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ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

7x42 (1 Viewer)

The EDG II and the Zen-Ray ED3. The latter is surprisingly good optically, in some respects the image is equal to and even better than the EDG, then again the EDG is so much better built, has an easier, more relaxed view with full edge sharpness (you can't even see the Zen-Ray's fuzzy fieldstop, and if you do, blackouts kill the view).
It's fine for a loaner or spare to store at the workplace.

//L
 
I have come into this topic late and I am not going to wade through all the posts.

1) Although the full moon is about the same surface brightness as the Earth and camera exposures are similar, a half moon is only 8% as bright as the full moon because the albedo is much less.
Even one day before and after full moon the moon is considerably less bright than full moon.
A thin crescent moon is very dim.
The full moon at the zenith and at perigee is much brighter than an average full moon, and rarely happens.
The average full moon is maybe 3 or 4 times less bright.

2) Magnification definitely shows increasingly fainter stars up to perhaps 30x per inch of aperture and maybe more at a high elevation selected site such as Hawaii or Chile.
This effect applies both in good skies or in light polluted skies.

A 20x50 binocular shows much fainter stars than a 10x50 binocular whatever opinions mentioned above.
For the same observer whose eyes are dark adapted.
That is for similar quality unvignetted binoculars.

It might be useful if experienced observational visual astronomers were consulted here.
Not photographers.

It is thought that the reason is the increasingly higher magnification steadily darkens the sky.

This effect applies both to direct vision and averted vision in healthy eyes.
 
I have come into this topic late and I am not going to wade through all the posts.

1) Although the full moon is about the same surface brightness as the Earth and camera exposures are similar, a half moon is only 8% as bright as the full moon because the albedo is much less.
Even one day before and after full moon the moon is considerably less bright than full moon.
A thin crescent moon is very dim.
The full moon at the zenith and at perigee is much brighter than an average full moon, and rarely happens.
The average full moon is maybe 3 or 4 times less bright.

When experience speaks, it's best to keep quiet and listen! :t:
- though it feels counterintuitive to me. If I had to take a guess, I'd say that a spot metering close to the edge of the half Moon that is turned toward the Sun, that the result should be similar as straight on the full Moon.
I can't seem to understand why the difference would be so pronounced.
That said, I don't question experience.


2) Magnification definitely shows increasingly fainter stars up to perhaps 30x per inch of aperture and maybe more at a high elevation selected site such as Hawaii or Chile.
This effect applies both in good skies or in light polluted skies.

A 20x50 binocular shows much fainter stars than a 10x50 binocular whatever opinions mentioned above.
For the same observer whose eyes are dark adapted.
That is for similar quality unvignetted binoculars.

It is thought that the reason is the increasingly higher magnification steadily darkens the sky.

Does not sound too far-fetched, in particular when supported by empiric findings. In general, I'm less technically interested and more interested in what happens in the real user position. But photography would yield a more consistent, reliable and quantifiable support to this than the human eye could.

I have this nagging feeling that there's a difference between looking at reflected light with the photopic vision and the night sky with the mesoptic to scotopic vision. I believe we finally established that one "too big aperture"/"unusable large exit pupil" doesn't mean an actual brightness reduction within the beam pencil.

I also believe that there must be formulas and theories explaining why more magnification reveals more stars, the darkening of the sky being one of them.
Thank you!

//L
 
- though it feels counterintuitive to me. If I had to take a guess, I'd say that a spot metering close to the edge of the half Moon that is turned toward the Sun, that the result should be similar as straight on the full Moon.
I can't seem to understand why the difference would be so pronounced.
That said, I don't question experience.

//L

Hello Lars,

At full moon there are no or few shadows on the moon. It is rather like an earthly area at high noon, in the summer.

Does that make it any clearer?


I found that my 7x42 Zeiss classic to be a good all around binocular, rather better than a Leica 7x42 BA. Because the Zeiss used Abbe-König prisms, there was eve a little stereopsis as the objectives were slightly farther apart than the oculars. The Zeiss also had better edge sharpness.

Happy bird watching,
Arthur Pinewood :hi:
 
Hi Arthur,

Past tense suggests that you no longer have the Zeiss Classic?

Ed

Hello Ed,

As I have not used it for some time, I wrote in the past tense. I should point out its shortcomings: it is large but easy to use, it does not focus very closely, and it is not waterproof.

