One last thing to do is to choose aperture size and sky annulus width.
A common rule of thumb is to use an aperture radius of about 1.5 - 2 times FWHM. So again
we move the mouse pointer over the aperture of a measurement in your image to open the information window. This will display the FWHM of that star:
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Figure 1: FWHM |
In this case FWHM = 2.7 so an aperture radius of 4 or 5 should be fine. The aperture
can be adjusted using the drop-down list in the toolbar above the image, and then clicking the refresh icon next to it:
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Figure 2: How to adjust the aperture and sky annulus settings |
Note that all measurements must have the same aperture size. So one should inspect all stars and use an average FWHM to establish an appropriate
aperture radius. Of
course, other factors influence this choice as well, such as how crowded the field is, and how faint the stars are.
The gap between the aperture and the annulus, and the width of the annulus can be changed for all stars simultaneously using the drop-downs in Figure 2.
But it can also be set for individual stars. To do that, open the information window, change the gap and/or width, and click the 'Update' link.
Next: Results
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Version 2.0
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