The observatory tries to obtain several objective estimates of image diameter each night, together with notes on prevailing conditions, to help in finding causes for poor seeing. These measurements also provide the observer with a quantitative measure of image quality. Normally the FWHM of the profile is quoted.
Two methods are currently used:
The eyepieces for both the prime focus and Cassegrain guiders have a double-cross graticule with a central box approximately 2" across (at f/8 for Cassegrain). Similarly, the autoguider displays a profile through the star while guiding, allowing relative measurements to be made (though an exact measurement is not possible from this display).
For systems where direct viewing of the focus is not possible, observers may be asked to examine their images to provide a seeing estimate.
This Page Last updated: Feb 21, 1996, by Chris Tinney.