To provide useful photometry of your star, you'll need to measure it relative to a star that we are pretty sure is not varying with time. First, let's choose this comparison star, which will allow you to determine whether any fluctuations in magnitude are unique to the star of interest.

The easiest way to find comparison stars is through the Variable Star Plotter (VSP) utility provided by the AAVSO. You can search for practically any variable star, and it should call up a chart with comparison stars numbered.

  1. The AAVSO has identified and compiled data on many comparison stars, which can be accessed at . Once there, hover over the "observing" tab, hover over "variable star charts", and then select "variable star plotter".
  2. In the following window type the name of your variable star into name box and choose F as your chart scale. In the box asking to plot a chart or a table of filed photometry, choose to plot a chart. If needed, in the advanced tab you can select the limiting magnitude and orientation. If you select "CCD" for "Chart Orientation", the finder chart should be oriented in the same way as the CCD images you observe. A magnitude limit of ~17 should serve you well. When ready, click on the "plot chart" button.
  3. A plot should appear with your star marked by cross-hairs in the middle. There should also be other stars in the field with numbers next to them; these are the comparison stars. Choose 2--4 comparison stars that are also in your images from the observatory (this chart has a larger field of view, so not all of them will be in your images)---you want more than one comparison star as a check that no one of the comparison stars is acting funny (i.e., varying) during your observations. You'll want to select comparison stars that are not near saturation in your images (<40,000 counts at peak). Ideally, they should have a similar brightness as your target star.
  4. Keep this tab open, as you will need it later.
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