Jump to content


Photo

Mirror, Dielectric, & Prism Performance Comparison

  • Please log in to reply
433 replies to this topic

#1 BillP

BillP

    Voyager 1

  • *****
  • Posts: 12048
  • Joined: 26 Nov 2006
  • Loc: Vienna, VA

Posted 13 January 2014 - 02:57 PM

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 :lol:

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)

Attached Files



#2 mgwhittle

mgwhittle

    Apollo

  • *****
  • Posts: 1378
  • Joined: 24 Aug 2011
  • Loc: Chattanooga, TN

Posted 13 January 2014 - 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!

#3 Lance1234

Lance1234

    Ranger 4

  • *****
  • Posts: 394
  • Joined: 01 Nov 2012
  • Loc: SoCal

Posted 13 January 2014 - 04:32 PM

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

#4 Fomalhaut

Fomalhaut

    Apollo

  • -----
  • Posts: 1058
  • Joined: 16 Aug 2008
  • Loc: Switzerland

Posted 13 January 2014 - 06:04 PM

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

#5 Paul G

Paul G

    Fly Me to the Moon

  • *****
  • Posts: 5162
  • Joined: 08 May 2003
  • Loc: Freedonia

Posted 13 January 2014 - 06:48 PM

Happy Birthday, Bill!

#6 BillP

BillP

    Voyager 1

  • *****
  • Posts: 12048
  • Joined: 26 Nov 2006
  • Loc: Vienna, VA

Posted 13 January 2014 - 07:10 PM

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 :lol:

Gus - Thanks :D

#7 Roy McCoy

Roy McCoy

    Apollo

  • *****
  • Posts: 1235
  • Joined: 13 Oct 2005
  • Loc: Glendale, AZ

Posted 13 January 2014 - 11:13 PM

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!

#8 Daniel Mounsey

Daniel Mounsey

    Vendor (Woodland Hills)

  • *****
  • Vendors
  • Posts: 5511
  • Joined: 12 Jun 2002

Posted 13 January 2014 - 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

#9 stargazer193857

stargazer193857

    Apollo

  • -----
  • Posts: 1266
  • Joined: 01 Dec 2013
  • Loc: Southern Idaho

Posted 13 January 2014 - 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.

#10 Mariner@sg

Mariner@sg

    Ranger 4

  • *****
  • Posts: 394
  • Joined: 27 Jun 2012
  • Loc: Little Red Dot

Posted 14 January 2014 - 02:31 AM

No William Optics' Dielectric? :(

#11 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44756
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 January 2014 - 05:30 AM

No William Optics' Dielectric? :(


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

Jon

#12 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44756
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 January 2014 - 05:42 AM

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

#13 Mariner@sg

Mariner@sg

    Ranger 4

  • *****
  • Posts: 394
  • Joined: 27 Jun 2012
  • Loc: Little Red Dot

Posted 14 January 2014 - 06:39 AM

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 :grin:

#14 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1984
  • Joined: 10 Dec 2007

Posted 14 January 2014 - 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

#15 BillP

BillP

    Voyager 1

  • *****
  • Posts: 12048
  • Joined: 26 Nov 2006
  • Loc: Vienna, VA

Posted 14 January 2014 - 09:11 AM

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 :lol: 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.

#16 Daniel Mounsey

Daniel Mounsey

    Vendor (Woodland Hills)

  • *****
  • Vendors
  • Posts: 5511
  • Joined: 12 Jun 2002

Posted 14 January 2014 - 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.

#17 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44756
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 January 2014 - 11:07 AM

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 :lol: 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

#18 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 23428
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 14 January 2014 - 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.

#19 BillP

BillP

    Voyager 1

  • *****
  • Posts: 12048
  • Joined: 26 Nov 2006
  • Loc: Vienna, VA

Posted 14 January 2014 - 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.

#20 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 17482
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 14 January 2014 - 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! :jump:

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

:grin:
Mike

#21 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44756
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 January 2014 - 02:02 PM

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

#22 Chucke

Chucke

    Vostok 1

  • -----
  • Posts: 101
  • Joined: 12 Mar 2010

Posted 14 January 2014 - 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

#23 schang

schang

    Apollo

  • -----
  • Posts: 1011
  • Joined: 24 Apr 2013
  • Loc: columbia, sc

Posted 14 January 2014 - 04:15 PM

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

http://www.cloudynig....php?item_id=82

http://www.cloudynig...hp?item_id=1854

#24 schang

schang

    Apollo

  • -----
  • Posts: 1011
  • Joined: 24 Apr 2013
  • Loc: columbia, sc

Posted 14 January 2014 - 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 :grin:).

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.

#25 Daniel Mounsey

Daniel Mounsey

    Vendor (Woodland Hills)

  • *****
  • Vendors
  • Posts: 5511
  • Joined: 12 Jun 2002

Posted 14 January 2014 - 05:03 PM

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.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics