Quote:
Originally Posted by ghsmith45
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.
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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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Actually, no that's not correct sorry. While it is true that the airy disk size depends on the aperture, it (as well as the size of the diffraction rings) are also affected by any obstructions in the entrance pupil. Larger central obstructions will not only result in more energy being distributed in the diffraction rings, but the angular size of the airy disk and the rings will decrease as well. Chapter 9 of Suiter's book descibes this ("The spot size is smaller in obstructed instruments, ... ").
I also happen to have written a Windows program that will provide a graphical display of the point spread function (ie. diffraction pattern) of a star based on user defined parameters (such as obstructions, wavelength, scale, etc) as well as measurements of airy disk size and diffraction ring diameters taken from the calculated data. The basic
algorithm is based on an article published in Sky and Telescope from 1987 and involves simulating thousands of random light rays to calculate the resultant phase difference at each pixel and convert to an intensity to form an image. I have attached a screenshot showing the results of 2 runs, one for an unobstructed aperture of 200mm, and the other
having a very large obstruction of 180mm. From the unobstructed results shown, you can see that the measurement of the diameter of the resultant Airy disk is 1.33 arc-seconds (in perfect agreement with theory), whilst the second (obstructed) run shows an Airy disk of only 0.89 arc-seconds. You can also see that the diameters of the diffraction rings are also smaller for the large obstruction (and that the relative intensity of the rings has increased as well).