>Because of diffraction, resolution in inversely proportional to aperture because the diameter of the airy disk is inversely proportional to the aperture.
... pardon me, but that explanation isn't complete without also explaining that
the actual size of the star image will be enlarged by atmospheric effects
so that the wavefront error scales with (aperture/Fried parameter)^{5/6}.
As many textbooks state, the Fried parameter is not so very small,
3/4 inch for 2 arc seconds seeing, so there is a penalty factor 1.7 for a
12 inch telescope with respect to a 6 inch telescope if I'm not making a mistake
in my math. The effective, time-averaged Strehl value for the 12 inch then deteriorates
to 0.25, getting progressively worse for longer exposure times of the detector. This penalty
will steadily increase as the seeing got progressively worse. At a certain point, a
perfect 6 inch telescope which is more rapidly acclimatized can in certain, possibly
rare cases outperform a poorly figured 12 inch telescope, particularly if the observer
is in a hurry. Not to say the Synta or GSO telescopes are bad, their quality can be quite good.
I would personally keep the 6 inch. One would want the best possible optics
at a given aperture. There are so few examples nowadays where an artisan
can produce an industrial product that can outperform a computerized mass-produced
one. I figure there is intrinsic value in that fact.
If you are going to quote me, quote everything I wrote that is relevant: I did say this:
"Getting the performance possible from a larger aperture is more difficult because of thermal issues and as aperture increases, seeing becomes more and more of an issue. "
The question I was asked was to explain to about why the wave front error is scaled relative to the aperture, why larger scopes have greater resolution. That's pure optics, there are times when the seeing will support the larger optic.
If the best you ever see is 2" and all you observe is the planets and double stars, first, you have my sympathy.. And in that case, the 6 inch might be a good fit but as I recall, Vlad's simulation said a larger scope was still advantageous. The actual diameter of the first minimum of the Airy disk of a 12 inch scope is twice the Rayleigh Criterion, that's about 2.2 arc-seconds. And as experienced planetary observers like Alan French will tell you, even in poor seeing, there are moments of stable seeing where the large aperture can be used advantageously.
In my world, 2" is rather poor seeing for viewing the planets, being south of the jet streams close to the Pacific ocean in one of the worlds mildest climates has it's advantages. The greater resolution and contrast possible with a 10 inch, 12 inch, 16 inch scope can be used to a good advantage.
Last night I spent some time on the star Jabbah, with my 10 inch. It's a pair of doubles, one is 1.3", the other 2.2" , both were wide clean splits, the seeing was well under 1". The views of Jupiter and Saturn were quite nice in my 10 inch.. Viewing the planets, I generally stop at 410x, the 0.6mm exit pupil is quite dim. That's where I stopped last night.
I did start the scope cooling with the fan running about an hour and a half before sunset. The scope just has generic Taiwanese optics, decent optics, I've split doubles slightly under 0.5", that won't be happening in a 6 inch.
Jon Isaacs.
