A network for solar optical observations in Bulgaria

The report presents a new initiative for the development of a network of ground-based stations for solar observations in the optical range. Three separate locations in Bulgaria have installed instrumentation for solar dedicated observations. The currently used telescopes (type, mounting, guiding systems) and designated filters (white-light, H-alpha) are described in detail. Test images are also included. Future plans for improvements are briefly discussed.

💡 Research Summary

The paper presents a new initiative to establish a ground‑based optical solar‑monitoring network in Bulgaria, comprising three separate stations that have already been equipped for dedicated solar observations. After a brief historical overview of solar observations dating back to the early 17th century, the authors emphasize the scientific importance of simultaneous white‑light (continuum) and H‑α (6563 Å) imaging, which together reveal sunspots, plages, filaments, prominences and flares.

The first station, located at the Astronomical Observatory in Belogradchik, uses a 279 mm Schmidt‑Cassegrain telescope (Celestron C11, f/10) on a VX AVX GoTo equatorial mount, a LUNT LS60F H‑α filter mounted on the optical axis, and a Common Vision Spark camera. A test image taken on 11 July 2025 shows active region AR 14136 with clear sunspot structure, but a pronounced interference pattern caused by the filter’s axial placement degrades image quality, forcing a central crop of the sensor.

The second station, at the Astronomical Center of Shumen University, employs a 40 mm Coronado ST‑4000 personal solar telescope (f/10) with an azimuthal mount and an automatic “HelioFind” solar‑guiding system. A NexImage 5 CCD camera records H‑α data after extending the focal length to 1200 mm using a projection eyepiece. The resulting image (23 September 2025) displays several sunspot groups, yet a faint interference pattern remains, indicating the need for better filter alignment and optical flatness.

The third station, the St. George International School and Preschool observatory in Sofia, uses a 120 mm Celestron Omni XLT refractor (f/8.3) on a 10 µm mount, an Atik Horizon Color CMOS camera, and a variety of metallized, glass and Baader AstroSolar Photo Film OD 5 filters. Full‑disk images obtained on 4 June 2025 successfully capture sunspot groups at the western limb (AR 14100 and AR 14099), demonstrating that the film‑type filter approach yields clean disk images without the interference seen at the other sites.

The authors discuss the current limitations of each setup—chiefly filter‑induced interference, focal‑length constraints, and the lack of a unified data‑transfer and processing pipeline. Their outlook outlines concrete steps to evolve the prototype network into an operational system: (1) upgrade optical components, acquire new telescopes and temperature‑stabilized H‑α filters where needed; (2) develop high‑speed data links, centralized storage, and automated image‑reduction pipelines (including flat‑fielding, stacking and feature extraction); (3) launch a public, online solar‑monitoring database for Bulgaria.

Beyond pure science, the network is intended to complement existing radio‑frequency solar facilities such as the Bulgarian LOFAR station, providing a rapid‑response trigger for multi‑wavelength campaigns. The authors also highlight the educational potential of the observatories, proposing student‑focused observing programs, outreach activities, and the integration of machine‑learning tools for automatic event detection (as exemplified by Ramos et al., 2023).

Funding for the project comes from a bilateral Bulgaria‑Austria joint‑observation grant, the Bulgarian National Science Foundation, and Austria’s OeAD, with additional support from the Bulgarian Ministry of Education and Science. The paper thus documents the hardware configurations, initial scientific results, and a roadmap for expanding Bulgaria’s capability in ground‑based solar physics.