ronharper
Carpal Tunnel
Reged: 02/14/06
Posts: 2065
|
|
I apologize for the impracticality of these remarks, in a largely consumer oriented forum. But I like to do experiments, and to try to learn something from them. The exit pupil gets a lot of discussion here, so I hope somebody will find my experiment interesting.
Tonight I stopped my Fujinon 16x70 down to 48, 32, and 16mm for exit pupils of 3, 2, and 1mm, and compared views with the 4.4mm exit pupil at 70mm. The idea was to see by how much, if any, the reduced light and larger focal ratio might improve apparent sharpness, and diminish the visibility of faint or low-contrast objects. The objects for comparison were double stars Gamma Delphinus (4.3 and 5.2 magnitutes, 9.2arcsec) and Gamma Aries (4.6 and 4.6 mag, 7.2arcsec), the now bloated comet Holmes, and the 12-hr past full moon. I have a trace (0.25 diopter) of astigmatism in one eye, and observe without glasses.
Results:
1mm exit pupil: gamma Del easily split as two dots, gamma Ari split but less distinct, comet visible as large but shapeless blur, moon completely devoid of chromatic aberration (usually a hallmark of this bino) but dim and lacking in contrast. The bino's edge correction was no better than at full aperture.
2mm exit pupil: Gamma Del even easier now, and Gamma Ari also nicely split as two dots, but extremely close (120 arcsec apparent) for my eyes, comet now shows one side to be better defined than the other, moon shows a trace of color but more contrast. I have split 120 arcsec apparent with a telescope, but never before with a bino. The view at 2mm had a telescopic look of perfection.
3mm exit pupil: Gamma Del still split, but stars start to look imperfect now. Gamma Ari no longer separated, but elongated and sometimes notched, comet about the same as at 2mm pupil, moon shows noticeable color but still more contrast.
Unstopped, 4.4 mm exit pupil: Gamma Del easily elongated but only marginally separated, did I momentarily see a black line or not?, star images appear noticeably imperfect. Gamma Ari showing elongation but suffering similarly. Comet now shows elongation of central brighter section, moon uncomfortably bright and suffering from intrusive color, but showing the greatest contrast yet.
Conclusions, broad-brush generalities, enlightments:
As I have read, 2mm exit pupil, which matches the eye's average daytime opening to which it has evolved maximum resolution, gives the sharpest view. But large pupils show greater contrast on extended objects. A bright-on-bright image seems better visible than dim-on-dim. The largish astro bino is not primarily intended for high resolution, but is a deep-sky machine, whose optical perfection has been compromised in pursuit of the ONE THING: dim, low contrast stuff. If I was stuck on a deserted island with nothing but this bino I would cut an aperture stop out of a palm leaf for viewing bright double stars. But, trying to split challenging doubles with a bino is the hang up of only a very few here. (A succesful close split gives me a rush of, if not exactly omnipotence, at least the dignity accorded to a minor deity.) Most of you wiser people apparently don't abuse your binos, nor your selves, in this way!
Ron
|
Mark9473
Postmaster
Reged: 07/21/05
Posts: 6459
Loc: 51°N 4°E
|
|
Thanks Ron, I found that very interesting. I'm concluding that edge imperfections are from the eyepiece and not the objective.
At some point I would still expect the full aperture - but with perhaps a neutral density filter fitted to reduce glare - to be better at resolving doubles. Or not?
Instead of using a palm leaf aperture mask, would you expect similar results with full aperture in a bright moonlit or twilight sky?
--------------------
Mark
Leica 8x20; Nikon 7x35; Vixen 8x42; Orion 15x63; Docter 15x60
WO Megrez II 80 FD / APM 107mm f/6.5 / Mewlon 210 on DM-6 + Berlebach Planet
|
ronharper
Carpal Tunnel
Reged: 02/14/06
Posts: 2065
|
|
Mark,
Thanks for your comments. My experience with telescopes is guiding my guesses as to "why" the things I see are happening. As a telescope, a 70mm f/4 achromat, two of which make a bino, is considered way too fast for its aperture to give very good images. Add in prisms, and all their possibilities for imperfections and misalignments, and a binocular starts to look like a very imperfect optical system, which can work OK only because the magnification is so low. The popular f/5 Short Tube 80, for example, is loved for its low power views, but despite its simplicity vs binos is almost never pushed near the 50x/inch that a longer achromat can use. So, I think that stopping the bino down gets it into the "long-achromat" regime, hence the very clean images I saw at 16x32, at about f/9. Testing these ideas by using filters instead of aperture stops would be a very useful experiment. Part of the aberration is bound to be scattering in my eye, which a filter would reduce as well as stopping down would. The observations were in fact made under a full moon, which would reduce apparent contrast of star against sky.
