where is the red stuff?

On Mon, 23 Jan 2006 20:27:24 GMT, "David Nakamoto"
wrote:

So obviously parts of the nebula were brighter through that telescope, otherwise
color vision wouldn't have kicked in.

I don't think so. I think the effect you are describing is related to
what Roger described, an enhancement of color sensitivity when more
cones are stimulated.

There is nothing a telescope can do to put more photons on a given area
of the retina than it receives seeing the same object without a
telescope. But there are all kinds of physiological phenomena that go
beyond the simple optics.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On 23 Jan 2006 11:58:44 -0800, "decaf" wrote:

How does this explain the color seen in the eyepiece of stars too
faint to be seen unaided?

Stars don't behave as extended sources. Their image size is determined
by diffraction, not simply by the magnification of the telescope. The
larger the telescope aperture, the brighter stars appear. That isn't
true for extended sources, however.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Chris L Peterson" wrote in message

There is nothing a telescope can do to put more photons on a given area
of the retina than it receives seeing the same object without a
telescope. But there are all kinds of physiological phenomena that go
beyond the simple optics.

I've heard this before but it is counter-intuitive (to me at least). The
telescope collects photons across the area occupied by its objective and
condenses them to a subpupil aperture, does it not?

Ed T.

Ed T wrote:
There is nothing a telescope can do to put more photons on a given area
of the retina than it receives seeing the same object without a
telescope. But there are all kinds of physiological phenomena that go
beyond the simple optics.

I've heard this before but it is counter-intuitive (to me at least). The
telescope collects photons across the area occupied by its objective and
condenses them to a subpupil aperture, does it not?

One of MOPFAQ answers this for the Moon...

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 pupil--actually gets in. The rest
of it falls uselessly (at least as far as image brightness is concerned)
on the surface of your eyeball. 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.

Of course, it *feels* as though the Moon is about to blind us, for two
reasons. One is that we typically observe the Moon by night. The same
phase by day is just as bright, but it doesn't feel blindingly bright
through the telescope because our eyes are then accustomed to daytime
light levels. Another reason is that the Moon *is* magnified by the
telescope, and at the same intensity throws more total light onto your
retina. By way of an analogy, if I shine a flashlight into your eye at
a distance of 10 cm, it's uncomfortably bright, whereas if I put a mask
on the flashlight that only lets through a tiny spot of light, it's
merely annoying. The total light output is much smaller, but the
intensity of that tiny spot is just as great as before.

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.)

Copyright (c) 2001-2006 Brian Tung

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

Ed T wrote:
"Phil Wheeler" wrote in message
...

pascal wrote:

Thanks all for these infos. I guess there is nothing more to expect
from a LX200 8" then (which I was planning to buy)



Not unless you do astrophotography.


And if you look at those photos in dim light, they'll still look black and
white g.
http://hyperphysics.phy-astr.gsu.edu...n/rodcone.html



Of course, there is always the "rose colored glasses" approach

On Mon, 23 Jan 2006 20:59:51 GMT, "Ed T" wrote:

I've heard this before but it is counter-intuitive (to me at least). The
telescope collects photons across the area occupied by its objective and
condenses them to a subpupil aperture, does it not?

Keep in mind that the actual number of photons you can get onto your
retina may be less than the aperture collects. Consider a telescope with
a magnification of one. Regardless of the size of its aperture, only
photons striking a subaperture the size of your eye's pupil will
actually make it into your eye. All the rest will fall outside your
pupil. Ignoring optical losses, the brightness (and size) of the object
will appear the same with or without the telescope. In order to utilize
more aperture, you need to increase the magnification of the telescope.
If the magnification is two, your eye's pupil now projects back to an
entrance aperture twice as large. That aperture, of course, collects
four times more photons. But now that your magnification is two, the
image is twice as large on your retina, and the brightness is exactly
the same as it was when the magnification was one (and the aperture was
smaller). You can continue to increase the magnification until your
eye's pupil projects back to the actual aperture of the telescope. As
you do this, the brightness will remain the same: you collect more
photons, but distribute them over a larger area of your retina. If you
want to use all the light passing your telescope's aperture, this
defines your minimum magnification. If you have a 300mm aperture, and
your pupil size is 6mm, the minimum magnification is 300mm/6mm=50. Any
less magnification and some light is missing your pupil.

As the magnification increases beyond the minimum, the brightness
decreases. That is because you are now taking a constant photon flux
(determined by the aperture size) and spreading it out over a larger and
larger area.

There really is no way to use a telescope to increase the photon density
over what you have with the naked eye.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Ed T" wrote in message
nk.net...
[snip]

And if you look at those photos in dim light, they'll still look black and
white g.

