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As usual, a first-pass sky subtracted frame is generated and launched in the xatv GUI. This time, use the space bar to select several points along the trace (rather than the one you selected above). Crosses will appear at the locations of your selections as shown here:
When you have enough points to feel happy fitting a trace, press capital 'P' to fit a polynomial to the trace points. The fit will appear as a solid line on the data:
The trace center is now defined. IF you don't like the fit, press 'C' to clear it and start over.
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 as dotted lines:
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.