Happy bird watching,
Arthur :hi:
 
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Following up on Binastros observations, I have a few comments. Most anyone with a zoom eyepiece in a telescope or a spotting scope can experience the sky background getting darker as you zoom to higher power either during the day or at night. As Lars said, astronomy is bound by the same optical rules. However the rules have slightly different implications for extended objects and point sources like stars. Looking at the rules:
  • The first rule is that the amount of light collected is proportional to the area of the clear aperture, so all things being equal (they never are but it is a reasonable stipulation to try to understand was is going on) an optic with objective lens diameter twice as large will collect four times as much light
  • The second rule is that the area of an extended object at the exit pupil increases like the square of the magnification. So for a given clear aperture, doubling the magnification will increase the area of the extended object four times.
  • The third rule is that the exit pupil is the clear aperture divided by the magnification. So if we double the clear aperture diameter and double the magnification we get the same exit pupil.

The first and surprising implication is that the maximum surface brightness you can get is when the exit pupil matches the entrance pupil of your eye. If the exit pupil is smaller than your entrance pupil, you could increase surface brightness by reducing magnification thus reducing the area over which the light is spread. If the exit pupil is larger than your entrance pupil, you are reducing the surface brightness because some of the light through the clear aperture never gets into your entrance pupil. Thus there is a minimum magnification for maximum surface brightness at any clear aperture

This is true regardless of the size of the instrument. Doubling the the clear aperture requires you to double the minimum magnification. We get four times the light spread over four times the area for an extended object. So while surface brightness of an extended object cannot be increased, magnification can help.

The sky background itself is an extended object. It has a certain surface brightness depending on how dark or bright your sky happens to be (usually measured in magnitudes per arc second^2). if you increase magnification with your zoom eyepiece at the same clear aperture, you decrease exit pupil, and the sky's surface brightness decreases.

Stars appear as point sources, at least until you magnify to the point that the diffraction effects dominate (something like 50x-60x per inch though depending on visual acuity people can notice the Airy disc around 30x-35x). So stars in binoculars at typical binocular magnifications will have a brightness determined only by clear aperture. It's not that the faint stars brighten with magnification, but they become detectable because sky background becomes dimmer and the contrast between a faint star and the sky background is better.

The figure of merit for binoculars that best matches my preference is a metric introduced by Alan Adler. It is the square root of objective size times the magnification. The Adler metric says that a 10x30 should be about the same as a 7x50 and I find that to be true comparing 10x30 IS to tripod mounted 7x50.

Gary Seronik talks about astronomy metrics for astronomy here. Roger Clark wrote an excellent book titled "The Visual Astronomy of the Deep Sky," where he examines the optimal aperture and magnification for different extended objects based on studies of human vision and the size, shape, and surface brightness of the object. He has a web site here.

Good Observing,
Alan
 
The first and surprising implication is that the maximum surface brightness you can get is when the exit pupil matches the entrance pupil of your eye. If the exit pupil is smaller than your entrance pupil, you could increase surface brightness by reducing magnification thus reducing the area over which the light is spread. If the exit pupil is larger than your entrance pupil, you are reducing the surface brightness because some of the light through the clear aperture never gets into your entrance pupil. Thus there is a minimum magnification for maximum surface brightness at any clear aperture

This is true regardless of the size of the instrument. Doubling the the clear aperture requires you to double the minimum magnification. We get four times the light spread over four times the area for an extended object. So while surface brightness of an extended object cannot be increased, magnification can help.

(---)
Good Observing,
Alan

Thanks for your thoughts, Alan! I'm sure we've barely begun dissecting the optics and vision physiology of nocturnal viewing.
If you read the part of the thread that deals with aperture and maximum brightness, you will understand why I question the notion that one too big an aperture will reduce the apparent surface brightness of an extended object.

I urge you to dig deeper into this and consider all the possible implications it would have. In my opinion, it's a factoid. This is why:
The actual amount of light from the imaginary area carried through the optical system will depend on the magnification, because the beams from the larger surface area will occupy a larger portion of the beam pencil exiting the telescope or binocular.
However, its surface brightness will remain the same as when it was observed with a configuration with an "unnecessarily large" aperture, yielding a beam pencil where some light falls outside the observer's pupil.
Yes, light is "wasted", but this does not mean the actual brightness getting reduced. It's not until the exit pupil is smaller than the observer's pupil brightness becomes reduced.
An analogy would be that a smaller AFOV with everything else being the same does not mean that the image brightness decreases, although light is "lost" when reaching outside the fieldstop.

//L
 
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Lars and Alan and Typo

This is a fascinating discussion which has me struggling to understand some aspects of it:

Alan, you posted: If the exit pupil is smaller than your entrance pupil, you could increase surface brightness by reducing magnification thus reducing the area over which the light is spread.

This sounds unexpected because as the magnification is reduced the EP gets larger and would seem to spread the light over a larger area.

Alan, you also posted: If the exit pupil is larger than your entrance pupil, you are reducing the surface brightness because some of the light through the clear aperture never gets into your entrance pupil.
And Lars you added to this with: Yes, light is "wasted", but this does not mean the actual brightness getting reduced.

