please keep in mind that I'm full of cold medicine and my head feels like a bubble, my mind is not fully firing on all cylinders right now.



The web is full of details. Read all the reviews and comments and take it all with a grain of salt. A lot of what's said is on the level of:
"It was 0.05% better, so it completely blew away the other one. My God, how could anyone put up with such a piece of junk as that only 99.95% perfect one!"

And they're ALL junk in 3" seeing.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6329273 - 01/22/14 08:01 PM

Thank you both Bill and Don, the info both of you provided did clear things up a bit. I have to be honest though, I feel a little misled, it just seems the claims from some rather big names are "possibly" misleading to one degree or another. It's not that I'm really fussing over it, I'm just trying to understand and it seemed to me that what I was reading here was in contradiction with some of what those big names were claiming. After Don's and Bill's response if I am understanding correctly it's not so much a contradiction as much as it is of not telling the whole story when stating such claims about a particular product.

I have the Maxbright diagonal along with the Baader/Zeiss prism and a number of other enhanced aluminum diagonals, I like them all and never really considered any differences until this thread, mostly used the Maxbright thinking the dielectric was the best. So it's been a learning experience for me.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6329385 - 01/22/14 09:12 PM Attachment (63 downloads)

Attached is a very preliminary scatter analysis I've done on 3 of the diagonals. It's been difficult to capture photographically the differences seen visually. These are afocal shots through a 5mm XO (163x) on the TSA-102 of a bright white-light source (Halogen lamp) thru a home made peep hole.

Note the difference in scatter. Also note the tonal differences. Can't swear yet to the colors as not sure the camera was doing any white balance adjusting. On next round I will manually set the white balance so it is set. In this pic the exposure is locked, as well as lens focus. So all is standardized and at same distances with only difference being the unknown on the white balance.

Anyway, easy to see the leftmost (common 99% reflective 2" diagonal) that there is a lot more scatter. Perhaps some CA also. Center shows much smaller scatter profile and darker background as well. Center is AP Maxbright. Right shows a slightly smaller scatter profile still than the AP Maxbright, a slightly darker background as well (visible in my hi res pic). Also interestingly the coffee tone!! Perhaps why the prisms seem to be excelling at planetary? Btw, the right pic is the Baader T2 Zeiss Prism.

Again, this is a preliminary set and things may change as I perfect the technique with regard to color. But the scatter is obvious, and aligns with impressions in the field.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6329421 - 01/22/14 09:34 PM

In the left image, does the lighter background suggest scatter across that FOV or did the camera auto expose the image? I ran into the same problem taking images of my exit pupil.

Maybe normalizing the background might show the true extend of the scatter compared to the others. The possible and very tiny chromatic effect is maybe what you "should" see. If a diagonal is showing you what you should see, wouldn't that make it a good diagonal?

I understand your work is preliminary, difficult to do, and interesting. It's very much appreciated.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Asbytec]
      #6329476 - 01/22/14 10:07 PM

Hi Bill:

In the right image from the Baader T2 Zeiss, am I correct in detecting a vertical oval shape instead of the more round shape of the other two? Does that indicate astigmatism?

-Dan


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6329550 - 01/22/14 10:48 PM

This is excellent information BillP. It is greatly appreciated that you have taken the time to do it.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RodgerHouTex]
      #6330044 - 01/23/14 08:15 AM

I can't help but think that the diagonal is overlooked for the most part in this hobby, we are always directing our attention to the latest eyepiece or the scope of our desire which is always one scope away regardless of how many scopes we get. You purchase a good diaganol and that's it, time to give your attention to the latest and greatest eyepiece scope or other accessory. I am kind of kicking myself in the butt because I have the Baader T2 Zeiss Prism and haven't given it the time that I have had opportunity for in the past.

"In the end, the cleaning advantage marketers say for dielectrics IMO is a solution searching for an issue"

There may be some truth to this, one of the reasons I never really reach for the other diaganols is my actions are indeed the result of the suggestions I have read about the dielectric coatings from this or that marketer, because the names of some of these marketers carry some weight I just figured what they suggested was the best way to go. That's not to say I didn't end up with a great product it just more or less unconsciously isloated me from really giving attention to some other worthy options.

I think I am your average Joe observer so my experience is probaly pretty common.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Asbytec]
      #6330061 - 01/23/14 08:22 AM

All - remember that these are just preliminary shots. so don't draw much of any conclusion rather than interesting at the moment. I am still in process of baselining the process.

Norme - That brighter background was from the diagonal. The camera was in complete manual mode, except for the white balance which I forgot about and that was still in auto mode.

Dan - I had some slight shake on that shot...but small enough that I didn't want to retake given this was a test.

Roger - you are welcome.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6330149 - 01/23/14 09:09 AM

Quote:

I can't help but think that the diagonal is overlooked for the most part in this hobby, we are always directing our attention to the latest eyepiece or the scope of our desire which is always one scope away regardless of how many scopes we get. You purchase a good diaganol and that's it,...
I think I am your average Joe observer so my experience is probaly pretty common.




You are not alone in this. When I started in this hobby 9 months ago, I specifically asked questions here for info for unbiased, technical/performance based reviews of telescope related equipment, like those you can get from camera/auto industries. I can say that I did not really get what I was looking for, and had to rely on my own technical knowledge and research/evaluation to get what I need. As you can see testing/comparing equipment is not as simple as one would have believed. Lots of reviews/first light reports can only be viewed as information only. You still need to do your own homework to sort out what will be the best choice for what you need. I am glad that Bill started this type of review, however daunting the task may be.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6330239 - 01/23/14 10:07 AM

Quote:

That brighter background was from the diagonal.




For a preliminary result, that's already quite impressive.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Asbytec]
      #6330465 - 01/23/14 11:56 AM

The prism is looking the best so far. The background is darker than the others with less scatter. I look forward to your next shoot with fixed white balance.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6330473 - 01/23/14 11:58 AM

Quote:

I can't help but think that the diagonal is overlooked for the most part in this hobby...




I agree. Odd too IMO as most Newt folks know the importance of a precision secondary, myself included. But still I never thought about the diagonal all that much over the years. Only thing that got my ears sensitive was those mentioning how prisms do better for planets...which caused me to start this diagonal testing journey


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6330592 - 01/23/14 12:43 PM

This thread has me looking at the StarRover StarChair, and settle the diagonal argument once and for all - no diagonal is best!!

I am not sure how well that chair would work for higher power planetary though.

Keith


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: skullpin]
      #6330662 - 01/23/14 01:11 PM

"Only thing that got my ears sensitive was those mentioning how prisms do better for planets"

And you most certainly passed that on to me by starting this thread, now I can't wait for the skies to clear so I can compare the prism to the dielectric but I have a feeling I will be waiting for some time as the winter weather by all indications is far from cooperating anytime soon. Regardless of how things turn out with the experiment it's a great idea and very interesting undertaking, after all we do give a crazy amount of time to discussing eyepieces and the benefits of one design vs another, why would the diaganol be any less important in transfering the image from the objective to the eyepiece?? especially since most of us are interested in the best possible viewing experience with what we own.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6331655 - 01/23/14 11:16 PM

A few weeks back I did a brief informal comparison with my Baader Dielectric and my Baader 2 inch prism looking at the rima hyginus with my Starfire 152. I was mostly using a 5 mm XW, but also 5 mm XO. Test was not too long, as it was about 10 degrees out. Differences were subtle, but every time I put the prism in I though "wow" as the images seemed a little bit more vivid and quite a few times noticed a new small sublet feature, that I had not noticed before. When, putting in the dielectric I could then also find the feature, however I kept noticing new subtle features (small craters, ejecta with different albedo) in the prism, not in the dielectric.

Looking forward to BillP's more structured and detailed comparison.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6331853 - 01/24/14 02:34 AM

Quote:

Quote:

Tammy,
I wonder how my Vernonscope Gold would do in the torture test.

Daniel



Daniel,
Gold has a very flat transmission in the infrared, where it is the preferred material for high transmission (no overcoating necessary, no corrosion!),
but it chops off most of the blue/violet end of the spectrum, so it really isn't a visual coating. It could have some specialty uses, like H-alpha photography, or searching for red giants in globulars, etc.
But for a normal visual coating, it's the wrong material.




Don,

I actually found the torture term amusing The gold diagonal is actually quite resilient and impressive on Mars. I will test it later with h-alpha solar. Since you haven't been into smaller, high quality optical systems, you may be missing out on the fun of testing all these different diagonals, yes?


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6332087 - 01/24/14 08:23 AM

Searching the internet for info on dielectric coatings I found this very interesting tid bit of information I was not aware of and hasn't been mentioned yet concerning the stressing effect of the dielectric coating process when they were first introduced. I'm wondering if this problem has been addressed by all manufacturers of dielectric diaganols??


"The Maxbright was
introduced to North America when Baader licensed the name to Astro-Physics and starting providing
the mirrors for AP's diagonals. Some years later, AP switched manufacturers for their dielectric
diagonals; Baader in the meantime developed new coating processes to address a dielectric mirror's
primary weakness - the dielectric coating process stresses the mirror substrate leading to optical
deformity. The 1/10 wavefront error specification stated in other manufacturer's marketing material
is before coating, even for those premium quartz mirrors. After the reflective dielectric coatings are
added, only the center of the mirror remains flat - the closer you get to the edge, the worse the optical
performance. In fact, one prominent equipment manufacturer recommends you only purchase 2" dielectric diagonals so that the sweet spot covers wider fields of view. For most eyepieces you will never see the difference, but for high quality wide
field eyepieces in a short to medium focal length
telescope, the distortion is noticeable."

Baader goes on to claim that now, "The Baader
Maxbright diagonal utilizes a dielectric coating
process with an equal number of layers on both
sides of the mirror substrate. This results in 1/10
wavelength performance after coating."

You can read the whole thing here,
http://www.baader-planetarium.de/sektion/s28/download/baaderClicklockDiagonal...


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Daniel Mounsey
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6332126 - 01/24/14 08:51 AM

John,
The issues regarding the edge are noticeable and can easily be seen visually while comparing with a corrected wide field. That's always been an issue as well.


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John Anthony
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6332149 - 01/24/14 09:07 AM

Quote:

John,
The issues regarding the edge are noticeable and can easily be seen visually while comparing with a corrected wide field. That's always been an issue as well.




I know some have spoken of the issues regarding edge correction, I just never knew why it was an issue until I read about the dielectric coating process stressing the mirror.