I get your drift, however. Sometimes telescopic doubles observers talk about stopping down for viewing bright stars. If the scope is long enough and good enough, however, aberrations aren't significant at full aperture, so the simple optical ideas that explain the behavior of a "perfect" telescope pretty much apply. Then, the only thing left to cause the smear of bright objects is scatter in the eye. In that case I think a filter, which reduces the light, but preserves aperture and resolution, is a better solution.
Finally, using telescopes has shown me and many others that eyepieces usually produce better images, including better edge correction, at larger focal ratios. The cheap and much-despised Huygens is fine at f/15, but an f/5 reflector requires more sophisticated multi-lens eyepieces for good edge sharpness. So, I was surprised that the edge correction was not noticeably improved at the much longer focal ratio of the stopped-down bino.
Ron
|
Jon Isaacs
Postmaster
Reged: 06/16/04
Posts: 32442
Loc: San Diego, California
|
|
Ron: I like you experiments with the various exit pupils. I will have to try them myself. It would be interesting to try to figure out if the issue is the size of the exit pupil or the resolution of the binoculars themselves.
One way to test this would be to use telescope rather than binoculars so that the focal ratio/optical quality was not so important. The other night I was noticed how well split Gamma Aries was in an 80mm F/7 FD at low mags but I don't honest remember if it was 13x (doubt it) or 23x...
Next clear night I will play around a bit and see what I see.
Jon
--------------------
|
ronharper
Carpal Tunnel
Reged: 02/14/06
Posts: 2065
|
|
John,
Thanks! It is widely, if not universally, conceded that binos do not rival the perfection of telescopes. BillC, in whatever state of forum limbo he may be now suffering, has repeatedly made this point. The findings of my experiment are not optics news, but have provided a personal revelation. I no longer view this imperfection as a matter of shame, and my once keen longing for a "perfect" bino is calming down, as I realize increasingly that a merely good bino beats a telescope in the one thing it is best at, namely the detection of large and very faint objects.
But, seriously, you gotta try stopping a big bino down to a 2mm exit pupil just once and looking at some stars. The view is what scope lovers call "refractor like"!
Ron
|
Mr. Bill
Postmaster
Reged: 02/09/05
Posts: 5219
Loc: Northeastern Cal
|
|
There is no reason that binoculars, especially those designated as "binocular telescopes" shouldn't be expected to perform like twin refractors at 15x/inch. That's 60x with a 100mm binocular. I'm thinking of my BT100s specifically.
If you were evaluating the binoculars as side by side refractors (which the BT100s are) this is certainly not an overly stringent criteria. The extrafocus diffraction patterns don't lie.
--------------------
The night sky is the palette....
My optics are the brushes....
The Milky Way is the masterpiece
Member IDA
|
Jon Isaacs
Postmaster
Reged: 06/16/04
Posts: 32442
Loc: San Diego, California
|
|
Quote:
But, seriously, you gotta try stopping a big bino down to a 2mm exit pupil just once and looking at some stars. The view is what scope lovers call "refractor like"!
Ron
I am aware that binoculars won't have the same perfection as a telescope. Imagine a binoculars made from a pair of Panoptics and a pair of 80mm F/7 FPL-53 Objectives.... Might be nice but pricy, not really worth it if you are only going to 25x.
My thinking was that by using a telescope, one could eliminate the "quality of the Optics" as a variable. So in a decent scope, if I could split gamma aries at 17x with a 2mm exit pupil but not with a 4.5 mm exit pupil, then it would imply the problem was with my eyes and not the scope optics.
And I will give it at try with both binoculars and a telescope. I have some 15x63 Mini-Giants so that should be about right.