Again this sounds unexpected because we are told to expect a brighter image from a larger EP (provided our pupils dilate enough to use it). If we all agree that a larger EP gives a brighter image than a smaller EP it would seem entirely logical to conclude (as Pileatus did) that if your pupil was not large enough to utilise the larger EP that the image will be not as bright as it would be if you could utilise the larger EP.

Two things occur to me here. Firstly that the amount light being transmitted through the bino might be different from the brightness as perceived by the viewer and secondly that the significant factor might be the area of the retina being stimulated by the EP rather than the density of photons in the EP.

Lee
 
If we all agree that a larger EP gives a brighter image than a smaller EP it would seem entirely logical to conclude that if your pupil was not large enough to utilise the larger EP that the image will be not as bright as it would be if you could utilise the larger EP.

Lee

It must be remembered that while the human pupil sizes fluctuates incessantly, its maximum size for the individual observer has to be included among the basic postulates. The bold "if you could" is the basic fault here, since it overthrows the basic postulate of the individual's maximum pupil size. It is indeed correct, but not applicable with the precondition that we're talking about the very same user.

You're not quoting somebody correctly here, I think (I removed the poster's reference ID from the quotes in my posts after he deleted his replies). There were claims about a too big clear aperture causing actual brightness reduction, and I believe we sorted out this misconception, along with the notion that a larger clear aperture will mean more brightness if/although the exit pupil size is constant.

//L
 
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Hi Arthur,
Post 64.
High Noon. A good film indeed :)
And yes, there are almost no shadows at high noon on the Moon.

The albedo of the full moon is 0.12, the albedo of the half moon is 0.02.

Lars,
Post 63.
Surprisingly there is little accepted theory or formulae.
The BAA, of which I have been a member for 60 years tried to scrap the formula they gave for limiting magnitude of a telescope based on aperture.
I told them not to scrap it as all telescope makers used this formula even though it was wrong, because it did not account for magnification.
So they modified it with more or less my suggestions.
However, in the latest Handbooks even this has disappeared.
In the 2009 Handbook, and I hope it is O.K. to mention this, it says, to paraphrase.

Visual limiting magnitude, 100% efficient optical system, = 2+5log(base10)D.
D mm.
In practice, it is likely the constant 2 could be replaced by a value between 3 and 4, particularly when higher magnifications are used.
 
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Lars and Alan and Typo

This is a fascinating discussion which has me struggling to understand some aspects of it:

Alan, you posted: If the exit pupil is smaller than your entrance pupil, you could increase surface brightness by reducing magnification thus reducing the area over which the light is spread.

This sounds unexpected because as the magnification is reduced the EP gets larger and would seem to spread the light over a larger area.

Lee

Lee,

As the magnification is reduced the exit pupil is larger but the image scale, i.e., the apparent size of any extended object is reduced so the light from that object is spread over a smaller area as presented to your eye.

Physiologically increasing the illuminated area on the retina has several effects. First it is easier to detect differences in contrast depending on size and shape of image. And also the dilation of the eye depends on the total illumination so a larger area at the same surface brightness might influence the size of entrance pupil.

Alan
 
I just took this photo with a record-breaking absence of artistic value.
The binocular appears to increase the illumination, mostly at the exit pupil distance.
This could indeed seem contradictory, but I guess that is because the distance to the lamp in the ceiling is so short. The illumination will decrease with the square of the distance. The binocular appears to shorten the distance with the inverted number of the magnification, hence concentrate and amplify the illumination to the paper.

I can only guess what would happen if the binoculars were directed towards the Sun. Decreasing the apparent distance with the inverted factor of the magnification would cause a dramatic increase of the illumination through the exit pupil. This is a special case, compared to customary exit pupil brightness cases.

A clear light source radiates light in all available directions, with strongly divergent rays close to it (I postulate it's a quite small one)
Farther away, a great share of the diverging beams don't go through where you stand, and if you measure them, their divergence has decreased and the illumination has decreased as well. With sufficiently long distances, the beams reaching you are almost perfectly parallel.

In every practical sense, the beams emanating from the Sun are parallel (the Sun is close enough not to appear point-shaped, but since it's so vastly bigger than the Earth, the beams coming from the small area on the Sun whose beams actually reach the Earth's surface, can safely be considered parallel).

The illumination power of a light source depends on its intensity and the distance to the surface being illuminated.
But the light source's surface brightness, assuming it's not point-shaped,is the same regardless of distance, just like with reflecting bodies like the Moon.
The same goes for any object reflecting light off its surface.

Now, do such objects with a fairly low reflection rate have an illumination power that possibly could increase through binoculars at terrestrial distances, and in particular differ with magnification - like in my picture?
I would tend to say that the illuminating power is decided by the area alone. The photographic experiment confirmed there's no brightness difference.