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6332150 - 01/24/14 09:09 AM

John,

here's an interesting link demonstrating the importance of the manufacturing method for depositing the coatings. The IBS method being by far the best. Don't know if any of the astro diagonal makers use this or not unfortunately. Thin-Film Coatings.

Also, from the same site, ready the last paragraph here... Link.

Backing up one page here's lots of good reading materials from EOU - Link.

Edited by BillP (01/24/14 09:26 AM)


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John Anthony
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6332153 - 01/24/14 09:11 AM

Forgot to mention I checked out your web site there Doctor D, I really like it, good reading for someone like me.

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John Anthony
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6332179 - 01/24/14 09:27 AM

Quote:

John,

here's an interesting link demonstrating the importance of the manufacturing method for depositing the coatings. The IBS method being by far the best. Don't know if any of the astro diagonal makers use this or not unfortunately. Thin-Film Coatings.

Also, from the same site, ready the last paragraph here... Link.




Thanks Bill, I appreciate the links you have given me, I have found it rather difficult to find anything on line thats a good read, while I have found some info on dielectric coatings most is to much technical mumbo jumbo and not really intended for the average Joe.


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Starman1
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6332273 - 01/24/14 10:30 AM

Quote:

Quote:

Quote:

Tammy,
I wonder how my Vernonscope Gold would do in the torture test.

Daniel



Daniel,
Gold has a very flat transmission in the infrared, where it is the preferred material for high transmission (no overcoating necessary, no corrosion!),
but it chops off most of the blue/violet end of the spectrum, so it really isn't a visual coating. It could have some specialty uses, like H-alpha photography, or searching for red giants in globulars, etc.
But for a normal visual coating, it's the wrong material.




Don,

I actually found the torture term amusing The gold diagonal is actually quite resilient and impressive on Mars. I will test it later with h-alpha solar. Since you haven't been into smaller, high quality optical systems, you may be missing out on the fun of testing all these different diagonals, yes?



Au contraire.
My eyepiece/barlow testing instrument of choice recently has been a TeleVue NP-101 (f/5.4) on a Gibralter mount. The Moon is a good test subject for chromatic aberration, small details, light scatter, et al.
I haven't tested the new diagonals from ES yet, but I could/should compare with TeleVue and Lumicon that are handy.
However, field testing of things like the new HR coma corrector from Explore Scientific has to wait until I get a night with the dob way out in the boonies.


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RodgerHouTex
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6332570 - 01/24/14 12:57 PM

I have a question about a particular prism. The Baader T2 says it uses a 37mm Zeiss prism. Since that is only 1.4 inches, won't it vignette the view when used as a 2 in diagonal?

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PeterR280
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RodgerHouTex]
      #6332578 - 01/24/14 12:59 PM

what would you need to use the Baader T2 with the 37mm Zeiss prism for visual? Are there 2 " components available?

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: PeterR280]
      #6332614 - 01/24/14 01:13 PM

If you're looking for a 2" I believe the one you're looking for is not the T2, which is actually for 1.25", but the one below. Unfortunately, it appears it's either backordered, or perhaps has been dropped. Maybe I'm wrong though.

http://agenaastro.com/baader-2-90-star-diagonal-zeiss-prism-2406010.html


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RodgerHouTex
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: The Mighty Mo]
      #6332780 - 01/24/14 02:19 PM

Yes. That's the same one that Alpine Astro is out of. It's misleading for the T2 though because they show it configured as a 2 inch diagonal here which it obviously is not.

http://alpineastro.com/Star_Diagonals/t-2_diagonal_configs/t2_diagonal_config...


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RodgerHouTex]
      #6332841 - 01/24/14 02:34 PM

Baader's own website still shows it as available. Maybe it's just a case this being a special order item due to lower sales numbers (and price).

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RodgerHouTex]
      #6332856 - 01/24/14 02:38 PM

My impression is that the T2 is definitely configurable to use 2" eyepieces. However, if the field stop of the eyepiece exceeds the clear aperture of the prism (34mm or something), then it will vignette. So a 27 Pan or 22 Nagler definitely ok, and maybe a 26T5 might just get by ok also.

Edited by BillP (01/24/14 02:39 PM)


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Fomalhaut
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: PeterR280]
      #6333336 - 01/24/14 06:00 PM

Quote:

what would you need to use the Baader T2 with the 37mm Zeiss prism for visual? Are there 2 " components available?




Just keep in mind that the bigger the prism => the longer the additional glass path => the more relevant the induced chromatic aberration!

As for my "white" 4-inch Apo, I think I'll return to my dedicated 1.25 inch Tak prism for planetary.

Chris


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Eddgie
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6333369 - 01/24/14 06:18 PM

Most people don't want to hear about the flaws in their test protocalls, but there is always that possibility.

Let's examine this test.

The reflectiviey of a diagonal to different wavelengths of light can be affected by the coatings used.

I am not saying that this is happening, but one cannot dismiss it.

It is possible that one diagonal will have more transmission in one wavelength than another might have.

If that were the case, it could be possible for a diagonal to pass more of the unfocused blue light from an APO or ED scope.

Also, the CCD camera chip itself can play a role here. In general it is more sensitive to a far wider range of wavelengths than the human eye.

Who is to say that perhaps what you are seeing is not more scatter, but just the different transmission characteristics of the diagonal itself perhaps showing more violet light from the source instrument???

I am not trying to dispute the result of your picture, but only to question whether the test is perhaps being influenced by a difference in the coatings that you are interpreting as scatter, when it could be a specific transmission attribute of the coatings used.


To eliminate this from consideration, I would recommend that a reflector be used for future tests.

But this still won't tell you if what you are seeing is scatter or just the result of better transmission of all wavelengths of light.

Eyepcice coatings are often this way too. They tend to have the greatest transmission in the green spectrum.

If these two other diagonals have lower tranmsision in the blue spectrum, all of the "Scatter" you are reporting could be the CA coming from the instrument itself, and the diagonal could just be showing this to you.

This would explain why the color is different. Maybe this diagonal is simply showing the CA from the scope while the others are attenuating it...


If you used an eyepiece with strong blue attenuation, it would make CA from the instrument seem less intense than perhaps it really is.

I would think using a reflector would eliminate this possibility. That way, all of the colors coming from the instrument are in perfect focus.

If you used a mirror and see the same thing, then you could say it is from the diagonal, but at this point it could be simply that the diagonal in question is doing a better job transmitting violet light.

The coatings on the prism may in fact be attenuating this part of the spectrum because this would better control the spherochromatism of the prism itself.

So, would seeing less violet light be a function of the coatings, or is it really that the diagonal in question is inferior.

I have my doubt. That is all I am saying.. Unless you eliminate this possibility, I personally would see the test as being inconclusive.

And this is why you really should be using a single wavelength source, preferably a green laser or green LED..
Otherwise, the test itself is suspect in my opinion...

Edited by Eddgie (01/24/14 06:20 PM)


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RodgerHouTex
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6333405 - 01/24/14 06:34 PM

And that was my point BillP. I almost dropped the close to $400 for the T2 thinking it was a 2 inch prism diagonal. Very misleading.

It also leaves me wondering if there are any 2 inch diagonals that use prisms or are silvered and protected. Other than the Baader which is not available or the Brandon Quartz which is $800.


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RodgerHouTex
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Eddgie]
      #6333419 - 01/24/14 06:41 PM

I'm afraid I have to disagree. Scatter manifests itself as a glow around the Airy disk. Increased transmission would not go outside of the Airy disk, but would instead would make the Airy disk brighter.

Plus because of what BillP is doing, I took my 1 1/4 dielectric diagonal out of my TSA-102 and replaced with the Tak prism diagonal and saw more detail on Jupiter's disk. It was quite astounding.


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Kunama
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RodgerHouTex]
      #6333428 - 01/24/14 06:46 PM

The Baader Zeiss 2" Prism is available and in stock at Telescope Service

http://www.teleskop-express.de/shop/product_info.php/info/p612_Carl-Zeiss-2--...


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Kunama
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RodgerHouTex]
      #6333434 - 01/24/14 06:49 PM

Quote:

I'm afraid I have to disagree. Scatter manifests itself as a glow around the Airy disk. Increased transmission would not go outside of the Airy disk, but would instead would make the Airy disk brighter.

Plus because of what BillP is doing, I took my 1 1/4 dielectric diagonal out of my TSA-102 and replaced with the Tak prism diagonal and saw more detail on Jupiter's disk. It was quite astounding.




I too found the Takahashi 1.25" Prism revealed more detail on Jupiter than a standard 2" dielectric mirror.

To Bill, I am thoroughly enjoying reading about this testing of yours, much kudos to you for undertaking it. Obviously there will be many who will disagree with your findings and to be fair, we all see differently, but I really appreciate your efforts and look forward to reading more of your reports.

My own mini comparison has been done using:
a. No diagonal
b. Tak 1.25" Prism
c. GSO Dielectric 2" &
d. Baader 2" ClickLock Mirror
e. William Optics 2" Dielectric

In the end I have settled on keeping the Tak 1.25" and the Baader 2" .

Cheers,
Matt


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Eddgie
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RodgerHouTex]
      #6333452 - 01/24/14 06:56 PM

Ok, lets pick this apart.

First, these pictures do not show an Airy Disk. They show a blob. I see no sign at all of an Airy Disk.

Second, an APO is usually only an APO when it is used at infinity focus.

In his book Star testing, Astronomical telescopes, Suiter suggests that to be safe when testing a APO, the pinhole needs to be placed at 40 times the focal length of the instrument. If it is not placed at this distance, the result can be that the outcome of the test will be affected by color error that is not present when the instrument is at infinity focus.

For star testing and APO, the pinhole has to be very carefully controlled in size, or the instrument can show spherical aberration.

Spehrical aberration has the effect of moving light further form the Airy Disk, and the light that is unfocused the most (almost always blue) will be diffracted the furtest.

So, we can disagree about it all day long, but to me, this test would be better conducted with a properly sized pinhole placed at least 20 times the focal length from a pinhole and using a reflector to eliminate the possibility that CA is contributing to the result.

But that is just me.... For me to believe in a test, all the variables have to be very carefully controlled, and I don't think sticking a light bulb behind a "peep hole" 10 or even 20 focal lengths from focus in an APO is a reliable test.

Believe what you like though. I won't convince anyone that does not want to be open to the possibility that the test is flawed.


And I am ok with that. I would not use the result to guide my own purchases though because I have my doubt that it is well enough controlled to give a reliable result, and at best case, is likely to exaggerate any true difference that might exist.

Perhaps I really don't belong on this thread though. Often I get the feeling that people just don't like my contributions and that is OK.

