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The HRD is the fundament for all theories of stellar evolution and will be discussed more in detail in other parts of AS1001 and other courses. The pre-requisite for the HRD is the measurement of stellar luminosities and temperatures.   \section{Telescopes}  Telescopes collect EMR from astronomical sources. We use telescopes for two reasons: a) They have a bigger \textit{collecting area} than the human eye, i.e. we can see fainter sources. b) They increase the \textit{angular resolution} compared with the human eye, i.e. we can see more details and measure more accurate positions.  An astronomical telescope is a system that brings parallel rays of light from a distant source to a \textit{focal plane}.  \subsection{Telescope types}  There are two fundamental types:  a) \textit{Refractors} use lenses to focus the light (objective lense and eyepiece). Largest refractor: 1.0\,m Yerkes telescope from 1888.   b) \textit{Reflectors} use mirrors (at least one, usually more than one) to focus the light. Largest reflectors:   11\,SALT in South Africa, 2x10\,m Keck in Hawaii, 4x8\,m VLT in Chile. Reflectors come in a variety of designs. Some of the most common ones are shown in Fig \ref{f12}.  \textit{Catadioptric telescopes} are telescopes that combine lenses and mirrors. An example is the Schmidt telescope which has the capability to make images with a very wide field of view. The James Gregory Telescope in St Andrews is a catadioptric Schmidt-Cassegrain telescope.  \subsection{Reflector optics}  The path of light through a reflector is shown in Fig \ref{f13}. Important concepts are the \textit{focal plane} (the plane where the light is focused) and the \textit{effective focal length} $f_e$ (the distance between objective lense and focal plane.  The focal ratio $f$ of a telescope is defined as $f = f_e/D$ (with $D$ being the diameter of the lens/mirror). The focal ratio describes how fast the beam converges to the focal plane. The focal ratio is usually written in units of the aperture. Example: A telescope with $D = 1$\,m and $f_e = 3$\,m has a focal ratio of $f/3$.   Small $f/$ means small image scale, but fast light collection (light is concentrated into a small area in the focal plane). This is ideal for wide-field surveys. Large $f/$ means large image scale, but slow light collection. This is ideal for seeing details and measuring accurate positions.  \subsection{Resolving power}  The \textit{resolution} of a telescope is the minimum angular separation of two sources on the sky, that can be seen separately.   The resolution is limited by the diffraction of light around the edges of the optics in the telescope. It depends on the aperture of the telescope and the wavelength of the light. For a circular aperture, each source of light causes an \textit{Airy pattern} of concentric rings in the focal plane. \textit{Rayleigh's criterion} provides an equation for the \textit{diffraction limit} of a telescope (the absolute limit of the resolving power);  \begin{equation}  \alpha = 1.22 \lambda / D \mathrm{radians} \sim 2.5 \cdot 10^5 \lambda / D \mathrm{arcsec}  \label{eq22}  \end{equation}  Here, $D$ is the diameter of the aperture of the telescope (i.e. the diameter of the objective lens or the main mirror). Examples for observations at optical wavelength ($\lambda = 500$\,nm): $\alpha = 1$\,arcsec for $D = 0.125$\,m. $\alpha = 0.03$\,arcsec for 4-m telescope.  \section{Synopsis}  \subsection{Properties of stars} 
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\textit{Astronomy means finding clever methods to get physical properties from observable properties.}  \subsection{What is a star?}