Ground experiment of a 50 mm balloon-borne coronagraph for near space project

We briefly report on the development of a 50 mm balloon-borne coronagraph and its recent ground experiment results made at the high altitude (4800 m above the sea level) site of Mt. Wumingshan in Daocheng, Sichuan of China. The main scientific purpose for developing this coronagraph is to investigate the morphology and dynamics of low-layer coronal structures before and during solar eruptions by observing at a float altitude of about 30 km from 1.08 Rsun to 1.5 Rsun at white light wavelength (centered at 550.0 nm, bandwidth 5 nm). The instrument is an internally occulted Lyot coronagraph developed by Yunnan Observatories in collaboration with Shangdong University (in Weihai) and Changchun Institute of Optics, Fine Mechanics and Physics. The coronagraph was designed with scattered light intensity level of better than 1×10-5 Isun in the inner field of view. A filter wheel system with linear polarizers and an sCMOS camera provided polarization and total brightness images of size 2048 x 2048 pixels. The first successful results were taken on February 27, 2021 in the Daocheng site. This coronagraph experiment obtained coronal images only showing obvious coronal structures very near limb. Furthermore, during the end of March and early April, after improving the polarizer filter system, higher-quality coronal images with pB coronal structures appeared in the full field of view were obtained in our ground-based experiments. Comparison between our results and the other coronal data in the world are discussed. The success of the 50 mm coronagraph in ground experiments is a milestone for us to develop the next-generation large-aperture coronagraph, as well as for future near space projects.