But without a highly reliable test method, you get less than highly reliable result.

I'll leave you gentlemen to it.

Enjoy the testing! I won't force my opinions on the effort any further.

Edited by Eddgie (01/24/14 07:01 PM)


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Kunama
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Eddgie]
      #6333484 - 01/24/14 07:13 PM

I doubt that Bill is trying to create the "Definitive Bible of Star Diagonals" here, he is doing what I consider a very interesting experiment and will no doubt give us HIS opinions at the end.
I also doubt that Bill is trying to sway your opinions on which diagonal to buy, seems his aim is to give his views so perhaps he ought to be allowed to run this comparison with any scope he chooses in any manner he chooses.
The same happened when recently Daniel offered to compare 2 scopes or similar aperture, suddenly we had people crying "foul" because one scope was a doublet the other triplet.



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urassner
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Kunama]
      #6333587 - 01/24/14 08:25 PM

Eddgies comment is very thoughtful. A monochromatic light source might be more scientific, however, the results would only apply to that particular wavelength. In real life, we have polychromatic light (unless a narrow band filter is used) and thus the observed scatter with a polychromatic light source may more closely resemble the real life experience. Thank you BillP for taking the time to do this extensive comparison.

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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: urassner]
      #6333616 - 01/24/14 08:50 PM

Quote:

A monochromatic light source might be more scientific, however, the results would only apply to that particular wavelength. In real life, we have polychromatic light (unless a narrow band filter is used) and thus the observed scatter with a polychromatic light source may more closely resemble the real life experience. Thank you BillP for taking the time to do this extensive comparison.




Bingo! Green light tests tell one nothing practical, unless that is how they observe. Neither do using 3 wavelengths. You can't presume specific wavelengths will not be problematic. A wide portion or all of the visual spectrum is the way to go and the only one that makes sense to replicate the *real* observing experience. Why I chose on purpose this method...I don't observe in green light. And in NO test are all the variables carefully controlled. I've reviewed plenty of research papers to know that. To think that all variables are well controlled and none were missed...well that's the first mistake of the researcher...

All tests conducted with an instrument, are a *system* test. To test just the diagonal means one cannot have other optics in the train. There are plenty of devices out there to specifically test for scatter, wavelength transmission, and the such, and none use commercial telescopes and eyepieces to do the test. But then when this is done, very difficult to extend to a system of components since the original test specifically tested it divorced from those influences.

As a reminder from the1st post, this test is for: "comparing these [diagonals] against each other in three main categories: CA, Planetary, Threshold Brightness.... the tests will be done in both short and fast focal ratio refractors (f/8 APO and f/6.25 APO)."

The indoor high intensity light tests (it's not a star test, but closer to a planetary extended source scatter test) are not even necessary, as the visual observations of Jupiter, Lunar Limb, and Bright Stars, have told the story that counts already. The outdoor tests are the only ones that really count as they don't try to "model" but ARE real observing. Indoor tests are hardly ever well extensible to predicting field results accurately, not in any of my experience from the countless ones I've conducted. That's because of course that the indoor tests forget that things operate as highly interactive and codependent systems. Anyway, now I'm just having some indoor fun Final outdoor tests I need to conduct are for threshold brightness. That's after I brave the cold tonight to try to see the supernova

And btw urassner, you are very welcome, as is everyone else interested. But really, the thanks go to all the contributors of the literally thousands of dollars of gear they loaned!!


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6333816 - 01/24/14 11:09 PM

Quote:

That's after I brave the cold tonight to try to see the supernova




WooHoo!! Just got a great view of the supernova in M82 XT10 using the vintage 14mm Meade 4000 UWA


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Eddgie]
      #6333931 - 01/25/14 12:17 AM

It's not that your opinions aren't valuable, it's just that some of us disagree with them. There is always the possibility that you might be wrong. Secondly,a few of us have empirical evidence of a prism being superior to a dielectric mirror on Tak refractors at least on Jupiter.

But if you want to "take your ball and go home", you are certainly free to do so.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Kunama]
      #6333945 - 01/25/14 12:26 AM

That looks suspiciously like the T2 Baader prism in the Telescope Service ad which has a too small for 2 inch 37 mm prism. It also seems to be a little low on the price side for the the larger 2 inch Baader that others are out of stock on. I think they're like $460 American dollars. But thanks.

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The Mighty Mo
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RodgerHouTex]
      #6333958 - 01/25/14 12:37 AM

No, I believe that's the right one Rodger. If you notice, the Zeiss prism numbers are the same, which, from what I've been able to figure, is the larger prism.

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Kunama
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RodgerHouTex]
      #6333966 - 01/25/14 12:48 AM

Quote:

That looks suspiciously like the T2 Baader prism in the Telescope Service ad which has a too small for 2 inch 37 mm prism. It also seems to be a little low on the price side for the the larger 2 inch Baader that others are out of stock on. I think they're like $460 American dollars. But thanks.




This is the larger prism Rodger, that price is 329 Euros or about $450 USD and they are in stock at TS online.


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RodgerHouTex
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Kunama]
      #6334324 - 01/25/14 08:36 AM

Okay thanks.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6334426 - 01/25/14 09:54 AM

Bill,

You've done well here. Two things you bring to the table. One is scientific data and the other is observational tests. I know many of us rely on data and I think it definitely is important to share it and thank those who do. I also think if others find this kind of topic interesting, they too should go out in the field and try to determine if they can see it for themselves. Since most others don't have access or funding for these diagonals, they are left to speculate on what they think is best, based on the information they see here. Are their conditions good enough in order to determine this for themselves?

I've tested many telescopes and accessories and made a few mistakes of my own, who hasn't ? In my opinion though, it's important to go out and observe with these telescopes and accessories and really see what appears to look best. As Jupiter makes its way over, I am constantly observing it. I observe with the FC60, FS102, FS152 and the Mewlon 250. I also had a few high quality dobs 10", 12.5" and 14". I have tested high end diagonals as well as low end ones. So far, the prisms I've tested have given me what I believe to be the most pleasing image quality on the planets.

Observational astronomy is special to me. I have spent countless evenings testing in sub arc second skies and bad skies as well and one thing I've definitely learned is that "seeing" is absolutely crucial in appreciating the differences between certain telescopes and diagonals if optical quality is the focus. An observer can have the finest optics, but if they don't have the proper platform to observe from, they may never really know what they're actually missing. I look at some observers and wonder how they've been able to observe for so many years with certain instruments and then I think well, maybe they've just never been exposed to nicer images, or maybe they just don't care, who knows.

For me, I want to see the finest and most detailed planetary images attainable and to me it's worth it. If the quality is good enough and the conditions are right, aperture wins and a good diagonal can enhance that experience whether it's a big dob, cassegrain or a refractor. I've explained it on my website, don't underestimate the secondary. Or, others can be like some of the Japanese and try to do without a diagonal and look straight through.

Certainly there are others who have tested many telescopes and accessories, but if others rarely achieve the seeing conditions needed in order to appreciate the differences, then it doesn't really matter how many telescopes or diagonals we use. It's just basically harder to conduct critical visual tests and comparisons. Bill, you've done a wonderful job sir and I am proud of the wisdom you have provided.


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Roy McCoy
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6334647 - 01/25/14 11:52 AM

Quote:

It's just basically harder to conduct critical visual tests and comparisons. Bill, you've done a wonderful job sir and I am proud of the wisdom you have provided.





And Thankful too.


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Jeff Morgan
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6334653 - 01/25/14 11:55 AM

Quote:

I have tested high end diagonals as well as low end ones. So far, the prisms I've tested have given me what I believe to be the most pleasing image quality on the planets.




I'm curious if you have ever run across a Newtonian using a prism diagonal?

In re-reading some of the old Gleanings For ATM articles from the S&T archive, this was done decades ago.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6334679 - 01/25/14 12:07 PM

I too applaud Bill for doing this perhaps ever deepening test. Whatever results he see's means more to me than bags of theory. As I'm in the market right now for a 2" diagonal for a TEC 140 I hope to have by the Spring, I'm all ears. I'm sure many here have seen this already but here's an article which in part does compare said telescope with a dielectric and prism.

http://www.astrosurf.com/laurent/apo140e.htm

I think I'm going to have to buy them both to see for myself as seeing is believing....though it doesn't come cheap.......Rod


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Pezdragon]
      #6334822 - 01/25/14 01:12 PM

You know...if I can swing it (time-wise as these all do belong to other folks), I will see if I can take the best performing Mirror, Dielectric, and Prism of the bunch (so just 3), and see if I can get some TEC140 seat time on Jupiter with BKBrown who lives not too far from me. Would be interesting to bump the aperture up 38mm and see if everything still holds.

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Starman1
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jeff Morgan]
      #6334922 - 01/25/14 01:45 PM

Quote:

Quote:

I have tested high end diagonals as well as low end ones. So far, the prisms I've tested have given me what I believe to be the most pleasing image quality on the planets.




I'm curious if you have ever run across a Newtonian using a prism diagonal?

In re-reading some of the old Gleanings For ATM articles from the S&T archive, this was done decades ago.



My first telescope, an Edmund Scientific 4.25" f/10.5, had a prism diagonal, but it was used in an odd way. I guess in 1963 1/8 wave 1" mirrors were uncommon, but 1/8 wave prisms were as common as fleas. So they mounted it on a stalk where the hypotenuse of the prism faced out, was aluminized, and served the same function as a diagonal mirror. The light reflected from the prism's exterior surface, and did not pass through the glass.
When I replaced the mirror with an upgraded mirror, I thought I'd try turning the prism over to let the light pass through the prism and reflect from its hypotenuse after passage through the glass. It didn't, to make a long story short, help the image quality at all. I ended up turning the prism back over and using its backside as a reflective mirror. Since the prism had square corners, I ended up replacing the prism with a normal quartz secondary from E&W Optical later on. And a few years later, the scope was replaced with a 4" f/15 Unitron refractor,
But the idea of using a prism as a mirror certainly holds merit when the mirror size needed is 1" or even less (3" newtonian reflectors were available then). Surfaces are usually quite accurate, even in fairly inexpensive prisms.

Fast forward several years, and a friends 8" Celestron came with a star diagonal prism. We were under the impression it was a poor quality move on Celestron's part so we "upgraded" the star diagonal from that plastic-body diagonal to a high-end all-aluminum mirror-type. We compared the two units, going back and forth and looking at globulars, double stars, etc. We could discern no difference in optical quality between the $49 prism diagonal and the $149 mirror diagonal.

