The Single-Star Photometry Tool provides a means of measuring the light from a single star and removing the sky background from it. You could use this tool to take measurements of a variable star and a star of constant brightness. By comparing their values you can then detect tiny changes in the brightness of the variable. This is differential photometry—the difference in brightness between two stars. Differential photometry is a powerful technique because thin clouds, haze, and atmospheric extinction may dim the stars, but their difference is barely affected.
Step 1: Open the Single-Image Photometry Tool. Close the two graphs, if they are still open, and close the Single-Star Photometry Tool. Click the Measure I Photometry I Single Image... menu item and the Single-Image Photometry Tool window will appear. Like the previous one, this tool provides a Settings tab that allows you to set the photometer radii.
Step 2: Select the Stars. Select the Result tab. Open the M82 image if it is not already open, and click on any star in the field to select the variable star. You will see that in the Variable and Comp Stars box, the Variable setting now reads "Yes," and the Raw Instrumental Magnitude box gives a magnitude reading for the star. Click the Clear button, and the setting will read "No."
Click on the star at X = 250, Y = 89. Next choose the first comparison star. Click the star at X = 83, Y = 71. You will now see that the Comp Stars setting reads "1." Each time you will see a display of the raw instrumental magnitude.
Click the Get Magnitude button, and AIP4Win will calculate the differential magnitude. The Differential Magnitude box shows that the variable is 0.563 magnitudes fainter than the comparison star.
To improve the quality of their data, astronomers select additional comparison star. As you add more comparison stars—clicking the Get Magnitude button as you select each one—note that the standard deviation decreases, meaning you are getting a more precise magnitude reading.
The information you obtain from these measurements is logged in the Data Log, but only when you click the Get Magnitude button. In normal practice you would select the variable star, select a series of comparison stars, and then click the Get Magnitude button to record one differential photometry reading.
If you wanted to monitor a supernova as its light faded, you could take a new image of it every clear night. You could then use the Single Image Photometry Tool to measure the difference between the supernova and a comparison star (V -CI) in each image. Over a period of a few months, you would obtain the supernova's light curve.
When you are finished, dismiss all the tools by clicking on the Window I Close all open Tool Windows menu item, and close the M82 image.
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