# Practice some simple aperture photometry

Let's practice some aperture photometry. We'll look at pictures of a field which has a very bright star near the center. Don't bother with it; instead, measure the fainter stars around it marked A, B, C and RX.

Each pair of astronomers should pick four of the images from the list below. To download an image, right-click on it and save to the desktop. Then read it into the MIRA program to measure instrumental magnitudes of the four stars.

Write down the instrumental magnitudes for each of the stars in each of the images, making a table like this:

```                         Raw magnitudes
Image         A         B        C        RX
--------------------------------------------------
rxa.024     12.33     11.41    12.37    11.98
rxa.025
rxa.026
rxa.027

```

Next, create a table of differential magnitudes by subtracting the magnitude of star B from each of the other stars:

```                      Differential magnitudes
Image       (A-B)     (B-B)    (C-B)    (RX-B)
--------------------------------------------------
rxa.024      0.98      0.00     0.94     0.57
rxa.025
rxa.026
rxa.027

```

When you are finished, write your differential magnitudes in the space provided on the blackboard.

So, here are the light curves for all the stars, based on measurements made by participants in the CLEA Workshop (with special thanks to Maria Barrios).