Many years later, when I started using an excellent 5" Maksutov, I compared a variety of star diagonals. I had access to about 10 different diagonals, and compared them all with the same eyepiece on the same night in that scope. The final 3, optically, were 1 dielectric mirror, 1 enhanced mirror, and a cheap $50 prism star diagonal. I didn't keep the prism because I just couldn't put up with its cheap plastic body and easily-stripped out insertion tube. Had it had a decent, machined-aluminum, body, I probably would have kept it. It was just as bright as the dielectric diagonal, too.

But now my primary use for a star diagonal is in an f/5.4 apo, and i simply don't trust a prism not to play havoc with the spectrum in that short a scope. My old f/15 Unitron, though, had a prism diagonal, and it worked just fine.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Pezdragon]
      #6334983 - 01/25/14 02:15 PM

Quote:

I too applaud Bill for doing this perhaps ever deepening test. Whatever results he see's means more to me than bags of theory. As I'm in the market right now for a 2" diagonal for a TEC 140 I hope to have by the Spring, I'm all ears. I'm sure many here have seen this already but here's an article which in part does compare said telescope with a dielectric and prism.

http://www.astrosurf.com/laurent/apo140e.htm

I think I'm going to have to buy them both to see for myself as seeing is believing....though it doesn't come cheap.......Rod




Hi Rod,

Thanks for posting the TEC140 review link. I stopped reading them when I started viewing with the scope several years ago. It looks like I'll have wait until summer to test a Baader prism on mine, since I'm wintering in Florida and my scope is up north. I'd like to read your results when you get the opportunity to view through both.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6335118 - 01/25/14 03:35 PM

Quote:

My first telescope, an Edmund Scientific 4.25" f/10.5, had a prism diagonal, but it was used in an odd way. I guess in 1963 1/8 wave 1" mirrors were uncommon, but 1/8 wave prisms were as common as fleas. So they mounted it on a stalk where the hypotenuse of the prism faced out, was aluminized, and served the same function as a diagonal mirror. The light reflected from the prism's exterior surface, and did not pass through the glass.




That was my first scope, also. IIRC, it was called a Palomar. I received mine for Christmas 1970. But, IIRC, by that time they were using a standard mirror diagonal, though still on a stalk. Or am I confusing the diagonal on the 4.25" with the diagonal for the 6" f/8 Newt I was building soon after?

Mike


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6335705 - 01/25/14 10:08 PM

Quote:

I've definitely learned is that "seeing" is absolutely crucial in appreciating the differences between certain telescopes and diagonals if optical quality is the focus.



I need more than two thumbs to adequately agree with this statement. Having stumbled into excellent conditions in retirement, it's so apparent how important seeing is especially on planets, lunar, and doubles. In my view, this is when your optic will actually approach theoretical performance and that diffraction limited performance is nothing short of stunning. Such conditions actually make me a believer in theory rather than one who disregards it as too optimistic or unrealistic. Splitting stars a few hundredths below Dawes, resolving smaller than normal craters on the moon, resolving detail on Ganymede, etc., can only be done in lab like conditions.

That said, I also greatly appreciate the empirical results testing in the field especially when it comes to scatter. Such testing probably does not require excellent seeing when simply looking at the amount of scatter, but it will probably swamp small scales and affect the results looking at Jupiter. It's hard to say whether their differences will be noted on Jupiter when seeing does not permit a stunning view. It might. But larger scale tests like the one above are certainly interesting.

So, yea, you really need lab like conditions to really evaluate performance, but if scatter is more apparent in one design we might safely assume the image is affected as well. You can see such performance with empirical studies under real world conditions provided those conditions are the same for each. But, one has to be careful. Scatter is not from the diagonal alone, it begins in the upper atmosphere and ends at the detector. But, if differences are noted in average seeing and transparency, they should be manifest in better conditions as well and probably in accord with theory (of optical coatings and chromatism with focal ratio) - effects probably easily overcome by less than perfect seeing.

Sorry for rambling, but I have a prism diagonal on my wish list as a result of this thread. Anything that will give Jupiter that tiny bit of an edge is of interest. I'd trust real world empirical results and believe they will improve on theoretical performance as a result.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6335900 - 01/26/14 01:26 AM

Quote:

I guess in 1963 1/8 wave 1" mirrors were uncommon, but 1/8 wave prisms were as common as fleas.




That never occurred to me - the prism equipped Newtonians were more a function of availability (WWII surplus) than any performance advantage. And once light cones started going sub-f/8, that would have been much harder on the prisms. I'm going to go back and re-read those articles more carefully.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jeff Morgan]
      #6336005 - 01/26/14 03:57 AM

Quote:

Quote:

I have tested high end diagonals as well as low end ones. So far, the prisms I've tested have given me what I believe to be the most pleasing image quality on the planets.




I'm curious if you have ever run across a Newtonian using a prism diagonal?

In re-reading some of the old Gleanings For ATM articles from the S&T archive, this was done decades ago.




I have not, however about a week ago a young man came into the store to ask about repairing an old 4" Edmund Newtonian. Upon removing the secondary, I discovered it was a prism! I think it would definitely make an improvement, particularly for slower Newtonians.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6336242 - 01/26/14 09:31 AM

Don,
That's interesting. After the post I just made about the Edmund, I just noticed you must have had the same model. It had the stalk holding the secondary prism just as you described.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6336347 - 01/26/14 10:40 AM Attachment (24 downloads)

Last night we had a blast at the Oak Canyon Star Party using a high quality 2" prism. Tamiji Homma was kind enough to deliver one to me at Woodland since I usually use the 1.25". I used it most of the night with my FS102 and went as low as an original 50mm Axiom of Japan which are no longer available. It's my favorite 50mm in the world. Exit pupil was about 6.25mm with a magnification of only 16x and I have no astigmatism. The stars in Stock 2, Melotte 31 and Collinder 70 using the 2" prism were pin pricks out to the field with beautiful contrast. Thank you Tammy.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6337623 - 01/26/14 09:24 PM Attachment (80 downloads)

Been experimenting with multiple methods to show via a picture, instead of just describing, scatter differences between diagonals. It has not been easy and spent all weekend conducting various experiments to determine which shows differences most discretely. Indoor star test inadequate since you get back reflection off the surface where the hole is drilled, Illuminating a flat reflective surface that is offset from a dark background also did not work well due to inability to get a bright enough reflection. No method where I did not incorporate the telescope worked at all (i.e., direct imaging using diagonal only of very bright targets). Finally, what seems to demonstrate scatter differences best in a lab setting is to photograph a clear christmas light bulb. The bulb is offset from a dark black bacground a few feet so no light from the bulb exposes the background. Camera settings were all fixed - focus, shutter, f-stop, ISO, white balance all fixed. The TSA-102 with the 5mm Pentax 5XO were used and afocally photographed the light bulb with a Fuji X-E1 camera. The telescope was focused for each shot on the filament of the bulb. Precise focus is easy with the Fuji X-E1 camera since the Electronic View Finder has it's own zoom function to facilitate precise focus. Plus it automatically adjusts its brightness to the camera settings, so a quick flip of the shutter to 1/4000th sec lets me see the filament not over exposed. Then backed it off to the test shutter speed (1/8 sec, 6400 ISO, f/1.4, WB=2700K).

Below is a pic through 3 of the 2" diagonals. The pic is of the black space just to the right of the right edge of the light bulb. 1st frame is straight through with no diagonal. I think this captures the differences well. None of this tells one how it will perform on real targets as only a field observation will reveal that the best. However, it's nice to see that the scatter levels shown in the pic are ligning up with the field impressions so far. Best if downloaded and viewed in full size btw. In the CN interface can't see any scatter from straight through, but there is some when viewing it after download. Interesting just how much scatter the diagonal adds. Unfortunately my neck will not put up with observing without one


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PowellAstro
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6337690 - 01/26/14 09:59 PM

Very telling and well done! I am liking the prism a lot and this is showing proof of your viewing reports!

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6337824 - 01/26/14 11:12 PM

Thanks for going to the trouble of capturing and posting that, Bill.

Dan


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6338023 - 01/27/14 02:02 AM

These shots are self- explanatory.The only way I guess to describe what one may experience using these diagonals.

Looking forward to the 1.25" group results!

Thanks for your excellent contribution to this relatively underestimated part of the telescopic optical light path!

George


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: george tatsis]
      #6338168 - 01/27/14 06:58 AM

Looks like no diagonal won.


Mike


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: george tatsis]
      #6338173 - 01/27/14 07:09 AM

Bill, thanks for going through the trouble to devise an interesting test. I see a huge difference between straight through and any diagonal, but very little difference between diagonals even after downloading and enhancing the image.

I'm pretty sure what you capture in the test would be manifest in the real world, as well, to some degree due to the behavior of the various coatings and substrates. The indoor "lab" test might be exaggerated, but that's fine. It shows us what we're looking for.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Sarkikos]
      #6338250 - 01/27/14 08:20 AM

Quote:

Looks like no diagonal won.





Yes...and by more than just a little! Might need a lounge chair though to observe that way with Juipiter being relatively high for best observing.


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Asbytec]
      #6338252 - 01/27/14 08:22 AM

Quote:

The indoor "lab" test might be exaggerated, but that's fine. It shows us what we're looking for.




Hey...stop dissing my lab! (kidding of course)


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leonard
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6338319 - 01/27/14 09:06 AM

Hello ,

Great job Bill .

Even with my ageing eyes,looking at the pic before reading your post I could spot the slight scatter in the pic without the diagonal. No wonder some people like to try viewing without a diagonal at all.
To me its obvious the prism is doing a better job at scatter control .
In an early ad ,Roland at Astro-Physics stated a reflector had (if I remember correctly) about 4 times the scatter compaired to refraction , seems to be close.

Very interesting thread , Leonard


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Asbytec]
      #6338436 - 01/27/14 10:21 AM

Quote:

I see a huge difference between straight through and any diagonal, but very little difference between diagonals even after downloading and enhancing the image.





Actually, there's quite a bit of difference, both in the level of edge brightening and in the distance the scatter spreads from the edge.

Your monitor may not be able to show you the differences as well as others can.


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Midnight Dan
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: rockethead26]
      #6338569 - 01/27/14 11:37 AM Attachment (26 downloads)

Just to get a little more analytical, I downloaded a free image analysis program called ImageJ. I drew a long thin rectangle across the middle of you scatter comparison image and plotted the gray value across the rectangle. They are the same test cases as yours from left to right:

No diagonal
Baader 2" Prism
AP MaxBright
AT 2" Dielectric.

