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Thanks for the info guys, very informative.
Glend, regarding your statement that one would need a refractor of more
than 140mm to equal my 200mm Newt's effective aperture; I am aware that the obstruction causes a small loss of contrast etc: but the light
gathering area of my 200mm mirror is 314 sq. cm minus 19 sq. cm
for the secondary holder, leaving 295 sq. cm. A 140mm ref has only
154 sq. cm. You would need a ref of 193.8mm [7.63 ins.] diameter.
My secondary is approx. 26% of the scope's diameter, but only 6.25%
of it's area.
raymo
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