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Most big telescopes are built in dry areas on the top of mountains. Examples: La Palma, Mauna Kea in Hawaii, Cerro Paranal in the Atacama desert in Chile  \section{Instruments}  Images are used to study structures of galaxies or nebulae, to measure brightnesses of stars (photometry), or to measure positions of objects (astrometry).  Before 1980s, the typical imaging device was the \textit{photographic plate}. Digitised versions of photographic plate material are still in use (for example, the Digital Sky Survey).  Nowadays, digital cameras are widespread in astronomy. For optical observations, \textit{CCDs} (charge-coupled devices) are used. CCDs are more sensitive than photographs, particularly in the red. They constitute a grid of 1000's of pixels, typical pixel size is $\sim 15\,\mu m$.  Spectra are a record of the emitted light as a function of wavelength. They are measured with a \textit{spectrograph}. The light from the telescope enters the spectrograph through a slit, then is dispersed on a grism or a prism, before being recorded with a camera.  An important parameter of spectrographs is the spectral resolution $R$:  \begin{equation}  R = \lambda / \Delta\lambda  \label{eq23}  \end{equation}  $R$ depends on slit width, grating properties, pixel size, seeing, etc.   For example: A Doppler shift of 10\,km/s needs to be measured. What spectral resolution is needed? Combining Eq \ref{eq7} and Eq \ref{eq23} we obtain: $R = \lambda / \Delta\lambda = c / v = 30000$. For $\lambda = 450$\,nm, this corresponds to a wavelength shift of $\Delta\lambda = 0.015$\,nm (which is very small).  \section{Synopsis}  \subsection{Properties of stars}  \textbf{Observable properties:} position, flux, magnitude, colour, spectrum, spectral type, lightcurve  \textbf{Physical properties:} luminosity, velocity, absolute magnitude, size, mass, temperature  \textit{Astronomy means finding clever methods to get physical properties from observable properties.}  \subsection{What is a star?}