Can Moon light blind you?
david
moon. Concern has been expressed that permanent damage to the eye can be
caused by viewing the full moon without a filter. Is that correct? Any
references would be welcomed.
David N.
Mike Simmons
100" f/30 with no problem. The Moon's surface brightness isn't
increased by the telescope, the image is just a lot bigger and covers
more of your retina. There won't be any permanent damage. I prefer
observing without a filter, but of course I don't expect to do any deep
sky observing with that eye very soon after lunar observing. ;-)
Mike Simmons
P. Edward Murray
though!:)
It can be dazzling enough in a 10 inch!
Ed
Eric Ng
through your telescope. If you use lower magnification, the exit pupil would
be bigger than your pupil, so that it is impossible for your eye to be
damaged by your eyes no matter how big the telescope for the moon
observation.
Best Regards,
Eric
Mike Simmons <ecli...@mwoa.org> wrote in message
news:3BEA4C2C...@mwoa.org...
Stephen Astro
The surface brightness of the moon in your Obsession is the same or
probably less per angular unit as the moon without a telescope. Less
light is entering your eye than enters it while driving in daylight.
People have expressed similar concern at my 17" Dob, expecially when
they see the eyeball of the observer lit up by the moon.
Steve O.
Atreju
St...@fastest-inc.com (Stephen Astro):
>People have expressed similar concern at my 17" Dob, expecially when
>they see the eyeball of the observer lit up by the moon.
It is quite a sight! Once that happened to me when I was looking at a
full moon, my friend went "wow!" and stood back a second. He saw the
bright spot on my eye and thought it was dangerous. After a brief
explanation he calmed down, but I have seen it too and it's pretty
cool.
Clear, dark skies to all.
-Atreju-
WJ Blanton
"Eric Ng" <eri...@cuhk.edu.hk> wrote in message
news:9se3nh$15...@imsp212.netvigator.com...
>it is impossible for your eye to be damaged by your eyes
I'm confused by this line....what do you mean?
-William
Bill Foley
their eyes to have them damaged by observing the Moon with a 25". That
said, I would STILL strongly advise a neutral or polarizing filter, to
preserve some night vision for other objects!!
--
Clear, Dark Skies
Bill Foley
Brian Tung
No references, just reasoning. From MOPFAQ:
Q. Is it true that looking at the Moon through a telescope will harm
your eyes?
A. You cannot harm your eyes by looking at the Moon through a
telescope. It may be uncomfortably bright, and you may can improve the
visibility of detail by either adding a neutral density filter (a gray
screw-on filter) to the eyepiece, or by increasing the magnification.
But there is no safety risk.
You may wonder how this can be, since the telescope gathers so much
more light than your eye. However, it also magnifies the Moon, so that
the extra light is spread out over a greater area. Each part of the
Moon's image is seen by just one portion of your eye, and as far as
damage is concerned, the critical factor is the intensity of light
falling, per individual portion of your eye. If your eye's pupil is
5 mm across, and your telescope is 100 mm across, then the telescope
gathers 20 squared, or 400 times more light than your eye alone. But
if you're using a magnification of 20x or greater, then that light is
spread out over an image at least 400 times larger, so that the actual
brightness seen by any portion of your eye is no greater, and usually
less, than when you observe the Moon with the unaided eye.
What if you observe the Moon at less than 20x--say, 10x? Shouldn't the
light be spread out over a smaller area, and thus more concentrated? At
10x, the 400 times more light is spread out over an image that is only
100 times larger, so it seems as though each part of the image should be
4 times as bright as when seen by the unaided eye.
However, consider that each portion of the Moon can be thought of as
pouring down light, out of which only a shaft 100 mm across--as wide as
your telescope--actually enters the optics. In the process of
magnification, that shaft is reduced to fit into your eye's pupil, and
the factor of reduction is equal to the magnification. In other words,
if you magnify by only 10x, the 100 mm shaft of light is shrunk down to
10 mm. The result is that only part of the light--a smaller shaft that
is 5 mm across--as big as your eye's pupill--actually gets in. Since
a circle 5 mm across has 1/4 the area of a circle 10 mm across, only 1/4
of the light gets into your eye, and this precisely compensates for the
extra intensity from lowering the magnification.
This is true of any object: The surface brightness--the amount of light
per unit "area" of the object's image--cannot be increased by a
telescope. It can only be kept equal or decreased.
Incidentally, some people may ask, why then is observing the Sun through
a telescope so dangerous? After all, although we don't stare at the Sun
(at least, we shouldn't), its light still comes through our eye. If
looking at the Moon through a telescope is no more dangerous than
looking at it without the telescope, why isn't the same true for the
Sun?
The answer is that the Sun is so bright that each portion of its image
is enough to create some heating in the eye. (So does the Moon, but its
light is about 400,000 times less intense and the heating is completely
negligible.) If any given part of your eye is subjected to that heating
for long enough, permanent damage will result. Your eyes avoid this by
moving around, so that the image of the Sun doesn't stay in place, and
the part of your eye that is getting heated by the Sun one moment has a
chance to cool down the next.
However, if you were to be so foolish as to observe the Sun through a
telescope, each portion of your eye gets heated the same amount, but now
moving the eye doesn't help, since it is still likely to be heated by
the Sun. Moreover, with a small image of the Sun (as when seeing it
with the unaided eye), the fraction of your eye being heated is small,
and it can dissipate heat rather easily to slow down the damage. With a
magnified image, the fraction of your eye being heated is much larger,
and there is now nowhere for the heat to go. You can as a result burn
out your retina with startling and tragic speed.
Bottom line: DON'T DO IT! DON'T OBSERVE THE SUN THROUGH A TELESCOPE
without proper safety precautions, such as an appropriate filter. Do
not use solar filters that screw onto the eyepiece. The focused heat at
the eyepiece is too intense and will crack the filter, sending all that
concentrated light and heat into your eye. The light must be filtered
before entering the telescope. (Exception: A Herschel wedge can be
safely used. If you don't know what a Herschel wedge is, though, don't
guess--just use a proper solar filter.)
Brian Tung <br...@isi.edu>
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
Paul Schlyter
david <sc...@bigpond.net.au> wrote:
> I have a 25 inch Obsession which is often used by the public to view
> the moon. Concern has been expressed that permanent damage to the
> eye can be caused by viewing the full moon without a filter. Is that
> correct?
No matter what aperture telescope you use and what magnification you
use on that telescope, a telescopic view of the full moon will never
ever appear brighter than the view of an ordinary sunlit landscape
during daytime - and such a view won't damage your eye, will it?
Your dark adaption will of course be destroyed by viewing the full
moon through a telescope, but that's a temporary, not permanent,
effect.
> Any references would be welcomed.
--
----------------------------------------------------------------
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch at saaf dot se or paul.schlyter at ausys dot se
WWW: http://hotel04.ausys.se/pausch http://welcome.to/pausch
Mark
What? A telescope doesn't increase the intensity of the image over
the naked eye? This seems like madness!
By my calculations my telescope gathers approximately 1275 times as
much light as my naked eye. So naturally, I'd expect that a 1275
increase in energy on a bright object (ie. the moon) might bring it to
the point where damage could be done.
Have I fallen off my rocker? It just seems silly to suggest that a
telescope couldn't amplify the moon's light to the point where eye
damage can occur.
Rest assured that everytime I look at the full moon through my 10"
scope, and my eyes are watering like at the end of Forest Gump, that I
am asking myself the same question that David brought up.
Mark
Brian Tung
> What? A telescope doesn't increase the intensity of the image over
> the naked eye? This seems like madness!
It might seem like madness, but it's true.
> By my calculations my telescope gathers approximately 1275 times as
> much light as my naked eye. So naturally, I'd expect that a 1275
> increase in energy on a bright object (ie. the moon) might bring it to
> the point where damage could be done.
Have you by any chance read my post? If not, take a look at the
question in MOPFAQ (link in my .sig below). In short, although your
telescope is gathering 1275 times as much light (or whatever the
factor is), it's also magnifying it by a factor of M, so that the
light is spread out over an area M^2 times as large.
If M is 36 or more, so that M^2 is greater than 1275, then the
light is spread out so thin that each bit of image is dimmer than
the Moon is to the unaided eye. If, on the other hand, M is 35 or
less, so that M^2 is less than 1275, then the exit pupil is larger
than your own eye's pupil and enough light falls uselessly on your
eyeball (uselessly so far as image intensity is concerned, at least)
so that the intensity is exactly unchanged from the unaided-eye view.
> Have I fallen off my rocker? It just seems silly to suggest that a
> telescope couldn't amplify the moon's light to the point where eye
> damage can occur.
Oh well.
> Rest assured that everytime I look at the full moon through my 10"
> scope, and my eyes are watering like at the end of Forest Gump, that I
> am asking myself the same question that David brought up.
As I said in my post, no one said it isn't uncomfortable. They only
said it wasn't dangerous.
Mike Simmons
>
> Greetings:
>
> What? A telescope doesn't increase the intensity of the image over
> the naked eye? This seems like madness!
It's true! It's true!
> By my calculations my telescope gathers approximately 1275 times as
> much light as my naked eye. So naturally, I'd expect that a 1275
> increase in energy on a bright object (ie. the moon) might bring it to
> the point where damage could be done.
