matching our stars against a sky-survey, so we know instrumental and calibrated magnitude for many stars.finding the stars in our image, and calculating instrumental magnitudes.astrometric solution of the image - so we know the RA and Dec of all the stars we measure in our image.
If you need calibrated photometry to better than a few percent, you will have to observe standard stars, and apply the rigorous method described in the previous lecture.Īpplying this method requires the application of four steps. Ignoring the secondary effects will probably introduce errors on a similar level. Typically, large sky surveys achieve accuracies of a few percent. Secondly, our photometry cannot be any more accurate than that in the sky survey we use. First of all we are not taking into account any secondary effects, such as 2nd-order extinction, or colour terms. The accuracy this technique can achieve is limited by two factors. If your data is covered by one of these surveys, and it provides calibrated magnitudes in the same filter as your data, you can apply this technique relatively quickly.
What makes this technique possible, is the existence of large sky surveys, which have provided calibrated magnitudes for many, relatively bright stars over a very wide areas of the sky. This intercept could then be added to all of our instrumental magnitudes to produce calibrated magnitudes. In the last lecture we saw that, for any star, the difference between the calibrated magnitude, \(m\) and the above-atmosphere instrumental magnitude \(m_ \).
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