Characterization of an Optical Transfer Function–based Phasing Technique for India’s Large Segmented Mirror Telescope

Abstract The upcoming 10 m class large Indian telescope is proposed to be built using segmented mirror technology. To achieve diffraction-limited image quality with a segmented mirror telescope, it is important to phase mirror segments with very high precision. We have explored an optical transfer function (OTF)–based technique for segment phasing. The major advantage of this approach is that it enables highly accurate phase measurements over a large capture range without requiring dedicated complex optics or mechanisms. Through extensive simulations and laboratory experiments, we have tested the feasibility of using this technique for the proposed large telescope. From extensive simulations, we found that by using optical filters of different bandwidths, the modulation transfer function allows a measurement of piston errors as large as 50–100 μ m and can reduce this down to 25–30 nm. Another advantage of using the OTF is that it inherently allows simultaneous measurement of phase errors in a large number of mirror segments without the use of any specialized optical elements. In this paper we present our results from simulations and laboratory experiments in relation to the proposed 10 m class segmented mirror telescope.