Curve of Growth

Curve of Growth

Aperture - 6.0

Aperture - 6.0

Single Image Photometry 0

Result } Details ) Settings Variable and Comp Stars Variable: Yes Comp Stars: 1

P Analysis Clear |

Variable Star Star* StarY: PVmax: Star-Sky: Sky:

249.998? 88.57937 2028.0 15567.9 180.81

Differential Magnitude 0.563 ±0.019

Get Magnitude || Close

Figure C.6 The Single Image Photometry tool compares the brightness of a variable star against the brightness of one or more non-variable comparison stars. Because it determines only the difference between stars, results from differential photometry are relatively unaffected by atmosphere and thin clouds.

era used to make the image. For the purposes of this tutorial, we will leave them set to their default values.

Step 4: Select and Measure a Star. Select the Result tab. Now click on the star at X = 250, Y = 89. You will see the results of measuring the star. The Measurement Log is updated as well, allowing you to export these values to a spreadsheet later. The raw instrumental magnitude is the sum of pixel values in the star image corrected for the sky background converted to magnitudes—not the "real" magnitude of the star.

Chapter 10 explains how astronomers convert raw instrumental magnitudes into accurate magnitude measurements. The Details tab displays information used to determine the standard deviation of the magnitude determination.

Try selecting and testing some other stars in this image. For an example of a star not well suited for photometry, try selecting one in the halo of the galaxy, or one that has another star within a few pixels of it. You will see that its profile will have a lot of extra light around it, adding uncertainty to the measurement of the star's brightness.

Finally, check the Data Log; you will see that it has faithfully recorded the position and magnitude of each of the stars you have clicked on.

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