can the moon damage eyesight?

In article ,
Mark McIntyre wrote:

So lets get this straight: you're happy that a tiny lens in an eye
will increase light intensity, but won't agree that a whopping big one
in a telescope will?

Telescope + eye produces about the same light energy per unit area as
eye alone.

Reality check: telescopes make dim things brighter by increasing light
intensity. If you're not certain about this, get a photometer and
measure it.

As you have done, no doubt.

For point sources, what you say is true. It's a point with or without
the telescope, and it has (say) 2000 times as much light energy. For
non-point sources it's false: it has 2000 times as much energy but
is spread over 2000 times as much area on your retina.

Look at the moon with 50x magnification through a 4-inch reflector.
Does its surface look brighter than with the naked eye? Or just
bigger?

-- Richard

In article ,
Mark McIntyre wrote:

In any case, as I said in my original message, it *is* much more
dangerous to look at the sun through a telescope than unaided. But it
is also true that the light per unit area in the image on the retina
is not higher.

These two statements are contradictions in terms.

No they are not. The reason it's much more dangerous is not that the
light per unit area is higher, it's that the area is much greater so
the eye can't be cooled fast enough.

-- Richard

Mark McIntyre wrote:
On 24 Oct 2006 18:00:17 GMT, in uk.sci.astronomy ,
(Richard Tobin) wrote:

If the diameter of the telescope is 50 times that of your pupil, the
amount of light focused is 50 squared times larger.

Sort of. Your fully dilated pupil is probably around 10mm in diameter.
With a 200mm scope, you'd have a ratio of around 20 if you looked at
the raw image.

If the magnification is 50, the light is focused onto an area of your
retina 50 squared times larger (in area) than the unmagnified image.

This isn't relevant.

It is extremely relevant. If you could use a passive telescope to form
an image brighter than the original source object then you can quickly
build a perpetual motion machine.

You look at a refocused image through an eyepiece. This creates a
small in-focus disc-shaped image around 6-10mm from the eyepiece. This
is what you look at. This disc has all the light from the original
aperture concentrated into a small area of say 10mm in diameter. So
the light concentration will be say 400x higher.

This is muddled. The eyepiece allows you to put your eye close to the
real image formed at the prime focus of the main objective, but it
creates a virtual image at infinity as far as the eye is concerned.
When the scope is focussed for normal use there is no real image formed
except at the back of the eye.

Only when using eyepiece projection onto film or CCD is there a real
image plane behind the eyepiece.

So the light per unit area on the retina is unchanged.

If this were true, then it would not be possible to set fire to a
piece of paper with a telescope by pointing it at the sun.

The total light coming out of the eyepiece is 400x more but it is also
diverging at 20x angular magnification so that conservation of energy
holds.

In point of fact a 200mm scope concentrates around 40W into a 10mm
diameter circle, certainly enough to burn your cornea.

As a flux concentrator there is a point in the eyepiece where all the
light going through the objective must pass through the field stop. It
diverges from that plane.

NB This restriction only applies to imaging systems and extended
objects.

Stephen Tonkins page has suitable details:
http://www.astunit.com/tutorials/telescope.htm

Unresolved point sources do get brighter when you use a larger
telescope, but extended objects do not.

Regards,
Martin Brown

Richard Tobin wrote:


No they are not. The reason it's much more dangerous is not that the
light per unit area is higher, it's that the area is much greater so
the eye can't be cooled fast enough.

If light *energy per square meter* is 200 W
then energy on 1/100 square meter is 200/100 = 2 W.
If you use a convex lens with an area 1/100 of a square meter and focus
these 2 Watts on an area of say 1/10000 of a square meter, then energy
input in this point is also 2 Watts. Right?

But energy per square meter in this very point is then
2 x 10000 = 20000 Watts *per square meter*

20000 is more then 200 the way I see it.

In article ,
Iordani wrote:

If light *energy per square meter* is 200 W
then energy on 1/100 square meter is 200/100 = 2 W.
If you use a convex lens with an area 1/100 of a square meter and focus
these 2 Watts on an area of say 1/10000 of a square meter, then energy
input in this point is also 2 Watts. Right?

But energy per square meter in this very point is then
2 x 10000 = 20000 Watts *per square meter*

But this is not the situation with a telescope. 2000 times as much energy
is focused onto an area of your retina 2000 times *bigger* than when
you look at the sun unaided.

-- Richard

On 25 Oct 2006 12:36:25 GMT, in uk.sci.astronomy ,
(Richard Tobin) wrote:

Look at the moon with 50x magnification through a 4-inch reflector.
Does its surface look brighter than with the naked eye? Or just
bigger?

Brighter. Thats why I end up using a polarising filter.

Next.
--
Mark McIntyre

On 25 Oct 2006 09:20:48 -0700, in uk.sci.astronomy , "Martin Brown"
wrote:


Mark McIntyre wrote:
On 24 Oct 2006 18:00:17 GMT, in uk.sci.astronomy ,
(Richard Tobin) wrote:

If the magnification is 50, the light is focused onto an area of your
retina 50 squared times larger (in area) than the unmagnified image.

This isn't relevant.

It is extremely relevant. If you could use a passive telescope to form
an image brighter than the original source object then you can quickly
build a perpetual motion machine.

Grin. To the point under discussion I meant.

--
Mark McIntyre

On 25 Oct 2006 12:36:25 GMT, in uk.sci.astronomy ,
(Richard Tobin) wrote:

For point sources, what you say is true. It's a point with or without
the telescope, and it has (say) 2000 times as much light energy. For
non-point sources it's false: it has 2000 times as much energy but
is spread over 2000 times as much area on your retina.

Whatever. I'm done with this discussion.
*threadplonk*
--
Mark McIntyre

Mark McIntyre wrote:
On 25 Oct 2006 12:36:25 GMT, in uk.sci.astronomy ,
(Richard Tobin) wrote:

Look at the moon with 50x magnification through a 4-inch reflector.
Does its surface look brighter than with the naked eye? Or just
bigger?

Brighter. Thats why I end up using a polarising filter.

You are wrong. It isn't. A telescope makes an extended object appear
bigger the surface brightness is very slightly less than naked eye
thanks to losses in the optics.

Why don't you look at one or two of the physics web pages that explain
this?.

Regards,
Martin Brown