The question has come up as to what RMS guide error is allowed to get round stars in a typical Astro image. I ran a series of tests with a 17" F8 astrograph on a 1600 mount on a reasonably steady night that produced 1.5 to 1.9 arc sec FWHM stars in a 5 - 10 minute exposure. Image positions are dithered after each exposure by 1 - 5 pixels.
Here is a screen shot showing Guider graph, Guider settings and resultant star size, roundness, etc. during period of steady seeing.
Guide star pixel scale is 1.32 arc sec/pixel, RMS error is .18 arc sec for X axis, .275 arc sec for Y axis. Stars measure flatness of .008 which is quite round even though X and Y axes RMS is quite different.
Rolando
Here is a screen shot showing Guider graph, Guider settings and resultant star size, roundness, etc. during period of steady seeing.
Guide star pixel scale is 1.32 arc sec/pixel, RMS error is .18 arc sec for X axis, .275 arc sec for Y axis. Stars measure flatness of .008 which is quite round even though X and Y axes RMS is quite different.
Rolando
Attached Image:
