...
There is a problem with the ViSP CDELT2 keyword (see also the following issue), which is the keyword giving the pixel plate scale along the slit. In the early data that was taken with ViSP this keyword provides a wrong value. We have not yet had the chance to fix it in the files that we made available to you but that is something that may be done in the future. Christian Beck, the ViSP Instrument scientist, has supplied the following correct values for CDELT2.
arm 1 630 nm: 0.0298"/pixel
arm 2 397 nm: 0.0245"/pixel
arm 3 854 nm: 0.0194"/pixel
The keywords NAXIS1 and NAXIS2 give the lengths of the spatial and spectral axis of the data array, respectively. However, the keywords CDELT1, CDELT1A, CRPIX1, CRPIX1A, CRVAL1, CRVAL1A, CTYPE1, CTYPE1A, CRDATE1, and CRDATE1A all refer to spectral quantities as opposed to spatial quantities. There may be other keywords that are "transposed" as a result of this that issue. This results in the WCS axis 1 referring to pixel axis 2 in the data array and vice versa.
There are issues with the calculated wavelength and dispersion based on the header information in ViSP as illustrated in Figure 1. No attempt is made to correct these at the moment. A wavelength calibration for ViSP data is being investigated for inclusion in a future version of the calibration pipeline.
Figure 1: Observed and Atlas Spectrum for Ca II 8542 A.
...
The ViSP instrument scientist has supplied a better approximate calibration of the wavelength in the 3 arms, show below in pseudo IDL code.
630nm = 629.495+findgen(1000)*1.285/1000. ; dispersion 1.285 pm /px
397nm = 396.418+findgen(1000)*0.77/1000. ; dispersion 0.77 pm / px
854nm = 853.182+findgen(1000)*1.882/1000. ; dispersion 1.882 pm / px
The ViSP data has incorrect pointing information in the WCS headers. This is not currently correctable.
...