next up previous contents
Next: Seeing Up: Title Page Previous: The ESO Very

The image quality of a telescope

The first requirement that astronomers will set for the design of a new telescope and its enclosure is that it should have the best possible image quality and the lowest possible seeing.

If the atmosphere behaved as a purely refractive medium, the image of a star as seen in a telescope with a perfect optical surface would consist of a right central spot surrounded by weak circles constituting the diffraction rings. This is due to the wave character of the light in combination with the effect of the finite diameter of the telescope mirror. In this case one says that the image is diffraction limited. In reality the optical image quality of a ground-based astronomical telescope is limited by several other factors, among which:

  1. Figuring and alignment errors of the optics which produce optical aberrations.
  2. Atmospheric seeing effects.
  3. Wind loading on the telescope which causes guiding control errors, and on the primary mirror, where pressure fluctuations may deform the figure of the optical surface.
It is interesting to note that all these factors will have similar effects on the quality of an exposure of the focused image: the diffraction rings are broken up and both they and the central spot loses its sharpness, so that the profile of the star image becomes larger and tends to takes the general form of a Gaussian.

Figuring and alignment errors of the telescope optics are unrelated to the scope of this work. The other two contributions depend chiefly on the conditions of the atmospheric environment surrounding the telescope and are generally quite influenced by the type and configuration of the telescope enclosure.

Lorenzo Zago,, Sun Feb 26 22:57:31 GMT+0100 1995