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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).
