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Equipment Discussions >> Equipment

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

Reged: 06/12/02

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

Reged: 06/12/02

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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John Anthony
sage
*****

Reged: 04/27/13

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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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
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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RodgerHouTex
Pooh-Bah
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Reged: 06/02/09

Loc: Houston, Texas, USA
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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BillP
Postmaster
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Reged: 11/26/06

Loc: Vienna, VA
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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The Mighty Mo
professor emeritus


Reged: 10/12/13

Loc: South of North, North of South...
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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Daniel Mounsey
Vendor (Woodland Hills)
*****

Reged: 06/12/02

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

Reged: 06/24/03

Loc: Los Angeles
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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Daniel Mounsey
Vendor (Woodland Hills)
*****

Reged: 06/12/02

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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Tamiji Homma
Carpal Tunnel
*****

Reged: 02/24/07

Loc: California, USA
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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schang
sage


Reged: 04/24/13

Loc: columbia, sc
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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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
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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Roy McCoy
Pooh-Bah
*****

Reged: 10/13/05

Loc: Glendale, AZ
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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Daniel Mounsey
Vendor (Woodland Hills)
*****

Reged: 06/12/02

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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Tamiji Homma
Carpal Tunnel
*****

Reged: 02/24/07

Loc: California, USA
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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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
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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Tamiji Homma
Carpal Tunnel
*****

Reged: 02/24/07

Loc: California, USA
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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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
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
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Mirror, Dielectric, & Prism Performance Comparison [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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