Some clarification and thoughts on Spot Diameters, etc.

Roland Christen

Hello Astro-imagers,

With the introduction of the 110 GTX I have made up some charts under technical design details that show various performance data. One chart that I initially hesitated to show was Spot Size. I eventually included it because some competing scope manufacturers use spot sizes to show imaging performance and perhaps use it for marketing purposes. Smaller numbers are always better than larger numbers, in the minds of consumers.

The reality is this:
1) There is no such thing as spot size. Diffraction effects determine the smallest spot diameter, not an artificial ray trace result.
2) Diffraction effects produce a minimum size star image, called the Airy Disc. This is the smallest size possible for white light.
3) The Airy Disc diameter is 1.35 x F-number of the optic in white light (red light slightly larger, deep blue slightly smaller).

Airy Disc diameter in microns for various telescope lenses, regardless of aperture:

F5:   6.8 microns
F6:   8.1 microns
F7:   9.5 microns
F8:   10.8 microns
F10: 13.5 microns

There is no way that any F7 telescope can produce a 1 micron spot diameter, no matter how well color corrected an optic is. It's an artificial number that comes out of an optical design program that uses geometrical ray tracing instead of wavefront analysis. It is not reality. For example, if you ray trace the on-axis spot diameter of a Newtonian parabolic mirror, the spot diameter comes out as zero microns. Which would mean that the optic has infinite resolution. For a 150mm F7 telescope that produces a 1 micron spot diameter, it would have a resolution equal to a 56 inch, 1.4 meter optic. Clearly impossible.

Ray tracing has value to the optical designer as a fast way to evaluate an optical design. It can come close, but only wavefront analysis can determine the best contrast and resolution. Ray tracing numbers (RMS Spot diameters) are really only accurate when the aberrations or spot sizes are larger than the Airy Disc diameter. Smaller numbers than the theoretical Airy Disc diameter are meaningless and don't occur in the real world.

Roland Christen

Roland Christen