The light doesn't all get into your eye, though, depending on the exit
pupil. And what does is spread out much more than looking with the
naked eye. Don't you use more than 1x at the telescope? Note that
visual observing is not analogous to using a camera. Visually, you've
got an eyepiece in there and a lens in your eye. You're stuck with
auxiliary optics. I met someone who viewed the Moon at the Nasmyth
focus of one of the Keck 10-meter scopes -- using both eyes on the image
formed at prime focus -- and he's not blind (yet<g>).
> Have I fallen off my rocker? It just seems silly to suggest that a
> telescope couldn't amplify the moon's light to the point where eye
> damage can occur.
Get back on your rocker, Grandpa! :-) See Brian's article. I know it
doesn't seem right at all but it is.
> Rest assured that everytime I look at the full moon through my 10"
> scope, and my eyes are watering like at the end of Forest Gump, that I
> am asking myself the same question that David brought up.
So you'd think I'd be blind in one eye after looking at the Moon in a
100" with 100 times the light gathering power of your 10". I can assure
you my eyes both work fine, though (well, with corrective optics these
days). There are several things that go into the comfort of observing.
I was with a group observing the Moon with a 60" last week and we left
the dome lights on low. The discomfort and apparent brightness comes at
least partly from the apparent angular size of the Moon being so much
larger than in the sky. It might cover your whole retina whereas it's
very small on the retina when viewed naked eye. Well, just go read
Brian's article. Repeat as needed. :-)
Mike Simmons
David Knisely
> What? A telescope doesn't increase the intensity of the image over
> the naked eye? This seems like madness!
>
> By my calculations my telescope gathers approximately 1275 times as
> much light as my naked eye. So naturally, I'd expect that a 1275
> increase in energy on a bright object (ie. the moon) might bring it to
> the point where damage could be done.
Yes, the scope gathers more light, but it also does *not* focus all of
that light into a single image point. It creates an image which is
considerably *larger* than that seen by the unaided eye, so the light
gathered by the instrument is spread out somewhat, rather than being
concentrated into very small dot. Although the view can be dazzling to
some people (and a few might want to use filters if they so desire),
there is no danger of eye "damage" when viewing the moon through a
telescope. Clear skies to you.
--
David Knisely KA0...@navix.net
Prairie Astronomy Club, Inc. http://www.4w.com/pac
Hyde Memorial Observatory:
http://www.blackstarpress.com/arin/hyde
******************************************************
* Attend the 9th Annual NEBRASKA STAR PARTY *
* August 4-9, 2002 http://www.nebraskastarparty.org *
******************************************************
Astronomer
> use on that telescope, a telescopic view of the full moon will never
> ever appear brighter than the view of an ordinary sunlit landscape
> during daytime - and such a view won't damage your eye, will it?
Let me add a very small dose of common sense to this discussion. If the
telescopic view of the Moon dazzles your eye enough to be uncomfortable or
even painful, don't look at it, at least not for very long. The discomfort
or pain is your body's way of warning you to back off.
Let me also add that an "ordinary sunlit landscape" may actually do real
damage to your eye, at least over a prolonged period. My eye doctor insists
that I wear sunglasses whenever I am outdoors in the sun. I think his
belief is that "ordinary" sunlight is a factor in the "aging" of our eyes as
we get older. I suppose the effect is similar to how "ordinary" sunlight
damages human skin.
Personally, I find viewing "ordinary sunlit landscapes" uncomfortable
without sunglasses.
Query: Does Moonlight contain ultraviolet light? I assume it does.....
Bill G.
Austin, Texas
Bryan Henry
sense. Clearly I can look at the sun briefly with my eye but would not dare
to look at the sun thru a scope.
"David Knisely" <KA0...@navix.net> wrote in message
news:3BEB1E5E...@navix.net...
William Hamblen
<willi...@earthlink.net> wrote:
>Query: Does Moonlight contain ultraviolet light? I assume it does.....
Reflectivity of the lunar surface at UV wavelengths is very low.
It's the ultraviolet light that contributes to cataracts, macular
degenration, et al. Be sure your sunglasses attenuate UV
sufficiently.
Mike Simmons
hold steady, the small image of the Sun focused on one part of the
retina will burn it. If you use a telescope, you'll burn your whole
retina instead of just a small part of it. The difference is the extent
of the damage caused by the larger image, not the brightness of the
image. The total amount of light is greater but not greater per unit
area.
Mike Simmons
Louis Boyd
telescope but the surface brightness cannot be greater. The moon (or
sun) subtends only about 1/2 degree while through a telescope with a
wide field eyepiece it might subtend 80 degrees. (a That's (80*2)^2 or
or 25000 times more energy through a large "rich field" telescope. Damn
right the sun through a telescope will damage your eye. Looking at the
sun without a telescope can too, just a smaller area of the eye.
Assuming your eye's smallest aperture is about 2mm in bright light,
looking at the moon through a 12" telescope at 160x magnification will
be about as bright as it's going to get. The moons image would be about
80 degrees and the exit pupil of the eyepiece would match your eye. A
full moon through medium to large telescope can be painful to look at
through a large telescope but the total energy won't be brighter than
looking at (for example) a sunny beach which also fills you field of
view. Just as on the beach, sunglasses may be desirable but a
reasonable viewing time isn't going to cause permanent damage.
Venus though a very large telescope can appear brighter (both total
brightness and surface brightness) than the moon because it has a much
higher albedo.
--
Lou Boyd
Fairborn Observatory.
--
Lou Boyd
Fairborn Observatory
Bill Nunnelee
of the moon is lower when viewed through a telescope. But if you want a
quick and easy way of demonstating the safety of lunar observing, try this.
Hold a piece of white paper behind the eyepiece so that the moon's image is
projected onto it. Now take a flashlight or an incandescent bulb or any
other common light source and shine it on the same paper. Most anything you
try will actually be brighter than the magnified moon was...and, of course,
none of them approach the brightness of a fully sunlit landscape!
"david" <sc...@bigpond.net.au> wrote in message
news:XNrG7.313280$bY5.1...@news-server.bigpond.net.au...
Mark Gingrich
> Venus though a very large telescope can appear brighter (both total
> brightness and surface brightness) than the moon because it has a much
> higher albedo.
Yes, that's the primary reason why Venus's surface brightness is
greater than the Moon's. There's also a secondary reason: sunlight
impinging upon Venus is more intense by about a factor of two.
--
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Mark Gingrich gri...@rahul.net San Leandro, California
Stephen Paul
news:UaFG7.25600$S4.23...@newsread1.prod.itd.earthlink.net...
> > No matter what aperture telescope you use and what magnification you
> > use on that telescope, a telescopic view of the full moon will never
> > ever appear brighter than the view of an ordinary sunlit landscape
> > during daytime - and such a view won't damage your eye, will it?
>
> Let me add a very small dose of common sense to this discussion. If the
> telescopic view of the Moon dazzles your eye enough to be uncomfortable or
> even painful, don't look at it, at least not for very long. The
discomfort
> or pain is your body's way of warning you to back off.
>
> Let me also add that an "ordinary sunlit landscape" may actually do real
> damage to your eye, at least over a prolonged period. My eye doctor
insists
This is probably commonly sensible to those of us who live in snow country
in winter. I would not mess around with looking at a very bright white moon,
any more than I would go out mid-day in winter without sun glasses. If you
don't have a filter, you might try sunglasses while observing the moon. ;-)
Stephen Paul
Zane
(snip)
>The discomfort and apparent brightness comes at
>least partly from the apparent angular size of the Moon being so much
>larger than in the sky. It might cover your whole retina whereas it's
>very small on the retina when viewed naked eye. Well, just go read
>Brian's article. Repeat as needed. :-)
An analogy that's close is walking out of a very dark movie theater onto a
sunlit parking lot during the day. The brightness is very similar to the
moon as seen through a largish telescope, and the discomfort is similar
too. Short exposures (less than years) won't damage your eyes, though.
Re your point about the size being a big factor-- If you walk out of the
movie theater at night and the lot is lit by very bright mercury arc
lights, there's not much discomfort from the lights. If you were to climb
a ladder so that one of the lights filled a big part of your field of view,
it would be very uncomfortable again.
Re the dangers of UV aging of the cornea and lens -- it probably takes many
thousands of hours of being outside in the bright sun before this becomes a
factor. If one's lifetime moon observing time is going to be on this
order, it would probably be wise to use a UV blocking filter or sunglasses.
Zane
Dave Mitsky
Mark,
A telescope does not amplify light, it merely collects it. There's a
big difference. A telescope can only increase the brightness of stars
and that's because they are point sources and not extended objects.
Have you ever exited a movie theater directly into bright sunlight?
Doesn't the light bother your eyes for a short time? The same thing
happens when you look at the full moon at low magnification with
relatively dark adapted eyes. If you were to turn on a white light to
reduce the diameter of your pupils, as I do when observing the moon
from a club observatory dome, there would be glare and no after image.
Dave Mitsky
Paul Schlyter
> Incidentally, some people may ask, why then is observing the Sun through
> a telescope so dangerous? After all, although we don't stare at the Sun
> (at least, we shouldn't), its light still comes through our eye. If
> looking at the Moon through a telescope is no more dangerous than
> looking at it without the telescope, why isn't the same true for the
> Sun?
no more dangerous than looking at it without the telescope". The
dark adaption of your eyes will be in much greater danger if you look
at the Moon through a telescope than without a telescope.
