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Telescope brightness and faint objects

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#1 Ziguy

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Posted 14 July 2016 - 10:39 AM

I just lean that a telescope can't increase brightness of extensive objects and that at best, brightness will be the same as with our naked eyes. I still don't fully understand the physic behind this but I'm wondering why we would then use a telescope to observe large faint objects if it does't increase brightness?


Edited by Ziguy, 14 July 2016 - 10:47 AM.


#2 paspat

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Posted 14 July 2016 - 11:02 AM

perhaps it has to do with intrinsic brightness and apparent brightness. just a thought



#3 havasman

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Posted 14 July 2016 - 11:10 AM

image size



#4 Davester9

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Posted 14 July 2016 - 11:26 AM

I don't know where you heard that, but, the average human eye has approximately 0.35 square inches of area in which to collect available photons (light). An 8" telescope aperture has 50 square inches of area. That is roughly 1,650 times more light gathering capacity. That's the math. Amount of light gathered is directly proportional to apparent brightness. I submit that you may have confused some other aspect of the science with brightness. IMHO.



#5 HenryB

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Posted 14 July 2016 - 11:28 AM

A larger aperture may produce an image of the same brightness, but as the previous posted mention it will be a larger image than the eye produces. If you could blow up the eye's image to the size produced by the larger aperture you see the advantage larger aperture gives. 



#6 Asbytec

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Posted 14 July 2016 - 11:55 AM

I like to think of it this way.

Imagine looking at a faint galaxy in a 150mm telescope at moderate 75 power and a 2mm exit pupil.

Now drop in your lowest 1x eyepeice. It'll have a 150mm exit pupil. Pretend, for a minute, your pupil can expand to 150mm, too. Your eye is gathering all the light from the scope and the view is 1x.

Now take your eye from the telescope and look at the same part of the sky. You may be surprised the view is the same with your 1x 150mm eye as it is in the telescope. (Ignoring absorption in the lenses and mirrors.)

The galaxies are never brighter than they really are. You are gathering more light from them and forming an image. But not making them intrinsically brighter.

As said above, telescopes gather enough light to make faint objects LARGER. Large enough to actually see them.

A 10th magnitude galaxy will never register at 1x with a 7mm pupil. Unless that 7mm 《exit》 pupil is formed by a larger light gathering optic.

It's still no brighter than it really is at a given pupil, it's just magnified. It's either 1x at 7mm pupil with our naked eye or 21x at 7mm exit pupil in a telescope.

The 1x image on the naked eye is small and faint. The 21x magnified image contains more photons but is larger and just as faint. The surface brightness is unchanged in either due to the inverse square law as it relates to the apparent angular size.

Telescopes gather enough light to make objects larger, not brighter.

Edited by Asbytec, 14 July 2016 - 12:03 PM.


#7 Jon Isaacs

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Posted 14 July 2016 - 11:56 AM

I just lean that a telescope can't increase brightness of extensive objects and that at best, brightness will be the same as with our naked eyes. I still don't fully understand the physic behind this but I'm wondering why we would then use a telescope to observe large faint objects if it does't increase brightness?

 

Giguy:

 

The physics behind it explains why the surface brightness explains can never be greater than naked eye.   A telescope captures more light than eye but it also magnifies the image. The surface brightness of an the image (really the intensity at a given point) is proportional to the area of the exit pupil, that beam of light you look at, the beam that enters your eye. The larger the exit pupil, the brighter the image.  The greater the magnification, the smaller the exit pupil.   The size of the exit pupil is easy to figure, it's the aperture divided by the magnification.  A 200mm telescope at 40x provides a 200mm/40 = 5mm exit pupil.  

 

You can actually see the exit pupil during the day, get back from the eyepiece and look at eyepiece with the objective uncovered, you will see a bright circle, that's the exit pupil. Swap eyepieces and you see that the size changes depending on the magnification.  

 

It is true a telescope captures much more light than the eye but it also magnifies more.  A 70mm telescope operating at 14 x captures 100 times as much light as a 7mm dark adapted pupil but the image covers 14 x 14 = 196 times the area, 100 times light captured, spread over about 200 times the area, the image is 100/200 as bright = 0.5 times.  The exit pupil is 70mm/14 = mm.   That is about (5mm/7mm)2 = 0.5 the area, the surface brightness is 1/2.  

