The Effect of Central Obstructions

When a reflector or a Schmidt-Cassegrain is considered, the resolution is slightly changed by the presence of the secondary mirror. The result is to slightly reduce the size of the Airy disk and reduce the radii of the bright rings, at the same time slightly broadening the width and increasing the intensity of the rings. The result is that, for equal pairs, the reflector is as effective as the refractor until the central obstruction is greater than about 33%; but for unequal pairs the wider diffraction ring makes it more difficult to see faint stars close to bright ones. Christopher Taylor will go into this in more detail in the next chapter, which will deal with the effect of alignment and aberrations on resolution for Newtonian reflectors.

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