# Photometry of SN 2013ej in M74

Let's put our aperture photometry skills to use by measuring the magnitude of supernova 2013ej in the galaxy M74.

Image courtesy of Martin Bernier

You can find a set of 5 images of the SN taken at the RIT Observatory in the directory called \$dd/sn. You can copy them all to your own directory like so:

```

cp \$dd/sn/*.fit .

```

Here's a chart, showing the SN and 5 nearby reference stars:

1. figure out the appropriate aperture size -- look at all the images and compute a single, average-ish value for all
2. set the apertures using
```
xlet aperture_innersky=
xlet aperture_outersky=
```
3. measure the instrumental magnitudes of the SN and all five stars in each of the five images. Write them in a neat table.

Then, you need to find the real, calibrated magnitudes of these comparison stars. I suggest you look in the UCAC4 catalog.

1. make a table which shows the B and V magnitudes from UCAC4 for each reference star

Now, to place the instrumental magnitudes onto the calibrated scale, we need to shift the instrumental values by a constant amount so that they match up with the calibrated values.

```

calibrated mag   =    instrumental mag   +   K

```
1. Use the star labelled B to compute the constant K for each image
2. subtract that K from the instrumental magnitudes all of the other reference stars, and from the SN

If all went well, your measurements of the reference stars should end up equal to their calibrated values .... or, at least, close to their calibrated values.

```

Q:  What is the magnitude of the supernova for each
of your images?   How well does your
value agree with the light curve shown below?

Q:  What is the typical difference between your
shifted magnitudes for the reference stars,
and the actual catalog magnitudes of the
reference stars?

```