Regarding the danger of permanently damaging your eyes: there some
threshold in light and heat upon your retina needs to be reached in
order to damage your eyes. Naked-eye as well as telescopic views of
the Moon are both well below this threshold. Naked-eye views of the
Sun is near the threshold (you'll probably get away with it if you
look at the Sun naked-eye only briefly, but if you stare at the Sun
naked-eye you'll probably damage small parts of your retina), while
telescopic views of the Sun is well above the threshold (large parts
of your retina can be damaged even from a brief telescopic view of
the Sun).
L.C.
ERIC K. CHEU
into a very small area of space, and it should be able to blind you.
Mark (mf...@cpsc.ucalgary.ca) wrote:
: Greetings:
Stephen Astro
> and that's because they are point sources and not extended objects.
This begs a question. Can you damage your eyes by looking at a very
bright star through a very large telescope? Are there stars that emit
wavelengths that might be dangerous?
Steve
Louis Boyd
The Hale telescope (200") gives a visual brightness increase of about
14.5 magnitudes. Sirius is about 11.1 magnitudes fainter than the full
moon, so looking at sirius will be around 3.4 magnitudes brighter than
the moon naked eye. That's about 23 times. It would be bright enough
you probably wouldn't want to stare at it, but your natural aversion
reflex would prevent damage. It wouldn't be anywhere close to as bad as
glancing at the sun with the naked eye which is 13 magnitudes brighter
than the full moon. Unless you used a very long focus eyepiece the
image would not appear as a point source because of seeing.
Some stars emit lots of ultraviolet but the earths atmosphere is a good
filter.
Brian Tung
> It's not quite true that "looking at the Moon through a telescope is
> no more dangerous than looking at it without the telescope". The
> dark adaption of your eyes will be in much greater danger if you look
> at the Moon through a telescope than without a telescope.
Boy, that is a *nitpick*.
That's not the danger being spoken of. If you look at the Moon, you
are also in danger of falling in love with amateur astronomy. Why not
object to that, too? Or the danger of wanting to buy new eyepieces.
How about that?
Come on, we're talking about dangers that are permanent. No one is
asking about damaging dark adaptation--they're talking about dangers
of damaging your eye permanently. The word "danger" isn't context
free.
Mark
First, I'd like to point out that my response to David's post was
delayed several hours by Google.
Ok, now this makes sense. I spent some time thinking about this
after I got home from work (no sense spending valuable brain cells at
work). Once I sat down and thought about how a telescope-eye system
works and started thinking in terms of area and magnification, I was
able to satisfy myself that looking at the moon at the magnifications
that amateurs use is completely safe. But woe unto thee that uses a
telescope that doesn't magnify!
I will give Brian credit for making available a nice summary of the
situation. Makes perfect sense now.
I still disagree with some of the other posts though. Mostly because
an area is just a set of adjacent points. Therefore, how the
telescope acts on a point will determine how it acts on an area. But
I'll just leave that alone for now, otherwise I might be able to
convince myself that my telescope doesn't actually do anything. ;)
Mark
la...@deleteme.math.kth.se
Sure. If you look at the sun in even a modest telescope, you
will damage your eyes.
Cheers -- Harald
Roman Svihorik
> The total energy entering your eye can be much greater looking through a
> telescope but the surface brightness cannot be greater.
Hmm, is this greater energy a possible reason for my personal feeling as
follows?
It seems to me (I am not sure, it is a feeling only) that during
observing Moon (at about 220x) my eye is heated - I feel it is getting worm.
Is this real feeling or just a fake?
Roman
**************************************************************************
Roman SVIHORIK
Mikoviniho 10
949 11 NITRA e-mail: gem...@ba.telecom.sk
SLOVAK REPUBLIC ISO 8859-2
Roman Svihorik
> observing Moon (at about 220x) my eye is heated - I feel it is getting
> worm.
Getting warm, of course. I am sorry.
Paul Schlyter
Louis Boyd wrote:
> Venus though a very large telescope can appear brighter (both total
> brightness and surface brightness) than the moon because it has a much
> higher albedo.
brightness of the Moon! Even at New Moon, when no sunlit part of the
Moon is visible, the earthshine alone causes the total brightness of
the Moon to reach approximately magnitude -3 (yes: minus three!)
And even with a very thin lunar crescent visible, the total
brightness of the Moon easily outshine the total brightness of Venus!
Paul Schlyter
ERIC K. CHEU <ekc...@supernova.uncg.edu> wrote:
> Build a big enough magnifying glass, concentrate all of the moon's light
> into a very small area of space, and it should be able to blind you.
Build a magnifying glass, as large as you wish, and point it towards
the full Moon and put your eyes in the focus of the lunar light. Your
eyes will only see what appears to be a full Moon spread over the entire
sky -- and the total brightness won't be much brighter than a blue sky
on an ordinary clear day.
Now, will looking at the blue sky blind you?
Paul Schlyter
>Paul Schlyter wrote:
>> It's not quite true that "looking at the Moon through a telescope is
>> no more dangerous than looking at it without the telescope". The
>> dark adaption of your eyes will be in much greater danger if you look
>> at the Moon through a telescope than without a telescope.
>
>Boy, that is a *nitpick*.
>
>That's not the danger being spoken of. If you look at the Moon, you
>are also in danger of falling in love with amateur astronomy. Why not
>object to that, too? Or the danger of wanting to buy new eyepieces.
>How about that?
>
>Come on, we're talking about dangers that are permanent. No one is
>asking about damaging dark adaptation--they're talking about dangers
>of damaging your eye permanently. The word "danger" isn't context
>free.
...and I explained that in the piece of my post which you removed...
True, the word "danger" isn't context free -- therefore one must
specify *which* danger...
Paul Schlyter
Stephen Astro <St...@fastest-inc.com> wrote:
>> A telescope can only increase the brightness of stars
>> and that's because they are point sources and not extended objects.
>
> This begs a question. Can you damage your eyes by looking at a very
> bright star through a very large telescope?
went supernova, it would probably shine as bright as the full Moon
while still appear as a point light source -- if that happened, one
should refrain from looking at Betelgeuze with a telescope.
> Are there stars that emit wavelengths that might be dangerous?
filtered out by our atmosphere already (OK, the near UV is somewhat
more dangerous than visible light). So it's more a matter of
intensity: if intense enough, ANY wavelength will become dangerous!
Paul Schlyter
Mike Simmons <ecli...@mwoa.org> wrote:
> Looking at the Sun with the naked eye can permanently damage it. If you
> hold steady, the small image of the Sun focused on one part of the
> retina will burn it.
really why our vision is saved when we birefly look at the Sun: the solar
image wander around at the retina all the time, and thus the solar heat
is spread over a larger area of the retina.
> If you use a telescope, you'll burn your whole retina instead of just
> a small part of it. The difference is the extent of the damage caused
> by the larger image, not the brightness of the image. The total amount
> of light is greater but not greater per unit area.
is insignificant compared to the apparent size of the solar disk as
seen through the telescope. Thus the retina gets heated all the
time, as opposed to when we view the Sun naked-eye.
Paul Schlyter
Bill Nunnelee <zo...@earthlink.net> wrote:
> Brian and others have done a good job explaining why the surface brightness
> of the moon is lower when viewed through a telescope. But if you want a
> quick and easy way of demonstating the safety of lunar observing, try this.
> Hold a piece of white paper behind the eyepiece so that the moon's image is
> projected onto it. Now take a flashlight or an incandescent bulb or any
> other common light source and shine it on the same paper. Most anything you
> try will actually be brighter than the magnified moon was...and, of course,
> none of them approach the brightness of a fully sunlit landscape!
Sun instead, you'll see that the illumination of the paper by the
solar image isn't that much brighter than the illumination by the
surrounding landscape, and you would then falsely conclude that
looking at the Sun through a telescope isn't that dangerous after all.
Here's how to do this experiment: hold the paper very near the
eyepiece so the exit pupil is projected onto it. Now the
illumination gets much stronger, right? If you do this with the Sun,
the paper will probably start burning after a little while! (don't
actually do this unless you don't care too much about your eyepiece:
the fire may damage the eyepiece glass).
Paul Schlyter
Astronomer <willi...@earthlink.net> wrote:
> Let me also add that an "ordinary sunlit landscape" may actually do
> real damage to your eye, at least over a prolonged period. My eye
> sun. I think his belief is that "ordinary" sunlight is a factor in
> the "aging" of our eyes as we get older. I suppose the effect is
> similar to how "ordinary" sunlight damages human skin.
It's the UV in sunlight which does this damage. The Moon's
reflectivity in UV is very low, much lower than in visible light.
And the optics of the telescope reduces the amount of UV even more.
Also we are often outdoors for many hours, but we rarely gaze at
the full moon through a telescope for many hours.
> Personally, I find viewing "ordinary sunlit landscapes" uncomfortable
> without sunglasses.
I guess this depends on which landscapes, and on whether the sunlight
is very strong. On a snow-covered mountain at high altitude I too
prefer to use sunglasses.
> Query: Does Moonlight contain ultraviolet light? I assume it does.....
Brian Tung
> True, the word "danger" isn't context free -- therefore one must
> specify *which* danger...
It was. In the original post, *and* in the title to my question,
which you conveniently snipped:
> Q. Is it true that looking at the Moon through a telescope will harm
> your eyes?
I assert that "impair your night adaptation" is not a reasonable
interpretation of "harm your eyes." Read more carefully, please.
Wiliam Hamblen
> This begs a question. Can you damage your eyes by looking at a very
> bright star through a very large telescope? Are there stars that emit
> wavelengths that might be dangerous?
is that they are so very far away. Sirius through a large telescope
will make you squint, but it won't hurt you.
