Airy discs are a troublesome phenomenon encountered with every telescope. If you magnify an object with a high enough magnification, in every telescope, you reach a stage where the object appears like a disc. This disc has nothing to do with seeing the object itself – like you would see a disc when viewing Jupiter, for example), but rather is side-effect of diffraction of the light coming from the object. This means that it is impossible to get a completely sharp image of a star through a telescope. The size of the airy disc is one of the main determinants of the resolution of a telescope. mybloggingplanet.com

The size of the airy disc is dependent on the size of the aperture of the telescope, and the wavelength of the light coming from the object. The larger the wavelength (more red) of the light, the larger the airy disc. This also means for radio telescopes, that “see” in relatively large wavelengths compared to optical telescopes, the airy disc is very large, and can sometimes be half a degree wide or more, severely limiting the resolution of these telescopes.

The formula for calculating the airy disc is:
sine(Airy disc) = 1.22 * Wavelength / Aperture
but since the angle of the airy disc is very small we can approximate by:
Airy disc = 1.22 * Wavelength / Aperture

		public static double CalcWavelengthAiry(double pdAiryDiam, double pdApertureAiry)
		{
			return pdAiryDiam * pdApertureAiry / 1.22;
		}

		public static double CalcApertureAiry(double pdAiryDiam, double pdWavelengthAiry)
		{
			return pdWavelengthAiry * 1.22 / pdAiryDiam;
		}

		public static double CalcAiryDiam(double pdApertureAiry, double pdWavelengthAiry)
		{
			return pdWavelengthAiry * 1.22 / pdApertureAiry;
		}
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