Informatics Publishing Limited

B. Raghavendra Prasad ¹, Dipankar Banerjee ¹, Jagdev Singh ¹, S. Nagabhushana ¹, Amit Kumar ¹, P. U. Kamath ¹, S. Kathiravan ¹, Suresh Venkata ¹, N. Rajkumar ¹, V. Natarajan ¹, Madhur Juneja ¹, Pawan Somu ¹, Vaibhav Pant ¹, Nigar Shaji ², K. Sankarsubramanian ², Asit Patra ³, R. Venkateswaran ⁴, Abhijit Avinash Adoni ², S. Narendra ², T. R. Haridas ⁵, Shibu K. Mathew ⁶, R. Mohan Krishna ², K. Amareswari ², Bhavesh Jaiswal ²

Affiliations
1 Indian Institute of Astrophysics, 2nd Block, Koramangala, Bengaluru 560 034, IN
2 ISRO Satellite Centre, Old Airport Road, Vimanapura Post, Bengaluru 560 017, IN
3 Space Applications Centre, Jodhpur Tekra, Ambawadi Vistar, P.O., Ahmedabad 380 015, IN
4 Laboratory for Electro-Optics Systems, First Phase, Peenya Industrial Estate, Bengaluru 560 058, IN
5 ISRO Inertial Systems Unit, Vattiyoorkavu PO, Thiruvananthapuram 695 013, IN
6 Udaipur Solar Observatory, Near Fatehpur Lake, Shilpgram, Udaipur 313 004, IN

Abstract

Solar coronagraph mimics total solar eclipse by blocking the solar disk and enabling the observation of extended coronal atmosphere of the Sun. Visible Emission Line Coronagraph (VELC), on-board Aditya-L1 space mission, is an internally occulted solar coronagraph capable of simultaneous imaging, spectroscopy and spectro-polarimetry close to the solar limb. This payload is designed to study the coronal plasma and heating of the solar corona. Studying development, dynamics and origin of coronal mass ejections and measurement of coronal magnetic fields over active regions are other important science goals. VELC is designed to image the solar corona at 500 nm with an angular resolution of 5" over a field of view (FOV) of 1.05-3 R_o. It also facilitates simultaneous multi-slit spectroscopy at three emission lines, viz. Fe XIV (530.3 nm), Fe XI (789.2 nm) and Fe XIII (1074.7 nm) with a spectral resolution of 28 , 31 and 202 mÅ/pixel respectively, over an FOV of 1.05-1.5 R_o. The payload has a dual-beam spectro-polarimetry channel for magnetic field measurements at 1074.7 nm.

Keywords

Coronagraph, Coronal Mass Ejection, Payload, Solar Corona.

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G. Suresh ¹, V. Natarajan ¹, C. Durga Prasad ²

Affiliations
1 Naval Physical & Oceanographic Laboratory, Kochi, IN
2 Naval Materials Research Laboratory, Mumbai, IN

Abstract

Acoustic imaging array systems generate an image by processing the acoustic waves backscattered from an underwater object. Generally these systems operate at higher frequencies and provide shorter imaging ranges. The lateral resolution is determined by the ultrasound beam width which is a function of aperture diameter and frequency. The diffraction pattern of an array accounts for all linear effects, such as, transducer size, side and grating lobes, focusing, and beam steering. Angular resolution enhance with increase in the number of elements or the frequency. The side lobes appear as a background noise in the image. The ability and usefulness of the algorithms depend on the quality (Sensitivity, bandwidth, and dynamic range) of the original echo signal, making the transducer the most critical component of ultrasound imaging systems. The present paper presents the design of a 128 element underwater acoustic imaging sensor linear array that operates at 500 kHz and with a horizontal beam width less than 1°. There are several sound field parameters that are critical in designing an ultrasonic transducer array, which have been discussed in the paper. The array evaluated underwater at 500 kHz shows a source level of 184 dB and a receiving sensitivity of -192 dB re V/μPa @1 m. The acoustic cross-talk and other acoustical parameters are also discussed.

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Bhaskar Prasad Saha ¹, Dulal Chandra Jana ¹, Prasenjit Barick ¹, V. Natarajan ², Suresh Venkata ², Abhijit A. Adoni ³, D. R. Veeresha ³, R. Venkateswaran ⁴, P. U. Kamath ², Roy Johnson ¹, K. V. Sriram ⁴, B. Raghavendra Prasad ², G. Padmanabham ¹

Affiliations
1 International Advanced Research Centre for Powder Metallurgy and New Materials, Balapur Post, RCI Road, Hyderabad 500 005, IN
2 Indian Institute of Astrophysics, 2nd Block, Koramangala, Bengaluru 560 034, IN
3 U. R. Rao Satellite Centre, Old Airport Road, Vimanapura Post, Bengaluru 560 017, IN
4 Laboratory for Electro-Optics Systems, First Phase, Peenya Industrial Estate, Bengaluru 560 058, IN

Abstract

A state-of-the-art Visible Emission Line Coronagraph (VELC) payload on-board India’s solar mission ADITYA-L1 was designed to study various solar pheno­mena. To maintain the thermal stability of the system, VELC design recommends silicon carbide (SiC)-based components because of their outstanding mechanical, thermal and optical properties. In particular, a SiC-based tertiary mirror (M3) was used for the collection of undesired sunrays and reflecting them out from the system, and a SiC radiator plate (popularly known as a cold finger) for efficient heat dissipation from the mirror and, in turn, from the system. This article describes the processing and evaluation of SiC-based M3 mirror and cold finger for VELC. The substrates for M3 mirror and cold finger were processed through dry pressing of SiC powder with the required formulation, followed by machining and temperature-assisted densification under an inert atmosphere. SiC components developed using powder metallurgical technique exhibited about 98.4% relative density (RD) and achieved the structural and thermal requirements of M3 mirror and cold finger. The optical requirement of M3 mirror was achieved through a coating of SiC substrate with 100% RD employing chemical vapour deposition followed by surface grinding and polishing. The final mirror achieved a surface flatness better than 20 nm, and microroughness data showed less than 5.1 Å root mean square surface roughness in a spatial scale of 0.02 to 0.9 mm

Keywords

Chemical vapour deposition, coronagraph, relative density, silicon carbide, solar mission.

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