Re: [ATM] Extended Object Brightness (projection)

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

Scott,
 That is a good question, basic to telescope optics.  Problem is most
telescope diagrams don't trace the rays all the way through the eye lens to
the retina.

The telescope system is "afocal" when you look through it visually.  This
means the rays from any point in the original object are exiting the
eyepiece as a bundle of parallel lines, as though coming from a distant
object. What is magnified by objective and eyepiece is the apparent angle
away from the optical axis of each point in the image.  IE the angle each
bundle makes with the optical axis.  Your eye lens then focuses the ray
bundles back into real image points on your retina, which is always the same
distance from  the eye lens.  You will notice as you move your eye back from
the exit pupil of the telescope, the image size does not change, but the
field of view gets smaller. The ray bundles with larger angles are missing
your eye pupil.

When you project the image onto a screen, you change eyepiece focus so the
rays in each bundle are not quite parallel but come slowly to focus at the
screen distance.  The off axis angle of the ray bundle for each point
remains the same, so the further the screen, the bigger the image.

I don't know of a good diagram to refer you to, but hope that verbal
explanation helps.

Stuart


----- Original Message ----- 
From: "Scott Ewart" <scott.e@quicksurfer.com>
To: <atm@atmlist.net>
Sent: Saturday, April 10, 2004 7:21 PM
Subject: Re: [ATM] Extended Object Brightness (projection)


> This tread reminded me of something I was trying to reconcile.
>     If you project an image of the sun, moon, or other bright object
through
> the eyepiece onto a screen, the image gets bigger the further away you
hold
> the screen.  But if you look through the eyepiece and back your head away
> several inches, looking at Jupiter, lets say, the field gets smaller, but
> the image stays the same size.  At least it looks that way to me.  Why
> doesn't the image look bigger?
>
> Scott Ewart
>
>
>
> ----- Original Message ----- 
> From: "Mel Bartels" <mbartels@bbastrodesigns.com>
> To: <atm@atmlist.net>
> Sent: Saturday, April 10, 2004 8:13 PM
> Subject: Re: [ATM] Extended Object Brightness
>
>
> | > down too slowly).  Is the answer here in the anatomy and
pathophysiology
> | of
> | > retinal burns, and not in the optics of extended objects?
> |
> | No.  The answer is that the projected size of the Sun onto the retina
when
> | unaided eye is very very small compared to the projected size of the Sun
> | onto the retina using a telescope.  Same with the Moon.  The surface
> | brightness or intensity on each portion of the retina remains the same.
> |
> | Mel Bartels
> |
> | _______________________________________________
> | ATM mailing list http://www.atmlist.net/
>
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