Sunspots have been systematically observed since 1610, after the invention of the telescope. In fact, the sunspot counting is considered like the world’s longest-running experiment. The sunspot number index is calculated from the number of sunspot appeared on the solar disc and it is the most used index to study the long-term solar activity. Recently, several problems were detected both in the sunspot number and the sunspot group databases. Therefore, they need a revision.
Nowadays, those problems are being addressed by the international community of space climate. In this presentation, several sunspot observation series made by amateur astronomer are presented highlighting their role in the recalibration of the sunspot number index in terms of their long-term stability in comparison with professional observatories working in a shift. Some of these observers with long observational series are David Hadden (U.S.A), Strach (United Kingdom) and Koyama (Japan). These contributions will play a key role to revise sunspot number and its extension to 14th century and beyond, on the basis of cosmogenic isotope with calibration of this index.