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For this activity, you will be measuring the intensity of the comparison star and supernova in the data files for the galaxy M100. These are labelled in the image below:
Below are the values for the magnitudes of the standard star in each filter. For this activity you only have the data files in the R filter, so you only need to look at the R magnitude value for the comparison star:
1. Open the photometry spreadsheet, ‘Photometry.xls’. Copy the R magnitude of the standard star as given above into the photometry spreadsheet into the relevant cell (R Mag).
2. The next step is to use SalsaJ to measure the intensity, or pixel counts in your FT image for the comparison star. To do this, open SalsaJ and go to: Analyse > Photometry Settings. Enter the aperture radius as 15 in the star radius box, as shown here.
3. Now go to Analyse > Photometry. A pop-up box will appear, with the photometry results in it.
4. Click on the comparison star to obtain the intensity value, and then enter this value in the relevant cell in the photometry spreadsheet (R Count).
5. Now, ensuring that you use the same star radius aperture value of 15, find the intensity value for the supernova in your image and enter it into the spreadsheet in order for the magnitude to be calculated. Make a note of the magnitude value for the supernova in the table below.
6. Repeat steps 4-5 for all the data files, for each date, and make a note of the magnitudes of the supernova in the table below.
(Occassionally, SalsaJ returns zero values for the intensity of objects - if this happens when you are measuring the intensity of your supernova in any of the images, close the image and just use the remaining ones for your plot.)
7. Open a new worksheet in Excel
8. Enter the values for the day of observation and magnitudes into this new Excel worksheet. (The day of observation was found by counting the number of days from the date that the first FITS file was taken. The dates for each image can be found in the FITS file header, which can be viewed in SalsaJ by going to Image > Show Info... )
9. Plot a graph with the day of observation on the x axis and the magnitude values on the y axis. When you plot your graph, make sure that the y axis is reversed - to do this, plot the graph and then double click on the y axis values. In the pop-up window that appears, select Scale > Values in reverse order. Select appropriate values for the minimum and maximum y values so that your curve covers the plotting area.
A typical supernova light curve looks like the one below:
Before carrying out this activity you must: