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#26 revans

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Posted 13 October 2019 - 05:48 AM

But, remember that a telescope can't make an object appear any brighter than as seen with the naked eye. Telescopes only magnify, and given that magnification the surface brightness of an object will either appear to be the same as seen with the naked eye (nearly, ignoring any transmission loss in the optics) or dimmer (but never "brighter," regardless of aperture).

 

Furthermore, the perceived brightness of an object is limited by the size of your eye's dark-adapted pupil and all a telescope can do is fill that pupil by some amount as determined by the size of the exit pupil from the optic system. So, while aperture does play a role in determining the size of the exit pupil (as a function of magnification) that doesn't allow a generalization to the effect that a larger aperture will produce a "brighter" image (under all situations). It really comes down to the size of the exit pupil (determined by aperture and magnification), your eye's dark adaption, and the apparent field of view of the eyepiece (the latter a consideration if the object/moon does or does not fill the entire field of view).

 

Having said that, it's generally true that given any particular magnification and eyepiece a larger aperture will produce a "brighter" looking image. But, if you change magnification or apparent field of view with a change of eyepiece or have an exit pupil that exceeds the size of your eye'e pupil then the determination becomes more complicated. In fact, once the size of the exit pupil from the eyepiece equals the dark-adapted size of your eye's own pupil the surface brightness of an object won't change with increased aperture (since the light from that increased aperture won't be able to enter your eye).

 

Also, for completeness I should add that all of the above applies specifically to the viewing of extended objects (like the moon), since point-source stars do appear brightened with increased aperture.

I have a question about this line of thought.  Surely a telescope concentrates light.  Obviously it cannot change the inherent brightness associated with any object, but it does concentrate the brightness of that object onto our retina.  That is why, although I am aware of the sun's extreme brightness and would not ever look at it directly with my unaided eye, I certainly would absolutely never look at it unfiltered in a telescope because it could literally burn a hole through my retina.  So, something tells me that I have just not understood you properly as you seem to me to be a very experienced and knowledgeable person even though I just don't think I've grasped your real teaching here.  I have always viewed a telescope as a concentrator of light. Where have I gone wrong with this thinking... I never was very good at physics and have to admit that my common sense is frequently misguided.  One last anecdote.... I once looked at the moon in a 25 inch F5 Obsession telescope and it was so bright compared to looking at the moon with my naked eye alone that I literally saw stars and was dazed... but I assume you might say that had something to do with my exit pupil diameter or something like that given your line of thought.  Anyway.... I'm open to more knowledge :)  Rick


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#27 james7ca

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Posted 13 October 2019 - 07:03 AM

In both cases you cite (the sun and the moon) the reason it would seem to be so much brighter than to your naked eye is that the telescope is magnifying the image and thus even though the surface brightness hasn't been increased the total area that has that brightness has increased (i.e. the object has become larger). It's somewhat the same as if you had a single light bulb in a dark room. If you add ten more bulbs that grouping of ten lights will appear brighter and the room will become brighter, but the surface brightness on each bulb hasn't changed.

 

So, it's the magnification that makes viewing the sun even more dangerous and in the case of the moon (at night) you have another factor which is that your surroundings are dark enough that the pupil of your eye has probably opened to adjust to the darkness but the telescope will magnify the image so even given the same surface brightness more total energy will be entering your eye. This is also why the field of view can play a role, since with a wider field more of the moon will be visible (assuming that the magnification is high enough that the moon will fill the entire field).

 

Another thing, if viewing through a telescope really made things brighter then it would probably be hard to use a telescope during the day. Just imaging all of that aperture making the image of a bird that is 50 yards away so bright that you couldn't even look at it! But, it's not a problem and in fact using a scope at high magnifications during the day can be hard because the image may become too dim to easily see in comparison to what your naked eye gathers from all of your surroundings (which are under daylight, and the pupil in your eye has probably contracted making the image in the telescope seem even darker -- kind of the opposite situation as with the moon when viewed at night).

 

But, once again, this is for extended objects. For stars, which are point sources, the brightness of the star does increase with telescope aperture (since all of that aperture is concentrated into a single "point"-- the Airy disk, which has a finite size but which can be approximated as a "point").