 

 At 10x, it captures 100 times as much light but the image covers 100 times the area so they are equally bright.  The exit pupil is 70mm/10 = 7mm, same pupil, same exit pupil, equally bright.  

So what happens if you chose an eyepiece that provides 7x? the telescope captures 100 times the light, it is spread over 49 times the area, 100x light/49x area = Twice as bright.. The exit pupil is 70mm/7 = 10mm.  

 

It is true that the exit pupil, the beam of light leaving the telescope is 10mm in diameter but your pupil is only 7mm so only a 7mm of the 10mm enters your eye, the surface brightness is no greater than it was naked eye.  

 

It is important to understand the concept of surface brightness, it is not the total brightness of the object, it is the intensity, photons per square millimeter.

 

As to what good is a telescope if it doesn't make extended objects brighter, it makes them bigger so that they eye can see them better.  Jupiter is about 30 arc-seconds in diameter, well below the ability of the eye to resolve any detail.  Magnify it 200 times and it will be large enough to see.

 

Jon 



#8 sg6

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Posted 14 July 2016 - 01:12 PM

Keep hearing this but then people say to get the biggest diameter you can to increase what you can see.

Seems odd as why buy as 14" dobsonian if a 6" can do the same.

So aperture is not king then. Just go get a nice 80mm apo. Apparently it shows as much.

Why Hubble and the VLT, all those up on Hawaii can be taken down as can Canary Islands and everything in the Atacama desert.

Seems odd they built those to see fainter objects and they didn't need to. :silly: :silly: :silly:



#9 Dave Mitsky

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Posted 14 July 2016 - 01:24 PM

Roger Clark's site discusses the effect of aperture and magnification on extended object detection.

 

Magnification can help you see detail in faint objects, like galaxies and nebulae, when viewed through your telescope. In my book Visual Astronomy of the Deep Sky, I show how the eye is more sensitive to fainter, lower contrast objects when they appear larger to your eye. As you increase the magnification of your telescope, an object increases in apparent size in the eyepiece in proportion to the magnification. But the surface brightness decreases because the light is spread out over a larger area. You can win up to a point.

 

http://www.clarkvisi...-mag/index.html

 

Telescope aperture has a large influence on the detail you can see in faint objects viewed through your telescope.

 

http://www.clarkvisi...pert/index.html

 

http://www.clarkvisi...mva1/index.html

 

A larger aperture allows one to view a DSO at higher magnification, making it easier for the eye to detect, and at the same time a larger exit pupil (i.e., a brighter image).

 

BTW, Jupiter varies in apparent size from a minimum of 29.80″ to a maximum of 50.59″ so a simple average apparent size would be 40.2".

 

Dave Mitsky



#10 Starman1

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Posted 14 July 2016 - 02:38 PM

I just lean that a telescope can't increase brightness of extensive objects and that at best, brightness will be the same as with our naked eyes. I still don't fully understand the physic behind this but I'm wondering why we would then use a telescope to observe large faint objects if it does't increase brightness?

Let's talk about exit pupil of a telescope, the size of the image formed behind the eyepiece.

The brightest it can be is when the exit pupil matches your pupil diameter.

 

For the sake of simplicity, let's assume your eye's pupil is 5mm, and your scope is f/5.

A 5mm exit pupil will be with a 25mm eyepiece.

In an 8" f/5 scope, the magnification will be 40.6x for the brightest image.

In a 16" f/5 scope, the magnification will be 81.3x for the brightest image.

But the object will be 4X as large by area as in the 8" scope.  Its light will cover 4X the area on your retina and be far more noticeable.

 

Let's talk about magnification. 

In the 16" scope, at the same power, the brightness will be 4X as great because the exit pupil is twice as large (as long as the exit pupil is not larger than your pupil).

For instance, at 203.2x in the 8", the exit pupil will be 1mm (5mm eyepiece).  But in the 16", at 203.2x, the exit pupil is 4x as large (twice as wide)(10mm eyepiece).

 

So the 16" is brighter at the same power, and the image is much larger at the same exit pupil.

