As I see it, as light comes through approximately same angular area of the lens, it should have less aberrations, and at the same time, it should still have high θ -> diffraction limit should stay as without this diaphragm.
What am I missing?
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Less spherical aberration, yes, but not less coma, nor less astigmatism. The diffraction would look like on a telescope with a very important central obstruction. It will not have the usual diffraction pattern, but the diffraction-limited MTF will be bad for low spatial frequencies (rapid decay of contrast) and then flatten out in such a way that the ultimate resolution (extinction of contrast) will be the same as for the full aperture. Edit: I found this page with nice simulations of the effect of a central obstruction on both the MTF and the images: http://www.damianpeach.com/simulation.htm Scroll down to "Part 2. Performance effects of different size central obstructions". As you can see, although the resolution (at MTF = 0) is the same, the image with 50% obstruction looks bad because of the loss of contrast at mid and low frequencies. |
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