Optical metrology on primary mirror of visible emission line coronagraph on board Aditya-L1 mission

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.

Ground calibration of visible emission line coronagraph on board Aditya-L1 mission

“…VELC consists of 44 optical elements in 18 sub assemblies. Each sub assembly is tested independently prior to integrating them with the payload 9 – 11 VELC consists of different channels and performs different modes of observations, and each channel should be calibrated to derive the achieved performance of the system.…”

“…Detailed studies (theoretical and experimental) are carried out to derive the fabrication specifications of the M1 and to define the limits of particulate and molecular contamination over M1 surface 5 – 9 …”

“…Increase in total integrated scatter over the FOV of continuum channel is mainly due to the higher surface microroughness on M1. Surface microroughness of the M1 required to achieve 5 ppm scatter is 0.15 nm, 5 whereas, the final achieved microroughness over M1 surface is 0.48 nm 9 . Due to this increase in scatter over the FOV, shorter exposures times (to avoid the detector saturation) and off-line binning (pixel/frame) are required to achieve the desired signal-to-noise ratio (SNR).…”

“…Detailed studies (theoretical and experimental) are carried out to derive the fabrication specifications of the M1 and to define the limits of particulate and molecular contamination over M1 surface. [5][6][7][8][9] 2.1 Performance Specifications of VELC Based on the optical design, performance, thermal, and tolerance analysis, achievable performance of the instrument is determined. Expected performance of VELC in different channels is given in Table 1.…”

“…Each sub assembly is tested independently prior to integrating them with the payload. [9][10][11] VELC consists of different channels and performs different modes of observations, and each channel should be calibrated to derive the achieved performance of the system. Hence, a detailed test and evaluation methodology is developed to determine the performance of the payload.…”