Both yield greater visibility in the larger scope.

 

Let's talk stars, which are just points.  In that case, the 16" concentrates 4X the light gathering power of the 8" in each little point, making every star 4X brighter in the 16".

That's about 1.5 magnitudes brighter, so the 16" will see stars about 1.5 magnitudes fainter than an 8".

 

So, the bigger scope is a WIN-WIN-WIN proposition: it sees fainter stars, at equal magnifications all objects are brighter, and at equal exit pupils all objects are larger.

Larger = more visible (has to do with our animal self-preservation from our past)

brighter = more visible.

 

And the larger scope would have the advantage in all those cases.

 

But wait, there's more:

Diffraction of the star point yields a finite size to the star images.  This is a "spurious" size, since all are far enough away they should appear as points to the eye.

The bigger the scope, the less diffraction of the star image, i.e. the smaller the size of the spurious disc.

This means the big scope resolves closer spaced dots, which means it sees closer double stars, smaller details on planets and moon, and more details in deep sky objects because

of improved resolution.

So the bigger scope resolves better in addition to the other advantages previously discussed.

 

So why a larger aperture?  Those are the reasons.  And they are pretty profound ones.



#11 GlennLeDrew

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Posted 14 July 2016 - 02:42 PM

A telescope effectively brings you nearer to the subject. Let's restrict to exit pupils equaling your iris diameter, in which this relationship is truest due to the image surface brightness remaining pretty much unchanged. And furthermore, we'll stick to binoculars, where both eyes are in use, just as for naked eye gazing. (A mono scope reduces signal to noise to 71% that obtained via a *true* binoscope.)

 

Your iris under the night sky is, say, 7mm. With the unaided eye you see the Andromeda galaxy (M31), at 2.4 million l-y, as a not large elliptical fuzz with a brighter center.

 

With a 7X50 bino you have a view of M31 that cuts the distance to 1/7, or about 350,000 l-y. Disregarding for the moment sky glow, you are effectively transported out into space to within 350,000 l-y of M31, enjoying the scene presented out there to your unaided eyes.

 

To make the comparison more relevant, we could suppose that a faint interior glow of lighting in your spaceship is reflecting off the glass, at a level of intensity equalling our earthly sky glow. I mention this because even at the darkest sites on earth the sky glow is fully 1.5 magnitudes, or 4X brighter than seen from orbit. And it's darker still in interstellar space, and yet a bit darker still in intergalactic space.

 

Now move up to a 20X140 binocular. You are brought to within 120,000 l-y of M31. That's a distance about equal to the diameter of the galaxy's disk system. Indeed, at 20X the 2.5 degree long disk will subtend 50 degrees, or twice the length of the Big Dipper. Again, assuming your porthole is reflecting Earth sky levels of light, you will see with eyes alone out there what your big 20X140 binocular delivers here on good ol' Terra.

 

I should mention the issue of perspective, in the name of completeness. A magnified view which has a subject subtending tens of degrees is not quite the same as being nearer to the object in proportion to the magnification. If the object has depth in proportion to width (that is, it occupies *roughly* a spherical volume), when you are located to within a few radii the near and far extents have sufficient difference in distance to affect resolution and brightness of its structures.

 

If you were to be positioned to within a disk diameter of M31 (which is tilted only some 15 degrees from edge-of-field for us), the far edge of the disk would lie 2X more distant than the near edge. Far edge clusters and nebulae would have half the diameter and integrated brightness 1.5 magnitudes fainter than near edge ones.

 

But if we overllook these effects of perspective, our telescopes are effectively spaceships, bringing our unaided eyes nearer in proportion to magnification.

 

When we employ exit pupils smaller than our iris, both object and sky are dimmed equally. This is like using (neutral tinted) sunglasses to dim the scene as one gets ever nearer.

 

To wit. Starting from a 7mm exit pupil, we double the magnification and thereby halve the exit pupil to 3.5mm. This doubling of magnification effectively halves yet again our distance to the object. The inverse square law says the integrated brightness of the object quadruples when the distance is halved. But our halving of the exit pupil dims the image to 1/4 its previous brightness. This 4X increase in integrated brightness is exactly countered by the 4X dimming by the smaller exit pupil. And so the effect is just like cutting the distance to the object in half, but at the same time putting on a pair of sunglasses which reduces the light transmission to 25%. Which is why I started out by restricting to iris-equalling exit pupils, which are more relevant to the you-are-out-there comparison effected via magnification.



