Abstract:
The visible emission line coronagraph (VELC) on board the Aditya-L1 mission is an
internally occulted reflective coronagraph. It is capable of simultaneous observations of the solar
corona in imaging, spectroscopic, and spectropolarimetric modes very close to the solar limb, to
1.05 R⊙ (R⊙ – solar radius). Primary mirror (M1) of the VELC receives the light from both the
solar disk and the corona up to 3 R⊙. In the VELC, occultation happens at the focus of the M1.
Secondary mirror (M2) with a central hole size equal to 1.05 R⊙ is mounted at the focal plane of
M1 and serves the purpose of an internal occulter. To meet the proposed science goals of the
payload, M1 surface should be super polished with good imaging characteristics. This results in
stringent requirements of the surface figure and microroughness on the mirror surface. M1 is an
off-axis parabola, so achieving the demanding requirements is quite challenging. At the same
time, testing of M1 after development is crucial for evaluating its performance. This paper provides the details of the optical metrology tests carried out on M1 along with the results obtained
and their implications on the performance of the VELC. © 2022 Society of Photo-Optical
Instrumentation Engineers (SPIE) [DOI: 10.1117/1.JATIS.8.4.044004]