Wiliam Hamblen
>
> Hmm, is this greater energy a possible reason for my personal feeling as
> follows?
> It seems to me (I am not sure, it is a feeling only) that during
> observing Moon (at about 220x) my eye is heated - I feel it is getting worm.
> Is this real feeling or just a fake?
>
You're not receiving enough energy to heat your eye. It would be nice
to warm your hands at the eyepiece on a cold night, but you can't. I
think the feeling comes from straining to see and your body heat warming
the eyepiece.
Stephen Astro
>
> The Hale telescope (200") gives a visual brightness increase of about
> 14.5 magnitudes. Sirius is about 11.1 magnitudes fainter than the full
> moon, so looking at sirius will be around 3.4 magnitudes brighter than
> the moon naked eye. That's about 23 times. It would be bright enough
> you probably wouldn't want to stare at it, but your natural aversion
> reflex would prevent damage. It wouldn't be anywhere close to as bad as
> glancing at the sun with the naked eye which is 13 magnitudes brighter
> than the full moon. Unless you used a very long focus eyepiece the
> image would not appear as a point source because of seeing.
>
> Some stars emit lots of ultraviolet but the earths atmosphere is a good
> filter.
But Louis, the full moon has its light scattered over a large angular
area where Sirius's light is concentrated at a single point. Not a
perfect point, but if you looked at it through the Hale scope (you'll
call me of course. I'll bring the beer) all that light will be on one
spot on the retina.
Steve
Stephen Astro
> Build a big enough magnifying glass, concentrate all of the moon's light
> into a very small area of space, and it should be able to blind you.
That's right. That point in a telescope's light path is in front of
the prime focus. That could be a dangerous place to place your eye.
Steve
Louis Boyd
>
> Louis Boyd wrote:
>
> > The total energy entering your eye can be much greater looking through a
> > telescope but the surface brightness cannot be greater.
>
> Hmm, is this greater energy a possible reason for my personal feeling as
> follows?
> It seems to me (I am not sure, it is a feeling only) that during
> observing Moon (at about 220x) my eye is heated - I feel it is getting worm.
> Is this real feeling or just a fake?
I get warm feelings when I observe the moon too. It's not from the
optical energy though.
The light from the sun is about 1kw/sq meter. The moon is 13 magnitudes
less or 6 milliwatts per square metere or 6 nanowatts per square mm..
With a 60 degree filled eyepiece you can get a maximum of (60/.5)^2
(increased apparent area) times 6 nanowatts time the area of your eye
pupil in square mm. In bright light that would be around 2mm diameter
or 3.2 sq mm. So the energy over that wide area of your retina would be
about 200 microwatts. I doubt thats enough to feel heating though it's
pretty bright. You'll get that at 120 x with a telescope of 10" or
larger aperture. You'll get a little more with a larger scope with a
wider field eyepiece, but not much.
--
Lou Boyd
Louis Boyd
>
>
> Louis Boyd wrote:
>
> > Venus though a very large telescope can appear brighter (both total
> > brightness and surface brightness) than the moon because it has a much
> > higher albedo.
>
> The total brightness of Venus is rarely greater than the total
> brightness of the Moon! Even at New Moon, when no sunlit part of the
> Moon is visible, the earthshine alone causes the total brightness of
> the Moon to reach approximately magnitude -3 (yes: minus three!)
> And even with a very thin lunar crescent visible, the total
> brightness of the Moon easily outshine the total brightness of Venus!
You seem to have missed the "through a very large telescope" part. You
can't increase apparent surface brightness with a telescope, only
increase its angular size while keeping the same surface brightness.
Once you fill your eye's field of view it won't get brighter with a
larger telescope. Venus has a higher surface brightness, and filling
your eye's field of view with that will be brighter than the moon can
ever appear. It takes a VERY large telescope to do that.
--
Lou Boyd
Zane
Actually, this is not true -- you couldn't really harm anything with the
moon at the focus of any lens, however large.
The reason is that a very large lens has a very long focal length, even if
it has, say, a focal ratio of 1. Since the moon has an angular size of
about 1/2 degree, the size of the image at the focus of the lens will
subtend an angle of 1/2 degree also. That is, it gets bigger as the lens
gets bigger.
An interesting feature of imaging (due to the above) is that the power
density in the image of an extended object at the focus of a lens doesn't
change as the lens diameter gets larger, as long as the focal ratio remains
the same. The power density in the image is always less than the power
density leaving the source, for any realizable short focal ratio. So the
watts per square centimeter at the lens focus with any lens would be less
than the watts per square centimeter hitting your hand (or eye) from light
reflected from the moon's surface if you were on the surface. In other
words, you could barely feel it, if at all.
So, it would be quite safe to put your eye, or skin, in the image of the
moon at the prime focus of a lens of any size, as long as you didn't leave
it there for thousands of hours (enough time to start to worry about
whether you might get cataracts from long-term UV exposure when you get
older).
Zane
WJ Blanton
> can't increase apparent surface brightness with a telescope, only
> increase its angular size while keeping the same surface brightness.
Help me understand this, I'm still not good with optics theory. If a dso ia
a given brightness in a say, f8 6" scope.....would it not be brighter in a
f8 8" scope with the same image scale (angular size)? Wouldn't it be same
angular size in the bigger scope but be brighter given the extra dia?
Thanks for explaining,
William
"Louis Boyd" <bo...@apt0.sao.arizona.edu> wrote in message
news:3BEC54F9...@apt0.sao.arizona.edu...
Brian Tung
> But Louis, the full moon has its light scattered over a large angular
> area where Sirius's light is concentrated at a single point. Not a
> perfect point, but if you looked at it through the Hale scope (you'll
> call me of course. I'll bring the beer) all that light will be on one
> spot on the retina.
True enough, the Moon is an extended object, whereas Sirius is a point.
But the retina can't get all the light, because the light has to get
through the pupil first. If you used a huge telescope without an
eyepiece, and placed your eyeball right at the focal plane, it *seems*
to me that you could damage your eyeball with a star, with a big enough
telescope. However, I haven't thought this through, and I have no idea
how big would be big enough.
Sam Wormley
>
> I have a 25 inch Obsession which is often used by the public to view the
> moon. Concern has been expressed that permanent damage to the eye can be
> caused by viewing the full moon without a filter. Is that correct? Any
> references would be welcomed.
>
> David N.
Background on huming eye (some quantitative information):
http://chppm-www.apgea.army.mil/dcpm/vcp/afopnet/ISSUES/NVG/NVG3-02.HTM
http://chppm-www.apgea.army.mil/dcpm/vcp/afopnet/ISSUES/NVG/TOC.HTM#CONTENTS
http://www.fandm.edu/departments/Psychology/Eyeprotection/Hazards04.html
ERIC K. CHEU
that will focus all of the light into a very small area. No exit pupil,
no eyepiece for magnifying (and hence spreading out the photons of light
over a larger area). Just a big enough lens that concentrates all of the light
into a small area.
Zane (zane...@sansnetcom.com) wrote:
Mario Mendes
even then you would only be able focus a small fraction of its reflected
light, because one of the laws of optics says:
"By definition, parallel rays entering an _ideal_ lens meet at its focal
plane"
That says that in order to focus all the light into one point, all of the
rays of light entering a lens must be parallel to each other.
The moon does not reflect all of its light in parallel rays, but in all
directions, and your lens would not be ideal. So you'd only be able to
focus a very tinny amount of the light from the moon.
The sun on the other hand, emits way much more energy from its surface than
it is required to burn your eyes.
Notice the difference. All of the light reflected from the moon, coming
towards you (whether directly to your eyes or through a telescope) does not
amount to the same amount of ENERGY that is required to burn your eyes, but
the sun delivers MANY MANY times more ENERGY than the minimum required to
burn your eyes.
"ERIC K. CHEU" <ekc...@supernova.uncg.edu> wrote in message
news:9si2nq$ott$1...@hypatia.uncg.edu...
William R. Meyers
Hello, all:
But have you considered the effect of moonlight when combined with roses? Much
more powerful. Be careful.
Bill Meyers
Zane
>I'm not talking about a telescope necessarily , just a magnifying glass
>that will focus all of the light into a very small area. No exit pupil,
>no eyepiece for magnifying (and hence spreading out the photons of light
>over a larger area). Just a big enough lens that concentrates all of the light
>into a small area.
That's what I'm talking about too -- just a big lens with a single focal
point.
Think about a magnifying glass 1 meter in diameter and 1 meter in focal
length put out in the yard so that it focuses the moon on a piece of paper
on the ground. Since the moon is 1/2 degree big, its image on the paper
will make an angle of 1/2 degree as measured from the lens. The full moon
will make a circle, then, about 9 mm in diameter on the paper.
Now use a glass 2 meters in diameter and 2 meters in focal length.
(Everything just scaled up by a factor of two.) The lens will collect four
times as much moonlight as the other one. But the image is now a little
less than 18 mm in diameter, so that the four-times-higher total power is
spread over a circle that's four times bigger in area than before. The
flux density tending to heat the paper (or your eye or whatever is placed
at the focus of the lens) is therefore the same.
And so forth for any size. The only way to increase the flux density is to
shorten the focal length and keep the diameter the same. A practical limit
is about f/0.5, though, as you can see if you trace some rays from an
imaginary lens.
