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

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.

Accepted for publication in MNRAS, 7 pages, 6 figures, 2 tables

References in corpus (4)

• Visualization of the challenges and limitations of the long-term sunspot number record
• Level and length of cyclic solar activity during the Maunder minimum as deduced from the active day statistics
• Sunspot positions, areas, and group tilt angles for 1611-1631 from observations by Christoph Scheiner
• Revisiting Kunitomo's Sunspot Drawings during 1835-1836 in Japan

Cited by in corpus (1)

• Thaddäus Derfflinger's sunspot observations during 1802-1824: A primary reference to understand the Dalton Minimum