-Dan


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6338591 - 01/27/14 11:52 AM

Quote:

Quote:

The indoor "lab" test might be exaggerated, but that's fine. It shows us what we're looking for.




Hey...stop dissing my lab! (kidding of course)




Were you wearing a white lab coat? If you're not wearing a lab coat, you might as well be working in your garage.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: SteveC]
      #6338613 - 01/27/14 12:02 PM Attachment (24 downloads)

After looking at the plots above from ImageJ, I realized it's pretty hard to compare them. So I saved the data, brought it into Excel and plotted them on the same graph. Better!

Note: Vertical axis is brightness of the image, horizontal axis is the location of the pixels from left to right on the image.

-Dan


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6338652 - 01/27/14 12:20 PM

Quote:

Been experimenting with multiple methods to show via a picture, instead of just describing, scatter differences between diagonals. It has not been easy and spent all weekend conducting various experiments to determine which shows differences most discretely. Indoor star test inadequate since you get back reflection off the surface where the hole is drilled, Illuminating a flat reflective surface that is offset from a dark background also did not work well due to inability to get a bright enough reflection. No method where I did not incorporate the telescope worked at all (i.e., direct imaging using diagonal only of very bright targets). Finally, what seems to demonstrate scatter differences best in a lab setting is to photograph a clear christmas light bulb. The bulb is offset from a dark black bacground a few feet so no light from the bulb exposes the background. Camera settings were all fixed - focus, shutter, f-stop, ISO, white balance all fixed. The TSA-102 with the 5mm Pentax 5XO were used and afocally photographed the light bulb with a Fuji X-E1 camera. The telescope was focused for each shot on the filament of the bulb. Precise focus is easy with the Fuji X-E1 camera since the Electronic View Finder has it's own zoom function to facilitate precise focus. Plus it automatically adjusts its brightness to the camera settings, so a quick flip of the shutter to 1/4000th sec lets me see the filament not over exposed. Then backed it off to the test shutter speed (1/8 sec, 6400 ISO, f/1.4, WB=2700K).

Below is a pic through 3 of the 2" diagonals. The pic is of the black space just to the right of the right edge of the light bulb. 1st frame is straight through with no diagonal. I think this captures the differences well. None of this tells one how it will perform on real targets as only a field observation will reveal that the best. However, it's nice to see that the scatter levels shown in the pic are ligning up with the field impressions so far. Best if downloaded and viewed in full size btw. In the CN interface can't see any scatter from straight through, but there is some when viewing it after download. Interesting just how much scatter the diagonal adds. Unfortunately my neck will not put up with observing without one



Was the Tak 1.25 part of this test Bill?


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: pga7602]
      #6338845 - 01/27/14 01:57 PM

Quote:

Was the Tak 1.25 part of this test Bill?




Yes. And still is. Will have scatter pics for all the diagonals in final report. Was just giving a progress report here.

Dan - Can I send you all the pics when I do them to compile a nice graph like this for the article?

SteveC - Thanks so much for the healthy


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6338887 - 01/27/14 02:18 PM

Quote:

Quote:

Looks like no diagonal won.





Yes...and by more than just a little! Might need a lounge chair though to observe that way with Juipiter being relatively high for best observing.




Great idea! Anybody have build plans for a cantilevered EQ mount I can use with my lounge chair?


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Midnight Dan
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6338935 - 01/27/14 02:46 PM

Quote:

Dan - Can I send you all the pics when I do them to compile a nice graph like this for the article?




Sure, I'd be happy to.
-Dan


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Midnight Dan]
      #6339027 - 01/27/14 03:36 PM

The AP MaxBright has pretty good performance.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: PeterR280]
      #6339443 - 01/27/14 07:10 PM


Bill,

Thank you for undertaking this project! I use an AP maxbright, and have been following your progress with great interest. First you had me contemplating a new prism diagonal, and now I'm thinking I need a new observing chair:

straight through viewing chair

Hermie

Edited by Hermie (01/27/14 08:14 PM)


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Hermie]
      #6339631 - 01/27/14 08:56 PM

Tough to fall asleep in that one......

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: herrointment]
      #6339637 - 01/27/14 09:00 PM

and a torture to my old bones...

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Hermie]
      #6339799 - 01/27/14 10:10 PM

Quote:

straight through viewing chair

Hermie






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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Hermie]
      #6339854 - 01/27/14 10:32 PM

Quote:


Bill,

Thank you for undertaking this project! I use an AP maxbright, and have been following your progress with great interest. First you had me contemplating a new prism diagonal, and now I'm thinking I need a new observing chair:

straight through viewing chair

Hermie




From what friends tell me, I ought to remember that position fondly.


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SteveC
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Midnight Dan]
      #6339872 - 01/27/14 10:42 PM

Quote:

After looking at the plots above from ImageJ, I realized it's pretty hard to compare them. So I saved the data, brought it into Excel and plotted them on the same graph. Better!

Note: Vertical axis is brightness of the image, horizontal axis is the location of the pixels from left to right on the image.

-Dan




Dan, can you plot the pixels from left and right of the center..................I suspect I'm looking for something that resembles a bell curve.


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Asbytec
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: SteveC]
      #6340209 - 01/28/14 05:05 AM

Hey, Bill, I know you're in jest, but just to be clear I was differentiating between "lab" (which I like) and real world tests (which others seem to prefer). I think you did just fine, with or without a lab coat.

Dan, okay, thanks for that analysis. Interesting. I can see the difference looking back at the image, just not as striking as I might have hoped - and not as striking as without. But, hey, every little bit helps.

Hermie, that was humorous and in a sense serious. Science is like that, you test a diagonal and realize the fix is a reclining deck chair.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Asbytec]
      #6340593 - 01/28/14 10:28 AM

This question just hit me this morning, don't know why it took so long...

What happens to the focus position using one of the 2" prisms vs a 2" dielectric? I doubt it stays the same. How far is it shifted and which direction?


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Asbytec
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: The Mighty Mo]
      #6340705 - 01/28/14 11:12 AM

Does scattered light come to focus?

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PeterR280
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Asbytec]
      #6340813 - 01/28/14 12:02 PM

Scattered lght will come into focus but it will take away from the contrast. It will also gray the background.

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Starman1
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Asbytec]
      #6340819 - 01/28/14 12:05 PM

Quote:

Does scattered light come to focus?



If it came to focus at the same place, i.e. the same plane, as the light from a star, it would expand the size of the rings (since the light wouldn't be in the spurious disk portion of the Airy Disk) and create a general glow around the star. That glow would defocus as you went either direction from focus (i.e. get larger).
If the scattering is random, the extra light might be found anywhere in the field, reducing contrast. It would be hard to tell, in that case, if it were focused or not.

Since we try to use our telescopes at focus, it is the contrast-reducing scatter that we are concerned with.

I've noticed scatter in fields around bright stars and planets, where the amount of light scattered is higher and therefore brighter. It appears to me, visually, to resemble the distribution of rods in the retina. I'll explain:

starting from one edge of the field and going across to the other edge, through the planet which is in the middle, I see a very slight brightening at first that accelerates (the gradient gets steeper) until there is a ring of haze around the planet at some distance away from the planet. Then the haze brightness drops precipitously until the area immediately around the planet appears black and free of haze. Then, immediately adjacent to the planet's edge is a very slight fuzziness that represents seeing issues and possibly corneal/lenticular/retinal issues in the eye.

Here is what I think is happening:
--when you first drive to a dark site and get out of your car, the sky in between the stars appears black. As you dark adapt, the sky seems to get brighter and grayer--even silvery.
--the really bright planet destroys the night vision in your eye, but its effect diminishes with distance away from the planet. Your eye is naturally wandering when viewing any target, so the area with completely destroyed night vision is a somewhat circular area . This is what creates the seemingly black area around the planet. It also will, of course, reduce sensitivity to the light of very faint moons, but that's the way it is.
--farther from the planet, the damage to night vision is not so intense. Plus, the concentration of rods in our retinas goes up so with a bit of peripheral vision we see the scattering of light that is everywhere in the field. And that scattering diminishes as we near the edge of the field with our peripheral vision.
--the light scatter is in the center, too, but we simply can't see it due to the reduced sensitivity of the retina anywhere near the planet. If we see it at all, then light scatter must be insanely large. And this is what happens when we view through dew on the eyepiece or optics or clouds in the sky. Then we see light scatter right up to the disc of the planet itself (and maybe on it).

So, obtusely, the answer to your question is that scattered light, almost by definition, is defocused light.
The question is what happens with light that is scattered only tiny amounts away from the points that make up any extended image, such as the extra diffraction caused by the presence of a secondary in the telescope's field. And we know this reduces contrast in a very close-in manner, and blurs the image sharpness. Of course, this damage to the image sharpness and contrast is a tiny pittance compared to the damge done by poor seeing (defined as any seeing which reduces the resolution of the instrument below what the optics could produce).
I've been lucky enough to see seeing that allowed my 12.5" to resolve to its limit, and all I can tell you is that at that time I was paying absolutely no attention to light scatter, and felt like reaching around for my helmet, because i was going down for a landing in the LEM. 608X on the Moon with absolutely no scintillation in the image whatsoever.

One of the things that occurs to me in this test of light scatter is, "How clean are the optical surfaces of the tested diagonals?" A tiny bit of dust and light scatter could become severe. A possible way around: test the diagonals, clean them and test immediately once more. Did any of the rankings change? If not, but the scatter merely reduced slightly, then the validity of the testing protocol holds up.


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Asbytec
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6340885 - 01/28/14 12:34 PM

Quote:

It will also gray the background.



Sure. Is that what we're seeing above?

I guess it all depends on whether the scatter is part of the image or not - which reflects from the diagonal and lands on the focal plane near the image. If the atmosphere scatters some light near a bright planet, then it comes to focus much as the planet does. It's part of the image forming light cone in front of the optic. If it enters the eyepiece field stop directly, as Don says, it can be anywhere. Maybe that's not so much scatter as it is veiling glare.

In any case, it gray's out the field. What we try not to have is any additional scatter in the optic possibly form the diagonal whether it comes to focus or not.

Quote:

So, obtusely, the answer to your question is that scattered light, almost by definition, is defocused light.



Makes sense. I'll have to really absorb your comments. It's interesting we see field brightness differently. I see it near Jupiter then falling off dramatically, but observing Jupiter I really don't pay much attention to the edge (unless I'm whipping some floaters out of the way.) Some image forming scatter it is atmospheric, some of it is optical, and some of it is in our not so perfect detectors.

Complicated...


