Hi Nick & All,
Quote:
Originally Posted by Wavytone
The last issue I have with all newtonians is diffraction spikes from spider vanes. Sure, most of you think it doesn't matter. But actually it does, as I found out when comparing the view of a tight double (separation 0.6") - the MK91 resolved the pair but the Mewlon could not thanks to the spikes. A design with 1 vane or two curved vanes could be better, I guess.
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I am not entirely surprised by your comparison between the Mk91 and the Mewlon on a pair of 0.6" separation. Was it the Mewlon 210 or 250 you were viewing with?
The Mewlon is
not a Newtonian but instead a Dall-Kirkham and is also a telescope (more correctly astrograph) optimised for photography/imaging -- not visual observing.
Sure, they do have first-class high-accuracy very smooth optics but again, 30% (or just over -- a bit more again than the MK91) central obstruction and the support vanes for the secondary mirror are about 4mm diameter. Not a problem for photography but they do make for thick, bright diffraction spikes.
I don't know which pair you were observing for making this comparison but just to add a bit of spice to the mix, here's a 1997 observation I made of Gamma Sextantis with my old 10" f/6.1 Newtonian that had a baffled tube 50mm secondary, three vane spider (with quite thin spider vanes) at x580. Gamma Sextantis at the time was at 0.54" separation with a difference in magnitudes of 5.1 + 5.4.:
"Well split with a dark hairs breadth between. Clearly split 90 - 95% of the time. In PA 70, diffraction rings surrounding both components virtually stationary. Another faint star 1' away magnitude11 in PA 340. "
The Dawes limit for a 10" telescope is 0.46" so I have split 'em almost down to the practical limit with nothing more than a common "light-bucket" -- as you like to call them.
Best,
L.