A general view of the instrument
The challenge faced by the GAMS spectrometers is managing to measure angles with a precision of less than one millionth of a degree (the angle of view of an apple seen from a distance of 100 km). In fact, the wavelength of gamma rays is exceptionally short and they are little deviated by matter.
GAMS are double monocrystal spectrometers. The photo shows the Laser interferometer measuring the rotation angle of these crystals which analyze incident gamma rays.