  The sunspot record is the only observational tracer of solar activity that
provides a fundamental, multi-century reference. Its homogeneity has been
largely maintained with a succession of long-duration visual observers. In this
paper, we examine observations of one of the primary reference sunspot
observers, Hisako Koyama. By consulting original archives of the National
Museum of Nature and Science of Japan (hereafter, NMNS), we retrace the main
steps of her solar-observing career, from 1945 to 1996. We also present the
reconstruction of a full digital database of her sunspot observations at the
NMNS, with her original drawings and logbooks. Here, we extend the availability
of her observational data from 1947-1984 to 1945-1996. Comparisons with the
international sunspot number (version 2) and with the group sunspot number
series show a good global stability of Koyama's observations, with only
temporary fluctuations over the main interval 1947-1982. Identifying drawings
made by alternate observers throughout the series, we find that a single
downward baseline shift in the record coincides with the partial contribution
of replacement observers mostly after 1983. We determine the correction factor
to bring the second part (1983-1996) to the same scale with Koyama's main
interval (1947-1982). We find a downward jump by 9% after 1983, which then
remains stable until 1996. Overall, the high quality of Koyama's observations
with her life-long dedication leaves a lasting legacy of this exceptional
personal achievement. With this comprehensive recovery, we now make the
totality of this legacy directly accessible and exploitable for future
research.
