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can you use roof prism binoculars for astronomy?

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#1 observing1

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Posted 04 November 2013 - 12:38 PM

I read that you couldn't because they would be too dark.

#2 Jon Isaacs

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Posted 04 November 2013 - 12:55 PM

The work just fine.

Jon

#3 Mark9473

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Posted 04 November 2013 - 01:10 PM

To expand on Jon's statement: the good ones work just fine. But if you're on a very tight budget then a porro prism binocular delivers more quality for the cost.

Something like an 8x42 roof prism binocular in the $250 range and up, is a very versatile instrument for daytime as well as night time. And you'll probably get suggestions for good ones that are even a bit cheaper.

#4 CSG

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Posted 04 November 2013 - 01:58 PM

I use both a Zeiss HT 8x42 and Nikon Monarch 7 10x42 for astronomy these days and find I like using the Monarch more than the Zeiss! The Zeiss beats the pants of it during the day but the 10x with decent glass, a good FOV, and light weight makes the Monarchs easy to use. Normally, I prefer lower mags for ease of holding but the Monarchs seem to work for me better than most of the 10x's I've tried. Of course, the Canon 10x30 IS work even better because of their stability but the Monarchs are pretty close when I use a ball cap brim to help steady them.

Bottom line is you can use pretty much any decent bin of any size for happy stargazing.

#5 KennyJ

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Posted 04 November 2013 - 02:10 PM

Observing1,

I'd like to know from what source you read such nonsense?

Kenny

#6 hallelujah

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Posted 04 November 2013 - 02:13 PM

http://www.cloudynig...ber/3409680/...

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#7 JustaBoy

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Posted 04 November 2013 - 02:18 PM

Too much Coffee, Stan?

-Chuck

#8 Mark9473

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Posted 04 November 2013 - 02:28 PM

:funny:

#9 hallelujah

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Posted 04 November 2013 - 02:40 PM

Too much Coffee, Stan?

-Chuck


It was not I (me), musta been a glitch in the system. :shrug:

Kaspersky detected a Trojan Downloader & cleaned my system. :crazy:

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#10 Shneor

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Posted 04 November 2013 - 03:04 PM

May well have been true prior to phase coating technology. But we have had that for years.

#11 Dave Mitsky

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Posted 04 November 2013 - 03:15 PM

I own several binoculars but the one that I use most often is an 8x42 Celestron. It's very compact and quite light and provides excellent views.

Dave Mitsky

#12 observing1

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Posted 04 November 2013 - 03:27 PM

I read it in a book called Astronomy Hacks, its a useful book.

"The optical design of roof prism binoculars requires that one surface of a roof prism be semi silvered to transmit the light and reflect part of it. This arrangement causes a light loss of 12 % to 15 %....a severe drawback for astronomy, where every photon counts when viewing dim objects." p. 34

They recommend only porro prism binoculars.

#13 hallelujah

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Posted 04 November 2013 - 03:33 PM

I read it in a book called Astronomy Hacks, its a useful book.

"The optical design of roof prism binoculars requires that one surface of a roof prism be semi silvered to transmit the light and reflect part of it. This arrangement causes a light loss of 12 % to 15 %....a severe drawback for astronomy, where every photon counts when viewing dim objects." p. 34

They recommend only porro prism binoculars.


http://www.cloudynig...hp?item_id=1268

Stan

#14 Rich V.

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Posted 04 November 2013 - 03:41 PM

They're behind the times when it comes to roof prism designs.

Abbe-Koenig prisms like Zeiss uses have total internal reflection and don't need silvered prism faces. Phase and dielectric mirror coatings have made roofs and Porros very close in light transmission.

Porro prism binos frequently have an edge when it comes to field illumination and the widest fields of view, though.

There are always tradeoffs...

Rich

#15 KennyJ

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Posted 04 November 2013 - 03:58 PM

Well, let's be honest here, obsy, even that greatly outdated statement of the light loss being between 12% and 15% isn't exactly the same as saying you couldn't use them, because they would be too dark, is it?

Kenny

#16 Rich V.

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Posted 04 November 2013 - 04:32 PM

At my age, I doubt I'd notice.

