Introduction - What is photometry?

Photometry is the measurement of the intensity or brightness of an astronomical object, such as a star or galaxy by adding up all of the light from the object. For example, a star looks like a point of light when you look at it just with your eyes but the Earth’s atmosphere smears it out into something that looks like a round blob when you use a telescope to look at it. In order to measure the total light coming from the star, we must add up all of the light from the smeared out star.

What is photometry used for?

Photometry is generally used to generate light curves of objects such as variable stars and supernovae, where the interest is the variation of total light energy output by the system over time.

It can also be used to discover exoplanets, by measuring the intensity of a stars light over a period of time. Deviations in the light output can indicate objects in orbit around the star.

Note: The technique of photometry (placing an aperture over a selected object in an image) is the same whatever type of object you are looking at, but there may be some slight differences in how you carry out the photometry and analyse your data, depending on whether you are just interested in looking at intensity values of objects in the image or if you are wanting to compare your values with others in the image, or known, published values.