Usoskin et al. 2016, "active-days" group number: description.
New group number reconstruction based on a new approach: the active-days method.
The key advantage of this method is that it does not rest on the determination on the mean k coefficient of the observer relative to other parallel observers, which is prone to a propagation of trends and becomes unusable when wide gaps without observations are present. The scale of individual observers is derived from a relation between the number of spotless days and the mean absolute number of sunspot groups. This relation is assumed to depend only on the acuity of the observer, i.e. his capability to detect the smallest sunspots.
The active-days/Gn relation was calibrated on the photographic catalogue of the Royal Greenwich Observatory, thus not on true visual observations. It assumes a constant time-independant scale of the observer and does not consider the difference in group splitting practices between observers, which can also influence the group number. Some aspects of this alternate series may thus be revised based on deeper validations.
This series was built only from 1749 to 1899. As it is calibrated in Greenwich data, it can be extended by simply appending the original group number (Hoyt & Schatten 1998), which is based on the Greenwich data over the 20th century. Like the original group number series, this composite series thus ends in 1995.