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\subsubsection{The Standard Stars}  To figure out the magnitudes of our stars we need to calibrate the  arbitrary flux units {\color{red}(`counts')}  we've measured into something more physical. In essence, we need ``$c$'' from the above equation.  This constant depends on the size of the telescope (big telescopes collect  more light, so you get more counts), the efficiency of the CCD (more 
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loose some of the light), and the atmosphere (haze or dust absorbs or  scatters the starlight before it reaches the telescope).  Astronomers use images of standard stars to perform this  calibration. These are stars of known (non-variable)  brightness, whose magnitudes and locations on the sky are catalogued in long lists.  In our case we don't need a separate standard star image, because we