Over the past four decades, remarkable advances in high-resolution studies of sunspots and active regions have been made through the use of new ground-based telescopes, space missions and associated instrumentation, and of new observing techniques. In this appendix we give a brief summary of these facilities and techniques. (Techniques for observing starspots are described separately in Chapter 9.) An excellent general introduction to modern solar telescopes and their instrumentation is given in Chapter 3 of Stix (2002). References to more thorough treatments of particular topics are provided in the sections below.
A1.1 High-resolution solar telescopes
Although there are more than 50 professional ground-based solar telescopes in regular operation around the world, we focus here only on the few large telescopes that are best suited to high-resolution studies of sunspots. These existing telescopes are listed in Table A1.1 along with two future telescopes, one nearing completion (GREGOR) and the other recently through its design phase and awaiting construction (ATST). There are also plans for a large European Solar Telescope, to come into operation around 2020. A comprehensive list of solar telescopes and their specifications has been compiled by Fleck and Keller (2003).
Ground-based observations are limited to electromagnetic radiation in the visible and near-infrared ranges, at wavelengths between about 300 nm and 2200 nm, and in a range of radio wavelengths. Space missions have allowed us to observe at shorter wavelengths (UV, EUV, X-ray and gamma-ray), revealing the properties of the higher-temperature chromospheric and coronal layers of the solar atmosphere, and have also provided long time series of seeing-free measurements in the visible for studying solar oscillations (helioseismology) and the evolution of solar magnetic fields.