Re: Galileo's telescope's f-ratio

From: Lewis Mammel (l.mammel_at_worldnet.att.net)
Date: 10/30/04 

Date: Sat, 30 Oct 2004 05:00:19 GMT

Jonathan Silverlight wrote:
>
> In message <eqsvn0hd54o24hc9sn2bp73rntj3lc3aan@4ax.com>, Jud McCranie
> <youknowwhat.mccranie@adelphia.net> writes

> Google is your friend, as usual, and there's a fascinating site at
> <http://www.pacifier.com/~tpope/index.htm> about imaging with a replica
> of Galileo's telescope, for instance.
> Until now, I didn't know that the exit pupil of a Galilean telescope is
> inside the tube!

This is the image of the object in the negative focal length eyepiece,
and with the eye at the eyepiece, it has the same angular diameter
as the objective would with the eyepiece removed. So the apparent
size of the field of view is the same as looking down an empty tube.

The part about the observer's eye pupil contributing to the field
of view is very interesting. You can demonstrate this by holding
a pinhole near your eye and observing the computer screen.
( You can form a pinhole between your thumb and index finger. )
If you make a small enough pinhole, your field of view is the
virtual pinhole image of the pupil of your eye.

I can actually read the screen this way without my glasses. At
the same distance, without the pinhole, I can barely resolve the
separate lines of text. That is, the text appears as fuzzy grey bars.

Lew Mammel, Jr.

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