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Plate scale and demagnification

The plate scale at the slit is equal to 4.44 arcsec/mm with the image derotator in the beam, or 4.51 arcsec/mm with it out of the beam.

The camera provides a demagnification factor along the slit (i.e. in the spatial direction) equal to the ratio of the focal lengths of the camera and collimator. Assuming no focal modifier lenses are in the beam, this is given by:

The demagnification factor perpendicular to the slit (i.e. in the spectral direction) is somewhat more complicated, since it is necessary to take into account the expansion of the beam caused by it not being normally incident on the echelle. The separation of the beam incident on the echelle and the reflected beam is 12.4 degrees, whilst the nominal blaze angle is 63.4 degrees; hence the angle of incidence is 57.2 degrees and the angle of reflection is 69.6 degrees. The demagnification factor is given by:

The plate scales at the slit, and at the detector both in the spatial and spectral directions, are summarised in Tab. 14. This assume that the focal modifier lenses are not used. An important number to remember is that a slitwidth of 1 arcsec projects to 43 microns at the detector, or about 2 detector pixels with an EEV CCD. This is often referred to as the 'matched slitwidth', since it is the minimum slitwidth which is properly sampled at the detector; using a larger slitwidth will degrade the instrumental resolution, whilst using a smaller slitwidth will result in the line profiles not being properly sampled. Clearly the matched slit will be different with different detectors.