From: "Frank Q" 
To: "John Sherman" , 
Reply-To: "Frank Q" 


Hi All

///////////  lots of snipping
----- Original Message -----
From: "John Sherman" 
> magnified, and so the disc still subtends the same angle on the sky. As
was
> pointed out, that angle is 2.44 lambda/D, usually given in arc-seconds. It
> is not dependent on the magnification, or even the f/#.
>
> Why not??
/////////// end of snip

The quantity (2.44 lambda/D) is related to the angular resolution of the
telescope. The resolution is 1.22 lambda/D. The angular resolution is
independent of focal length, F/# etc etc. It only depends on the diameter
of your mirror. All that FL or F/# do is specify where and how big the
final airy disk will be. If two point sources are located such that the
angle they subtend at the mirror is less than the angular resolution of the
mirror, then
their airy disks will overlap. Varying the FL or F/# will not separate
them. Varying the FL / F/# will just give a bigger (or smaller) image (at
the focal plane) of two overlapping airy disks.

If lambda and D are specified in the same units then (1.22 lambda/D) will
be the angular resolution of the telescope in radians when it is viewing
something at a wavelength of lambda. This will have to be converted to
arc-seconds by someone at some stage.

Incidentally, the resolution of a mirror will be about twice as good for
objects viewed in blue light (lambda = 0.4 microns) compared to objects
viewed in red light (lambda = 0.7 microns approx).

Cheers
Frank Q

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