Visible emission line coronagraph (VELC) is the prime payload on board India’s first space solar observatory Aditya-L1. VELC is a unique payload with simultaneous observational capabilities in imaging, spectroscopy, and spectro polarimetry modes. VELC is capable of achieving high spatial, spectral, and temporal resolution closer to the solar limb 1.05 R ⊙ compared to the existing space and ground-based solar coronagraphs. VELC consists of a total of 44 optical elements in 18 groups, which are custom designed and developed to meet the desired performance requirements. In addition, it consists of four mechanisms out of which two are multioperational with expected life cycle of million operations. Four detectors (three sCMOS and one InGaAs) are used to record the data. The performance of the payload depends on the performance of individual element, subsystems, and the system level performance of all the elements (such as optics, mechanism, and detectors) together. To ensure the desired performance levels are achieved, each element/subsystem should be tested prior to integrating them together. Evaluation of performance of the integrated system is essential to validate the payload capabilities to meet the proposed science goals. This paper summarizes the calibration tests carried out on the integrated system and compares the results obtained with respect to the design requirements to meet the proposed science goals.
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.
Scattering of light from a surface depends on its microroughness. In general, total integrated scattering of a surface is expressed in terms of root-mean-square microroughness (σrms) using empirical relations. These empirical relations assume that the σrms is measured over the spatial scales ranging from 0 to surface length/diameter (L). In reality, σrms will be measured over finite spatial lengths/scales that result in band-limited microroughness (σbl) and hence band-limited integrated scatter (BLIS). BLIS depends on the microroughness of surface over the finite spatial bandwidths rather than infinite spatial scales. Scatter from super polished optical mirrors peaks in specular direction and falls off exponentially. Hence, BLIS measurements in near-specular direction provide crucial information in understanding the relation between surface microroughness and scatter. Our work gives a detailed description of BLIS measurements carried out on super polished optical surfaces with different surface microroughness and a comparison between the scatter and surface microroughness measurements.
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