Re: where is the red stuff?

On Mon, 23 Jan 2006 20:59:51 GMT, "Ed T" <reply@xxxxxxxxxxxx> 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
.

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