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Main
Date: 26 May 2007 19:19:49
From: Dennis Woos
Subject: Apparent Size of Stars
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In another thread, Chris L Peterson writes:
> even perfect optics will make stars look larger than they
> would naked eye. That's simply a consequence of the fact that
> diffraction and seeing prevent stars from being true point sources, and
> therefore they have an apparent diameter that is proportional to their
> brightness. A telescope makes any star brighter at the retina or camera,
> and therefore larger.
I am confused by this statement. Certainly, the diameter of the central
peak of the Airy disk is inversely proportional to the diameter of the
objective, as is the Rayleigh Criterion and the Dawes Limit. Futhermore,
the brightness of a star is proportional to the diameter of the objective.
How then can the apparent diameter of a star in the eyepiece be proportional
to brightness, and therefore proportional to objective diameter, as opposed
to being inversely proportional?
Dennis
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Date: 26 May 2007 23:35:46
From: Chris L Peterson
Subject: Re: Apparent Size of Stars
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On Sat, 26 May 2007 19:19:49 -0400, "Dennis Woos" <dpwoos@gmavt.net >
wrote:
>I am confused by this statement. Certainly, the diameter of the central
>peak of the Airy disk is inversely proportional to the diameter of the
>objective, as is the Rayleigh Criterion and the Dawes Limit. Futhermore,
>the brightness of a star is proportional to the diameter of the objective.
>How then can the apparent diameter of a star in the eyepiece be proportional
>to brightness, and therefore proportional to objective diameter, as opposed
>to being inversely proportional?
Because you rarely see a star as the Airy disk. Seeing broadens the star
into a Moffat or modified Gaussian, with wide tails. The brighter the
star, the farther out on these tails you can see, and the larger the
star appears.
Of course, even under perfect conditions the apparent size of the star
will increase with magnification, since you are magnifying the central
Airy disk (and with bright stars, one or more surrounding rings will
also become visible, increasing the apparent size).
Surely, you've noticed that in any telescopic field, regardless of the
seeing conditions or optical quality, bright stars look bigger? This is
true for naked eye observing, too. It's also obvious in any image (more
so, since seeing averages out the star profile more with a long
exposure).
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 26 May 2007 19:41:17
From: William Hamblen
Subject: Re: Apparent Size of Stars
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On Sat, 26 May 2007 23:35:46 GMT, Chris L Peterson
<clp@alumni.caltech.edu > wrote:
>Because you rarely see a star as the Airy disk. Seeing broadens the star
>into a Moffat or modified Gaussian, with wide tails. The brighter the
>star, the farther out on these tails you can see, and the larger the
>star appears.
There also is irradiation, which make bright objects appear larger.
Bud
--
The night is just the shadow of the Earth.
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Date: 26 May 2007 22:12:57
From: Dennis Woos
Subject: Re: Apparent Size of Stars
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> There also is irradiation, which make bright objects appear larger.
>
How about some more detail on this?
Dennis
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Date: 27 May 2007 01:53:11
From: Chris L Peterson
Subject: Re: Apparent Size of Stars
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On Sat, 26 May 2007 19:41:17 -0500, William Hamblen
<wrhamblen@comcast.net > wrote:
>There also is irradiation, which make bright objects appear larger.
I'm not sure what you are referring to as "irradiation". Are you talking
about an effect in the eye (which does indeed make bright points appear
larger), or something else?
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 26 May 2007 22:25:52
From: William Hamblen
Subject: Re: Apparent Size of Stars
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On Sun, 27 May 2007 01:53:11 GMT, Chris L Peterson
<clp@alumni.caltech.edu > wrote:
>On Sat, 26 May 2007 19:41:17 -0500, William Hamblen
><wrhamblen@comcast.net> wrote:
>
>>There also is irradiation, which make bright objects appear larger.
>
>I'm not sure what you are referring to as "irradiation". Are you talking
>about an effect in the eye (which does indeed make bright points appear
>larger), or something else?
Irradiation is the optical illusion that brighter objects are larger.
Bud
--
The night is just the shadow of the Earth.
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Date: 27 May 2007 05:14:44
From: Chris L Peterson
Subject: Re: Apparent Size of Stars
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On Sat, 26 May 2007 22:25:52 -0500, William Hamblen
<wrhamblen@comcast.net > wrote:
>Irradiation is the optical illusion that brighter objects are larger.
I'm not familiar with the term used that way, but I'm certainly familiar
with the illusion. So we can add this eye/brain processing effect to the
physical effects I've already mentioned.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 28 May 2007 12:08:39
From: Stuart Levy
Subject: Re: Apparent Size of Stars
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On 2007-05-27, Chris L Peterson <clp@alumni.caltech.edu > wrote:
> On Sat, 26 May 2007 22:25:52 -0500, William Hamblen
><wrhamblen@comcast.net> wrote:
>
>>Irradiation is the optical illusion that brighter objects are larger.
>
> I'm not familiar with the term used that way, but I'm certainly familiar
> with the illusion. So we can add this eye/brain processing effect to the
> physical effects I've already mentioned.
Yes -- it's a term of art. For example, the effects that makes a
crescent moon look larger than its earthshine-illuminated mate,
or a geometrically-almost-first-quarter moon look as though it's
just past first quarter, are said to be due to "irradiation".
Likewise the black drop effect, as e.g. at
http://metaresearch.org/home/Viewpoint/blackdrop.asp
But I don't know its mechanism -- whether it's due to scattering
within the eye, or what.
cheers
Stuart in Champaign, IL
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Date: 26 May 2007 20:01:16
From: Dennis Woos
Subject: Re: Apparent Size of Stars
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> Surely, you've noticed that in any telescopic field, regardless of the
> seeing conditions or optical quality, bright stars look bigger? This is
> true for naked eye observing, too. It's also obvious in any image (more
> so, since seeing averages out the star profile more with a long
> exposure).
