Star Testing you Eyes

Al Alkoloid wrote:
I imagine that this might actually be a good way to self
diagnose the condition of one's eyes.

Suiter's 1994 classic, _Star Testing Astronomical Telescopes_
(Willman-Bell), suggests the following test (on p. 240), which I have
not personally tried:

"Your eye also suffers from medium scale roughness. Take aluminum foil
and perforate it with a pin. Hold the foil about 8 to 15 cm in front
of your eye and look through the pinhole at a frosted incandescent
light bulb. Try to focus your eye on the lamp, not the pinhole, and
cover the other eye. If you have punch the right size hole in the
foil, you should see a mottled disk that roughly approximates the
out-of-focus patterns seen in this book. . . . . The appearance may be
cleared up slighty by placing a colored [telescope lens] filter between
the lamp and the pinhole.

As you blink, horizontal lines appear briefly on the defocused disk. .
.. . [Y]ou may also see some dim radial spikes outside the disk. These
spikes may be caused by diffraction . . . or streaks in the roughness.

The roughness is visible as coarseness in the expanded field. This
coarseness does not vary from blink to blink. . . . .

The human eye is not even close to diffraction-limited. An eye with a
3-mm iris opening . . . can theoretically resolve lines separated by
0.6 arcminutes, but a person who resolves lines only 1 arcminute apart
is deemed to have excellent vision."

Making a good small pinhole in foil takes some practice. You may have
to try several tries before making a suitable small hole. The idea is
to just punch the tip of the pin through the other side of the foil in
order to make the smallest hole possible.

Enjoy.

- Canopus56

P.S. - I have posting in this thread that seems to have been corrupted
by the usenet web posting interface. My apologies.

On 27 Dec 2004 11:06:09 -0800, "canopus56"
wrote:

Al wrote,
I imagine that this might actually be a good way to self diagnose the
condition
of one's eyes.

Suiter's 1994 classic, _Star Testing Astronomical Telescopes_
(Willman-Bell), suggests the following test (on p. 240), which I have
not personally tried:

"Your eye also suffers from medium scale roughness. Take aluminum foil
and perforate it with a pin. Hold the foil about 8 to 15 cm in front
of your eye and look through the pinhole at a frosted incandescent
light bulb. Try to focus your eye on the lamp, not the pinhole, and
cover the other eye. If you have punch the right size hole in the
foil, you should see a mottled disk that roughly approximates the
out-of-focus patterns seen in this book. . . . . The appearance may be
cleared up slighty by placing a colored [telescope lens] filter between
the lamp and the pinhole.

As you blink, horizontal lines appear briefly on the defocused disk. .
. . [Y]ou may also see some dim radial spikes outside the disk. These
spikes may be caused by diffraction . . . or streaks in the roughness.

The roughness is visible as coarseness in the expanded field. This
coarseness does not vary from blink to blink. . . . .

How about people who have had their cornea's replaced? Is the
roughness still present in the skin that covers the replacement lens?
-Rich

RichA wrote:
On 27 Dec 2004 11:06:09 -0800, "canopus56"
wrote:snip and Rich wrote:
How about people who have had their cornea's replaced?

I do not know that much about it. Considering the transplanted cornea
is a donated organ from someone who has passed-on, and the replacement
is not a manufactured lens, I would assume everyone's biological cornea
has similar roughness. See -
http://www.eyesite.org/corneatransplantation.html

- Canopus56

On Sun, 26 Dec 2004 23:37:49 -0600, Rolo wrote:

Are Fresnel rings the same as Diffraction rings?

The diffraction rings you see around a star is an example of
fraunhofer diffraction.

To get a fresnel pattern you need a linear feature. My guess is the
screen has something to do with it.

On 27 Dec 2004 11:06:09 -0800, "canopus56"
wrote:

Suiter's 1994 classic, _Star Testing Astronomical Telescopes_
(Willman-Bell), suggests the following test (on p. 240), which I have
not personally tried:

"Your eye also suffers from medium scale roughness. Take aluminum foil
and perforate it with a pin. Hold the foil about 8 to 15 cm in front
of your eye and look through the pinhole at a frosted incandescent
light bulb. Try to focus your eye on the lamp, not the pinhole, and
cover the other eye. If you have punch the right size hole in the
foil, you should see a mottled disk that roughly approximates the
out-of-focus patterns seen in this book. . . . . The appearance may be
cleared up slighty by placing a colored [telescope lens] filter between
the lamp and the pinhole.