The formula for the power density from a diffuse object, like the moon, at
the focal point of a magnifying glass (or camera lens) is
Hf = (Ho x T)/(1 + 4f^2)
where
Hf is the watts/cm^2 at the focus
Ho is the watts/cm^2 leaving the surface of the object being imaged
T is the transmission of the lens
f is the focal ratio (focal length/diameter) of the lens
You can see that the diameter of the lens is not important. For example,
for any perfect lens with a focal ratio of 1, the power density at the
focus is 1/5 as high as the power density _leaving_ the imaged surface. In
this case, the moon's surface, which is lower than the power density
leaving a sunlit concrete driveway, for example -- a very safe power
level.
This subject came up once before in the context of whether one could fry an
ant using the full moon and a big enough magnifying glass. If you plug in
the power density leaving the moon's surface in the above equation, you
find that the ant will just think it's getting daylight and feel no
discomfort at all.
The sun is a different story, of course. An f/1 lens gives a power density
1/5 (ignoring atmospheric losses) of that leaving the sun's surface! Hot
enough to fry most anything.
Zane
Mike Simmons
>
> Paul Schlyter wrote:
> > It's not quite true that "looking at the Moon through a telescope is
> > no more dangerous than looking at it without the telescope". The
> > dark adaption of your eyes will be in much greater danger if you look
> > at the Moon through a telescope than without a telescope.
>
> Boy, that is a *nitpick*.
Consider the source, Brian. It's to be expected.
Mike Simmons
Chris Marriott
"Mark" <mf...@cpsc.ucalgary.ca> wrote in message
news:3e7575bf.01110...@posting.google.com...
> Ok, now this makes sense. I spent some time thinking about this
> after I got home from work (no sense spending valuable brain cells at
> work). Once I sat down and thought about how a telescope-eye system
> works and started thinking in terms of area and magnification, I was
> able to satisfy myself that looking at the moon at the magnifications
> that amateurs use is completely safe. But woe unto thee that uses a
> telescope that doesn't magnify!
No, looking at the Moon through ANY telescope at ANY magnification is safe.
If you consider the "surface brightness" of the Moon - ie, the amount of
light which reaches your eye from each "square degree" of what you're
looking at - then no "passive" imaging system can increase that beyond what
you see with the naked eye. The very best that a telescope can do is to make
the Moon appear to fill more of your field of view while retaining its naked
eye surface brightness - it cannot increase that surface brightness.
The Moon is merely a sunlit landscape. Surely your own everyday experience
tells you that looking at a sunlit landscape here on Earth through a
telescope or pair of binoculars doesn't make that landscape "brighter"!
Regards,
--
Chris
---------------------------------------------------------------
Chris Marriott, SkyMap Software, UK
e-mail: ch...@skymap.com Web site: http://www.skymap.com
Astronomy software written by astronomers, for astronomers
Mark Gingrich
> But have you considered the effect of moonlight when combined with
> roses? Much more powerful. Be careful.
I believe that leads to a malady called "stars in your eyes" disease.
Very scary stuff....
--
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Mark Gingrich gri...@rahul.net San Leandro, California
Louis Boyd
>
> You
> > can't increase apparent surface brightness with a telescope, only
> > increase its angular size while keeping the same surface brightness.
>
> Help me understand this, I'm still not good with optics theory. If a dso ia
> a given brightness in a say, f8 6" scope.....would it not be brighter in a
> f8 8" scope with the same image scale (angular size)? Wouldn't it be same
> angular size in the bigger scope but be brighter given the extra dia?
> Thanks for explaining,
> William
I wasn't trying to say that looking though an F8 8" scope couldn't give
greater surface brightness than a f8 6" scope. I just said that
neither can give a surface brightness greater than that of the unaided
eye. It's a matter of how much of the light which enters the
telescope actually makes it into the eye. A telescope forms an image
of the entrance pupil (the primary mirror or objective lens) at the
point you place your eye called the exit pupil. It is the same shape as
the lens or mirror, usually round. It will be the size of the
objective lens divided by the magnification. The diameter of the exit
pupil gets smaller as the magnification is increased, but is linearly
larger for an increase in objective aperture. For eyepieces with a 60
degree field that would start to occur at 120x magnification looking at
the 1/2 degree moon and it's independent of the diameter of the mirror.
If the exit pupil is exactly the size of eye's entrance pupil, objects
will have the same apparent surface brightness as viewed with the naked
eye minus reflection and transmission losses in the telescope optics.
If the exit pupil is larger than the eye, light is lost which would
otherwise go toward making the image brighter. The surface brightness
will be the same as for the optimum case, but the object will be
smaller. If the magnification is higher, the exit pupil will be smaller
than the eye's pupil, but the light will be spread out over a larger
area. As magnification is increased the surface brightness decreases
while the total brightness of the object remains constant up to the
point it fills the field of the eyepiece. Beyond that the total
brightness will drop off too as magnification increases.
Look at two cases. One where Assume we're looking at a full moon with
30 arcminute diameter and that the eye has a 1/4" diameter.
6" 20x 8" 20x 6" 50x 8" 50x
exit pupil dia 0.3" .4" 0.12" 0.16"
Apparent dia of moon 10 deg 10 deg 25 deg 25 deg
surface brightness * 1.0 1.0 .23 .41
total brightness * 400x 400x 576x 1024x
* relative to naked eye (transmission losses not included)
I purposely picked magnifications on either side of an exactly filled
pupil for both size mirrors.
A third case is where the exit pupil just fills the 1/4" eye pupil.
for a 6" telescope that would be at 24x and for a 8" telescope it would
be at 32x.
exit pupil diameter 6" 24x 8" 32x
apparent image dia 0.25" 0.25"
apparent diameter of moon 12 degrees 16 degrees
surface brightness * 1.0 1.0
total brightness 576x 1024x
That doesn't mean that having the telescope's exit pupil exactly match
the eye's entrance pupil is desirable, because any lateral motion of the
eye changes the image brightness. That's not the case with an
underfilled or overfilled pupil.
To see the exit pupil of your telescope aim it at the clear sky and hold
a piece of ground glass (or waxed paper) where your eye would normally
go. Adjust it in and out to get a sharp image of the primary. If you
compare the ratio of the diameter the objective to the diameter of the
exit pupil it will have the same ratio as the objective focal length to
the eyepiece focal length. Both ratios will be the telescope
magnification.
The above calculations aren't very realistic since when looking at the
moon the eye's pupil will be a lot smaller than 1/4".
--
Lou Boyd
Martin Brown
Paul Schlyter wrote:
> In article <3BEB3AD6...@mwoa.org>,
> Mike Simmons <ecli...@mwoa.org> wrote:
>
> > Looking at the Sun with the naked eye can permanently damage it. If you
> > hold steady, the small image of the Sun focused on one part of the
> > retina will burn it.
>
> Most people find it very hard to hold their eyes that steady.
The first person to do it seriously for science was the Belgian, Joseph
Plateau - discoverer of persistence of vision and inventor of the stroboscope.
He went partially blind at 28 and totally at 40 as a result of staring fixedly
at the sun in an experiment to see what happened. Try:
http://www.geometry.net/Scientists/Plateau_Joseph.htm
Staring deliberately at the sun even with the naked eye is not a smart move.
> And that's really why our vision is saved when we birefly look at the Sun:
> the solar
> image wander around at the retina all the time, and thus the solar heat
> is spread over a larger area of the retina.
Indeed normally the image is always shifting about. If you jig things so the
image tracks the motion of the eyeball exactly the perceived image gradually
fades out.
Afterwards you see a negative image of the scene when looking at a blank white
sheet.
Various demos of this trick exist on the web.
Regards,
Martin Brown
Martin Brown
Paul Schlyter wrote:
> Here's how to do this experiment: hold the paper very near the
> eyepiece so the exit pupil is projected onto it. Now the
> illumination gets much stronger, right? If you do this with the Sun,
> the paper will probably start burning after a little while!
I remember some of the older observers at my first astronomical society had a bad
tendency to light their cigarettes off the exit pupil of a 4" scope being used for
solar projection.
This practice meant that the eyepiece used for solar work had to be cleaned rather
often.
> (don't actually do this unless you don't care too much about your eyepiece:
> the fire may damage the eyepiece glass).
If it is a cemented eyepiece it may well crack under the thermal load.
The same problem that befalls those wretched "sun" filters that were in toy scope
kits.
Regards,
Martin Brown
David Knisely
> If you apply this same argument to viewing the sun it does not seem to make
> sense. Clearly I can look at the sun briefly with my eye but would not dare
> to look at the sun thru a scope.
>
No, it isn't quite the same. You *can* damage your eyes by looking at
the sun even without a telescope. In the brief glance mode, there isn't
enough time to do much permanent damage, but if you stared and kept the
eye fixed on it, you would eventually see damage in one spot. Your eye
can move around or look away, but in a telescope, a large portion of the
field of view is saturated by intense light, so addition to having to
deal with more light, your eye would have no place to look away to, and
the damage would be over a large portion of the retina. In any case,
you cannot hurt your eye by looking through the bright moon through a
telescope. I have been doing so for over 30 years, and my eyes still
can reach past 7th magnitude unaided from a dark sky site. It may be
too dazzling for some people, and they might want to use filtration to
reduce this, but it will *not* damage your eyes. Clear skies to
you.
--
David Knisely KA0...@navix.net
Prairie Astronomy Club, Inc. http://www.4w.com/pac
Hyde Memorial Observatory:
http://www.blackstarpress.com/arin/hyde
******************************************************
* Attend the 9th Annual NEBRASKA STAR PARTY *
* August 4-9, 2002 http://www.nebraskastarparty.org *
******************************************************
Mike Simmons
>
> I remember some of the older observers at my first astronomical society had a bad
> tendency to light their cigarettes off the exit pupil of a 4" scope being used for
> solar projection.