So you think "a severe drawback for astronomy" is pretty severe, Kenny? :D

I hate pointing out to the OP that his 70mm Porro binos have a 19% light loss due to restricted aperture and the 20x80s a 17% loss. There's plenty of places for things to go wrong in bino design and execution... ;)

Rich

#17 KennyJ

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Posted 04 November 2013 - 04:48 PM

< So you think "a severe drawback for astronomy" is pretty severe, Kenny? >

My severe lower back pain is more of a severe drawback than anything else I can think of, Rich! :-)

Kenny

#18 Chuck Hards

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Posted 04 November 2013 - 07:29 PM

Even a cheap binocular is better than none at all. I keep cheapie roof prism 10x25s in the glove compartment of my cars (not wanting to risk a better bino to theft) and many times have been glad they were there.

Kenny, look into a TENS unit for that back pain. Keeps me sane.

#19 Pinewood

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Posted 04 November 2013 - 08:00 PM

I read it in a book called Astronomy Hacks, its a useful book.

"The optical design of roof prism binoculars requires that one surface of a roof prism be semi silvered to transmit the light and reflect part of it. This arrangement causes a light loss of 12 % to 15 %....a severe drawback for astronomy, where every photon counts when viewing dim objects." p. 34

They recommend only porro prism binoculars.

Hello Observing1,

I would guess that dielectric mirrors have eliminated that difference.

Clear skies,
Arthur Pinewood

#20 SMark

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Posted 05 November 2013 - 12:07 AM

I read it in a book called Astronomy Hacks, its a useful book.

"The optical design of roof prism binoculars requires that one surface of a roof prism be semi silvered to transmit the light and reflect part of it. This arrangement causes a light loss of 12 % to 15 %....a severe drawback for astronomy, where every photon counts when viewing dim objects." p. 34

They recommend only porro prism binoculars.


It typically all comes down to how much you spend. "Dollar for dollar" when compared to porro prism binoculars, I have found this statement to be true, and I have had two examples of this myself.

The first is with a pair of Bresser binoculars that I purchase for the express purpose of comparing to each other. Both were 8x56, one was a porro and one was a roof, and both carried roughly the same retail price which, as I recall, was around $125.

The differences were extremely obvious. The roof images were significantly darker and duller. Then did a similar comparison between slightly cheaper Bushnell binoculars. The roof images were even worse in this example.

So I would say that the statement is valid when you compare in the less expensive price ranges.

Just MHO.

#21 Jon Isaacs

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Posted 05 November 2013 - 04:57 AM

The differences were extremely obvious. The roof images were significantly darker and duller. Then did a similar comparison between slightly cheaper Bushnell binoculars. The roof images were even worse in this example.



Did you verify that the loss of image brightness was due to the roof prisms and not a difference in the effective aperture?

In any event, a 12%-15% loss of brightness is relatively insignificant, for a 50mm binocular it corresponds to an effective aperture of about 46mm...

Jon

#22 Shneor

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Posted 05 November 2013 - 06:46 PM

Cheaper roofs don't have phase coating, and probably lack dielectric reflective surfaces as well. I'm just delighted with my 8X42 Zhumell ED roofs; $94 at Hayneedle when I purchased them, discounted 80% or more from suggested retail. I suspect this may have been a result of poor QC, but my pair is excellent, with an 8* well-corrected apparent field with bright, crisp images. Also very light, with a good soft case, etc. I should test them against my 10x50 porros, but those have been on loan to someone for a few months now.

#23 SMark

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Posted 05 November 2013 - 07:43 PM

The differences were extremely obvious. The roof images were significantly darker and duller. Then did a similar comparison between slightly cheaper Bushnell binoculars. The roof images were even worse in this example.




Did you verify that the loss of image brightness was due to the roof prisms and not a difference in the effective aperture?

In any event, a 12%-15% loss of brightness is relatively insignificant, for a 50mm binocular it corresponds to an effective aperture of about 46mm...

Jon


No, I didn't do any aperture tests or anything like that. But I would have expected these pairs to be more similar than different with regard to build quality. And the lack of brightness and sharpness in the roofs was more significant than that anyway. I expect the lack of coatings and phase correction contributed the most to the poor relative performance of the roofs.

#24 Rich V.

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Posted 05 November 2013 - 08:06 PM

I won a Barska 9x63(59) in a raffle at a star party; it was a lower end non-PC roof prism design.

Upon close comparison with my 10x35 Nikon Porros, I was surprised that faint fuzzys like M1 and M78 were brighter and had more contrast in the little Nikons than the much larger 9x roofs. I'm sure the extra 11% mag. helped but I didn't expect the difference in performance between a 6.5mm and 3.5mm exit pupil would be so lopsided towards the smaller bino.

Rich


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