Rotation of the detector unit is used to align the slit image with the CCD pixel line. When the blaze wavelength of the echelle grating is set to the center of the detector, the inclination of the slit image to the CCD pixel line is minimized by using a rotation angle of −1 degree (nominal value). Fine tuning of the rotation angle may be required for more accurate alignment (e.g., for long-slit observations).
Since the inclination of the slit image to the CCD pixel line is large for the wavelength region far from the blaze, a rotation of the detector unit may be required (this may be the case for observations in the near-infrared range).