MIDSSAR: MId Dispersion Spectrograph for Stellar Activity Research

Abstract

We have developed and commissioned MIDSSAR (MId Dispersion Spectrograph for Stellar Activity Research), a novel fiber-fed, dual-band spectrograph optimized for high-cadence monitoring of stellar superflares and magnetic activity phenomena, mounted on the 3.8-m Seimei Telescope at Kyoto University Okayama Observatory. Utilizing a custom-designed volume phase holographic grating and a commercially available volume binary transmission grating, the instrument achieves simultaneous observations of key chromospheric activity indicators: the CaII H&K band (382.5-406.5nm) at a resolving power of R=13,800, and the H$\alpha$ band (641.1-667.6nm) at R=18,200. MIDSSAR is strategically integrated with the existing high-dispersion spectrograph GAOES-RV to enable concurrent photosphere-chromosphere monitoring. The control software coordinates high-speed dual-camera exposures down to 1ms while incorporating real-time telescope telemetry into the data headers. We also developed a dedicated data reduction pipeline that automatically executes calibration, spatial reconstruction of the 61-fiber bundle, and one-dimensional spectral extraction with a wavelength precision of $\sim$ 0.002nm. Initial on-sky test observations conducted from March to May 2026 successfully verified the system's quantitative readiness. The observed signal-to-noise ratio characteristics perfectly align with modeled theoretical limits under varying seeing conditions, confirming that the entire instrument complex successfully meets its primary design requirements and operational expectations for systematic stellar astrophysics surveys.