To: "John Sherman" ,
From: Mark Suchting 
Reply-To: Mark Suchting 


At 06:02 PM 7/21/2003 -0700, John Sherman wrote:


>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??


The angle subtended by the Airy Disc at the focal plane of a telescope  is
dependent only on aperture and wavelength as has been set out.     The
angular size  of the Airy Disc  always stays the same regardless of
telescope F#, because for a given aperture, as the F# increases so does the
physical diameter of the disc , so the _angular_ size, and hence angular
resolution of the scope  always stays the same.  Similar magnifications on
equal aperture scopes with different F#'s are of course obtained just by
using different focal length eyepieces.

Regardless of the F#, the amount of magnification it takes, with any given
mirror  for the average eye to resolve the Airy Disc  ( assuming perfect
`seeing' ) , is around the same as the aperture `in MM'  eg.  300X for a
30cm telescope . In inches the rule is 50X per inch. An  easy rule of thumb
for maximum useful eyepiece is to choose an eyepiece focal length in MM,
equal to the telescopes F #  .   This rule will always yield the required
magnification as mentioned above ( ie. 50X per inch ).    This is where the
50X per inch rule comes from. Once you are seeing diffraction structure,
further magnification   yields no further information.   I often read about
people claiming that they use 100X per inch on a good mirror before `the
image breaks down'. I suspect that they have less than average visual
resolution or the seeing was not so good.

When the seeing is very good, I always back off from 50X per inch because
the visibility of diffraction discs spoils the illusion of looking at
`stars'. Planetary observing is best done at these times anyway!

~Mark  Suchting

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