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Star Test Results w/line

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Old Monday 12th April 2010, 02:30   #1
statestat
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Star Test Results w/line

At the prompting of the many wise contributors to BF I have slowly become more adept at doing the star test, but not as good at interpretation of the results. Seems most web sites have examples for telescopes and not bins. What does it mean if one barrel has a distinct horizontal line that is not black but faint greenish in color across the 'airy disc'. At focus the star is a dot and perfect circle close-to-infinity with one outer ring. I seem to recall this is from the roof-prisms line.
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Old Monday 12th April 2010, 06:07   #2
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Statesat,
Are you "boosting" the magnification with a second telescope behind the eyepieces, or using the binocular in the natural way?
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Old Monday 12th April 2010, 22:38   #3
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Using a 3x scope to boost the image, is that enough?
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Old Tuesday 13th April 2010, 03:16   #4
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I recently suggested, in another thread, trying binocular on bright stars as a rough test of optical quality. To try to go beyond this can get pretty complicated.

I have some experience with astronomical telescopes, but really very little at critically testing binoculars. Binoculars do not stand up well to boosted tests on point sources. Their aberrations are a messy zoo, difficult to interpret. I hope Henry Link, Surveyor, and some of the bona fide binocular experts will jump in here and help. I can share a little something useful, and I'll try.

At 3x boost, there will still be features that are resolved by the binocular optics, but appear so small, that they depend on you having exceptional visual acuity to see. Unless you are 20/10-correctable, like some lucky guys are, 8x or 10x boost would be nice.

I doubt that a normal eye could see the in focus first diffraction ring at 3x boost. 40-50x per inch of aperture is usually needed for a decent view of this feature. Also, I believe that it is very unusual for a binocular optic to be of the near perfection necessary to produce a neat textbook-like diffraction pattern in the first place. That requires nearly perfect optics, which just isn't necessary at bino powers, and would be very difficult with the complex prisms. Binocular opticians aren't foolish enough to over engineer at huge expense, for little improvement. That kind of near-perfection is the territory of astronomical telescopes, which are optically simpler than binoculars, and routinely required to operate at up to 50x per inch of aperture.

So the circular ring you are seeing puzzles me. There is of course a chance that your eyes are that exceptional, and also that your binocular is exceptional too, and you really are seeing the "first diffraction ring". For now, let us hope.

I would suggest higher boost power, and testing both barrels.

Regarding the horizontal green line seen when out of focus, I don't know, but it isn't normal. Streaks on bright point sources can result from a roof prism not having a good knife edge cut to the roof apex, but the streak is not colored any differently form the point source itself, and is best seen when in focus. A smattering of this is very common and mostly benign as far as daytime usage. What you describe is different. Still, I'm guessing, only guessing, it's due to some kind of prism effect/defect.
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Old Tuesday 13th April 2010, 03:55   #5
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Not sure I described the outer ring that well. It seemed like a halo/haze around the outer edge, with a dark, narrow in-between region separating the larger lighted disk. This appears when I near the end of the out-of-focus range, not at the pin-point in-focus spot. The bins seem great, no problems with clarity or resolution, but did the start test to be complete in my evaluation of them. I do use glasses and had an eye exam two weeks ago and did beat 20/20, I think the doc said 20/15, I'll check Thursday. I will try higher magnification, the bins are 8x and the scope was 3x. Horizontal line is only in the left barrel, and was not like a drawn thin line but seemed more like cutting a circle in half and pulling the two half's apart very slightly, letting the greenish light through. Wished I could take a pic, might have something rare.
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Old Tuesday 13th April 2010, 06:04   #6
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I can tell you right now that I will never be able to say "aha, pinched objective lens", "aha, off center prism" or aha anything else. Despite my earlier optics-kid enthusiasm for looking deeper, I have rethought this matter, and believe it could rapidly turn into a downward spiral of questionable diagnostics, and fretting over what is in fact a very fine binocular.

The true high-magnification star test is relevant to astronomical telescopes, hey, they are used to look at stars at high magnification. The test is so sensitive, that large variations from ideal appearance of the diffraction pattern can reveal aberrations that are so small that they are essentially insignificant in use. And, it is so sensitive that the larger aberrations of binoculars will send it flying all over the place.

So I now change my tune, and recommend retreating to the basic unboosted "star test". If stars look neat and compact and not too amoeba like, call it good and be happy.

The hardest test for a daytime bino is the bird test. Honestly, I don't know of anything but birds that offer such amazing color variations, fine detail, and difficulties of observation.
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Old Tuesday 13th April 2010, 17:42   #7
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Doesn't look like you need any help to me, Ron. I agree that binocular star tests are usually a big mess with lots of chromatic and spherical aberrations and some sphero-chromaticism thrown in just to confuse you, but some defects shouldn't be there, like astigmatism, pinching, coma or a poorly formed roof prism. Those are pretty easy to diagnose with the magnification boosted to an exit pupil of around 1.5mm or smaller.

Of course, even if a binocular has one or more of those defects the image may still be fine at normal magnification, especially when the aperture is stopped down in daylight, but the star test gives you a heads up to look very carefully at the offending barrel. It's always helpful to have one "good" barrel to use as a reference. If you see no problem with the defective barrel compared to the good one at normal magnification, then no harm is done.

Statestat seems to be describing a bad roof prism edge, although the dark line through the out of focus diffraction pattern is usually on the diagonal rather than horizontal in a binocular. My Zeiss 8x56 FL has a bad prism like that. Fortunately the other barrel is "perfect", so I was able to satisfy myself that no problem resulted at 8x. As for the "halo/haze" and central spot on one side of focus instead of neat diffraction rings, that's the typical outside of focus diffraction pattern from an undercorrected binocular, which is most of 'em.

Henry

Last edited by henry link : Tuesday 13th April 2010 at 19:19.
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Old Tuesday 13th April 2010, 22:43   #8
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Thanks for critique of the results. I do agree that the bins are keepers, just have that curious line. One thought it may be from the cross-hairs, since I am using a very old Colt 3x20 rifle-scope from the early 70s that fit well behind the bins, but only appeared in one barrel. I will try another scope.

Actually I was pleased that the out-of-focus shapes really maintained a circular shape, and yes the un-boosted image is fine. I have learned that conducting the star test does reveal problems or reasure us that everything is good.
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Old Monday 19th April 2010, 22:37   #9
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Boosted Start Line Remains

Took the star test up to 64x and the horizontal line is still there, but now when going to infinity the line is greenish and at close focus it darkens to black. Also at focus what us expected to be a circle is a distinct triangle or arrow-head pointing left to right. All this only in the left barrel. Right barrel seems fine.

The bino worked well when birding this weekend saw a blue bird and a migrating palm warbler, both had brilliant colors.

I know that if I get lucky enough to purchase another top-end pair I will star test first thing and probably rely on the BF experts for more advice, Thanks
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Old Tuesday 20th April 2010, 00:11   #10
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statestat;

Sounds like that may be a classic pinched element. If you want to get a little more into star testing, I recommend you download a free program that will let you simulate conditions (if you use a Windows machine). It is called Aberrator and can be downloaded at http://aberrator.astronomy.net/

Star testing is very sensitive and you will see errors that you may never see at low power without a lot of boost. Attached is a picture of 1/2 wave of pinch with no other error. Chances are pretty good that the other side of focus of your binoculars might look like a ball of cotton because of some field curvature. A half wave is very decent for low power binoculars.
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Old Tuesday 20th April 2010, 23:32   #11
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Many thanks surveyor, that is what it looks like. I am downloading the program. Greatly appreciated.
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