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You can specify multiple locations, although there is no particular advantage to doing so. The software will simply take the median x-value of the selections and use that as an initial guess for the trace finding code, which fits a polynomial for trace x-position as a function of y-pixel. Once you have selected your object, press 'q' or click 'Done' and the code will trace your selection. When it has finished, it will plot up the results:
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If you are happy with the result, move on to object extraction. If not, then you can try your hand at fully manual apertures, described next.
Fully Manual Object Finding and Trace Definition
If you absolutely MUST put the trace in a certain region of the slit and are having trouble getting the automated finder to bow to your will, you can run trace finding in a fully manual mode by choice in the Preferences pane.
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.