The SGM-EI is a water-cooled grating chamber, incorporating the latest in precision UHV mechanism design to promote high accuracy and ease of use. The UHV design allows adjustment of grating pitch, roll, and yaw over ±180 arc seconds (900 microradians) while under vacuum, or ±2° outside vacuum. Unprecedented accuracy is obtained by utilizing laser interferometer angle readout and an absolute angle reference. Grating scans are computer controlled. Gratings may be exchanged at any scan angle.