Ritchey–Common sparse-aperture testing of the Giant Steerable Science Mirror

The Giant Steerable Science Mirror (GSSM) is the tertiary mirror of the future large telescope, the Thirty Meter Telescope. However, the mirror is too large to be tested using only one aperture, and using many apertures will increase the cost of testing. To accomplish testing at a low cost, the number of apertures should be reduced. The Ritchey–Common (R–C) testing method, commonly used for testing large flat surfaces, uses only a reference spherical mirror and avoids the use of large planar interferometers. Additionally, only the low-spatial-frequency mirror figure is relevant in the system assembly and alignment. Hence, the applicability of sparse-aperture testing is investigated in this paper. Sparse-aperture testing and the R–C method were combined to lower the cost. Using this method and the normalized point source sensitivity (PSSn), the mirror figure can be specified in a simple and accurate manner. Moreover, as fewer subapertures are under test, the efficiency can be improved. An error analysis is conducted, focusing on the shifting error, irregularity error, tipping error, tangential/sagittal error, and seeing. For the testing of the GSSM prototype, the error analysis showed the total error in PSSn is 0.9701.