I have a corneal opacity in my right eye that I can see when I look
through a pinhole at a moderately bright light. Because of the
opacity my right cornea is "lumpy". I can look at the Moon and see
several fainter, false moons clustered around the real one.

When you look at the sunlit face of the moon at extremely high power
some of what you see is actually the structure of your eye illuminated
by the highly collimated rays of light coming through the eyepiece.

Glenn Holliday wrote:

Rolo wrote:

Stephen Paul wrote:

The Fresnel rings

Is this Fresnel pattern the same thing as a diffraction ring pattern?
If yes, when and why did the terminology change, or is this a news
group dialectical preference not shared at major universities?

A Fresnel lens is different than a diffraction grating, and
has a different purpose.

What lens and grating? Nobody said anything about lenses and gratings?
Theatrical lighting? Hmmmmmm. I think you did not read
the words I printed out to ask a question. Maybe you need fresnel
Lens?



The lens is not made with one curvature
across the surface of the lens, but is made in concentric rings,
each ring with a slightly different curvature. The purpose is
to get a better focus with a light source that is not a point.
In theatrical lighting, the effect is to get a more uniform
illumination of the area lit by a light that has a Fresnel lens.

--
Glenn Holliday

Well... In or out of focus it still would be a diffraction pattern?
Because diffraction, ie. a diffracted wave front, is what the pattern
derrives from in the first place? Maybe it has something to do with
the fact a man named Fresnel did early research on "diffraction
patterns" (diffraction of light), and last year some famous amateur
discovered this historical fact, and then renamed diffraction patterns
to be Fresnel Patterns ... maybe it was Brian Tung ... and now we
are all supposed to start using that term and known wnat is meant?
Im just guessing. Or maybe it has always been Fresnel patterns in
physics but in amateur astronomy we have talked about diffraction
patterns, and now amateurs are becoming more updated.

I just like to know what people are really talking about - thats the
only reason I ask. Last week I ordered a Mickey and nobody
outside of Homeland Security seemed to know what iwas talking
about. A Mickey of course is 7.62mm semi automatic pistol.

John


Stephen Paul wrote:

Rolo wrote:


Are Fresnel rings the same as Diffraction rings?


As I understand it, diffraction rings are what you get in focus, and
Fresnel rings are what you get in a star test (out of focus).

(Please keep in mind that this is a newsgroup for amateurs, not
professionals only. It's okay to be wrong. Someone will invariably
correct wrong knowledge or assumptions, PDQ.)

Are Fresnel rings the same as Diffraction rings?

You could say that. Fraunhofer diffraction, which is at work when we
star test a telescope, is an idealized case of Fresnel diffraction. In
Fresnel diffraction, a spherical light wavefront encounters an obstruction
(such as the circular opening of our pupil). In Fraunhofer diffraction,
the wavefront is flat. In truth, a star does of course emit a basically
spherical wavefront, but the star is so far away that by the time it
reaches our puny little telescope, it is as good as flat.

Fraunhofer diffraction is easier to analyze because of the flat wavefront,
and it gives useful results such as Cor Berrevoets Aberrator program. For
accurate results on nearby light sources, however, one must resort to the
more complex Fresnel treatment.

Qualitatively, they are very similar indeed. Also, I am not responsible
for any change in terminology--these names are much older than I am.

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

"Rolo" wrote in message
...


I just like to know what people are really talking about - thats the
only reason I ask. Last week I ordered a Mickey and nobody
outside of Homeland Security seemed to know what iwas talking
about. A Mickey of course is 7.62mm semi automatic pistol.


Just what is a Mickey ???

Tokarev, CZ 52, Broomhandle ???
Dont tell me it's a Luger, I wouldnt believe ya








































--
Only A Gentleman Can Insult Me And A True Gentleman Never Will

Just what is a Mickey ???


a Mickey is a barbituate added to a drink (alcoholic, of course).