Ferdinand Ellerman -- one of the people George Ellery Hale brought with
him from Yerkes Observatory to the new Mount Wilson Solar Observatory --
used to use the Sun to light his pipe when he had an audience. When
visitors were watching as he made his twice-daily ascent to the top of
the 150-foot solar tower
(http://www.astro.ucla.edu/~obs/images/tower1.jpg) he would stop the
elevator halfway up, take a pipe from his pocket and load it with
tobacco (with great flair and expectation), pull a small lens from his
pocket and form an image of the Sun on the tobacco to light it.
Joe Hickox -- a long-time solar observer at Mount Wilson following
Ellerman -- used to hike up to the observatory from his home in the
valley below when he was young. He was always disappointed at not
seeing anyone that looked like an astronomer to him. He described
seeing Ellerman's little pipe-lighting show one day, at which point he
declared, "Ah, there's an astronomer!"
There was someone cooking a hot dog at the focus of a large refractor at
RTMC this year. It seemed to be doing well but I didn't stick around
for the taste test (not that I was invited anyway!).
Mike Simmons
ERIC K. CHEU
Okay thanks...
apparently there is no practical method for the moon to blind you.
What if in theory, there was a way to make an optical system so that the
focal length could go less than .5, however? like .0000000001?
For example, take a trillion lenses, aim them at a certain (same spot)
using the moonlight.
Zane (zane...@sansnetcom.com) wrote:
: ekc...@supernova.uncg.edu (ERIC K. CHEU) wrote:
: Zane
William Hamblen
wrote:
>There was someone cooking a hot dog at the focus of a large refractor at
>RTMC this year. It seemed to be doing well but I didn't stick around
>for the taste test (not that I was invited anyway!).
It is not hard to make a solar oven from cardboard, aluminum foil and
wire. A typical Boy Scout stunt.
Stephen Astro
"Moonglow" Now that's the stuff that can blind you!
It can make you forget about astronomy--for a while.
Voodoo Ink
William R. Meyers wrote:
Oh yes, be very careful. I somtimes forget to use punctuation ;)
over you
while you sleep
silent darkness
the moon lingers above in sweet
suspended honey
lighting
the garden of wild roses
where
together
my thoughts
visions
descend you
me opening upon you like the petals of a rose
I tell you
of moon burn
whispers with my
eyes
Cathy
--
http://www.VoodooInk.net
William R. Meyers
Thank you for the lovely poem.
Bill Meyers
WJ Blanton
increase brightness.....but the eye is a scope. Also I don't understand how
a scope does not make an object brighter than the unaided eye.... If I
can't see but mag1+ stars lets say with my eye but when I use binos I can
see mag 5 or 6+ stars....isn't that making the stars apparently brighter
than unaided? Sorry I'm still not catching on.
Thanks for your helping me understand,news:3BECEF2F...@apt0.sao.arizona.edu...
WJ Blanton
> you see with the naked eye.
This is similar to an above dicussion I'm having about same sublect. I
don't understand this. With unaided eye I can see mag3+ stars lets say from
my yard, but with binos I can see mag6+ stars which seem to say the binos
are increasing apparent brightness over the unaided eye. Also, with my 70mm
binos I can see in dimm dusk light much better than with my eye alone. I'm
not sure I grasp your or the above posters concept, I'm sure ya'll are
correct but it 2+2 is not equaling 4 for me.
Thanks for explaining,
William
"Chris Marriott" <ch...@chrism.nospam.demon.co.uk> wrote in message
news:1005376846.12071....@news.demon.co.uk...
William R. Meyers
WJ Blanton wrote:
> Boy, you blinded me with science! So you meant unaided eye vs. scope can't
> increase brightness.....but the eye is a scope. Also I don't understand how
> a scope does not make an object brighter than the unaided eye.... If I
> can't see but mag1+ stars lets say with my eye but when I use binos I can
> see mag 5 or 6+ stars....isn't that making the stars apparently brighter
> than unaided? Sorry I'm still not catching on.
> Thanks for your helping me understand,
> William
Hi, WJ,
You are not catching on because this thread has been a miracle of
obfuscation. Exactly how this obfuscation has been achieved I don't know but I
have some ideas, as follows.
Semantic confusion has been introduced by the use of " brighter" in several
very different senses, as one of the posters has pointed out. Specifically, the
same word has been used for "surface brightness" ( which seems to be emission
per unit of area) and "total brightness" of the entire object (which seems to
be all the light emitted by or reflected from it.).
Some of the obfuscation may have been intentional. That is, some may have
taken pleasure in puzzling the reader through creating paradoxical formulations
by using "brightness" in two different senses at once. This puts people who have
taken college physics "one up" on those who have not.
Others of the posters undoubtedly meant well but just aren't very good at
putting complex physical and biological phenomena into words.
Some posters, such as Tony Flanders, and many others too, do have a flair
for explaining physical phenomena clearly, and I look forward to their posts.
And Roland Christen and Tom Back are very clear on advanced optical issues.
Oddly, the posts on saa explaining computer issues, such as the nature of
viruses, how to achieve virus protection, or the use of filters to screen out
trolls, are almost always very lucid and helpful. I don't understand why the
posts explaining computer issues tend to be so much more lucid than the posts
explaining physical optics.
I can see needing up to, say, half a dozen responses to explain a point
in answer to a question, such as we get when we have a query about computers. It
is puzzling to me why it takes 50 responses to explain that the full moon, when
it is entirely in the eyepiece field of a large telescope, is unpleasantly
bright but does not damage your retina. S.a.a. is given to using fifty
responses to discuss something that one person could have explained clearly the
first time, or at least that could have been explained completely in half a
dozen responses. (I am not referring to threads giving opinions or personal
experience, such as the "refractors versus reflectors" threads.)
As a teacher, I explain things for a living, and am consequently
impatient with those who seem to enjoy obfuscation. But I can't explain things I
myself don't understand , so I can't lend any clarity to threads such as the
present one, but I do get irritated when people contradict each other endlessly
on matters of fact.
The above post is just my opinion. Maybe everyone else finds this thread
very clear, and I am reacting only out of my own intellectual limitations. Could
be.
Clear skies,
Bill Meyers
William R. Meyers
This thread is very unclear. You are not at fault for not understanding it.
WJ Blanton wrote:
> then no "passive" imaging system can increase that beyond what
> > you see with the naked eye.
>
> This is similar to an above dicussion I'm having about same sublect. I
> don't understand this. With unaided eye I can see mag3+ stars lets say from
> my yard, but with binos I can see mag6+ stars which seem to say the binos
> are increasing apparent brightness over the unaided eye.
Here I think the issue is that a star is a point source, not an extended object,
and even these benighted posters can agree that a telescope does make a point
source brighter than it would be naked eye. They are arguing about extended
objects, like the moon, not about point sources. That is, if I understand this
crazy thread at all.
>
> Also, with my 70mm
> binos I can see in dimm dusk light much better than with my eye alone.
Yes, this is the key issue, not well explained in this long confusing thread.
> I'm
> not sure I grasp your or the above posters concept, I'm sure ya'll are
> correct but it 2+2 is not equaling 4 for me.
> Thanks for explaining,
> William
You are not at fault for not understaning these "explanations."
Clear skies,
Bill Meyers
>
>
Zane
The geometries involved will frustrate doing this. One can show by more
basic physical principles than ray tracing that it would violate the
fundamental laws of physics if you could make an image brighter than its
source.
Zane
WJ Blanton
point source vs extended object.....would my same example not be true of say
m57 or a dusk landscape....both are extended and while I'm not sure about
m57 and like dso's being brighter in my 70mm binos, I do feel that a dusk
lit landscape is brighter through them....does this qualify as an extended
object?
Thanks for your answers,
William
"William R. Meyers" <william...@uc.edu> wrote in message
news:3BEE0BA5...@uc.edu...
Louis Boyd
eye is a telescope. It's not. Try thinking about what I posted instead
of just being scared by it.
The case of point sources is a bit different than for extended sources.
Note the word "Moon" in the subject line. The concept still applies
however. If the magnification of a telescope is low enough that they
eye's entrance pupil blocks part of the light, increasing the
telescope's aperture will not produce ANY apparent brightness increase
if the magnification is kept constant. Most telescopes and
magnifications used for stargazing have high enough magnification that
doesn't happen. If you actually analyze the image which forms on the
eye the same principle still applies. The surface brightness won't be
any brighter than the surface brightness of the star as if you viewed it
close up with your eye. Since your eye can't resolve either image it
will appear brighter both for surface brightness and total brightness
through the telescope. Once the magnification of the telescope is high
enough to fill the eye's entrance pupil, additional magnification will
not make the star appear brighter. If you had an absolutely huge
telescope a class g star could appear to have as high of surface
brightness as looking at the sun naked eye. A class O star could be
much brighter. Such a large telescope does not exist.
--
Lou Boyd
--
Lou Boyd
Fairborn Observatory
WJ Blanton
>Try thinking about what I posted instead
> of just being scared by it.
>
Maybe I did think about it but still didn't understand your
explanation....thanks for pointing out my stupidity....I've learned my
lesson and will no longer ask for clarification so I can understand
something better...SORRY!
