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

## TL;DR

This study investigates how telescope size, scattered light, and human visual detection thresholds affected early sunspot observations, providing models to normalize historical sunspot data for better comparison.

## Contribution

It introduces a method to model and correct historical sunspot counts by considering telescope and human eye limitations, improving the accuracy of long-term solar activity records.

## Key findings

- Smaller apertures lead to undercounting sunspots.
- Scattered light reduces contrast, affecting detection.
- A functional model to normalize historical data is proposed.

## Abstract

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