Star Testing you Eyes

There it was, mid-day, and the sun shining on the south facing window of
the bathroom. I peer out through the glass and the screen beyond to get
a glimpse of the sky, and the condition of the backyard. As I'm looking
out I see this reflection that looks like a star test pattern. What the
heck? I move my eyes around, to and fro and realize that it must be a
Fresnel pattern from my cornea, reflecting off the window.

I'm still uncertain about how this phenomenon occurred, but boy did I
get a good glimpse at what is causing the astigmatism in my eyes at
large exit pupils. There was a flat spot on one portion of the outer
circles in the pattern. Up and left in the reflection.

The Fresnel rings near the center on the other hand, looked absolutely
stunning, perfect. I wish it was that easy to get a good Fresnel pattern
out of my night time skies. I could then dial in the scope to
perfection, and get some decent planetary views. :-)

Stephen Paul
Shirley, MA

Stephen Paul wrote:

There it was, mid-day, and the sun shining on the south facing window of
the bathroom. I peer out through the glass and the screen beyond to get
a glimpse of the sky, and the condition of the backyard. As I'm looking
out I see this reflection that looks like a star test pattern. What the
heck? I move my eyes around, to and fro and realize that it must be a
Fresnel pattern from my cornea, reflecting off the window.

I'm still uncertain about how this phenomenon occurred, but boy did I
get a good glimpse at what is causing the astigmatism in my eyes at
large exit pupils. There was a flat spot on one portion of the outer
circles in the pattern. Up and left in the reflection.

The Fresnel rings

Are Fresnel rings the same as Diffraction rings?

near the center on the other hand, looked absolutely
stunning, perfect. I wish it was that easy to get a good Fresnel pattern

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?






out of my night time skies. I could then dial in the scope to
perfection, and get some decent planetary views. :-)

Stephen Paul
Shirley, MA

You can do the same thing with tiny spots on sunglasses Allow the sun to
reflect off of the spots while wearing them, and you'll see diffraction
patterns. I first noticed this only a couple of weeks ago and had the same
thoughts you did about the perfect telescope. My eyes are 20/15 or better,
so very good patterns.

I imagine that this might actually be a good way to self diagnose the
condition of one's eyes. I do know that shining a flashlight into the eyes
to see the retina reveals several things.... I read an article several years
ago telling parents to shine lights into their kids eyes to reveal a serious
problem, just what it was I don't know now.

Al


"Stephen Paul" wrote in message
...
There it was, mid-day, and the sun shining on the south facing window of
the bathroom. I peer out through the glass and the screen beyond to get
a glimpse of the sky, and the condition of the backyard. As I'm looking
out I see this reflection that looks like a star test pattern. What the
heck? I move my eyes around, to and fro and realize that it must be a
Fresnel pattern from my cornea, reflecting off the window.

I'm still uncertain about how this phenomenon occurred, but boy did I
get a good glimpse at what is causing the astigmatism in my eyes at
large exit pupils. There was a flat spot on one portion of the outer
circles in the pattern. Up and left in the reflection.

The Fresnel rings near the center on the other hand, looked absolutely
stunning, perfect. I wish it was that easy to get a good Fresnel pattern
out of my night time skies. I could then dial in the scope to
perfection, and get some decent planetary views. :-)

Stephen Paul
Shirley, MA

"Al Alkoloid" wrote

You can do the same thing with tiny spots on sunglasses Allow the sun to
reflect off of the spots while wearing them, and you'll see diffraction
patterns. I first noticed this only a couple of weeks ago and had the
same
thoughts you did about the perfect telescope. My eyes are 20/15 or
better,
so very good patterns.

I believe you can have "20/15" eyesight and still have tons of astigmatism.
Therefore, just because you have 20/15 doesn't mean the pattern of fresnel
rings will be good.

causing the astigmatism in my eyes at

I have a crazy thought.I've had astigmatism for years. I also need correction
for far and near sightedness. I'm sure you know what that means for astronomy.
Recently, I had two cateracts removed and guess what. The lenses they put in
are not astimatic and perfectly focused at infinity. Although I have some
trouble reading paper sized print, I no loger need to wear glass while
observing. Fortunately, my retina is in great shape. I live in the greater
Phila delphia so that I have not had a chance to check out my new eyes. I'm
going to the WSP and I'll report later

"Al Alkoloid" wrote:

I imagine that this might actually be a good way to self diagnose the
condition of one's eyes. I do know that shining a flashlight into the eyes
to see the retina reveals several things.... I read an article several years
ago telling parents to shine lights into their kids eyes to reveal a serious
problem, just what it was I don't know now.

Given the scientific igorance of the general public, I think eye examinations
are best left to trained professionals.

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. 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

"Howard Lester" wrote in message
...

"Al Alkoloid" wrote

You can do the same thing with tiny spots on sunglasses Allow the sun
to
reflect off of the spots while wearing them, and you'll see diffraction
patterns. I first noticed this only a couple of weeks ago and had the
same
thoughts you did about the perfect telescope. My eyes are 20/15 or
better,
so very good patterns.

I believe you can have "20/15" eyesight and still have tons of
astigmatism.
Therefore, just because you have 20/15 doesn't mean the pattern of fresnel
rings will be good.

That's probably true, although my diffraction patterns were about as
circular as they get. On the other side of the coin though, I had an SCT a
year ago that had severe astigmatism!

Al

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.)

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. . . . .

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