Astronomical Image Formation

The image formed by an astronomical telescope of a distant unresolved (point source) object has a distribution of energy on the detector which is established by several factors.

• If only geometrical (ray tracing) optics were involved, all the light from such an object would fall at the same point on the detector and we would observe a "delta function" response.
• The optics of the telescope introduce a diffraction pattern resulting in the classical "diffraction rings". The actual function is (in the ideal case) a Bessel Function of the 1st kind. This spreads the light from every object out over a range of pixels.

• Since the objects are all observed through the Earth's atmosphere, the image of every object (under typical conditions) is further spread out into a "seeing disk".