Ronchi :-) question..... (pun)

Before I start on the construction of a Ronchi screen for the Celestron CR150 tomorrow, does anyone know what the effect of its accuracy will be from normal "shimmering" of atmospheric turbulance?.

e.g., will heavy "flaring" of a star cause line distortion in it self, that will PREVENT measuring how much the objective is undercorrected or overcorrected?.

I am in the pits for a viewing location.

do you have anything distant to use as an 'artificial' star? I normally just use an artificial star for collimation purposes, but for doing a good analysis of optics, the 'star' must be very distant; focusing on targets not in the 'infinity' range can introduce false spherical correction results. Shouldn't make that much of a difference on your refractor, but SCT's and maks for example have horrendous consequences in using a close artificial star for testing.