#12 GlennLeDrew

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Posted 14 July 2016 - 02:49 PM

Dan,

When you state that the exit pupil is "the size of the image", the uninitiated might wonder, "What image?"

 

It should always be clarified that this "image" is that of the objective itself, which nominally is the entrance pupil. In this way the beautiful symmetry of the entrance pupil (objective aperture), magnification and exit pupil is never lost sight of.



#13 Tony Flanders

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Posted 14 July 2016 - 04:38 PM

I just lean that a telescope can't increase brightness of extensive objects and that at best, brightness will be the same as with our naked eyes.


That statement is true in one narrow, not very useful sense of the word "brightness" - namely, what's technically called "surface brightness" and might more colloquially be described as "intensity." This is the object's total brightness divided by its apparent area. A telescope increases both the total brightness and the apparent area, leaving the intensity unchanged.

More commonly, "brightness" means either how bright something seems to you subjectively or what I just called "total brightness" above: the total amount of light from the object that's received by your eye. In both of those senses, telescopes do indeed increase objects' brightness.

Edited by Tony Flanders, 14 July 2016 - 04:39 PM.


#14 Jon Isaacs

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Posted 14 July 2016 - 05:34 PM

 

I just lean that a telescope can't increase brightness of extensive objects and that at best, brightness will be the same as with our naked eyes.


That statement is true in one narrow, not very useful sense of the word "brightness" - namely, what's technically called "surface brightness" and might more colloquially be described as "intensity." This is the object's total brightness divided by its apparent area. A telescope increases both the total brightness and the apparent area, leaving the intensity unchanged.

More commonly, "brightness" means either how bright something seems to you subjectively or what I just called "total brightness" above: the total amount of light from the object that's received by your eye. In both of those senses, telescopes do indeed increase objects' brightness.

 

 

Tony:

 

When you look at the sky, do you look at the surface brightness or the total brightness? Does the sky somehow become much darker if you look through a tube that restricts your vision to 30 degrees?  Looking at the night sky, a telescope cannot make it brighter than it is naked eye.  

 

Surface brightness is a very important concept and not in the least bit narrow. 

 

-  M33 with a visual magnitude of 5.7,  M57 with a visual magnitude of 8.8.  M33 has a total integrated brightness that is 17 times that of M57 and yet M57 is so much more easily seen under light polluted sky.  The reason M57 is so much easier under light polluted sky is explained by surface brightness.  

 

- The reason surface brightness is important is that it is what the individual rods and cones see, they do not see the total integrated brightness unless the object is a point source.  And importantly, contrast is a function of surface brightness, not total integrated brightness.  

 

Jon



#15 GlennLeDrew

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Posted 14 July 2016 - 06:05 PM

Tony,

Yet it is crucial to ensure the tyro is clearly aware of the importance of surface brightness. In the same way that my living room wall gets no brighter as I cross the room and put my face up to it, neither does a nebula get brighter as I employ ever larger apertures at given exit pupil. Things just get bigger.

 

Once an object is enlarged to cover a few tens of degrees in angular extent on the retina, just how meaningful is the increase in *integrated* brightness when switching to a larger scope? The object has already been well enough magnified for its global detection. And it makes no real difference in the discrimination of detail of scale a degree or a few degrees in frequency (that is, much smaller than that the object in its entirety.)

 

For smaller objects of size up to about 10 arcminutes, certainly with typical amateur equipment the overall visibility continues to improve up to pretty large aperture. But for objects about the size of the Moon, considerable retinal coverage occurs at pretty low magnification (and hence small aperture.) A 30 arcminute object subtends 30 degrees at only 60X. For a fairly bright 5mm exit pupil, the aperture to provide that is 300mm. If one jumps up to a 600mm aperture, at the same 5mm exit pupil the object now subtends 60 degrees. To me, in such case the impact of the visibility here does not accrue as the area ratio of the aperture increase, or a factor of four. The integrated brightness indeed is 4X greater, but visually the surface of the object over which discernible detail is sought is materially unchanged.