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Asbytec]
      #6340953 - 01/28/14 01:01 PM

Perhaps I should rephrase my question you responded to Norme, in case it was mistunderstood. I'm using a 2" Astrotech dielectric now. If I decide to buy a 2" prism, will I need a tube extension, and if so how long? I understand the question about how scattered light is affected, and is interesting in and of itself. But I was literally asking where the physical focal plane is of a prism wrt a mirror.

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Fomalhaut
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: The Mighty Mo]
      #6340992 - 01/28/14 01:17 PM

Quote:

I was literally asking where the physical focal plane is of a prism wrs a mirror.




If being used with a prism diagonal, the telescope's focal plane is shifted a bit more outside of the original focal plane (the one with mirror diagonal).
The shift can be calculated by this formula:

s ~ t*(n-1) / n

[for: t = thickness of flat plate or additional glass path through prism; n = refractive index of glass]

Chris


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: The Mighty Mo]
      #6341003 - 01/28/14 01:22 PM

I believe the light is taking a similar reflection in a prism as in a mirror so it should not make the light path longer. For erecting prisms, the light bounces arouns a few times so it is a longer path.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: The Mighty Mo]
      #6341023 - 01/28/14 01:31 PM

Hi Daniel,

Quote:

I was literally asking where the physical focal plane is of a prism wrt a mirror.




I am not exactly sure how to answer to this but I am guessing you are asking light path difference between diagonals?.

If so, look at toward end of the thread, message # 3566473.

Shortest 2" mirror diagonal l

The Baader/Zeiss 2" Prism Diagonal is one of the shortest light path diagonals that I tested without special modification to eyepiece holder/nosepiece.

Tammy


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The Mighty Mo
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6341065 - 01/28/14 01:49 PM

Thanks Tammy. Am I reading that post of yours right in the other thread, that the prism actually needs a bit of infocus compared to a mirror of the same size in the same size housing?

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: The Mighty Mo]
      #6341256 - 01/28/14 03:14 PM

Hi Daniel

Prism diagonal light path is shorter than mirror diagonal in general.

You would need outward (away from objective lens) focus distance to come to focus to infinity (or eyepiece focal plane to be placed at correct distance to come to focus to infinity) with prism diagonal than mirror diagonal.

So if you are running out inward focus (focuser racking into OTA, toward to objective lens) with mirror diagonal, prism diagonal could be a godsend

It happened to me several times with scopes whose back focus is limited.

Tammy


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The Mighty Mo
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6341268 - 01/28/14 03:22 PM

Thanks Tammy, that's what I thought was the case, more outward travel will be needed.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6341276 - 01/28/14 03:25 PM

Quote:

So if you are running out inward focus (focuser racking into OTA, toward to objective lens) with mirror diagonal, prism diagonal could be a godsend




Yup. And of the prisms I am evaluating, the Tak 1.25 is the most bin0-friendly having the shortest light path.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6341624 - 01/28/14 06:41 PM Attachment (37 downloads)

Here's another short one that I use regularly...

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Fomalhaut
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6342320 - 01/29/14 03:46 AM

The elongation of glass-path by the prism made it impossible for my 100/640 to be used with a binoviewer. That was why I bought a (very short) Baader Maxbright which allows the TV binoviewer (with 2x correcting lens system) to reach focus. This definitely brings me a contrast-enhancement over mono-viewing with the same Maxbright.

However yesterday, mono-mode with my old Tak-1.25 prism (at 160x) distinctly showed slightly more Jovian detail than in bino-mode (at 142x) with Maxbright.

Chris


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6344931 - 01/30/14 11:20 AM

Quote:

Quote:

So if you are running out inward focus (focuser racking into OTA, toward to objective lens) with mirror diagonal, prism diagonal could be a godsend




Yup. And of the prisms I am evaluating, the Tak 1.25 is the most bin0-friendly having the shortest light path.




I agree. I was able to reach focus without an OCS on my Istar using my Tak prism diagonal. I had about 10mm of travel remaining and all my eyepieces came to focus.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RGM]
      #6344950 - 01/30/14 11:33 AM

In the measures I did of the diagonals I'm testing, the Vixen actually came in 1mm shorter than the Tak. So the inexpensive Vixen is actually the shortest path. Would not be surprised if the Vixen and Antares prisms were from the same OEM.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6352407 - 02/03/14 01:17 AM

Quote:


I have not, however about a week ago a young man came into the store to ask about repairing an old 4" Edmund Newtonian. Upon removing the secondary, I discovered it was a prism! I think it would definitely make an improvement, particularly for slower Newtonians.




I have been digging through the old Sky & Telescope Gleanings For Amateur Telescope Makers columns in connection with my 8" f/9 project and have found a few mentions of prism diagonals in Newtonians. Specifically: September 1944, September 1954, November 1954, and October 1969. I am just starting the 1970's issues.

The November 1954 article seemed to address the scatter issue, albeit indirectly. That being the prism diagonal may be frequently cleaned with no degradation, while the mirror diagonal will suffer coating damage.

I find it difficult to believe they were not aware of the concept of scatter. Indeed, reading the old articles with respect to Newtonian performance the amateurs of that era seemed every bit as knowledgable as today's amateurs. For example, reading the April and May 1944 columns comparing refractors to reflectors they were discussing everything we do today, including thermal management and fan cooling. In fact, the articles often applied a mathematical rigor to problems one rarely sees in todays publications. One may even get the impression they were smarter on the fundamentals than we are today!

The 1969 article seems to address the point most directly. The author calculated total spherical aberration, primary axial color, secondary axial color, coma, and focus displacement in f/22 and f/4 systems. While the slower Newtonian handled the prism significantly better the overall conclusion was that prisms should be avoided altogether in the Newtonian.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jeff Morgan]
      #6352859 - 02/03/14 09:51 AM Attachment (32 downloads)

Jeff,

Nice work on the research! I intend to follow up and check those articles and yes, I'm not surprised with regard to the prisms use with faster Newtonians. Also, I spent the entire Saturday evening comparing Tammy's 2" prism to other models here and when I get time shortly, I'll follow up with a constructive report. Comparisons were made on M82, extremely faint stars at the center of M35 and more.


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Lance1234
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6353930 - 02/03/14 06:20 PM

Daniel,

It will be very interesting to see your results.

Lance


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Lance1234]
      #6354281 - 02/03/14 08:49 PM

+1 on the results

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John Anthony
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: B. Cook]
      #6354570 - 02/03/14 11:05 PM

Finally a clear night and I got out with the AP Maxbright and Baader T2 prism, using both with my TMB 92SS APO, 11 degrees and poor to average seeing conditions. Aaaaand the winner is.... need more time under better seeing, it seemed like the prism showed finer detail on the moon but it may have been the seeing. on Jupiter it was the same, to close to call probably because seeing was poor. I let everything cool for two hours before using it, but my initial impression was detail was better in the prism ever so slightly but scatter was better controlled in the Maxbright, does this make sense ?? Need more time under better seeing, for 92mm's just not sure theres a big difference.

For what its worth just my two cents and it might not be worth one cent.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Lance1234]
      #6354608 - 02/03/14 11:27 PM

Quote:

Daniel,

It will be very interesting to see your results.

Lance





Certainly but as a courtesy I would like to let Bill have an opportunity to share all his final results and then I could share mine. Bill and I have actually corresponded by phone regarding our own personal impressions and I think in the end, this thread should help others make more informed choices if this topic is of concern to them. Bill has gone to great efforts to sort out his conclusions I'm sure.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6355186 - 02/04/14 10:03 AM

Quote:

Quote:

Daniel,

It will be very interesting to see your results.

Lance





Certainly but as a courtesy I would like to let Bill have an opportunity to share all his final results and then I could share mine. Bill and I have actually corresponded by phone regarding our own personal impressions and I think in the end, this thread should help others make more informed choices if this topic is of concern to them. Bill has gone to great efforts to sort out his conclusions I'm sure.




Daniel,

I am actually fine if you push your results first. I would actually like to reference other compares done in my write up, so would mention yours if you give us results before I'm done. btw, I am still several weeks away from a final write up anyway.


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Lance1234
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6355951 - 02/04/14 04:59 PM

Well, there are a lot of us anxiously awaiting BOTH of your reviews. While there is never any substitute for observing things with one's own eye through one's own telescope, it is nearly impossible for most of us to make these kinds of large scale comparisons. So the ability to have the benefit of comparisons such as these, made by experienced observers such as you and Daniel, is invaluable. To loosely paraphrase a quote from another post (which was stated much more eloquently), it helps inform our decisions.

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Daniel Mounsey
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Lance1234]
      #6355997 - 02/04/14 05:24 PM

Lance,
Results won't be long. It would also be nice if Tammy posted his thoughts. He has a wonderful eye for detail and a valuable opinion of the diagonals.


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John Anthony
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Lance1234]
      #6356295 - 02/04/14 08:16 PM

Quote:

Well, there are a lot of us anxiously awaiting BOTH of your reviews. While there is never any substitute for observing things with one's own eye through one's own telescope, it is nearly impossible for most of us to make these kinds of large scale comparisons. So the ability to have the benefit of comparisons such as these, made by experienced observers such as you and Daniel, is invaluable. To loosely paraphrase a quote from another post (which was stated much more eloquently), it helps inform our decisions.




I know I'm looking forward to these reviews, for some of us its impossible to make small scale comparisons let alone large scale comparisons, I'll be the first to admit I probably don't what I'm doing with what I have other then determining if I like one over the other and even then I'm not sure I would be able to accurately describe why. I do my best to apply what other more experienced and knowledgable observers post to my own observing experience but when it come to the science behind some of it I'm as thick as a brick.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6356793 - 02/05/14 12:25 AM

What he said!

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: herrointment]
      #6357805 - 02/05/14 02:16 PM

I think all of us following this thread are anxious to read the conclusions and observations. But this thread has also made me think about my experiences with diagonals and observing with different telescopes.

In 2000 I began observing regularly with a TV 85 and 2" dielectric diagonal with TV Plossls and Naglers. It provided good views to this beginning amateur astronomer. About 2003, I started using an older Tak FC 60 with a 1.25" Tak prism diagonal and Tak LE eyepieces. I liked the combination so much that it became my most used scope. I thought the views through the little 60mm objective were excellent and initially, I attributed it solely to the objective lens and the different eyepieces used. Later, I began to understand that the smaller objective helped reduce scatter by limiting the amount of light transmitted to my eye. Then I learned how atmospheric conditions with moisture and particulate pollution played their part in providing that annoying glow of light around bright planets and stars… and how the relationship between one's cornea and retina can also effect our perception of scatter. But I never considered that the prism diagonal might have played a part in providing memorable, if smaller scale, images with very well controlled scatter.