Jon
--------------------
|
ronharper
Carpal Tunnel
Reged: 02/14/06
Posts: 2065
|
|
Bill and John,
There's no reason but price keeping a big bino from being of excellent optical quality, and great ones are sure to be out there. The BT100 is often mentioned as one. But, it's hard for me to get beautiful star images with large exit pupils, even with excellent optics, because my eyes just won't cooperate. My eyes aren't that bad, so I suppose most other people's experience must be similar. I think most fixed-power, large exit pupil bino manufacturers make a sensible engineering choice to save money and relax the optical standards for such typical binos. Well, some overdo it for sure. Below 3mm exit pupil, where only binoscopes can go, average eyes can start to appreciate fine quality, and I'm envious of those with such instruments. But, large exit pupils, with their almost necessarily inferior-looking star images, offer unique capabilities in terms of contrast on large very faint objects.
The two-eyed, double-barreled advantage ought to put the superb binoscope ahead of a telescope, in terms of resolution as well as contrast sensitivity, according to EdZ's recent refresher on the binocular advantage.
Jon, I'd be interested to hear the results of stopping down either your fine refractor or your bino. It's bound to be fun, and we stand to learn something from it.
Ron
|
patter1
professor emeritus
Reged: 01/19/05
Posts: 597
Loc: Canada
|
|
Ron thanks for sharing the results of your experiment. In my limited testing, I've found the same results as you. I'm going to do some more in-depth testing and I'll buy several spare objective lens caps and cut holes in them to various diameters so I can quickly and easily reduce/change the apertures/exit pupils of my binocs (and the focal ratio), and compare the views on various objects.
--------------------
Patrick
8" f/6 NewStar dobsonian
Orion Starblast 4.5" f/4 mini dobsonian
42mm SuperView, 17mm Nagler T4, some other cheapies
Omcon 7x50, Oberwerk 11x56, Olympus DPS-R 7x35, Olympus Magellan 8x25
homemade 50mm right-angle bino-scope prototype
|
Tony Flanders
Postmaster
Reged: 05/18/06
Posts: 8212
Loc: Cambridge, MA, USA
|
|
Quote:
Tonight I stopped my Fujinon 16x70 down to 48, 32, and 16mm for exit pupils of 3, 2, and 1mm, and compared views with the 4.4mm exit pupil at 70mm ...
Interesting. I thought I was the only one who did weird stuff like this. Most recently, I actually tried stopping down my own unaided eyes to see if I'd be able to see more Pleaids at smaller pupils. Answer: no. Any increase in sharpness was greatly outweighed by loss in brightness.
Quote:
Conclusions, broad-brush generalities, enlightments:
As I have read, 2mm exit pupil, which matches the eye's average daytime opening to which it has evolved maximum resolution, gives the sharpest view.
Presumably, that's going to vary a lot depending on the individual. It's a fair bet that the binoculars themselves are not at fault. Even a really lousy 70-mm telescope delivers sharp images at 25X, so it's unlikely that pricey Fujinons are going to show a lot of optical aberration at a much lower power.
But eyes are a whole different matter. Even wearing my glasses, which correct for astigmatism, the view of a bright point source like Venus has all manner of interesting flares and points coming off it -- especially when I'm dark adapted. Stopping the pupil down helps a lot in suppressing the human eye's aberrations.
But some people (S.J. O'Meara comes to mind) seem to be able to form picture-perfect images even at a 7-mm pupil. These are the people who can see down to mag 8 naked-eye under dark skies -- a feat that's far beyond me.
--------------------
Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
|
ronharper
Carpal Tunnel
Reged: 02/14/06
Posts: 2065
|
|
I'd rather have perfect eyes and an imperfect instrument than the other way around. Then I could just save up my money, buy perfection, and see perfection. But, at larger exit pupils, my eyes don't deserve perfection, and couldn't recognize it. My bino might be perfect, duh, who knows?
Ron
|
Jon Isaacs
Postmaster
Reged: 06/16/04
Posts: 32442
Loc: San Diego, California
|
|
Quote:
Jon, I'd be interested to hear the results of stopping down either your fine refractor or your bino. It's bound to be fun, and we stand to learn something from it.
Ron
Ron:
It was clear for a while this evening so I made as set of aperture masks for my WO 80mm MegrezII FD and spent some time looking at Gamma Aries... To duplicate your work, I choose a 32mm Plossl and also used a 15mm TV Widefield and a 9mm Generic Synta Widefield. I made 3 masks, 25mm, 35mm and 50mm. With the 32mm Plossl this provided exit pupils of 4.6mm, 2.9mm, 2.0mm and 1.4mm.