 

The best way to put this into perspective is to consider a somewhat limited and consistently applied field stop which has an AFoV of, say, 30 degrees (which the early telescopists hardly beat.) If the object already fills the FOV with a smaller aperture scope, further increases in aperture only overfill the field to a greater degree. That visible portion within the 30 degree limit, at the same exit pupil, has just the same unit and integrated brightness. If there was no discernible detail at all the two views would be utterly indistinguishable.

 

So. For tinier objects the visual impact does tend to accrue as, or nearly as the area ratio of the aperture increase. But for larger objects, once subtending a few tens of degrees on the retina the impact of further areal increases via increased aperture is rather smaller. At least from the standpoint of the scale of what is resolvable, which is certainly less than 10 degrees on the retina, even at near threshold levels of low brightness and contrast.

 

I stress this so as to make it very clear to those less experienced that integrated brightness and unit surface brightness are very different things (although intimately interdependent), and that the varied response of the human visual system can and does impart illusory effect that one might naively attribute to the (inviolable) laws of optics.


Edited by GlennLeDrew, 14 July 2016 - 06:08 PM.


#16 Tony Flanders

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Posted 14 July 2016 - 06:31 PM

Surface brightness is a very important concept and not in the least bit narrow.


You bet! I have devoted a great deal of my total writing to explaining that fact.

But using the word "brightness" without any qualifier to mean "surface brightness" is not doing anybody a service. In terms of visibility, surface brightness is very important, but total brightness is arguably even more important.

#17 Asbytec

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Posted 14 July 2016 - 06:51 PM

Keep hearing this but then people say to get the biggest diameter you can to increase what you can see.

Seems odd as why buy as 14" dobsonian if a 6" can do the same.

So aperture is not king then. Just go get a nice 80mm apo. Apparently it shows as much.

Why Hubble and the VLT, all those up on Hawaii can be taken down as can Canary Islands and everything in the Atacama desert.

Seems odd they built those to see fainter objects and they didn't need to. :silly: :silly: :silly:

 

Large apertures gather more light and focus it into the object. Stars are brighter and you can see more of them. They are point sources with an integrated brightness as mentioned above. They contain all the light gathered in a single diffraction point. 

 

For extended objects, aperture still gathers more light just as it does for point sources. Except that the light is not focused into a point. All that light is spread across the surface area of the image - it's surface brightness per unit of area, whether it's small and bright or large and dim on the focal plane. 

 

Whether you look at an object with a 80mm refractor or an 18" Dob, it will appear to be as bright as the naked eye with an 80mm or 18" iris. It is as bright in the telescope as it is in the sky to the naked eye with an imaginary iris of those same diameters. Nothing makes the galaxy brighter than it normally is unless it suddenly erupts with billions of supernovae. 

 

Really, a telescope is nothing more than a mechanism for expanding the human iris to the aperture. The magnification and the exit pupil stop down the beam and squeeze that light into your eye basically giving you either an 80mm or 18" iris. The object is as bright as the light gathered whether viewed through an imaginary large human eye or through the telescope. (Actually, the telescope is a little dimmer due to the additional lenses and mirror coatings.) 

 

A telescope is like a large prosthetic attached to the human eye. But object brightness is the same whether viewed through that prosthetic or with an equally (and impossibly) large human eye. The object is as bright in either and both large iris and telescope. The rest is a function of magnification and exit pupil in the telescope relative to the 1x human eye.

 

At 1x in the telescope, it is as bright as a large human eye and just as small. In the telescope it is just as bright, except that it's much larger with a surface brightness equal to its angular area due to exit pupil (that pumps the gathered light through your normally small iris) and resulting magnification. 

 

It may not be technically correct, or maybe it is, but imagine looking at the sky with an 18" Iris. Then imagine looking at it with a 7mm iris through an 18" aperture that provides a 7mm exit pupil - effectively you have an 18" iris your eye is incapable of. In either, the galaxy is the same surface brightness brightness and at 1x they are the same apparent size. 


Edited by Asbytec, 14 July 2016 - 07:18 PM.


