The "coma-free" zone is that zone where the distortion of the star image does not yet exceed the size of the Airy disc.
The formula I use, and have seen often quoted is:
0.0007" x f/ratio³
or, in millimeters
0.01778mm x f/ratio³
A less-stringent application uses a different figure:
0.022mm x f/ratio³
In neither case is coma likely to be visible to even the pickiest observer.
The difference between them is small.
In my f/5 newtonian, it's 2.22mm versus 2.75mm
If I'm using an eyepiece with a 36mm field stop, whichever figure is used, only the very center of the field is without coma.
Remember that linear size of a comatic image and apparent size of a comatic image are two different things.
The linear size is important to know, but it is the apparent size (which also takes magnification into account) that determines
what we see. For example, the linear size of coma at the edge of the field in a 10mm 100° eyepiece is 1/2 the size of the comatic star
at the edge of the field in a 20mm 100° eyepiece. But since the magnification is doubled, the apparent size of the comatic stars
at the edge are the same in both eyepieces.
Edited by Starman1, 09 December 2014 - 03:33 PM.