Study on low-order aberration measurements of large-aperture flats based on scanning pentaprism technology

Pentaprism scanning technology (PPS) is an absolute testing method that has the advantages of a simple structure and absolute testing without an extra reference flat, as well as being able to provide in situ surface measurements, and more. It plays an important role in the manufacturing process of large flat mirrors. For calibrating the PPS's uncertainty, this paper describes a multi-mode scanning method to implement the measurement of low-order aberrations and introduces the concept of an autocorrelation coefficient to evaluate the data processing progress. These improvements were applied to the measurement of a large flat mirror (1630 mm in diameter), which demonstrates that the measuring uncertainty of PPS can be about 20 nm rms. Furthermore, in regard to the special requirements of M3MP, the prototype mirror of M3M (the tertiary mirror) in the Thirty Meter Telescope project with a non-circular aperture, we analyzed the slope distribution of low-order aberrations, power, and astigmatism. The sample route lines of PPS are then reorganized and a new data process algorithm is implemented. This work was performed in order to improve the PPS's performance in measuring low-order aberrations of large flat mirrors.