Quote:
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Originally Posted by CoombellKid
If you had not realise why or did not know, why say anything? I would
of thought being a moderator on here, doesn't give you the right to
just say anything. thats 2 times now in a couple of weeks
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Goodness me, Rob! You're a worry...
The facts:
- Geoff obviously meant the reciprocal of what he said, i.e. obstruction diameter divided by primary mirror diameter, so 2"/10" = 50/250 = 0.2 = 20%.
- GSO Dobs have about 25% central obstruction, i.e., the diameter of the secondary obstruction is about 25% the diameter of the primary mirror.
- The difference between light gathering area of two equal-aperture scopes with 25 and 40% CO is about 10%, which would correspond to an effective aperture difference of only 5%. Not enough to make too much of a fuss about. The effect of the larger CO on the point transfer function of the instrument is a much more important factor as it impacts on contrast.