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Early telescopic observations of sunspots were conducted with instruments ofrelatively small aperture. These instruments also suffered from a higher levelof scattered light, and the human eye served as a "detector". The eye's abilityto resolve small details depends on image contrast, and on average theintensity variations smaller than $\approx$ 3\% contrast relative to backgroundare not detected even if they are resolved by the telescope. Here we study theeffect of these three parameters (telescope aperture, scattered light, anddetection threshold of human vision) on sunspot number, group number, and areaof sunspots. As an "ideal" dataset, we employ white-light (pseudo-continuum)observations from Helioseismic and Magnetic Imager (HMI) onboard of SolarDynamics Observatory, and we model the appearance of sunspots by degrading theHMI images to corresponding telescope apertures with an added scattered light.We discuss the effects of different parameters on sunspot counts and derivefunctional dependencies, which could be used to normalize historicalobservations of sunspot counts to common denominator.

The effect of telescope aperture, scattered light, and human vision on early measurements of sunspot and group numbers