Edited by james7ca, 13 October 2019 - 07:11 AM.

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#28 revans

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Posted 13 October 2019 - 07:16 AM

In both cases you cite (the sun and the moon) the reason it would seem to be so much brighter than to your naked eye is that the telescope is magnifying the image and thus even though the surface brightness hasn't been increased the total area that has that brightness has increased (i.e. the object has become larger). It's somewhat the same as if you had a single light bulb in a dark room. If you add ten more bulbs that grouping of ten lights will appear brighter and the room will become brighter, but the surface brightness on each bulb hasn't changed.

 

So, it's the magnification that makes viewing the sun even more dangerous and in the case of the moon (at night) you have another factor which is that your surroundings are dark enough that the pupil of your eye has probably opened to adjust to the darkness but the telescope will magnify the image so even given the same surface brightness more total energy will be entering your eye. This is also why the field of view can play a role, since with a wider field more of the moon will be visible (assuming that the magnification is high enough that the moon will fill the entire field).

 

Another thing, if viewing through a telescope really made things brighter then it would probably be hard to use a telescope during the day. Just imaging all of that aperture making the image of a bird that is 50 yards away so bright that you couldn't even look at it! But, it's not a problem and in fact using a scope at high magnifications during the day can be hard because the image may become too dim to easily see in comparison to what your naked eye gathers from all of your surroundings (which are under daylight).

 

But, once again, this is for extended objects. For stars, which are point sources, the brightness of the star does increase with telescope aperture (since all of that aperture is concentrated into a single "point"-- the Airy disk, which has a finite size but which can be approximated as a "point").

Thanks for taking the time to explain this to me.... I think that now I understand what you have been saying.  It is a concept that I hadn't considered before and it seems to make sense to me now :)  I had always viewed brightness as simply the concentration of light on the retina... the more photons hitting the retina per unit area per unit time then the brighter the image (in my mind).  I never considered magnification per se, only the concentration of photons but your way of describing things is much more subtle and the concentration of photons is the result of magnification (in extended objects).  Or did I get it wrong again :)))  Rick.



#29 james7ca

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Posted 13 October 2019 - 07:26 AM

... the more photons hitting the retina per unit area per unit time then the brighter the image (in my mind)...

Well, that's obviously true, but that's not what is happening with a telescope. The photons per unit area when viewed through a telescope appear no "brighter" than it would be without the telescope. But, since the object appears larger even the same number of photons per unit area would be perceived as "brighter." It's a difference in total area, not in photons per unit area (which will always be equal to or lower than if viewed with the naked eye).


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#30 revans

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Posted 13 October 2019 - 08:06 AM

Well its not a trivial thing to really understand all this.  I was brought up being taught that large telescopes were light buckets that harvested more light.  The more aperture the more light.  But then there is the concept of stars vs extended objects vs terrestrial viewing.  I was also taught that the larger the aperture the finer the visible detail possible.  And as for magnification, I was taught that the brightness of an object decreases by the square of the distance of separation from it.  So the further I walk away from a light bulb the fainter it seems until I finally can't see it at all at some substantial distance.  Conversely, the close I walk toward it the bright it appears to me, and by walking toward it I'm increasing the magnification.  But when I look at the moon with really high magnification everything gets dim and I suppose this is mostly because I'm looking at a smaller and smaller area of the moon and therefore a smaller and smaller portion of its overall photonic reflectance.  

 

Not trivial stuff at all.  By the way.... do you have a background in optics or physics... so know this stuff backwards and forwards.  I suspect that like quantum physics I'll always be in a state of some uncertainty about even this Newtonian concept we are talking about.  I wonder how many other folks really grasp it as you do.

 

Rick



#31 james7ca

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Posted 13 October 2019 - 01:05 PM

A larger aperture can be useful for visual work because of the increased resolution and the fact that a large aperture can deliver a larger exit pupil size at the same or even higher magnifications than can be had with a smaller scope (thus, the "light bucket" concept). However, once the exit pupil has grown to the same size as the pupil in your eye the surface of the object won't be perceived as being any brighter (regardless of aperture). However, a larger aperture means you can use more magnification to deliver a given "brightness." A faint and small extended object is easier seen at a larger size, that's just one of the ways our visual system works.

