Optimal Photometry
Advantage 1. Signal-to-noise.![]() This is a colour-magnitude diagram of all the stars within X-ray error boxes from an dataset for NGC2547, reduced with aperture photometry. Below is the same dataset, reduced with optimal photometry (now the brightest stars in each X-ray error box are circled). Note how much cleaner the PMS is around V-I=2.5, and the detection of a source at V=21. We find the optimal reduction of the same data reached 0.45 magnitudes deeper for the same signal-to-noise. In principle the aperture photometry could be improved by using a smaller aperture, but as we show below, in practice this would degrade the photometry. ![]() Advantage 2. Robust error estimates.![]() Sometimes we measure the same star in several different images. This gives us the opportunity to check that our error model is correct, by calculating the chi-squared for each measurement of a star with respect to its average brightness. The resulting distribution of chi-squared is shown as the histogram in this figure, and the theoretical distribution as a line. Note that the line is not a fit to the data, there are no free parameters! The slight excess in actual chi-squared at high values is due to genuine variability. |