surface brightness and photons

Brian Tung wrote:
chemstudcou wrote:
I take the "virtual image" to mean the eye relief distance
where all the parallel rays from different angle meets. Anyway.

You can if you like, but you'll be using that term very differently
from how everyone else uses it

The virtual image is not floating in space in back of the eyepiece.
there is no image in back of the eyepiece at all, except for that
formed at your retina by your eye lens.

In my conversation with chemstud, he and I have been using an ambiguous
general reader defintion of "virtual image" that does not correspond to
the terms "real image" and "virtual image" as it is used in optics and
physics, as you point out. In common English, one use of "virtual"
means "[e]xisting or resulting in essence or effect though not in
actual fact, form, or name . . ."

Since the common English and optical definitions of virtual image and
real image can lead to misunderstanding when comparing this thread with
astronomy and optic texts, I appreciate you honing on in my ambiguous
use of the term.

Although you know all this, it may help other readers of the thread to
repeat some basic definitions.

For most non-optical general readers, the only "real image" is the one
that comes from the object and hits your eyeball. Everything else is a
"virtual," in the sense of "without substance" or "without essence."
The only "real image" of the Andromeda galaxy, in its common English
usage, is the one that I see with my naked-eye at a dark sky site. In
common usage, everything else - the view in the eyepiece, the digital
bits in an astrophotograph, and dots on a film photograph as simply
"virtual" (but improved) representations of the real thing, but are not
the real thing itself - as in "virtual reality" or "virtual community"
- as in this newsgroup. A telescope can viewed as a light filter that
creates and modifies a series of virtual images of the real object
before the final virtual representation of the object reaches the eye.

In optics, a "real image" and "virtual image" have different and more
precise meanings - the definitions as you have been using them.

"Real image is a term used in optics and physics. It is a
representation of an actual object (source) formed by rays of light
passing through the image."
http://en.wikipedia.org/wiki/Real_image

"Virtual image . . . is a representation of an actual object (source)
formed by diverging rays of light which seem to originate from the
image, but in reality do not cross at that position."
http://en.wikipedia.org/wiki/Virtual_image

The prime focus image is a real image created in the telescope by
objective from light rays coming from the celestial object at infinity.
It is a "real image" in the optical sense. When telescope books label
the prime focus image as a "real image," that is what the author means.


If I understand this correctly, when I remove the eyepiece from my 5"
Apogee refractor and put my eye at the end of tube at prime focus, I am
looking at an optical "real image."

When I put the eyepiece back in, another "real image" forms on the
observer side of the eyepiece, but its rays start to diverge after it
passed the eye relief distance. When those divergent rays enter my eye,
they create the illusion of a virtual image of a size equal to the AFOV
of the lens covering about 52 degrees of my vision's field of view. If
I am viewing the Moon, there is no "real" Moon 52 degrees wide in front
of my face, anymore than there is a "real" Moon with a linear height of
1/8" inside the eyepiece tube at "real image" at prime focus.

The Wikipedia entry for a "telescope" also uses "virtual" in this
ambiguous everyday language sense, with respect to the prime focus
image - as in the "virtual image at prime focus is magnified by the
eyepiece":

"The basic scheme is that the primary light-gathering element, called
the 'objective lens', focuses light to a focal plane where it forms a
bright virtual image. An 'eyepiece' then magnifies the virtual image."
http://en.wikipedia.org/wiki/Optical_telescope

But isn't the image formed on the otherside of eyepiece at the eyepiece
relief distance (where light initially converges after passing through
eyepiece), in the optics sense, a "real image" - it is formed by
converging rays traceable back to the object. After the light passes
through the eye relief distance, it starts diverging before entering
the eye pupil - at which point we perceive it as a virtual image? Just
asking.

Thanks for pointing out the ambiguity in my use of language.

