SOPHIE : the instrument PSFThe instrument point-spread-function (PSF) is derived from the profiles of the unresolved emission lines in the Thorium spectrum used for wavelength calibration. The first plot shows a small region of the 27th order (region near 5539 Å) in extracted pixel space. Due to a difference in the geometrical location of the projected image between the HE and the HR fibers, the lines do not fall on the same "pixels" in the two extracted spectra. A gaussian fit for this particular line at 5539.265 Å gives a FWHM of 2.64 pixels in HR mode and 4.98 pixels in HE mode (red curve).
The second plot shows the region around this Thorium line now plotted in wavelength space. Note how the HE profile (red curve) is slightly "flatter" than the HR profile (a consequence of the full 100 micron fiber aperture in HE mode, compared to the 40 micron exit slit used in HR mode). For the pixel size of 0.0275 Å which applies here, the spectral resolution at this wavelength is ~76500 in HR mode and ~40500 in HE mode. The resolution decreases slightly near the order edges in HR (down to 72000) and varies from 43000 to 37000 across the order in HE. In order to allow for the inclination of the lines with respect to the orders, which is typical of cross-dispersed echelle spectra, the HR slit has been tilted to eliminate this effect near the center of the CCD. However, no such compensation exists in HE mode, hence a slight variable mis-match between the numerical extraction aperture and the lines.