Re: Simple question from a newbie

"John S" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> Why doesn't the shadow of a secondary mirror in a reflector telescope
block
> part of the image being viewed?

Hi John,

Every part of the primary mirror is used in forming every part of the image
of the object being viewed. In other words, at the surface of the primary,
nothing is focused. So the outer rim (zone) of the primary is being used to
form the image of the center of the object and the outer part of the object.
Therefore nothing shows up as missing (a shadow). Likewise, the center
portion which is blocked by the secondary would otherwise be forming the
image of both the center of the object and the outer portions of the object
being observed. Since the rest of the mirror is also forming the image of
these same areas, there is no area that is missing more light from the
center of the mirror than any other part of the image is. So in a real
sense, there is a shadow, but it is across the entire image. And a shadow
can only be seen when parts of the nonshadow (brighter) area is also
visible. Hope this is clear.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/