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Now you must define the aperture width. Press lowercase 'a' to bring up a widget for this purpose (do not use capital 'A', this keystroke is reserved for echelle mode).
The default is to define the aperture in units of arcsec (FWHM) where the pixel scale is 0.15"/pixel. You may specify a radius in pixels as well.
At extraction, this width will be used as the aperture boundary in boxcar mode. In optimal extraction mode it is used to define a masking region around the object to exclude from the sky model.
Once you have set this and clicked "accept" the boundaries will be plotted on the image: 
If you are not happy with the result, you can hit 'a' again to revise the aperture or 'C' to clear and start over.
Click 'Done' or press 'q' if you are satisfied and ready to move on to object extraction.