3.2 Echelle Grating

  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° ($\tan \theta = 2$) 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.