"John Carruthers" wrote in message
oups.com...

James Harris wrote:
Can anyone explain how light really passes through a telescope???
Opinions I've heard seem contradictory so I'd appreciate some
clarification. In particular (and this is the practical reason for
asking the question) when light exits an eyepiece does it emerge as a
cylinder or as a cone? If a cone is it narrowing - i.e. focussed on a
point after the eyepiece - or is it diverging - i.e. already past the
point of focus?


Hi james, check it out yourself, try focussing on the moon then hold a
piece of frosted glass or tracing paper behind the eyepiece. Does the
image diameter increase as you move the screen further from the
eyepiece ?
jc

James
The light from the distant object should be emerging in a parallel sided
cylinder when the telescope is correctly focussed at a distant object.

John
I'm not sure what 'image' you would get using the method described - I
suspect the nearest to an image you could bring to focus would be that of
the Objective behind the eyepiece.