David Knisely wrote in message ...
Edward posted:

Isn't Dawes an empirical limit, a rough description of what we are typically
able/not able to resolve? Seems to be quite a few who claim to have
surpassed its "limits".

Yes, but the duplicity of close doubles can be detected at separations which
are a little less than that listed by Dawes. In fact, an elongation of the
star image of a close double star is observable at a separation which is
noticably less than Dawes. I prefer to use the Sparrow criteria which has the
brightness level constant along a line running from the center of one
diffraction disk to the center of the adjacent one (in English units, the
Sparrow limit is about r = 4.47/D, where D is the aperture in inches and r is
the separation in arc seconds). At that separation, the "notching" is still
visible, but perhaps not quite as prominent as at the Dawes level (4.56/D).
Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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An obstructed scope may offer an advantage in splitting close doubles,
especially those of nearly equal magnitudes. The central obstruction
redistributes the power from the central dot of the Airy disc to the
first ring with respect to the unobstructed scope. This can give the
obstructed scope an advantage when viewing point sources. Of course
there is a corresponding disadvantage when observing extended targets.

The Physics of the obstructed scope is treated in texts like Suiter
and Rutten & van Venrooij.

Clear skies,

Dennis Persyk
Igloo Observatory Home Page http://dpersyk.home.att.net
Hampshire, IL

New Images http://home.att.net/~dpersyk/new.htm