Broadly speaking, astronomers do two types of photometry: differential photometry and all-sky photometry. Differential photometry is focused on monitoring one "target" to observe how it changes. The target may be a variable star, an asteroid, a star with exoplanet transits, or an exotic target like a quasar or the nucleus of a Seyfert galaxy. All-sky photometry is aimed at establishing accurate magnitudes for an object or objects relative to standard stars with well-established magnitudes, and generally requires careful observation and rigorous data reduction with no shortcuts allowed.
Differential photometry is easy because the target and a comparison star are usually in the same field of view, and are observed at the same time, through the same atmosphere, with the same filters—and all that matters is an accurate comparison between the target and the comparison. All-sky photometry is difficult because the target objects have been captured in different images from the standard stars, have been observed at different times, through a different atmosphere, and usually have been through multiple filters.
However, observers know that making CCD integrations at the telescope and extracting magnitudes from the resulting images is just the beginning. Raw instrumental magnitudes are strongly affected by three factors:
• Filter(s) used in making the image(s). Although some observing programs do not require filtered images, many do. A standard set of filters includes U, B, V, R, and I, although just two filters, V and R, are necessary to get started.
• Atmospheric extinction that dims stars. Although this dimming is not particularly apparent to the eye, for photometry it is significant and must be corrected.
• A unique set of instrumental magnitudes defined by the peculiarities of your particular set of filters, your CCD, your observing site and your telescope. To combine your observations with those of others, your magnitudes must be transformed to a standard photometric system.
The section following describes how astronomers measure and then compensate for these factors.
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