Quote:
Originally Posted by pgc hunter
The airy disk doesn't actually decrease in size with increasing central obstruction, but rather less percentage of light is concentrated inside the airy disk and more in the diffraction rings which translates into a loss of contrast.
The size of the disk only decreases as you increase the aperture and vice-versa.
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Yes, that's correct--its just that you can't see the fainter bits, so it looks smaller, so it makes splitting equal brightness double stars easier. In fact, some DS observers actually put a mask over the spider to block out the central area in order to enhance this effect
Note added: I dug out this pic from Suiter's "Star Testing Astronomical Telescopes" which gives a nice illustration of the effect of increasing the central obstruction. It is easy to see why we get a loss of contrast. Also easy to see why it will be easier to split a close equal double. As I said above, almost all aspects of telescopes involve some type of trade-off