Quote:
Originally Posted by Satchmo
The general rule of thumb for the equivelent refractor ( at least in the ability to reproduce low level contrast features is to subtract the diameter of the secondary obstruction. I verified this under good seeing conditions when a stock 10" F6 Newt with 2" secondary clearly gave better images of Jupiter than a 7" F9 Starfire Apo ( and both instruments were well aclimatised. A 14" RC would have about a 6" obstruction at the baffle so a 7" Apo would theoretically be a match for a 14" RC , not in light gathering of course.
Mark
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Interesting.
I had a Nexstar 11 GPS and it gave slightly brighter views than a Tak FS152. But the Tak gave brighter views than an LX90 (8 inch SCT) that I had. So hence the rough conversion - a bit subjective as well.
Small central obstruction seems to be the go for visual.
Its no good for imaging though as it would give too small a corrected field to be useful with today's large chipped cameras hence the 50% obstructions a lot use for astrographs. This makes them less useful as visual instruments.
Greg.