- Canopus56

P.S.-

Other lurker background link for more reading on the difference between
the optical real and virtual image:

http://howthingswork.virginia.edu/supplements/telescopes_and_microscopes.pdf#search='telescope%2 0virtual%20image%20real'

Kurt (canopus56) wrote:
In my conversation with chemstud, he and I have been using an ambiguous
general reader defintion of "virtual image" that does not correspond to
the terms "real image" and "virtual image" as it is used in optics and
physics, as you point out. In common English, one use of "virtual"
means "[e]xisting or resulting in essence or effect though not in
actual fact, form, or name . . ."

Since the common English and optical definitions of virtual image and
real image can lead to misunderstanding when comparing this thread with
astronomy and optic texts, I appreciate you honing on in my ambiguous
use of the term.

It's actually not the use of "virtual" that is the problem, but "image."
There is no image, real or virtual, that is formed in back of the
eyepiece, at the eye relief distance.

Note to Mike Simmons: I think I'm going to propose that book on optics
for telescope users after all. I haven't seen any news of the book that
you mentioned lo these many moons ago.

You repeat this error below:

When I put the eyepiece back in, another "real image" forms on the
observer side of the eyepiece, but its rays start to diverge after it
passed the eye relief distance. When those divergent rays enter my eye,
they create the illusion of a virtual image of a size equal to the AFOV
of the lens covering about 52 degrees of my vision's field of view.

No. No image at all is formed in back of the eyepiece (that is, outside
the telescope). The rays that originated from any object in the field
of view (Betelgeuse, let's say) are parallel when they leave the eyepiece.
A telescope is an afocal arrangement.

The real image is only formed by your eye lens, which intercepts that
bundle of light rays, and causes it to converge into an image on your
retina. Your retinal receptors convert the light energy of that image
into electrochemical impulses that are interpreted by your brain as a
"picture" of Betelgeuse.

A litmus test for whether something is a real image is if you put a
piece of ground glass there, do you see a miniature of the object? If
you do, it's a real image. If you take out the eyepiece and put a
piece of ground glass (or translucent plastic) at the focal plane, you
will see a miniature of the object. This miniature is the real image,
and it is magnified by the eyepiece, acting as a magnifying glass on
that real image.

If you put the eyepiece back in and try putting a piece of ground glass
there, you will not see a miniature of the object. At best, depending
on how brightly illuminated the field is, you may see the exit pupil.
That's all.

If
I am viewing the Moon, there is no "real" Moon 52 degrees wide in front
of my face, anymore than there is a "real" Moon with a linear height of
1/8" inside the eyepiece tube at "real image" at prime focus.

There is, however, a virtual image formed, at infinity, of the magnified
Moon. Virtual images at infinity have no linear size, only angular
size. It would be 52 degrees wide (or whatever is determined by the
magnification).

The Wikipedia entry for a "telescope" also uses "virtual" in this
ambiguous everyday language sense, with respect to the prime focus
image - as in the "virtual image at prime focus is magnified by the
eyepiece":

"The basic scheme is that the primary light-gathering element, called
the 'objective lens', focuses light to a focal plane where it forms a
bright virtual image. An 'eyepiece' then magnifies the virtual image."
http://en.wikipedia.org/wiki/Optical_telescope

No, I think they are trying to be precise, and using the wrong term.
The sentence is exactly correct if "virtual" is replaced by "real" in
the two places where it occurs. Doubtless they are influenced by the
everyday sense of the word "virtual" (meaning non-corporeal?), but in
the context of an optical discussion, it is not merely ambiguous to use
virtual in that way; in this case, it is wrong.

But isn't the image formed on the otherside of eyepiece at the eyepiece
relief distance (where light initially converges after passing through
eyepiece), in the optics sense, a "real image" - it is formed by
converging rays traceable back to the object.

Yes--you must say that the light rays pass through the image. That is
why the Wikipedia entry is wrong. It is a real image, although it is
formed at a distance from the eyepiece equal to the eyepiece's focal
length, not the exit pupil. The two are often related, especially when
considering eyepieces within a single design, but they are not the same.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

X-No-archive: yes
Brian Tung wrote:
snip all
Brian, thanks for the clarification - Canopus56