It is well known that off-axis testing of a spherical surface introduc
es aberrations, mainly primary astigmatism. The magnitudes of these ab
errations are discussed and analytically calculated. Their influence i
n several testing configurations is discussed. A case of particular in
terest is when a linear carrier fringe pattern is used in a Fizeau int
erferometer. (C) 1996 Society of Photo-Optical Instrumentation Enginee
rs.