High precision photometry involves a measurement of the light coming from different sources in a field and a calculation of the difference between their instrumental and apparent magnitudes. This is done using a number of standard stars, mainly from globular clusters, with known magnitudes and a source of varying flux.
We develop a methodology which calibrates SEO and Yerkes 24″ photometry to the Sloan Digital Sky Survey (SDSS) photometric system. We do this by correcting the instrumental magnitudes of these standard stars to match their magnitudes as measured within SDSS, adjusting unknown sources by this same shift factor, or zeropoint. This allows us to isolate the varying source and measure its true variability without interference from conditions that change from night to night. This kind of photometry is important for projects dealing with measurements of fluctuating sources, as it can accurately give a measurement of the change in brightness over time.
High precision photometry is difficult to obtain, however, and there is a need to accurately and systematically perform these measurements on our telescopes. The methodology we create involves using the Aperture Photometry Tool and Topcat programs to perform this cross-photometry calibration. Once getting these magnitudes, we focus on improving precision. Ultimately, we achieve less than 5% external precision, and less than 2% internal precision images from both the Stone Edge telescope and the Yerkes 24”.
Currently, we are continuing to explore different methods of analyzing data and improving photometric conditions with the intention that our methodology will be used for future projects dealing with variable stars, supernova and other objects too bright for larger and more sensitive telescopes.