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Equipment Discussions >> Equipment

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BillP
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Reged: 11/26/06

Loc: Vienna, VA
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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Reged: 06/24/03

Loc: Los Angeles
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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SteveC
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Reged: 06/15/06

Loc: Sunshine State & Ocean State
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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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
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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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
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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Jeff Morgan
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Reged: 09/28/03

Loc: Prescott, AZ
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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Daniel Mounsey
Vendor (Woodland Hills)
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Reged: 06/12/02

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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Daniel Mounsey
Vendor (Woodland Hills)
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Reged: 06/12/02

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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Daniel Mounsey
Vendor (Woodland Hills)
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Reged: 06/12/02

Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6336347 - 01/26/14 10:40 AM Attachment (23 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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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6337623 - 01/26/14 09:24 PM Attachment (77 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
scholastic sledgehammer
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Reged: 10/14/09

Loc: Tennessee
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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dcoyle
professor emeritus


Reged: 10/11/05

Loc: Turbulent but dark skies, N.M.
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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george tatsis
professor emeritus
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Reged: 11/20/08

Loc: Flushing, NY - Europe
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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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
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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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Mirror, Dielectric, & Prism Performance Comparison [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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BillP
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Loc: Vienna, VA
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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Reged: 11/26/06

Loc: Vienna, VA
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
scholastic sledgehammer
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Reged: 10/19/07

Loc: West Virginia
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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rockethead26
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Reged: 10/21/09

Loc: Arizona, USA
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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Loc: Hilton, NY, Yellow Zone (Bortl...
Re: Mirror, Dielectric, & Prism Performance Comparison [Re: rockethead26]
      #6338569 - 01/27/14 11:37 AM Attachment (25 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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