>
Sure, Deneb looks bigger than does Sadr. However, I still don't see how
this relates to the fact that big scopes split closer doubles than do small
scopes, and at lower magnification, even though stars are brighter in big
scopes and therefore "look bigger".
Dennis
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Date: 27 May 2007 00:41:11
From: TMA
Subject: Re: Apparent Size of Stars
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"Dennis Woos" <dpwoos@gmavt.net > wrote in message
news:135hiiga997p9a4@corp.supernews.com...
>> Surely, you've noticed that in any telescopic field, regardless of the
>> seeing conditions or optical quality, bright stars look bigger? This is
>> true for naked eye observing, too. It's also obvious in any image (more
>> so, since seeing averages out the star profile more with a long
>> exposure).
>>
>
> Sure, Deneb looks bigger than does Sadr. However, I still don't see how
> this relates to the fact that big scopes split closer doubles than do
> small scopes, and at lower magnification, even though stars are brighter
> in big scopes and therefore "look bigger".
>
> Dennis
They are ALL point sources only. Intuitively the implications of the
propogation
of incoherent light rays emminating from a source that puts out energy that
pretty much dwarfs
the sun at such extreme distance always boggles my mind. Thank God for the
the inverse square law.
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Date: 27 May 2007 00:33:05
From: Chris L Peterson
Subject: Re: Apparent Size of Stars
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On Sat, 26 May 2007 20:01:16 -0400, "Dennis Woos" <dpwoos@gmavt.net >
wrote:
>Sure, Deneb looks bigger than does Sadr. However, I still don't see how
>this relates to the fact that big scopes split closer doubles than do small
>scopes, and at lower magnification, even though stars are brighter in big
>scopes and therefore "look bigger".
When you're splitting really tight doubles, you're generally doing it
under good seeing conditions, right? That means you are better able to
take advantage of the smaller Airy disk you get with a large aperture.
Of course, even if you have great seeing and are resolving the Airy
disk, bright stars look bigger because you see farther out on the wings
of the central disk (and maybe even see some rings).
The thing is, a star in a small aperture scope will appear larger than
the same star in a large aperture scope- that's why the large aperture
is better for splitting tight doubles. But each scope will still make
bright stars look bigger than dim stars. The apparent size range will be
different, though. That make sense?
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 26 May 2007 22:10:47
From: Dennis Woos
Subject: Re: Apparent Size of Stars
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> Of course, even if you have great seeing and are resolving the Airy
> disk, bright stars look bigger because you see farther out on the wings
> of the central disk (and maybe even see some rings).
>
I wonder if it would ever be advantageous in splitting tight doubles to cut
the brightness so as to get the intensity of the outer portion of the
central disk and rings below the visible threshhold?
> The thing is, a star in a small aperture scope will appear larger than
> the same star in a large aperture scope- that's why the large aperture
> is better for splitting tight doubles.
Is this true even in poor seeing? Might not the increased brightness of the
large aperture image, when distorted by bad seeing, actually produce a
larger apparent star diameter than that produced by a smaller aperture
scope?
Dennis
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Date: 27 May 2007 02:49:50
From: Chris L Peterson
Subject: Re: Apparent Size of Stars
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On Sat, 26 May 2007 22:10:47 -0400, "Dennis Woos" <dpwoos@gmavt.net >
wrote:
>I wonder if it would ever be advantageous in splitting tight doubles to cut
>the brightness so as to get the intensity of the outer portion of the
>central disk and rings below the visible threshhold?
Not sure. It might help reduce effects in the eye itself. You can
actually see this when driving at night- squinting down your eyes can
make it easier to detect headlights as a pair. Of course, part of that
may be because of increased acuity with a smaller aperture, but maybe
not all.
>> The thing is, a star in a small aperture scope will appear larger than
>> the same star in a large aperture scope- that's why the large aperture
>> is better for splitting tight doubles.
>
>Is this true even in poor seeing? Might not the increased brightness of the
>large aperture image, when distorted by bad seeing, actually produce a
>larger apparent star diameter than that produced by a smaller aperture
>scope?
I'd say that as the seeing gets worse, it becomes the dominant effect.
So in really bad seeing, there's not much difference between star sizes
in a small and large telescope (as measured by FWHM, or a similar
invariant metric), so it would seem to make sense that the brighter
stars (that is, in the larger scope) would also have larger _apparent_
diameters. I've never tested this experimentally; it sounds like
something fun to try.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 27 May 2007 15:23:13
From: Curtis Croulet
Subject: Re: Apparent Size of Stars
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> You can
> actually see this when driving at night- squinting down your eyes can
> make it easier to detect headlights as a pair.
If you're doing this, you need glasses!
--
Curtis Croulet
Temecula, California
33°27'59"N, 117°05'53"W
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Date: 28 May 2007 17:22:09
From: Mike Ruskai
Subject: Re: Apparent Size of Stars
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On or about Sun, 27 May 2007 15:23:13 GMT did "Curtis Croulet"
<calypte@_NO_SPAM_verizon.net > dribble thusly:
>> You can
>> actually see this when driving at night- squinting down your eyes can
>> make it easier to detect headlights as a pair.
>
>If you're doing this, you need glasses!
Not necessarily. Just like telescopes and camera lenses, our eyes
have more aberrations off axis. Squinting kills some of the off-axis
light, consequently removing some of the aberrations.
For someone who's myopic, the difference is just much larger, since
you're also removing light that's simply out of focus.
Beyond that, squinting reduces brightness, which reduces lens flare
(in your eye's lens, that is).
--
- Mike
Ignore the Python in me to send e-mail.
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