1. ADF (active-day fraction) method of sunspot number calibration
We wanted to study how the PSD at different frequencies behaves at long time scales.
I. Usoskin 1, G. Kovaltsov 2, M. Lockwood 3, K. Mursula 1,
M. Owens 2, S. Solanki 4, T. Willamo 5
1 University of Oulu, Finland Ioffe Phys-Tech. Inst., Russia
3 University of Reading, UK Max Planck Institute for Solar System Research, Germany
5 University of Helsinki, Finland

2. Daisy chain linear regression (Ri, Rg)
Calibration
Daisy chain linear regression (Ri, Rg)
’Backbone’ daisy chain (SS15)
Can we propose something better? 2 2

3. New independent calibration
Reference data set
Active day fraction  No daisy-chain.
All calibrations independent.
* Another daisy-chain-free method by Andres Munos-Jaramillo 3 3

4. 1. Calibration curve
Reference data set (RGO, ), sunspot groups with area.
For each month  ADF: A=Nactive/Nobs
Apply a threshold Ss  Cumulative pdf P(A, SS)

5. 2. Finding SS
For each observer we find P and compared with the calibration curves to find the observational threshold SS

6. 3. Correction matrix
Using the reference data set (RGO, ) we build a correction matrix for each observer, using SS

7. Individual corrections
Each observer is calibrated independently to the reference data set.

8. Summary series with 95% c.i.

9. Final series

10. Test

12. Conclusions
A new sunspot group series is constructed using the direct calibration method since 1749, normalized to the reference data set (RGO, ).
It lies between low GSN (Hoyt & Schatten, 1998) and high (Svalgaard & Schatten, 2016) series.
The Grand Modern maximum is confirmed, but the high period in 18th century is not.
The method may sightly understimate high cycles and strongly overestimate low cycles, and is not applicable to very low cycles (DM or MM).