--
-- William.... Dallas, Tx --
---------------------------------------
Louis Boyd
distributed than the cones. Higher magnification is required to give
equivalent resolution when your eye is dark adapted. You can almost
always see more detail at a high magnification than the one which gives
the greatest surface brightness although the surface brightness is
lower. The human eye has variable sensitivity depending on a number of
things including surface brightness and the total brightness in the
field. As an example it's much easier to see the central star of the
ring nebula if you increase the magnification to the point the ring is
entirely outside of the field of the eyepiece. It doesn't make the star
a bit brighter doing that, but it looks a lot brighter because the eye
increases it's sensitivity in the reduced brightness field.
--
Lou Boyd
--
Lou Boyd
Fairborn Observatory
Louis Boyd
>
> "Louis Boyd" <bo...@apt0.sao.arizona.edu> wrote in message
> news:3BED6EC7...@apt0.sao.arizona.edu...
> >Try thinking about what I posted instead
> > of just being scared by it.
> >
>
> Maybe I did think about it but still didn't understand your
> explanation....thanks for pointing out my stupidity....I've learned my
> lesson and will no longer ask for clarification so I can understand
> something better...SORRY!
I wasn't intending offense. Perhaps you could as a question specific
about the parts you don't understand. Surely some of what I said made
sense.
--
Lou Boyd
WJ Blanton
me understand. I'm sorry but the technical mumbo jumbo didn't make alot of
sense to me, maybe I'm denser than most. Someone pointed out nicely that
the statement I did not understand did not apply the same to a point source.
That I understood. But more basically then, why does a dusk lit landscape,
which is not a pointsource, look brighter to me through my 70mm binos than
with the unaided eye? I'm not arguing with you, just trying to understand
things that *seem* to confict with the statement made by you and others that
a scope does not make things brighter.
Thanks,
William
"Louis Boyd" <bo...@apt0.sao.arizona.edu> wrote in message
news:3BED7F4C...@apt0.sao.arizona.edu...
Enyo
create higher intensity than the source but they don't form
images.
"Zane" <zane...@sansnetcom.com> wrote in message
news:3bed6cd7...@nntp.ix.netcom.com...
Steve Tyler
>Astronomer <willi...@earthlink.net> wrote:
>> Let me also add that an "ordinary sunlit landscape" may actually do
>> real damage to your eye, at least over a prolonged period.
...
>> Personally, I find viewing "ordinary sunlit landscapes" uncomfortable
>> without sunglasses.
>I guess this depends on which landscapes, and on whether the sunlight
>is very strong. On a snow-covered mountain at high altitude I too
>prefer to use sunglasses.
Snow blindness is a real danger.
Ultraviolet Keratitis
"... unprotected exposures to the sun or solar eclipses or
exposure to the sun on highly reflective snow fields at high elevation
can lead to direct corneal epithelial injury.
The latter clinical scenario is known as snow blindness."
http://www.emedicine.com/emerg/topic759.htm
"snow blindness"
A usually temporary loss of vision
and inflammation of the conjunctiva and cornea,
caused by exposure of the eyes to bright sunlight and ultraviolet rays
reflected from snow or ice.
http://www.bartleby.com/61/
"snow blindness"
Date: 1748
inflammation and photophobia caused by exposure of the eyes
to ultraviolet rays reflected from snow or ice
http://www.m-w.com/
--
Steve Tyler
Peter Rathmann
...
> That I understood. But more basically then, why does a dusk lit landscape,
> which is not a pointsource, look brighter to me through my 70mm binos than
> with the unaided eye?
This is an easy experiment to try. Hold your 70mm (or whatever)
binoculars so one eye looks through one side of them and the other eye
looks directly at the same landscape. Pick out an area of fairly even
brightness and position the binoculars so you see this area with both
eyes simultaneously.
Whenever I have done this it confirmed that the surface brightness as
seen with the unaided eye was at least as bright as when seen with an
optical aid. If your binoculars are around 10-15 power the surface
brightness should be almost equal to the unaided eye view (reduced only
through transmission losses in the optics) since the exit pupil is as
big as your eyes pupil. With greater magnification as is typical for
telescopes, the surface brightness would be less than with the unaided
eye.
WJ Blanton
1x power, would they not make things seem brighter? The explanations are in
confict (in *my* mind) with the common description of a telescope
"collecting" more light than the eye or smaller scope. Ya'll have made a
vallent attempt to explain this though, Thanks.
-William
"Peter Rathmann" <prat...@home.com> wrote in message
news:3BEDB158...@home.com...
Mike Simmons
pupil will also be the same size as the objective -- 70mm. The pupil of
your eye will be less than 7mm. The rest of the light in the 70mm image
is wasted shining on your face rather than going into your eye. The
amount of light per unit angular size is the same.
What you may be thinking of is a system that doesn't form an image with
additional optics so that your eye can view it. If you're taking photos
with your 70mm lens then the light is all formed in an image on the
focal plane. The shorter the focal length, the smaller the image and
the greater the amount of energy per unit area at the focal plane. But
using an eye with a lens in it as a detector rather than a flat CCD chip
or piece of film, you're limited as to what you can do because you have
to use an eyepiece to produce an image that can be viewed with the
optical system you're born with. If you could remove the lens from the
eye and build your own optical system for imaging with the retina then
there are lots of great things you could do! Or, as to burning your eye
with moonlight, I suppose a large and fast enough lens or mirror could
produce an image at prime focus that could burn your cornea if you put
your eye there. You wouldn't be viewing the image (unless you got your
eye in just the right position) but you would accomplish your goal of
permanent eye damage! ;-)
Mike Simmons
Louis Boyd
>
> So pulling together everything on this subject....If I had 70mm binos with
> 1x power, would they not make things seem brighter? The explanations are in
> confict (in *my* mind) with the common description of a telescope
> "collecting" more light than the eye or smaller scope. Ya'll have made a
> vallent attempt to explain this though, Thanks.
> -William
anything look brigher. It's called a picture window. I could improve
it by getting the surfaces anti-reflection coated. No optical telescope
can do any better for providing a bright image at "times one
magnification" If it did it would violate the basic laws of
conservation of energy.
A picture window collects a whole lot of light just like a telescopy,
but when you look through it at low magnification (in this case 1.0)
your eye restricts the amout of light it can receive. The same is true
when looking though a telescope. Reread the post about entrance and
exit pupils.
--
Lou Boyd
Louis Boyd
>
> So pulling together everything on this subject....If I had 70mm binos with
> 1x power, would they not make things seem brighter? The explanations are in
> confict (in *my* mind) with the common description of a telescope
> "collecting" more light than the eye or smaller scope. Ya'll have made a
> vallent attempt to explain this though, Thanks.
It's amazing that if you tell a group of people on the Internet that
there are no naturally occuring purple cows some will say you said cows
don't exist and a few others will claim you said there is no color
called purple.
--
Lou Boyd
WJ Blanton
Well, at least the poster above you provided me a simple explanation with
out the sarcasim.
-Williamnews:3BEDD2A6...@apt0.sao.arizona.edu...
Bill Nunnelee
"Paul Schlyter" <pau...@saaf.se> wrote in message
news:9sh9sn$g2t$1...@merope.saaf.se...
> In article <rkIG7.26968$hZ.24...@newsread2.prod.itd.earthlink.net>,
> Bill Nunnelee <zo...@earthlink.net> wrote:
>
> > Brian and others have done a good job explaining why the surface
brightness
> > of the moon is lower when viewed through a telescope. But if you want a
> > quick and easy way of demonstating the safety of lunar observing, try
this.
> > Hold a piece of white paper behind the eyepiece so that the moon's image
is
> > projected onto it. Now take a flashlight or an incandescent bulb or any
> > other common light source and shine it on the same paper. Most anything
you
> > try will actually be brighter than the magnified moon was...and, of
course,
> > none of them approach the brightness of a fully sunlit landscape!
>
> That's not a good way to do it! If you repeat the experiment on the
> Sun instead, you'll see that the illumination of the paper by the
> solar image isn't that much brighter than the illumination by the
> surrounding landscape, and you would then falsely conclude that
> looking at the Sun through a telescope isn't that dangerous after all.
>
> Here's how to do this experiment: hold the paper very near the
> eyepiece so the exit pupil is projected onto it. Now the
> illumination gets much stronger, right? If you do this with the Sun,
> the paper will probably start burning after a little while! (don't
> actually do this unless you don't care too much about your eyepiece:
> the fire may damage the eyepiece glass).
>
> --
> ----------------------------------------------------------------
> Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
> Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
> e-mail: pausch at saaf dot se or paul.schlyter at ausys dot se
> WWW: http://hotel04.ausys.se/pausch http://welcome.to/pausch
>
Steve Tyler
>Thanks, that help me understand it all, it is all so obvious now.
>Well, at least the poster above you provided me
>a simple explanation with out the sarcasim.
If your current newsreader doesn't support
"filters" or "killfiles", here is a solution
to the problem of sarcasm without explanation:
--
Steve Tyler
Martin Brown
William Hamblen wrote:
Two such designs for solar powered cookers featured recently on a BBC popular
science show and are on the web at
http://www.bbc.co.uk/science/scienceshack/articles/requests/r_oilrunsout01.shtml
Perhaps not surprisingly they didn't cook very well in the late October
sunshine of latitude 50N.
These simple designs ought to work pretty well in midsummer though.
Regards,
Martin Brown
Paul Schlyter
>> then no "passive" imaging system can increase that beyond what
>> you see with the naked eye.
