We’ve received some questions on our recent Setting the Wedge Factor post regarding measurement in autocollimation.
The first question concerns the relative quality of the optics M1 versus M2. The mirror that will serve as the reference must be of significantly higher quality than the mirror to be tested; if the two mirrors are of similar quality then a measurement of either mirror will lose fidelity, as it is impossible to distinguish which mirror is contributing to the departure.
The second question regards measuring the surface height of M1 (which, in our Wedge Factor chart is shown as “N/A, not applicable”). It is not an uncommon practice to measure M1 in the arrangement shown, using a Wedge Factor of 1/4 (A=1/2, B=1/2 and C=1) when M1 has a larger F/#. However, in this arrangement, the angle of incidence varies across the parabolic surface, and this variation cannot be accounted for with a single Wedge Factor value. Instead, the system would need to be modeled and the resulting function applied to accurately determine surface height.
Determining surface height is typically only necessary for applications such as providing feedback in computer controlled polishing. The reflected wavefront measurement adequately characterizes the optical performance of the mirror, and thus, should be sufficient for determining if an optic is within a specified tolerance (e.g. < 1/10th wave).