After the slit tilt, the next instrument feature to be corrected for is the orbital variation of the line centroid. (Note that it is important to do the tilt correction first, then the orbit correction.) The orbit correction is performed using the actual spectra - there is currently no independent way of performing the orbit correction.
If 'fit' is the output from eis_tilt_correction, then the orbit correction is done as:
IDL> newfit=eis_orbit_spline(fit)
You will see plotted the variation of the line centroid over time, averaged along the slit, with the spline fit to the variation over-plotted as a thick line. 'newfit' is identical to 'fit' other than the 'cen' and 'vel' tags have been modified for the orbital correction.
Note that the orbital variation seems to be approximately the same in terms of angstroms (not velocity) for all lines. This allows the possibility for taking the Fe XII 195 orbit variation and applying it to other lines - see the spline= keyword mentioned below.
/setzero Forces the average velocity to be zero and the average centroid to be refwvl over the raster.
spline= As an output, this is a structure containing the spline fit to the orbit variation. It can also be used as an input. E.g., if you have done a spline fit for Fe XII 195, you can then send the spline back to eis_orbit_spline in order to correct Fe XV 284.
If you try using eis_orbit_spline for data partly obtained during orbital night, then generally the spline fit will not be good. The solution is to flag any data obtained during night as missing so that it is not included in the fit. This can be done as follows:
IDL> newfit=eis_flag_night(fit)
You will see the average intensity along the slit plotted as a function of time, and night periods should be clearly seen. A default intensity threshold is used by the routine and shown as a thick horizontal line. Any data columns that lie below this threshold will be flagged as missing. You can adjust the threshold with the threshold= keyword.
Note that the jump from night to day is not immediate and the line intensities will increase over a 2-3 minute period coming out of the eclipse. It is up to the user to determine how much of this period they want to retain.