A new paper appeared on arXiv today, discussing the dependence of Sunspot group sizes on the level of solar activity and its influence on the calibration of solar observation. A nice analysis on k-factors is included. The paper can be accessed through:
Best wishes - clear skies,
Here's a recent open Springer pub on a 'Revised Collection of Sunspot Group Numbers'. What's neat about this article is that the revised database has included over 28,000 solar observations from the AAVSO archives on Luft and Cragg data. Thanks to Leif Svalgaard, and Mike Saladyga for digitizing Luft's note books and Cragg's drawings.