Hi George
The flat field used in astro photography is a check on the sensitivity of the ccd over its entire surface and is used to correct the luminance of the exposure. It normally has nothing to do with collimation.
Correcting the collimation of the OTA is done to make sure that the focal plane of the light path is at the position where all parts of the image are in the best focus.
With a refractor telescope it is not normally done as most telescopes do not have facilities for adjustment although it is necessary for best results.
On a Newtonian or SCT It is very important and it is done in two stages. The first is to get the off focus image of the secondary mirror centred in the eyepiece. The second is to then do a very fine adjustment using diffraction rings caused by the secondary "obstruction" on a highly magnified star off focus to make them concentric.
This is a very brief description perhaps over simplified. Others may give you more detailed information.
Thierry Legault used to have has a very informative article on collimation on his web site. I think it is still there.
http://perso.club-internet.fr/legault
Barry