At 17x, I think my observations were quite similar to yours except I think the best split was with the 3mm exit pupil and that it was definitely split with the 4.6mm exit pupil.
At the greater magnifications, all combinations provided clean splits though the 1.5mm and 2.0mm were obviously fainter and reminded me somewhat of the double double at 62x unmasked. This split at 62x (9mm) at full aperture was a beauty with the brilliant points one can get.
Here's a few thoughts/conclusions:
Optically this test platform is very good, the 80mm F/7 FD with a TV Everbright diagonal is capable of reaching the Dawes limit splitting doubles. What this means is that the Airy discs at the optical plane are the appropriate size for an 80mm aperture. This means that any decrease in the size of the star at the eyepiece is not caused by the telescope's optics.
So, as you noticed, when the aperture was reduced/exit pupil, the apparent size of the star was also reduced. This means that something is causing the image to be larger than it ought to be, either the eyepiece, which I doubt, or most likely IMHO, the eye/brain combination.
I think we all know that a brighter star also looks bigger, Rigel next to it's companion is quite large. This really is happening and it affects the observers ability to split tight doubles at large exit pupils. I think this raises the question as to whether the cause is the overall brightness or aberrations associated with the exit pupil size.
My guess is that it is related to brightness, maybe similar to "blooming" in a CCD chip where a full well will overflow to surrounding wells. Support for this comes from my observation of how perfect those tiny stars in M11 are in a big scope at large exit pupils. At 46x (6.8mm exit pupil) in my 12.5 inch those tiny stars are beautiful bright diamond pinpoints. Were they brighter, they would probably be bigger...
I think there is something to be learned here. Idea that the smaller exit pupil provides better resolution is often interpreted to mean at a constant aperture, so the smaller exit pupil corresponds to a greater magnification. But your experiment suggests that the even at a constant magnification, a smaller exit pupil can provide better resolution... My contribution here is just to provide some proof that it is not the optical quality of the telescope that gives this result.
I did want to try a neutral filter to see if that provided similar results but the high thin clouds were building and my filter was too aggressive. One thing I did notice, which I have noticed on other occasions, is that at full aperture, through the clouds, Gamma Aries did look cleaner.
Jon
--------------------
|
ronharper
Carpal Tunnel
Reged: 02/14/06
Posts: 2065
|
|
Jon,
Now even a moderator is talking about single-barreled scopes, in this case a very nice one. This place is going to the dogs. Ban yourself from the forum immediately!
Good work Jon. That you could split Gamma Aries at 17x at larger exit pupils than me means either your scope is better than my bino, and/or you eye is better than my eyes, but the general flavor of your results is the same. One of my favorite tricks is to use a filter on the double-double and split it at 50x. I first noticed this effect through clouds. Why I was looking through clouds, well I must be crazy!
So, why do bright stars look big? If we knew, we might fight this demon. One obvious possibility is scatter/poor focusing by an irregular or cloudy eye lens or cloudy intereyeball fluid, which would of course happen with dim stars too, but the scattered light would be too dim to notice. You also suggest a new one to me, an effect at the retina, where one cone cell might spill light over to another. Experiments with cadaverous eyeballs are all that come to mind to try to settle that issue, one thing I don't have in my junk drawer I'm afraid, so I'll call the distinction moot, and lump it all as "the eye's fault".
You have used a very nice telescope, and gotten better results than me with my bino, but is it yet possible that the telescope optics is a contributor to the bloating you see? The eye can't tell. A CCD, or film, would have a hard time because of the blooming effect. Working out brightness at given distances from the center of the diffraction pattern would be a mathematical pain, and you'd still have to ask what is the threshold of visibility.
But, I have an idea. Look at brightly backlit pinholes, and try to compare their apparent size to that of similarly bright stars seen through an instrument. The pinhole is so simple, I claim it is an essentially perfect point. So, what you see is your eye's point spread function. If what you see is fully as large as through your instrument, the spread is all in your eye. If it is smaller, some is from the instrument. I don't claim it would be easy to quantify. I say, does this make a shred of sense?
Ron
|
ronharper
Carpal Tunnel
Reged: 02/14/06
Posts: 2065
|
|
On second thought, my pinhole idea seems kind of loopy, why not just use the stars instead of pinhole sources? They usually aren't bright enough. My 70mm bino shows obvious bloating to my eyes for stars of 3rd magnitude or brighter. It provides a roughly 5 magnitude brightness boost, so a star of -2 magnitude would be needed. But Venus at times would do the trick, wouldn't it? How bright does it get? It would sure beat trying to rig pinholes of known brightness!
