Photometry is the measurement of the amount of light from an object. Accurate photometric measurement is a fundamental and important means for understanding the physics behind astronomical objects and events.
Prior to CCD cameras, photometric measurements were made visually, photographically, or using photomultiplier tubes. The former two methods suffer from accuracy problems; whereas the latter is rather difficult and time-consuming to use. CCD cameras permit the simultaneous capture of quantitative data for both the target star or object and other comparison stars.
Typically photometric measurements of stars are performed using filters. Several filter systems are in use, including Johnson UBVRI, Kron/Cousins UBVRI, and Bessell.
Comparing different filter bands allows the determination of a color index for a star. Fundamental physical parameters for a star can be determined using these methods. Many stars vary in brightness; analyzing the light curves can be used to set constraints on physical models, measure distances, etc.
MaxIm DL includes two photometric measurement tools, both based on aperture photometry - measuring the total light within a circular aperture. The simplest is the Information window, which can be calibrated to a star of known magnitude on an image. Then other stars can be measured using that calibration.
The Analyze menu Photometry command can be used to perform more accurate measurements and plot them over time. This command performs a careful background subtraction using median-mean techniques, and also takes partial pixels into account when integrating the light inside the measurement aperture. Data can be plotted for measured stars, comparison stars, and check stars.
The aperture used for the measurements is fully adjustable. An annular region for background measurements is also fully adjustable. This allows you to include the total light from a star, and pick the best region for use for the background measurement.
For details on the photometry command please see Photometry Quick Start Guide.
A detailed explanation of photometry is beyond the scope of this manual. A good place to start for further research is the American Association of Variable Star Observers, http://www.aavso.org.