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. Following testing it will be released to SSW. In the meantime 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

One routine that has been added to SSW to deal with the oribital correction is eis_orbit_correction written by Peter Young.