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The next component in the optical path is the echelle grating. It is
the dispersive element that, in combination with the long focal length
of the parabolic cross disperser, gives IMAPS its high resolution. The
grating is a replica of an original Bausch & Lomb grating
that was ruled at 79 grooves mm-1 with a blaze angle of
63.4° () over a 200×400 mm area. The
echelle is mounted in a way that the incident and diffracted beams are
offset on either side of the plane perpendicular to the rulings by an
angle of 3.5°. While this off-plane configuration gives the
highest possible efficiency at the blaze center, it also results in a
rapid drop in response toward the edges of the free spectral range of
each order (Bottema 1981).
The surface of the echelle grating has a small cylindrical error, and
this results in some astigmatism that broadens the orders when an image
is formed. Over most of the format the image points are distorted
nearly perpendicular to the orders, so there is no loss of resolution.
12/15/1998