Re: I'm want to beat this XT-12 vs. Lightbridge-12 horse a little longer.


Starboard wrote:

Are there any principle differences between ORION and MEADE
optics? MEADE brags "diffraction limited". This means nothing to me
without knowing what the limits actually are. ORION's optics are
diffraction limited too (I would imagine); although they don't say so.

Astronomics has this to say about what the limit is:

Diffraction-Limited:

A telescope is said to be "diffraction limited" if its optics are made
with enough accuracy so that all the light rays from a star fall within
that star's Airy disk and diffraction rings, with no excess light being
scattered out of the disc and rings by defects in the mirrors. Optics
that bring all light rays to a focus within 1/4th of a wavelength of
light of each other at the final focus are considered to be diffraction
limited.

Technically, a telescope is diffraction limited if it meets the
Rayleigh limit - which specifies the separation in arc seconds of two
equally-bright binary stars which appear to be just touching as being
equal to 140 divided by the aperture in mm. The Rayleigh limit, which
deals with a telescope's ability to separate closely-spaced stars,
should not be confused with the Rayleigh criterion, which deals with
how accurately an optical system is made. Note that the visual Rayleigh
limit for an 8" (203mm) aperture telescope is 0.69 arc seconds (140 /
203), a less-stringent specification than the Dawes' limit of 0.57 arc
seconds.

Telescopes meeting either limit can resolve more detail than the
Earth's atmosphere will allow us to see under average seeing
conditions, as our atmosphere typically limits the seeing to no better
than one arc second resolution (the resolution of a 6" scope) on even a
very good night. Five arc second resolution or worse is more typical of
an average night.

Errol
PASNOLA

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