Monday, January 27, 2014

Basic Photometry

Since a lot of my students are getting into 'research mode' and require a knowledge of the means to analyze data, I thought it would be good to make a post on some basic info here. Among the easiest and most accessible is doing photometric analysis. [Forgive me if I once again go about some technical info in this blog, I will try to make it as basic as possible]

Photometry or the 'measurement of light' allows you to do a variety of things, such as identify magnitudes, sky brightness, limiting magnitudes, render light curves, etc. A variety of research subjects can be tackled by photometry.

Your digital camera can act as a detector which can be used to gather photometric data for analysis. You also need a list of standard stars (stars with fixed magnitudes) if you wish to make a comparison of magnitudes. Here is a sample list of standard stars (A. U. Landolt, 1992). And lastly you need a program that can do photometric analysis like IRIS or IRAF [I personally recommend Iris since it's more user friendly].

We can compare magnitudes by using the equation
m1 - m2 = -2.5 log (b1 / b2)
where m1 and m2 are the magnitudes of two stars and b1 and b2 are their corresponding brightness.

First you have to acquire an image with a standard star in your field of view. I then can set point 1 as my standard star and set point 2 for any object I wish to compare. Using IRIS or IRAF, I can measure the pixel count/intensity for both points and use them as b1 and b2 in the equation. By doing so I can identify the magnitude of any other point in the image in contrast to the standard star. 

----Side note----
This type of measurements become more accurate when standard data reduction is done (lights, darks, flats, bias) and also when the appropriate filters are used (standard Johnson filters). However, I'll leave those out for another post or for you to find out on your own.

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