That's actually a pretty good way to discern how some of the brightest M
objects will appear with small apertures:

1) Take your favorite color photograph of any M object.
2) Place it against the sofa in your room, so it subtends to the correct
angular size (of course if you know this, you've already seen or simulated
the object :-)
3) Lower the window blinds until you can barely see to walk your way around
the room.
4) Sit in the dark for 10-15 minutes.
5) Look at the photo.

It will look black and white and very dim. Very close to how it will look
under a small apperture.

[snip]

Ed T.
--
Ioannis --- http://ioannis.virtualcomposer2000.com/

Chris L Peterson wrote:
On Mon, 23 Jan 2006 20:27:24 GMT, "David Nakamoto"
wrote:

So obviously parts of the nebula were brighter through that telescope, otherwise
color vision wouldn't have kicked in.

I don't think so. I think the effect you are describing is related to
what Roger described, an enhancement of color sensitivity when more
cones are stimulated.

I am not so sure that is the entire explanation. The only time I have
seen colour in the Orion nebula core was one particularly dark clear
night with an 18" scope inside the city limits of Manchester UK. The
bright rectangular core looked a desaturated pale apple green.

By comparison quite a few OIII planetary nebulae always look green to
turquoise depending where you draw the line. I see turquoise/cyan.

There is nothing a telescope can do to put more photons on a given area
of the retina than it receives seeing the same object without a
telescope. But there are all kinds of physiological phenomena that go
beyond the simple optics.

It can for linear wisps of emission that remain unresolved in smaller
scopes. For an unresolved feature the surface brightness on the retina
increases with aperture (until it is resolved).

I suspect the first thing that triggers seeing in colour is the long
edge shock wave and then the brightest bit near the trapezium. Seeing
colour near the bright stars is harder. I have a hunch that the brain
shades in the rest to match. The HST core shot shows what I mean:

http://www.naic.edu/~gibson/m42/m42_HST.jpg

One other curious thing was that (on the small sample available at an
astronmy event) females were more likely to see the red emission.

Regards,
Martin Brown

Paul Winalski wrote:

You're not going to see any red color in M42, except possibly in the
very very largest telescopes such as Keck or the Hale 200" at Palomar.
Amateur scopes, even the large ones, don't gather enough light for
the red color to be perceived with the unaided eye.

That's not my experience. I never saw red in M42 before it was
explicitly
pointed out to me while observing at my astro club's suburban site.
But as with so many astronomical observations, having seen it once,
I now find it hard *not* to see it. And that's in my "little" 12.5-inch
scope.

The red in question is streaked through the Huygenian region -- the
brightest
part of the nebula, right around the Trapezium. It's a faint overlay on
the
predominant green.

However, the huge swath of red that shows in the photos, extending all
the
way out to Iota Ori, is undoubtedly far too faint for the human eye to
see
under any circumstance. I bet the Huygenian region's surface brightness
is nearly 100X times that of the faint outer regions of the nebula. And
even
there, the red color is marginal.

- Tony Flanders

On Tue, 24 Jan 2006 08:48:22 +0000, Martin Brown
wrote:

I am not so sure that is the entire explanation. The only time I have
seen colour in the Orion nebula core was one particularly dark clear
night with an 18" scope inside the city limits of Manchester UK. The
bright rectangular core looked a desaturated pale apple green.

By comparison quite a few OIII planetary nebulae always look green to
turquoise depending where you draw the line. I see turquoise/cyan.

I'm not sure how these examples argue against the idea that color is
more likely to be perceived if you spread the signal across more retina.

We sometimes see green in the Orion nebula because it is so bright, even
though it isn't primarily an OIII source. Under dark skies, I even see
the green color when viewing the nebula unassisted. I've never seen red
in a telescope of any size, however.

OIII nebulas also are bright enough to stimulate the cones. Generally
some magnification is required- either to get small planetaries large
enough to even be seen, or small dense knots in larger but generally
diffuse nebulas above the resolution threshold.



There is nothing a telescope can do to put more photons on a given area
of the retina than it receives seeing the same object without a
telescope. But there are all kinds of physiological phenomena that go
beyond the simple optics.

It can for linear wisps of emission that remain unresolved in smaller
scopes. For an unresolved feature the surface brightness on the retina
increases with aperture (until it is resolved).

Agreed. Although that is still related to getting the light spread out
over more retina. You haven't changed the brightness with the telescope
(taking brightness as the photon density), but you probably have
increased the total number of photons.

I suspect the first thing that triggers seeing in colour is the long
edge shock wave and then the brightest bit near the trapezium. Seeing
colour near the bright stars is harder. I have a hunch that the brain
shades in the rest to match...

That seems very reasonable.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com