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The philosophy is to trace an aperture down the order center, and then cross correlate this trace against an archived solution to identify lines and make a rough fit. Then the lines are re-centroided and and arc solution is written to disk.
Manual Arc Solutions: The X_IDENTIFY GUI
If you wish to inspect the solutions - and you should, at least for the first several tries - you can select "User Reidentify" from the preferences menu. If this mode is called, a GUI will launch to allow the user to check the line centroids and evaluate the fit manually.
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The initial screen of the line ID GUI (based on x_identify from xidl) is shown below:
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Sometimes it is more instructive to view a plot of the fit residuals when judging whether to reject a point by hand. To view a residuals plot, type capital 'R' in the 1D fitting window. Zooming and rejection work as before. To change back from residuals mode to the actual fit values, type capital 'V' in the residuals plot window. A view of a residual plot is shown below.
When you are happy with the fit, type 'q' or click 'Done' to return to x_identify. Now, you will see a refreshed plot of the arc trace, but now the x-axis shows wavelengths derived from your fit:
At this point, you may go back and identify more lines by hand and iterate on the 1D fit. With your working model of the wavelength map it is also possible to have the code attempt an auto-id of all lines from the line list, although in our experience this has been spotty. If you are satisfied with the final product type 'q' or click 'Done' to complete the wavelength fit. For completeness, we show below a zoomed-in version of the wavelength plot to illustrate the appearance of positively identified lines.
From 1D wavelength fit to 2D wavelength Map
After you complete the 1D wavelength fit, the code projects this onto a 2D wavelength map, using information about the curvature of the slit image in detector coordinates. As a check, the code plots up an image of the slitillustrating the quality of the slit curvature fit:
The fit is shown as black lines tracing the profile of each arc line used in the solution. If these do not trace the curves properly there is a problem. In this event you should try another arc exposure, and if this doesn't work, contact the instrument team for help.
You are now finished with the arc solution. A file named Arcs/ArcImg****.fits.gz is generated in the reductions directory. This contains the 2D wavelength map, along with additional extensions encoding information about the fits to wavelength and spatial curvature. Time to move on to generating the flat field.