 

As for distance from an extended object, things generally don't get "dimmer" with increased distance (unless the medium that is between you and the object is blocking some of the light -- dust, moisture, etc.). Take a white piece of paper and view it from just a few inches, then move the paper to the other side of the street. Does it appear to be less bright? If the surface brightness varied by the square of the distance then when the paper was close to your eye it would appear radically bright (example, at six inches we'd have 6"^2 = 36, but at 16 feet we'd have (16 x 12")^2 = 36864 and a ratio of 36864 / 36 = 1024X. Does the paper appear 1024 times as "bright" when viewed at six inches? No, of course not. The key here is that while the apparent size/area of the object varies by the square of the distance the perceived surface brightness remains the same. Of course, if you move the light SOURCE to a greater distance then the amount of light falling on the object will vary by the inverse square law (for the distance between the object and the light source, not the distance between the object and the viewer).

 

With a camera it's the f-ratio that determines the irradiance at the sensor plane (the "brightness"), aperture alone doesn't matter (except for point sources and for when a change in aperture results in a change in f-ratio). Then, for imaging you have to consider image scale and how the pixel size of the camera affects both that image scale and the number of photons collected per unit time per pixel (since a large pixel can collect more photons than a small pixel).

 

When imaging it can be useful to remember that for any given image scale a larger aperture will generally produce a "faster" system (either from a change in the f-ratio or by a change in the pixel size of the sensor or a combination of both). This is why large professional telescopes can image very faint objects (the cameras have very large pixels for the given image scale, but the imaging optics can also be very "fast," f/3 or /4 because they can be that and still deliver good image scale because of their long focal length).


Edited by james7ca, 13 October 2019 - 01:08 PM.


#32 j.gardavsky

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Posted 13 October 2019 - 04:09 PM

Re: Moon and filters

 

I must admit that at the exit pupil of 1mm and smaller, the Moon is on my 6" achro stil blinding my observing eye.

O.K., I have old Omegon polarizing filters allowing to adjust the optimum brightness, but since some time I am using the green narrow pass-band Solar Continuum filter on a good reason: it increases the contrast of the finest details on the Moon. It also kills the CA of the achromatic refractor.

 

Best,

JG



#33 Magnetic Field

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Posted 15 October 2019 - 01:12 PM

Re: Moon and filters

 

I must admit that at the exit pupil of 1mm and smaller, the Moon is on my 6" achro stil blinding my observing eye.

O.K., I have old Omegon polarizing filters allowing to adjust the optimum brightness, but since some time I am using the green narrow pass-band Solar Continuum filter on a good reason: it increases the contrast of the finest details on the Moon. It also kills the CA of the achromatic refractor.

 

Best,

JG

Even in small as my Vixen VMC 110L the moon is still quite bright at high magnifications (> 150x) and I often use a moon filter.

 

I cannot imagine observing the moon in a 8" telescope without the help of a moon filter. Cranking up the magnification and dimming the view only works to a point if one doesn't live next to the ESO site in the Atacamo desert.

 

My I ask you why you don't observe say with a 6" Newtonian or 8" SCT  or 5" Maksutov etc. What is the point of  trying to remedy  all these problems related to false colour (which must be terrible in a 6" achromat)?



#34 j.gardavsky

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Posted 16 October 2019 - 08:51 AM

...

 

My I ask you why you don't observe say with a 6" Newtonian or 8" SCT  or 5" Maksutov etc. What is the point of  trying to remedy  all these problems related to false colour (which must be terrible in a 6" achromat)?

Hello Magnetic Field and thank you for asking

 

My answer is easy: Because I don't have any Newt, SCT, or Mac.

 

Of my principal interest are the large faint fuzzies, and here is a fast achro good and comfortable enough.

I take the Moon and planets just for fun, and found my way to enjoy the views.

 

Best,

JG


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