Here's my (hopefully qualified) guess:

An overcast sky, as well as most reflective material found on Earth short of water, will scatter the light in all directions. When we look at such things, there's no clear vergence like from clear light sources. There's still another vergence related to the distance, coming from the object's form and structure, and parallax. That's what we pick up when we focus on it.

The scattered light with its absence of vergence cues will not make the object's apparent surface brightness change with distance or magnification.
Its only illuminating power comes from its apparent area. So long as it is safely comparable with the adjacent image element's brightness, very little will happen when it enters the field of view.

All in all, I have tried to concentrate on terrestrial viewing throughout this particular discussion. Let me assure you that I learn the most myself and am still learning.

I dare not exclude the possibility that other physical and physiological phenomena decide brightness and detectability of faint astronomical objects with and without optical tools.

Recovered from the flu, going back to work tomorrow and I believe I might hear a quiet sigh of relief from the forum members ;)

//L
 

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I've been busy and haven't had time to go through all the recent posts, but I think it's worth picking up on this point from #68.

"It's not that the faint stars brighten with magnification, but they become detectable because sky background becomes dimmer and the contrast between a faint star and the sky background is better."

Contrast is the ratio of signal to noise. In this case star to night sky. Increasing the magnification and reducing the exit pupil dims the whole view, both signal and noise so the ratio would be unchanged. That is, until the instrument, or the eye, becomes diffraction limited and the point source spreads when the signal to noise, or contrast would decrease. It doesn't seem to explain why increased magnification making stars more visible.

I need some sleep. I'll have a look at the rest tomorrow.

David
 
I've been busy and haven't had time to go through all the recent posts, but I think it's worth picking up on this point from #68.

"It's not that the faint stars brighten with magnification, but they become detectable because sky background becomes dimmer and the contrast between a faint star and the sky background is better."

Contrast is the ratio of signal to noise. In this case star to night sky. Increasing the magnification and reducing the exit pupil dims the whole view, both signal and noise so the ratio would be unchanged. That is, until the instrument, or the eye, becomes diffraction limited and the point source spreads when the signal to noise, or contrast would decrease. It doesn't seem to explain why increased magnification making stars more visible.

I need some sleep. I'll have a look at the rest tomorrow.

David

Hi, David:

You probably shouldn't talk too much about defraction because if the "A" type personalities knew just how bad even the "Alpha" binos were, they would probably just say to heck with it and take up ... needlepoint! :cat:

Bill
 
Bill,

I'm not going to defend the optical industry on that one. I think the ISO14133 standards used by the industry, including the big three, are woefully inadequate. Fortunately, most of the time for terrestrial use the instrument resolution value is pretty much irrelevant. By accident, if not design there are binoculars about that will deliver 116/D when it matters. They are worth looking out for.

I really don't need to remind you that terrestrial observation is quite different to astro viewing. Even the most exotic astro refractor on the planet would have a practical magnification limit of about 12x per inch for some users. My eyesight isn't that good. It's about 14x for me.

David
 
..............

Two things occur to me here. Firstly that the amount light being transmitted through the bino might be different from the brightness as perceived by the viewer and secondly that the significant factor might be the area of the retina being stimulated by the EP rather than the density of photons in the EP.

Lee

Lee,

I'll answer those two components separately.

"the amount light being transmitted through the bino might be different from the brightness as perceived by the viewer ".

I think there is a strong case to be made that perception is critical to this story. As I mentioed above, reducing the EP and dimming the view wouldn't increase real contrast, but perceived contrast is quite different. That's dependant on the biochemisty and physiology of the eye and the neuronal processing of the electrical signals from the receptors. I've no more than the sketchiest knowledge of what happens there, but I'm sure it's the key to answering some of these questions.

".... the significant factor might be the area of the retina being stimulated by the EP rather than the density of photons in the EP"

The area covered by the projected image of the star? Almost certainly. Not the angle of view passing through exit pupil and projected on the retina.

David
 
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Bill,

I'm not going to defend the optical industry on that one. I think the ISO14133 standards used by the industry, including the big three, are woefully inadequate. Fortunately, most of the time for terrestrial use the instrument resolution value is pretty much irrelevant. By accident, if not design there are binoculars about that will deliver 116/D when it matters. They are worth looking out for.

I really don't need to remind you that terrestrial observation is quite different to astro viewing. Even the most exotic astro refractor on the planet would have a practical magnification limit of about 12x per inch for some users. My eyesight isn't that good. It's about 14x for me.

David


Ah, but just think how often "... is pretty much irrelevant" could be used concerning so many things observers sweat about. :cat:

B
 
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