The names of all reference images to be used for wavelength calibration.
The number of supplied images should be the same as the value of NREF_FRAME. The image names should be separated by commas, for example:
reffrm1,reffrm2
Note that in most circumstances there will only be one or possibly two reference frames.
If you do not wish to provide an arc frame at all then you may reply NONE in response to this prompt or, alternatively, set the parameter TUNE_NOARC=YES.
If no arc is available ECHOMOP can not currently be used to build wavelength scales and they must be provided by other means if required.
Martin Clayton