In recent years, I have gathered 4 expensive planetary eyepieces, in a pursuit to address the issue of scatter, ghosting and other stray light induced aberrations, when observing the bright planets and especially double stars… never questioning the use of a dielectric diagonal in my refractor. The use of a prism diagonal to gain additional advantage in splitting close doubles and seeing even finer detail on Luna, Jupiter and Saturn never crossed my mind until reading this thread.

I've tried observing without a diagonal… one time was enough for my neck to rebel. Straight through observing is not in my future. Perhaps a prism diagonal will once again grace my equipment to help refine visible detail. If Bill's initial findings are supported with followup results, Baader and Tak might have to ramp up production!

Thanks so much for your talent and time which benefits us all. And, Bill, you might have to do a revision of your book to include a synopsis of findings about diagonals, upon which eyepieces and observers depend.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: GeezerGazer]
      #6358006 - 02/05/14 04:09 PM

Quote:

Later, I began to understand that the smaller objective helped reduce scatter by limiting the amount of light transmitted to my eye. …and how the relationship between one's cornea and retina can also effect our perception of scatter.




Pardon the pun, but this diagonal test has been a real eye opener for me related to this. As for scatter, and its impact on contrast and attainging planetary details, there is definitely an interplay between the scatter from the optical system and the eye. In terms of scatter, what the eye sees and what the camera will see through the same optical chain, are two very different things!


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6383874 - 02/18/14 11:59 PM Attachment (25 downloads)

Okay, after doing some extensive visual comparisons using my 6" apo alongside a buddy of mine who had a very good quality 8" F-9.75 modified, open tube cassegrain, I've drawn some conclusions.

I carefully observed a faint star known as GSC 1877-0458 in the middle of a darkened area at the center of M35 which happened to be near the visual threshold on this particular night. Threshold objects are a great way to detect even the most subtle of possible differences in brightness.

I tried to discern any visual brightness differences between a very high quality 2" 99% dielectric diagonal as well as a high quality 2" prism and a smaller high quality prism. Based carefully on what I saw, it was impossible for me to detect any difference in brightness between all three of them. Regardless of what color any star was, I could not see or detect even the slightest difference in brightness.

Later I decided to slew the 6" apo over to M82 to check out the brightness of the galaxy and observe the supernova as well. Once again while going back and forth between the diagonals, I could detect absolutely no differences with the brightness of M82. I was thinking that a slight difference in tone with the diagonals might have a slight effect, but that didn't appear to be the case. With certain eyepieces, I have unquestionably seen that difference.

I then decided to slew the telescope over to Jupiter and this is where the differences immediately became noticeable. I used an extremely high quality 10mm eyepiece with a magnification of just 122x. After switching back and forth between the diagonals, there was no question which was which. The surface of Jupiter looked about 15% cleaner and crisper in my visual estimation in the prisms. It was like it just had a smoother surface appearance. After switching back and forth, every time I went back to the prisms, Jupiter's details were noticeably cleaner with less light scatter. It was absolutely wonderful to see how beautiful Jupiter appeared in the prisms.

I could easily tell which image had less light scatter and the prisms were the winners hands down. I asked my observing buddy Darren to have a look. After he tried the diagonals, he looked at me and said man! I gotta get myself a good prism. The 99% dielectric diagonal which belonged to him bothered him a little after the comparison. It's hard to ignore something nicer once you've actually seen it for yourself.

Note, that this high quality dielectric diagonal has been time tested and used again and again. Unless you sat down and conducted a test like this, you wouldn't know any better. According to Tammy, the 2" prism exhibits just a slight hair of false color compared to the smaller prism but I don't recall at what F-ratio that was but he did say at F-5.6 the 2" prism was at its limit as far as false color. I personally don't use any telescope that fast, especially a refractor, so that may be a different case all together and Tammy may have to chime in to offer his opinion. Perhaps at F-8 it just wasn't an issue in my case.

I've spent many years observing variable stars and one thing you have to remember is to keep an unbiased and opened mind because if you don't, you can actually convince yourself you're seeing something you're actually not. It's important to gain field experience. That means you go out and use this stuff and literally see for yourself what all the paper boils down to because ultimately the field is where it's going to be used, and you have to try to keep an open mind.

We observed M35 and Jupiter using Darren's 8" modified cassagrain, but the results were exactly the same thing as with the 6' apo. Another thing you may find interesting is that the brightness of GSC 1877-0458 was exactly the same in the 8" cass as it was in the 6" apo. The only thing is that the stars looked tighter and crisper in the 6" apo but as far as brightness, they were dead nuts. The edges in the 6" apo were also crisper.

Now I realize in Bill's images, the smaller prism appears to exhibit a slight bit of warmth or tone to it. During my visual comparisons using the smaller prism, I can not say for sure that I noticed this slight effect which is quite ironic. I admit that this whole thing about coffee tones was a curse I would dread in later years since it went viral.

I then decided to set the smaller prism aside and have another go at the entire M35 cluster with just the 2" prism and the 99% dielectric 2" diagonal to compare the over all image. Immediately another thing I noticed right off the batt was that the 2" prism had a noticeably crisper image around the outer periphery. This was a test I did using an original and discontinued 50mm Axiom from Japan, my favorite 2" 50mm eyepiece in the world.

After conducting these tests and later testing a silver coated 2" diagonal of mine from a separate location, I would put my ratings in the following order of diagonal performance.

A.) prism and silver coatings in 1st place.

B.) aluminized in 2nd place.

C.) 99% dielectric in last place in fact I don't even use these type of dielectric diagonals.

Daniel


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6383928 - 02/19/14 01:03 AM

Thanks for your very interesting report, Daniel. I use a mirror diagonal with my Mark V bino and Tec 160FL but have always wondered what a prism would be like. Baader used to include the prism and then switched to the mirror and I don't think anyone knew for certain why. I believe I read Roland C said there would be no difference other than some focus travel. Maybe it depends on the speed of the scope? So after your findings I still wonder what a prism would be like in my bino. Did you compare the prism and mirror with your Mark V?

Thanks!


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Daniel Mounsey
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Kent10]
      #6383942 - 02/19/14 01:18 AM

I did several years ago with a friend using an 8" apo, however because of fluctuations in seeing on that particular night and the time required to switch the diagonals, it was not conclusive. I use the prism in my Mark V now though.

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6384177 - 02/19/14 07:32 AM

"you have to remember is to keep an unbiased and opened mind because if you don't, you can actually convince yourself you're seeing something you're actually not. It's important to gain field experience. That means you go out and use this stuff and literally see for yourself what all the paper boils down to because ultimately the field is where it's going to be used, and you have to try to keep an open mind."

Interesting stuff, I have an open mind and I'm ready and anxious to play, I got the info and I got the tools, now if you could just do something about the seeing and the clouds for me I'll be all set.

Still very anxiously waiting for Mr Bills test report.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6384430 - 02/19/14 10:38 AM

Daniel,

Thanks for the test results! FWIW, everything you mentioned is what I am seeing in my tests. At f/6 the prisms do show a very minor amount of color on brightest objects, but IMO it is inconsequential it is so minimal. At f/7 and f8, no color. While the prisms in my tests are showing a marked level of better performance on planetary, all diagonals are showing objects just as brightly, and even threshold stars that are barely visible with adverted vision remain that way across all diagonals. So in terms of brightness, they all perform on-par. Planetary the prism holds a big advantage. On lunar it is very subtly better...so subtle that inconsequential. Star fields, nebula, galaxies, clusters, all are showing just as well, only exception being that one does notice the lesser scatter using the prisms. So depending on what you are atuned to when you observe, you may feel the picture looks a little more aesthetically pleasing with a prism on star fields. So just depends on how sensitive you are to noticing a bit more or less scatter arounf bright stars. For me I saw it, but didn't feel even though obvious, that it mattered all that much for me. But then again, maybe I feel that way because my passion is not with that type of observing, but with planetary.

Bottom line for my testing of the 12 diagonals I have on hand bubbles out the same as your list. The Prism and Ag Mirror (Vernonscope) really distinguish themselves on everything. I can't say which I prefer though, but they are definitely preferred to the Al Mirrors or Dielectrics for me now. I do get a sense of a view difference between the Ag Mirror and the Prisms, but just can't put my finger on it. So both perform fabulously, but still have their own distinct character. Been an eye opening experience about this overlooked item in the optical chain (for me anyway).


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Midnight Dan
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: John Anthony]
      #6384458 - 02/19/14 10:58 AM

Quote:

"you have to remember is to keep an unbiased and opened mind because if you don't, you can actually convince yourself you're seeing something you're actually not. It's important to gain field experience. That means you go out and use this stuff and literally see for yourself what all the paper boils down to because ultimately the field is where it's going to be used, and you have to try to keep an open mind."

Interesting stuff, I have an open mind and I'm ready and anxious to play, I got the info and I got the tools, now if you could just do something about the seeing and the clouds for me I'll be all set.




As I get older, my mind is becoming more open. I can tell because things keep falling out.

-Dan


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B. Cook
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6384487 - 02/19/14 11:10 AM

So bottom line which brand prism in your own minds were the best tested? There are quite a few out there but which one to choose regardless of price for the best performance?

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Tamiji Homma
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6384517 - 02/19/14 11:27 AM

Hi Bill,

Quote:

At f/6 the prisms do show a very minor amount of color on brightest objects, but IMO it is inconsequential it is so minimal. At f/7 and f8, no color.




Your observation matches mine. I've tested various mirror/prism diagonals with f/4, f/5.5, f/6.3, f/7, f/8 refractors.

From f/6.3 or faster, I see color cast (toward yellow-ish) but not that bad. At f/5.5, it is clearly visible. At f/4, I decided not to use the prism diagonal

I used the 2" prism diagonal with f/4 scope because I ran out of inward focus distance with 2" mirror diagonal. The prism diagonal worked.

However the color cast annoyed me enough so that I decided to make custom adapter to shorten the light path to Takahashi 2" mirror diagonal. With the adapter, most 2" eyepieces come to focus to infinity.



Tammy


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6384580 - 02/19/14 12:05 PM

Quote:

Hi Bill,

Quote:

At f/6 the prisms do show a very minor amount of color on brightest objects, but IMO it is inconsequential it is so minimal. At f/7 and f8, no color.




