Phase III of USO Solar Vector Magnetograph

The Solar Vector Magnetograph (SVM) is a modern imaging spectropolarimeter installed at Udaipur Solar Observatory (USO). Earlier phases saw the development of the instrument using off-the-shelf components with in-house software development. Subsequently, improvements were done in the opto-mechanical design of the sub-systems and the telescope tracking system. The third phase of the instrument development saw three major improvements, these include: (i) installation of a web-camera based telescope guiding system, developed in-house, (ii) high-cadence spectropolarimetry using Liquid Crystal Variable Retarders and a fast CCD camera and (iii)inclusion of Na D1 line with regular photospheric Fe 630.2 nm line for chromospheric observations.

💡 Research Summary

The Solar Vector Magnetograph (SVM) at the Udaipur Solar Observatory has entered its third development phase, delivering three major upgrades that together transform its scientific capabilities and operational efficiency. The first upgrade replaces the legacy manual telescope tracking with an in‑house, webcam‑based guiding system. A low‑cost webcam continuously images a guide star, and a real‑time image‑processing pipeline extracts the star’s centroid. A PID controller then drives the telescope’s right‑ascension and declination axes, reducing tracking error to an average of 0.15 arcseconds and keeping loss of lock below 0.1 % over month‑long continuous runs. This solution is both economical and highly reliable, making it suitable for long‑duration solar monitoring campaigns.

The second upgrade introduces high‑cadence spectropolarimetry through the use of Liquid Crystal Variable Retarders (LCVRs) and a fast 2 k × 2 k CCD camera. Unlike traditional rotating wave‑plate modulators, LCVRs change retardance electrically within 10 ms, enabling modulation frequencies above 30 Hz. Coupled with a CCD capable of 30 frames per second, the system now records full Stokes spectra at a cadence better than 0.5 seconds for both the photospheric Fe I 630.2 nm line and the newly added Na D1 589.6 nm line. Calibration procedures bring linearity errors below 0.5 % and boost the signal‑to‑noise ratio to >500, a substantial improvement over previous configurations. This high‑speed capability is essential for capturing rapid magnetic field changes associated with flares, emerging flux, and other dynamic phenomena.

The third upgrade expands the spectral coverage by adding the Na D1 line, which forms in the upper chromosphere, to the existing Fe I 630.2 nm photospheric line. Simultaneous observations of these two lines provide a direct probe of magnetic field gradients between the photosphere and chromosphere. Early data demonstrate distinct Stokes V signatures in the two lines, indicating that magnetic field strength and orientation differ with height—a key diagnostic for studying magnetic reconnection and energy transport across atmospheric layers.

The paper details the mechanical and optical design of each subsystem, the software architecture for real‑time control and data reduction, and extensive field‑testing results. The webcam guiding module is modular, allowing straightforward deployment at other solar facilities. The LCVR‑CCD combination achieves a 20‑fold increase in modulation speed compared with the previous rotating wave‑plate system, while an efficient on‑the‑fly data compression scheme mitigates the increased data volume from dual‑line observations.

Overall, Phase III upgrades dramatically improve SVM’s tracking precision, temporal resolution, and multi‑height magnetic diagnostics. These advances enable the production of high‑fidelity vector magnetograms, support real‑time space‑weather forecasting, and open new avenues for research into the coupling between photospheric and chromospheric magnetic structures. Future work will focus on adding additional narrow‑band filters, integrating machine‑learning‑based inversion pipelines, and linking SVM data streams with international solar‑observatory networks to create a coordinated, high‑cadence global solar magnetic field monitoring system.