Harvard University Astronomy Lab and Clay Telescope

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Photometry with MaximDL

MaximDL makes photometry very easy! In a few steps you can determine the magnitudes of your star(s) and even make a lightcurve!

• Open one (or 100 at one time!) that you would like to do photometry on. Images must be of the same field and filter!
• From the top menu bar, click on Analyze --> Photometry and this opens a photometry window.
• Check the following boxes
  • Act on all image - When this is checked, it will then search for the objects and reference stars in every image by just clicking on the first image
  • Use star matching - This will use the stars in the image to find the object and references stars in the image
  • Snap to centroid - This will center the annulus on a star when you click on it
• Right clicking on a star will adjust the aperture radius, gap width, and the annulus thickness
  • The aperture radius is the inner circle and it should be large enough that it gets all the photons from the star.
  • The gap width in the middle circle and it is meant as a spacer between the inner and outer circles to make sure we are not polluting the outer circle with scattered light from the star.
  • Finally, the annulus thickness is the size of the outer circle and it collects sky counts between the outer and middle circle.
• In the photometry window, set mouse click tag to new object
  • Click on the star (or stars) in the image that you would like to analyze.
  • LABEL (by X, Y coordinates) the object star in a table or image printout
• Now, in the photometry window, change mouse set tag to new ref star
  • Click on a reference star
  • In photometry window, enter in the ref mag box the known magnitude of the reference star. If you don't know the magnitude or are only interested in relative magnitudes as a result than you can just enter 0 for all your ref mags.
  • LABEL (using X, Y coordinates) the reference stars in a table or image printout
• When done, click on the view graph plot in the photometry window. This will pop up a lightcurve window and from there you can save the data as a .csv file (Excel compatable). Once you have the data in an Excel format we can easily make nice plots such as a lightcurve or HR Diagram.

NOTE: Make sure to LABEL all object stars and reference stars. The output .csv file will have multiple columns. Your columns will be labeled Julian date, object star 1, reference star 1, reference star 2 ... and unless you noted which reference star you used you will not be able to reproduce your results in another filter! The Julian date is a fancy way Astronomers keep track of time. The object star and ref star columns will be in magnitude or relative magnitude.