#18 CounterWeight

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Posted 14 July 2016 - 07:30 PM

Simplistically all a telescope does is magnify. Add your eyepiece to focus and now you can see what is (magnified) there.

The eyepiece becomes an integrated part of the optic... and the eyepiece will decide how that object is displayed.

That only leaves your eye and how it works.

So in order of understanding there is

-Absolute brightness or magnitude -or- extended brightness /magnitude of the object in outer space (astronomy)
-Telescope optics, how that telescope magnifies the size of any object (optics)
-Eyepieces and magnification and exit pupil, how eyepieces work (optics)
-The human eye and how it works (anatomy / physiology)

Each one of these things has a say in what you see with a telescope, and how at best you might see it, if that is possible.

Example being a fellow with a very dim flashlight. You will see it differently as you walk away - though the flashlight and it's light have not changed... as you get farther and farther away it will become more difficult to actually see that light, and at some distance you will no longer see it at all no matter how hard you look. The brightness of the flashlight never changed.

After you can no longer see the flashlight you look at the area with a telescope or binoculars and see a faint light again. The aperture (and back to eyepieces the exit pupil it yeilds for the scope) is the main factor in how bright it might appear, but it can never be brighter than the flashlight is to start with.

Edited by CounterWeight, 14 July 2016 - 07:33 PM.


#19 Ziguy

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Posted 14 July 2016 - 10:38 PM

I tried with my binocular and even if they capture 49 times more light than my naked eyes, nothing in my yard was brighter with the binocular.

 

There is a gray satelite dish with the company name writted in black on it. In daytime, I can easelly read the name. But when only lit by moonlight, I can not even see there is something writting on it. I just see a very dark uniform oval. But if I use my binocular, the dish is as dark as before but now a can see the writting. Nothing is brighter but because it is magnified, I can now see the contrast between the disk and the text.

 

Probably the same thing with large faint object. Nothing is brighter but you get better contrast between the object and the sky after magnifying. 


Edited by Ziguy, 14 July 2016 - 11:10 PM.


#20 GlennLeDrew

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Posted 14 July 2016 - 11:01 PM

Norme,

A fuller accounting of the interfacing of eye to objective considers the iris diameter.

 

The complete picture:

 

1) A telescope at a certain magnification effectively replaces the eye's own lens with one having a focal length equal to the eye lens effective focal length multiplied by the magnification. Example: If we take the eye's lens f.l. as 22mm, at 100X the eye is effectively equipped with a replacement lens of focal length 22 * 100 = 2,200mm.

 

2) The effective aperture of this 'replacement' lens equals the lesser of the iris diameter or the exit pupil multiplied by the magnification. If the exit pupil is the limiter, the 'replacement' lens diameter follows as usual from the exit pupil multiplied by the magnification. But when the exit pupil is 'oversized', the eye's iris becomes the exit pupil, and the effective aperture is reduced as the ratio of the iris to exit pupil. Example: The magnification is 100X, the exit pupil is 10mm and the observer's iris diameter is 5mm. The 'replacement' lens diameter is thus 100 * 5 = 500mm. If the iris could somehow open up to 10mm, the 'replacement' lens diameter would be 1,000mm, which of course would equal the actual aperture.

 

This last paragraph above says nothing new, of course, for it merely repeats the well known relationship between objective aperture, magnification and exit pupil diameter. The only potentially new bit for the neophyte is the matter of the effective  aperture being reduced when the exit pupil exceeds the iris diameter.

 

The *really* interesting part of all this has to do with image brightness as determined by the geometry of the 'replacement' lens. In a nut shell, the f/ratio of the 'replacement' lens is the same as for the unaided eye when its iris equals or exceeds the exit pupil.

 

Harken back to our assumed eye lens f.l. of 22mm. For a 7mm iris this makes the eye's lens operate at just a bit faster than f/3. Any instrument operating at a 7mm or larger exit pupil is furnishing the eye with a 'replacement' lens that's also a tad faster than f/3. And so it is as found for prime focus imaging. Irrespective of aperture, at given f/ratio extended objects present the same surface brightness on the detector (here, the retina, as opposed to film or CCD.) Your naked eye at given iris diameter is illuminated at given intensity as set by its own iris aperture, which controls the f/ratio of the light converging to focus. The 'replacement' lens has its f/ratio set by either the eye's iris or the exit pupil, whichever is the smaller.

 

An even better way to understand the interfacing action of the exit pupil.... Just as the eyepiece projects an image of the objective behind it, and which we call the exit pupil, so too does the eyepiece project upon the objective an image of the iris (when correctly in the plane of the exit pupil.) And so it literally the case that for a telescope the iris is enlarged by factor the magnification and effectively placed at the objective. If the iris is smaller than the exit pupil, it is similarly smaller in proportion to the objective. If the iris is larger than the exit pupil, only that portion of the iris accommodated by the exit pupil contributes; the unused outer annulus is projected as larger than the objective.

 

We see, then, the crucial action of the eyepiece as coupler of objective and iris. And in the end, as far as surface brightness for extended objects goes, it all comes down to the f/ratio as set by the eye's lens f.l. and either its iris diameter of the exit pupil, whichever is smaller.

 

And I might add this. Diffraction is also introduced by the eye's iris or the exit pupil diameter, whichever is the smaller. In any optical system, that restrictor which defines the aperture as 'seen' from the focus is that which diffracts light.

 

For an afocal instrument (which has an ocular delivering more or less collimated light to the eye), the objective is *not* producing a diffracted image, which the eyepiece then magnifies! Want proof? Install a small hole in some sheet material behind the eyepiece, as near to the exit pupil as permitted by the eye's requirement for its placement in order to have not too large a restriction on FOV. If this hole is as large as or larger than the exit pupil (or does not impinge upon the cylindrical light bundle for some star or other image point under examination), no change to diffraction is seen. But make this hole smaller than the exit pupil, then will diffraction expand in extent. With no change to magnification. Why? Because the now smaller aperture as 'seen' by the retina increases the size of the Fresnel pattern.



#21 Asbytec

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Posted 15 July 2016 - 12:25 AM

Glenn, I haven't read your comment fully yet, but I always do. Right after I comment...

This whole basic concept became crystal clear to me considering the view through a 6" telescope at 1x and a naked eye with a 6" iris also at 1x.

The views are the same, the galaxy is not brighter in the telescope than the naked eye.

Its just that we have to squeeze that incomming 6" beam into a 7mm exit pupil so it can fit inside our iris.

Doing so magnifies the image hense the relationship between the entrance and exit pupil being magnification and a larger image with the aperture's light grasp spread across that larger image.

As I recall Jon saying, telescopes make images larger. Not brighter. Well said...once it clicks.

Edited by Asbytec, 15 July 2016 - 12:37 AM.


#22 Asbytec

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Posted 15 July 2016 - 12:35 AM

Glenn, yes, we discussed some of that in the Focal Ratio thread. Its intetesting to note, it sounds like anyway, an alien looking down our scope could see our retina magnified, too.

You have such a grasp on these topics, normally I read them twice. And slowly each time. Thank you for your contribution to our hobby.

#23 Jon Isaacs

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Posted 15 July 2016 - 09:23 AM

Bottom line:  A telescope cannot make an extended object brighter than it is naked eye.  It can make it much larger without a loss of contrast. it can actually increase the fine scale contrast.

 

Don't pass up an opportunity to look at the Veil Nebula in a large aperture scope, don't pass up an opportunity to look at M51 or NGC-1999 in a large aperture scope.. At the optimal magnification, they are dimmer than they would be naked eye but they sure seem bright.

 

Jon



#24 Asbytec

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Posted 15 July 2016 - 09:46 AM

Hi Jon, NGC 1999 is, to this day and beyond, one of those memorable observations. Something new to observe. Never knew it was there. Maybe a symbol on a map one might not hope to see and overshadowed by the great Orion nebula.

I dont recall the magnification, pretty high I believe. But it is a wonderful example of the fine detail contrast and of using the appropriate magnification even sacraficing surface brightness.

Thanks for the tip on observing that one. Memorable and productive observation.

These discussions are interesting, including the focal ratio thread. They speak to the function of our instruments and put to rest myths and misunderstandings. I always appreciate your input. Learned a lot from you over the years.

So, to the OP, is the topic easier to grasp?


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