>
> This is similar to an above dicussion I'm having about same sublect. I
> don't understand this. With unaided eye I can see mag3+ stars lets say from
> my yard, but with binos I can see mag6+ stars which seem to say the binos
> are increasing apparent brightness over the unaided eye. Also, with my 70mm
> binos I can see in dimm dusk light much better than with my eye alone. I'm
> not sure I grasp your or the above posters concept, I'm sure ya'll are
> correct but it 2+2 is not equaling 4 for me.
light sources with extended light sources.
But you can view it like this: anything we view will be an "extended"
light source, with a size of either the source itself, or the smallest
source we can resolve, whichever is bigger.
Now, if you view a star naked-eye, it will appear perhaps a few
arcminutes large, because you cannot see details smaller than that.
Now, point a telescope towards the star, and it will appear only one or
a few arcseconds large, because the telescope can see details as small
as that. In effect, this will tremendeously increase the apparent
surface brightness of the star -- and that's why you see stars in
the telescope which are invisible with your naked eye.
But if you point your telescope towards an object large enough to be
truly resolved both with your naked eye and the telescope, the
situation gets different. Such objects are the Moon, the Milky Way,
the Andromeda Galaxy, the Aurora, the Zodiacal Light. And then the
telescope won't be able to increase the surface brightness of the
object at all. Which means if you don't see the object naked-eye,
you won't see it in a telescope either. Try it on e.g. the
Gegenschein (the part of the Zodiacal Light opposite to the Sun in
the sky): it's hard to see and you probably cannot see it. Now,
try to point a telescope to it -- doing so it will still be just as
hard to see!
Paul Schlyter
Martin Brown <martin...@pandora.be> wrote:
> Paul Schlyter wrote:
>
>> Here's how to do this experiment: hold the paper very near the
>> eyepiece so the exit pupil is projected onto it. Now the
>> illumination gets much stronger, right? If you do this with the Sun,
>> the paper will probably start burning after a little while!
>
> society had a bad tendency to light their cigarettes off the exit
> pupil of a 4" scope being used for solar projection.
>
> This practice meant that the eyepiece used for solar work had to be cleaned rather
> often.
Sometimes when I show some kids the Sun by eyepiece projection, some
of them asks if they can look directly through the telescope instead.
Then I sometimes tell one of them to put a finger at the eyepiece
instead -- after a few seconds it hurts and the kid removes the
finger. I think that's a pretty efficient way to teach them why they
shouldn't attempt looking at the Sun through an unfiltered telescope.
Paul Schlyter
Louis Boyd <bo...@apt0.sao.arizona.edu> wrote:
> Paul Schlyter wrote:
>
>> Louis Boyd wrote:
>>
>>> Venus though a very large telescope can appear brighter (both total
>>> brightness and surface brightness) than the moon because it has a much
>>> higher albedo.
>>
>> The total brightness of Venus is rarely greater than the total
>> brightness of the Moon! Even at New Moon, when no sunlit part of the
>> Moon is visible, the earthshine alone causes the total brightness of
>> the Moon to reach approximately magnitude -3 (yes: minus three!)
>> And even with a very thin lunar crescent visible, the total
>> brightness of the Moon easily outshine the total brightness of Venus!
>
> You seem to have missed the "through a very large telescope" part. You
> can't increase apparent surface brightness with a telescope, only
> increase its angular size while keeping the same surface brightness.
> larger telescope. Venus has a higher surface brightness, and filling
> your eye's field of view with that will be brighter than the moon can
> ever appear.
True; however then you're comparing the surface brightness, not the
total brightness, of the Moon and Venus. To get the total brightness
of an object you cannot use a magnification that's too large to show
the entire object in the field of view.
> It takes a VERY large telescope to do that.
It takes high magnification, true, but not necessarily a very large
telescope.
When I was in my teens and had got my first telescope (a 60 mm
refractor), I once replaced the eyepiece with a microscope, and then
I obtained a magnification of perhaps 4000X ! With difficulty I
managed to bring Jupiter into the very narrow field of view, and it
did indeed almost fill the field of view. In a way the view was
impressive, but the image detail was of course horrible -- I saw less
detail on Jupiter using 4000X than using the more common 50X to 150X.
Paul Schlyter
ERIC K. CHEU <ekc...@supernova.uncg.edu> wrote:
> Okay thanks...
> apparently there is no practical method for the moon to blind you.
> What if in theory, there was a way to make an optical system so that the
> focal length could go less than .5, however? like .0000000001?
> For example, take a trillion lenses, aim them at a certain (same spot)
> using the moonlight.
(Zane), and you'll see that going from an f-ratio of 0.5 to
0.0000000001 increases the illumination at focus by only a factor of 2:
# The formula for the power density from a diffuse object, like the moon, at
# the focal point of a magnifying glass (or camera lens) is
#
# Hf = (Ho x T)/(1 + 4f^2)
#
# where
#
# Hf is the watts/cm^2 at the focus
# Ho is the watts/cm^2 leaving the surface of the object being imaged
# T is the transmission of the lens
# f is the focal ratio (focal length/diameter) of the lens
#
# You can see that the diameter of the lens is not important. For example,
# for any perfect lens with a focal ratio of 1, the power density at the
# focus is 1/5 as high as the power density _leaving_ the imaged surface. In
# this case, the moon's surface, which is lower than the power density
# leaving a sunlit concrete driveway, for example -- a very safe power
# level.
Paul Schlyter
>> Mike Simmons <ecli...@mwoa.org> wrote:
>>
>>> Looking at the Sun with the naked eye can permanently damage it. If you
>>> hold steady, the small image of the Sun focused on one part of the
>>> retina will burn it.
>>
>> Most people find it very hard to hold their eyes that steady.
>
> The first person to do it seriously for science was the Belgian, Joseph
> Plateau - discoverer of persistence of vision and inventor of the stroboscope.
> He went partially blind at 28 and totally at 40 as a result of staring fixedly
> at the sun in an experiment to see what happened. Try:
>
> http://www.geometry.net/Scientists/Plateau_Joseph.htm
>
> Staring deliberately at the sun even with the naked eye is not a smart move.
We're in full agreement about this.
Once, back in 1971, I viewed a partial solar eclipse. A friend of
mine was going to visit me and view it with me. But he was late:
when the eclipse had started he hadn't appeared yet. He knew this
and didn't want to miss the eclipse, so he stared at the Sun with his
naked and unprotected eye until he arrived (when he got access to
safe solar filters of course). Some time later he discovered "dots"
in his central field of view which didn't go away. He visited an eye
doctor who told him they were due to permanent damage to small parts
of his retina.
WJ Blanton
--
-- William.... Dallas, Tx --
---------------------------------------
Chris Marriott
"WJ Blanton" <wjb1******@ev1.net> wrote in message
news:3bee9fc1$1...@newsa.ev1.net...
> Then a huge dob will *not* show m31 brighter than the unaided eye?
As has been explained on this thread many times, the work "brighter" is
highly ambiguous and needs to be used very carefully.
When talking about extended objects such as the Moon or a galaxy, the most
useful definition is "surface brightness" - ie, the amount of light which
reaches your eye from each square degree of the object and the answer to
that is no, no telescope will make the surface brightness of M31 greater
than its naked eye surface brightness.
Obviously a telescope will make it a lot bigger, so more light in total will
reach your eye from it, so its "integrated magnitude" will be much brighter
than with the naked eye.
From my light polluted back garden, for example, M33 (which has a very low
surface brightness) cannot be seen due to light pollution. I can't see it
through any telescope from there, whether it be a 3", 8", or 30". No
telescope can increase its surface brightness, so it just plain can't be
seen, no matter how large the telescope.
Regards,
--
Chris
---------------------------------------------------------------
Chris Marriott, SkyMap Software, UK
e-mail: ch...@skymap.com Web site: http://www.skymap.com
Astronomy software written by astronomers, for astronomers
Louis Boyd
For magnifications less then the diameter of a telescope's diameter
divided by the aperture of the eye of the human using it, the surface
brightness (the amount of light from a fixed size portion of the image
of the object) will appear just slightly less through the dob than the
viewing the same object naked eye. The total brightness will be the
magnification squared greater. It will appear much brighter because the
image of the object covers a much larger area of your retina.
For higher magnifications the surface brightness will decrease as the
magnification is increased. The total brightness will remaing constant
at the square of the ratio of the diameter of the telescope divided by
the diameter of the observer's eye.
Of course the object looks brighter through a telescope but that's only
because it appears bigger, equivalent to the object being closer. It's
just like a light bulb looks brighter when it's placed 1 foot in front
of your face compared to one 10 feet away. The one a 1 foot will have
100 times the apparent area so it total brightness is 100 times
greater. It's surface brightness will be the same.
You are simply confusing the concept of surface brightness with total
brightness. At no point did I claim the total brighness could not
exceed that of the unaided eye. It does. The surface brightness cannot.
--
Lou Boyd
--
Lou Boyd
Fairborn Observatory
Richard G Amirault
: I have a 25 inch Obsession which is often used by the public to view the
: moon. Concern has been expressed that permanent damage to the eye can be
: caused by viewing the full moon without a filter. Is that correct? Any
: references would be welcomed.
I don't have time to read *all* the posts answering this ...
My $.02 is that observing the moon in my 8 inch Dob without a filter is
*painful* Maybe I'm more sensitive than most ... but letting random folks
view thru your scope it would be prudent to add a moon filter just to be
safe.
Richard in Boston