Ron
|
Jon Isaacs
Postmaster
Reged: 06/16/04
Posts: 32442
Loc: San Diego, California
|
|
Quote:
You have used a very nice telescope, and gotten better results than me with my bino, but is it yet possible that the telescope optics is a contributor to the bloating you see?
Ron: This is the way I see it: The telescope produces an image at the focal plane. Whether I am looking at that image with a 32mm eyepiece or a 4.8 mm eyepiece, that image is still the same with it's Airy Disk and Diffraction rings.
To me, this means that the increase in size of the star as the exit pupil is increased has to be a result of either the eyepiece or the eye/brain. The fact that this increase in size happens with all eyepieces and other aberrations of the eyepiece are clearly seeing in eyepieces leads me to believe this increase in size is a function of the eye/brain.
Jon
--------------------
|
ronharper
Carpal Tunnel
Reged: 02/14/06
Posts: 2065
|
|
Jon,
Yes, I believe I get it and you are correct. Your scope is known to be of high quality, from its high-power diffraction pattern. Yet at low power, stars start to look somewhat messy. Therefore, the messiness is from the eye.
At the same 4.4mm exit pupil, my 5" D&G makes a tighter image than my 70mm Fujinon bino, and that's with over 3x the light to focus! This proves to me the bino is imperfect, not surprising for an f/4 double achromat, with all kinds of nice features, costing a "mere" $600. But lots of the fault is in my eyes. I think most bino users must accept this state of affairs, and use them for what they're best at. After a lifetime of using telescopes, I am slowly getting there. It requires a quality I'm kind of short on usually-"grace".
Thanks to all who contributed to this discussion, especially Jon, who actually went out and did something, and made sense out of it!
Ron
|
Jim Rosenstock
Postmaster
Reged: 07/14/05
Posts: 6543
Loc: MD, south of the DC Nebula
|
|
fyi, I often stop down my inexpensive Chinese 25X100s to 50mm.....not for double stars, but for the Moon and other bright objects. CA and edge sharpness issues are both much improved.
At full aperture and ~f/4, the Moon is painfully bright and glaringly fringed with bright yellow CA, and noticeably "soft" 1/3 of the way from the edge of FOV; stopped down to 50mm ~f/8, the image is less glaring, less "colorful", and MUCH sharper, nearly to the edge. Jupiter and its moons are more pleasingly resolved; large, bright open clusters (e.g. the Pleiades) are also cleaner looking when stopped down.
Of course, these big binos are FAR from Fujinons, so the stopping-down has a much more noticeable effect, I think, than on binos that have sharper optics to begin with.
Clear skies,
Jim
--------------------
QUESTION AUTHORITY!
"errr....sez who??"
|
DJB
Carpal Tunnel
Reged: 02/23/05
Posts: 1742
Loc: Lisle NY
|
|
Hi Ron,
For your information Venus can get a bit brighter than magnitude -4 in some years.
This is why some folks prefer to observe Venus in the day. I have done so infrequently.
Best regards,
Dave.
|
Mr. Bill
Postmaster
Reged: 02/09/05
Posts: 5219
Loc: Northeastern Cal
|
|
Quote:
fyi, I often stop down my inexpensive Chinese 25X100s to 50mm.....not for double stars, but for the Moon and other bright objects. CA and edge sharpness issues are both much improved.
At full aperture and ~f/4, the Moon is painfully bright and glaringly fringed with bright yellow CA, and noticeably "soft" 1/3 of the way from the edge of FOV; stopped down to 50mm ~f/8, the image is less glaring, less "colorful", and MUCH sharper, nearly to the edge. Jupiter and its moons are more pleasingly resolved; large, bright open clusters (e.g. the Pleiades) are also cleaner looking when stopped down.
Of course, these big binos are FAR from Fujinons, so the stopping-down has a much more noticeable effect, I think, than on binos that have sharper optics to begin with.
Clear skies,
Jim
HUGE difference in theoretical performance between f/4 and f/8 achromats in terms of CA and edge of field sharpness....that's why I like the BT100s...f/6 is a nice compromise.
--------------------
The night sky is the palette....
My optics are the brushes....
The Milky Way is the masterpiece
Member IDA
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
