Growth curve, i.e. the number of counts as a function of aperture radii. We measured the aperture photometry (integrated counts) for reference star three with increasing aperture size in 1″ increments. The aperture photometry from two different STELLA images is shown. The pixel size of STELLA is 0.322″ per pixel. The uncertainties shown are the square roots of the counts. These images have seeing values on the low and high end of the range in our data set. The air mass is moderate and both have an exposure time of about 1200 s. The two horizontal lines indicate the count level at an aperture size of radius 10″ – the final aperture size chosen. The count number does not increase significantly above this value.