Your observation matches mine. ... I used the 2" prism diagonal with f/4 scope because I ran out of inward focus distance with 2" mirror diagonal. The prism diagonal worked. However the color cast annoyed me enough so that I decided to make custom adapter to shorten the light path to Takahashi 2" mirror diagonal. With the adapter, most 2" eyepieces come to focus to infinity.





Really scarey that we are all agree

Good work (as usual) on adapting the diagonal...necessity is the morther of invention


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Jeff Morgan
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6384618 - 02/19/14 12:21 PM

Quote:

Really scarey that we are all agree




The scary part is now I have something else to buy …..


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Pinbout
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6384652 - 02/19/14 12:38 PM

Quote:

necessity is the morther of invention





does she live in cleveland?


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Sarkikos
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Tamiji Homma]
      #6384697 - 02/19/14 12:57 PM

Quote:

From f/6.3 or faster, I see color cast (toward yellow-ish) but not that bad. At f/5.5, it is clearly visible. At f/4, I decided not to use the prism diagonal




So I should use a prism diagonal with my 70mm f/12.9, 102mm f/9.8 and 50mm f/7.2. The 70mm f/5.7 and 80mm f/5 need a mirror diagonal.

And from what I read previously in this thread, if I view deep sky either type of diagonal will be OK. (I have noticed this also.) But the prism diagonal shows a better image for planet/lunar, at least with the slower refractors.

However, no matter what the results of these comparos I'll continue to use Amici prisms in my fast achromat finder scopes. I've got to have that correct-image non-reversed view for star hopping!


Mike


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Fomalhaut
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: B. Cook]
      #6384703 - 02/19/14 01:01 PM

Quote:

So bottom line which brand prism in your own minds were the best tested? There are quite a few out there but which one to choose regardless of price for the best performance?




My 1.25" Tak-prism introduces not much, but still perceptible spherochromatism to my (straight-through) "white" f/6.4 fluorite-triplet - fortunately it does so to a substantially smaller amount than my 45mm Tak-prism.
The 1.25-prism also shows planets with tiny little but nevertheless perceptibly better detail than the 45mm-prism (which on planets is still better than my Maxbright-mirror).
Conclusion: My beloved 0.96" Tak- 4mm and 2.8mm Hi-Orthos are now missing their (sold ) original 0.96"-Tak prism ...!

Chris


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Sarkikos]
      #6384863 - 02/19/14 02:19 PM

Quote:

However, no matter what the results of these comparos I'll continue to use Amici prisms in my fast achromat finder scopes. I've got to have that correct-image non-reversed view for star hopping!




All of these comparos only point out where advantages are or disadvantages are. Up to each of us to decide how much weight to give an advantage or disadvantage. The indivudual's weighting is what makes "better". In my case, when I am not doing an evaluation, my observing of stars and DSO is quite casual, so for me any old diagonal works just fine. But for planets, I don't settle or observe casually, so that I weight very much. For you, as you said, correct view is a trump card. It's all good. The value of comparos is that at least we get to know better how things work on different scope for different targets so we can make a real informed decision of what will be best for us!

btw, the other lesson learned in this adventure I set out on was the marked difference between the visual system and the imaging system. They are NOT equal by a long shot. So as I developed better and better techniques using the camera to ferret things out, I realized that what the camera wass seeing was nothing at all like what my human perception system was seeing. So the two just do not equate. When I do the final write up, there will therefore be no imaging compnent to the evaluation. The visual system adds a whole lot of other variables to the mix, and the way they interact makes any conclusions based soley on imaging invalid IME from this exercise. So back to trusting those good ole eyes


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6385115 - 02/19/14 05:03 PM

Thanks for continuing to pursue and follow through with these comparisons Bill. Now a quick question for you. Do you feel the performance of the prisms are closer to that of dielectrics or of no diagonal?

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: The Mighty Mo]
      #6385242 - 02/19/14 06:29 PM

Thanks to Daniel and Tammy as well for their corroborating observations...that was invaluable! On planets, the Dielectrics were least favorite. On other objects, no problem. I have not done the "no diagonal" observation yet. That will be an interesting compare for sure...especially with Jupiter so high overhead

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6385382 - 02/19/14 07:46 PM Attachment (18 downloads)

My 32mm Baader Prism arrived yesterday and I'm looking forward to this Prism vs. Dielctric myself (once the worst Winter on Record leaves!).

But better Planetary views and possible improved scatter control wasn't my main reason for getting the Prism, It was the 37mm optical path.
Combined with my TV BinoVues and AP Visual Back, I should be well under the allotted 200mm Back Focus and thus keep the full 14" Aperture (or so I hope!).

Edited by junomike (02/19/14 07:47 PM)


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Daniel Mounsey
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6385625 - 02/19/14 09:46 PM

Quote:



All of these comp
btw, the other lesson learned in this adventure I set out on was the marked difference between the visual system and the imaging system. They are NOT equal by a long shot. So as I developed better and better techniques using the camera to ferret things out, I realized that what the camera wass seeing was nothing at all like what my human perception system was seeing. So the two just do not equate. When I do the final write up, there will therefore be no imaging compnent to the evaluation. The visual system adds a whole lot of other variables to the mix, and the way they interact makes any conclusions based soley on imaging invalid IME from this exercise. So back to trusting those good ole eyes




Bill,
I would have to concur with you there sir. Wise decision IMO.


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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6386408 - 02/20/14 09:58 AM

Thanks for the very interesting report. Ever since the thread began I have found a similar increase in detail on Jupiter when using a prism as opposed to dielectric diagonal.

Thanks,


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RodgerHouTex
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6386417 - 02/20/14 10:03 AM

Time to lay on your back to observe.

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Sarkikos
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: RodgerHouTex]
      #6386455 - 02/20/14 10:23 AM

I have a box full of 1.25" diagonals, also a few 0.965". I need to take out that box and see which diagonals are prism. I used to have all my diagonal info in a spreadsheet, but it was erased at some point years ago.

Mike


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Jeff Morgan
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6386710 - 02/20/14 12:30 PM

Quote:

While the prisms in my tests are showing a marked level of better performance on planetary, all diagonals are showing objects just as brightly, and even threshold stars that are barely visible with adverted vision remain that way across all diagonals. So in terms of brightness, they all perform on-par.




You and Daniel have provided a lot of food for thought here.

One aspect of performance neither of you mentioned however - star testing for differences in aberrations. The star observations seemed to center on transmission (faintest visible).

The planetary performance would strongly imply the prism is holding it's own, but ….


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Starman1
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jeff Morgan]
      #6386790 - 02/20/14 01:13 PM

Quote:

Quote:

While the prisms in my tests are showing a marked level of better performance on planetary, all diagonals are showing objects just as brightly, and even threshold stars that are barely visible with adverted vision remain that way across all diagonals. So in terms of brightness, they all perform on-par.




You and Daniel have provided a lot of food for thought here.

One aspect of performance neither of you mentioned however - star testing for differences in aberrations. The star observations seemed to center on transmission (faintest visible).

The planetary performance would strongly imply the prism is holding it's own, but ….



Indeed. Intra- and extra-focal star images indicate......?


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SteveG
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6386827 - 02/20/14 01:33 PM

Without having to read all 12 pages of this thread, is there a consensus on 2 or 3 good quality 2" prism diagonals?

I know the Baader is liked, but how about some others?


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B. Cook
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: SteveG]
      #6386895 - 02/20/14 02:07 PM

+1 SteveG

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Hermie
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: SteveG]
      #6386914 - 02/20/14 02:12 PM

Steve,

That is like asking for the season finale of [insert your favorite drama]! Bill has only given hints of what he is seeing. Us onlookers favor the usual suspects: Baader, Zeiss, Tak. You will have to keep following to get your answer.

Hermie


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Daniel Mounsey
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: SteveG]
      #6386931 - 02/20/14 02:20 PM

I've conducted tests over the past few years with several different high end prisms and I still come up with the same conclusions.

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Pinbout
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6386970 - 02/20/14 02:39 PM

how did you test the prisms?

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Daniel Mounsey
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Pinbout]
      #6386988 - 02/20/14 02:55 PM

Several of us tested them on some flowers using a few high quality 60mm and 4" apos during the day and then more tests during the night on the Moon, stars and planets.

We also did blind tests and all of it was conducted visually.


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Pinbout
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6387573 - 02/20/14 08:41 PM Attachment (10 downloads)

but you didn't test them like tex has in his book by defocusing on a star reflecting off the diag. thru a scope?

I recently wanted to make a H.wedge and took my meade diagonal apart to see, oh wow 1/2~, I think I'll make a test with this and use my other prism for a good H. wedge.



Edited by Pinbout (02/20/14 11:53 PM)


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6387739 - 02/20/14 10:10 PM

Quote:

Several of us tested them on some flowers using a few high quality 60mm and 4" apos during the day and then more tests during the night on the Moon, stars and planets.




I bet it was very much easier to see any contrast differences when viewing daytime nature vs astronomical targets since your eye/brain is much more atuned to normal daytime viewing.


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leonard
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6387907 - 02/20/14 11:41 PM

Hi Bill ,

Just curious Bill ,have you used your ball eyepieces with the prisms yet ?


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SteveC
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6387910 - 02/20/14 11:42 PM

Quote:

Several of us tested them on some flowers using a few high quality 60mm and 4" apos during the day and then more tests during the night on the Moon, stars and planets.

We also did blind tests and all of it was conducted visually.




Isn't that cute - flower power, soooo 60's. Anybody want to join me in singing, "All You Need Is Love"?


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Roy McCoy
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6388028 - 02/21/14 01:21 AM

Quote:

We also did blind tests and all of it was conducted visually.




hmmm . . .







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Mariner@sg
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Roy McCoy]
      #6388083 - 02/21/14 02:59 AM

Quote:

Quote:

We also did blind tests and all of it was conducted visually.




hmmm . . .










So... how many blind guys managed to see?

Just kidding


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BillP
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: leonard]
      #6388276 - 02/21/14 08:27 AM

Quote:

Hi Bill ,

Just curious Bill ,have you used your ball eyepieces with the prisms yet ?




No. Have just been using my ZAO and XO and TMB Supermono. Would not expect the Ball to perform differently.


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SteveG
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6388618 - 02/21/14 11:48 AM

Quote:

I've conducted tests over the past few years with several different high end prisms and I still come up with the same conclusions.




And what were your conclusions?


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Bill Barlow
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6388661 - 02/21/14 12:13 PM

I was wondering why the prism diagonals perform better on planets than the dielectric ones? Do you or anyone else have any ideas why this is so? Thanks for the interesting comparisons.

Bill


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leonard
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6388736 - 02/21/14 12:57 PM