There is a shift of the spectral line position during the orbit. This is due to the thermal changes occuring across the instrument during the orbit and was an expected effect. We are currently collecting data that will allow us to model this accurately for correction. As well as orbital variation, there will be seasonal variation as well. In the meantime, especially when dealing with rasters, you must correct for this. An uncorrected velocity map looks like the attachment, where you can clearly see the change in red and blue shift during the orbit. Software is being produced to correct for this but is not within eis_prep. It is now released to SSW (eis_orbit_correction). You can correct for the variation by modelling the line position along the time direction and subtracting that component which is sinusiodal in shape. Orbitfiles
I've been working with the 2" slit. I took different data from different days (13/April 17-22h and 19/April 18-22h) and I've plot the mean value of the position of the Fe XII 195.12 line. As you can see in both days there is a kink (marked with a blue circle). I've been looking for the best way to solve it, and the dashed line that is in the plot is a sinusoidal fit that I've tried but that is not perfect. In my opinion that kink is produced by thermal effects, since it is repeated in the other day and. The best way I've found to fix that is using splines every 5 time-steps as it is used in EIS_ORBIT_CORRECTION.PRO for 1" slit. But in that program it uses as reference wavelength the mean value between the maximum and minimum of the file, whereas I used the min and max of the all dataset ( I think that's important because if you have different files they will be corrected with different reference wavelength).
-David Pérez-Suárez, 2007-Sep-21
