Re: [ATM] Extended Object Brightness

["Stuart Hutchins" <stuart452@earthlink.net>]

Hi Dan,

Seems like in the daytime outdoors, when you are most likely to glance at
the sun, the iris of your eye is stopped down to a pinhole -- less than a mm
for most people in bright sunlight.  Even at that setting, more than a
second or two will burn a hole in your retina, as numerous people can
attest.  Pain causes most people to blink or look away before permanent
damage occurs.

In the magnified telescopic image, all the light energy collected by the
objective is concentrated at the exit pupil, which at high magnification
will be small, say 1 or 2 mm diameter.  All this energy passes through the
pupil of your eye. The image formed on your retina is not brighter in terms
of luminous flux per square mm, but it is much larger, and can cook a
substantial region of the retina.

For example, suppose your eye pupil is stopped down to 1mm diameter.  Direct
light energy from sun's rays is very roughly one kilowatt per square meter
or one militate per square mm at earth's surface.  So only about 1 miliwatt
total enters the eye.  But a 100mm telescope collects 10,000 times that flux
which is about 10 watts and concentrates the power into a 1mm exit pupil (at
100x magnification) so it can all get inside the eye.  A larger objective
will do the job even faster.  A twenty incher (500 mm) gives you about 200
watts to work with.

A little fresh parsley on that boiled eyeball, sir?

Stuart


----- Original Message ----- 
From: "Daniel Reinders" <danreind@shaw.ca>
To: <atm@atmlist.net>
Sent: Saturday, April 10, 2004 12:13 PM
Subject: [ATM] Extended Object Brightness


> Hi All,
>
> So I've gone through the archives and read what I could about why you
can't make an extended object brighter than would be seen by your eye (just
bigger).  So you drop the magnification below a certain point and all that
extra light you gathered just spills out onto the non-light receiving parts
of the eyeball - got it.
>
> My newbie question then is: why is it so dangerous to look at the sun
through a telescope (Note - please don't flame me with excited posts about
not looking at the sun, I'm only raising it as a point of discussion, not an
action plan).  As an extended object, the telescope should therefore make it
no brighter (photons per retinal area) than with direct vision.  Now staring
at the sun is a terrible idea even with the naked eye, but who hasn't looked
up at the sun at some point in their lives for a second or two and not gone
blind?  Or even just had it in your field of view?  Is it some subtle effect
relating to the dark tube causing the iris to open up and then bang - you
find the sun and your eye is not ready for it? Or is there some total amount
of light received by the eye that cannot be surpassed?
>
> Other question - when does an object transition between extended and point
source (where telescopes can and cannot improve brightness).  Can I assume
it is something to do with when the "disk" becomes smaller than the angular
limit of resolution of the scope?  Along these lines, since a galaxy's
diffuse light is a collection of point source points, it would seem to
follow that there is some aperature of vast size (unimaginable?) where
adding additional aperature does increase brightness since the stars become
resolvable as point sources - true or false?  In fact, was not the
resolution of variable stars in other galaxies the key to initially
estimating inter-galaxy distances?
>
> Thanks,
>
> Dan Reinders
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>


_______________________________________________
ATM mailing list http://www.atmlist.net/