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The Littrow spectroscope has two "distortions" - Slant and tilt.
The spectrum should always be set up horizontal.
When this is done the lines will appear tilted in the spectrum by about 4 degrees (caused by the pick off mirror positioning in the Littrow) and is very constant.
The slant of the spectrum band across the CCD chip does change with increasing wavelength.
When the 600 l/mm grating is used in the 2nd order, for instance, the spectrum may appear well down in the CCD frame.
The testing I'm doing with the 1800 l/mm grating show it will be difficult to get the zero order and the far IR on small chips! (without changing the tilt of the grating holder!!) IRIS can correct for both.
(I'm assuming the image you published was just the 5000A section...)
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