A compound common-path interferometer (10, 130, 150) comprising: a first interferometer having a first measurement arm, a second interferometer having a second measurement arm, a base (124, 132, 152) for supporting the first and second measurement arms (42, 44), each of the first and second measurement arms (42, 44) including a reference optic (54, 56) and an optical pathway for conveying both a test beam (66, 68) from a test object and a reference beam (62, 64) from each of the reference optics (54, 56), and a coupling (102, 104) between the reference optics (54, 56) of the first and second measurement arms (42, 44) forming together with the reference optics (54, 56) a monolithic measurement cavity for maintaining reference surfaces (58, 60) of the reference optics (54, 56) at a fixed spacing and orientation, characterized in that the reference optic of the first measurement arm is structurally disconnected from a rest of the first measurement arm for limiting transmission of disturbances between the reference optic of the first measurement arm and the rest of the first measurement arm, the reference optic of the second measurement arm is structurally disconnected from a rest of the second measurement arm for limiting transmission of disturbances between the reference optic of the second measurement arm and the rest of the second measurement arm, and the monolithic measurement cavity is connected to the base (124, 132, 152) independently of the rest of the first measurement arm and independently of the rest of the second measurement arm, wherein the coupling (102, 104) between the reference optics (54, 56) of the first and second measurement arms (42, 44) includes a thermally stable spacer (102, 104) between the reference optics (54, 56) of the first and second measurement arms (42,44). a first interferometer having a first measurement arm, a second interferometer having a second measurement arm, a base (124, 132, 152) for supporting the first and second measurement arms (42, 44), each of the first and second measurement arms (42, 44) including a reference optic (54, 56) and an optical pathway for conveying both a test beam (66, 68) from a test object and a reference beam (62, 64) from each of the reference optics (54, 56), and a coupling (102, 104) between the reference optics (54, 56) of the first and second measurement arms (42, 44) forming together with the reference optics (54, 56) a monolithic measurement cavity for maintaining reference surfaces (58, 60) of the reference optics (54, 56) at a fixed spacing and orientation, characterized in that the reference optic of the first measurement arm is structurally disconnected from a rest of the first measurement arm for limiting transmission of disturbances between the reference optic of the first measurement arm and the rest of the first measurement arm, the reference optic of the second measurement arm is structurally disconnected from a rest of the second measurement arm for limiting transmission of disturbances between the reference optic of the second measurement arm and the rest of the second measurement arm, and the monolithic measurement cavity is connected to the base (124, 132, 152) independently of the rest of the first measurement arm and independently of the rest of the second measurement arm, wherein the coupling (102, 104) between the reference optics (54, 56) of the first and second measurement arms (42, 44) includes a thermally stable spacer (102, 104) between the reference optics (54, 56) of the first and second measurement arms (42,44).