A schematic of the MTR2 spectrograph is shown here.

The spectrograph is made of a predispersor (“SP1” first one on the left part), forming a lowres spectrum on the wavelength selector (so called the “mask plate”). From there , only the selected bandwith travel to the highres echelle spectrograph (right part), ending over the MTR cameras at the SP2 output.

The optical layout along Y is fixed (no changes, never). The ruling of the gratings are along Y only and the dispersion of both spectra is along Z only. The entrance slit is fixed (but for its width), and such are the collimators and camera mirrors.

The moving parts are:

• Predisperser gratings selection table (EM35) . There are 3 selectable predispersers, although only one (the central slot) is used for MTR operation.
• Predisperser can rotate about the Y axis to change the grating incidence angle a (EM37). This allows shifting the low res spectrum along Z over the masks stop plate.
• The mask plate can translate in the Z direction (EM39)

  • The echelle grating can rotate about the Y axis to change its grating incidence angle a (EM37). This allows shifting the highres spectrum along Z at the SP2 output.
• Legacy “DPSM engage” mirror (EM71, “out” in normal operation.
• Each of the 4 spectrograph mirrors has a shutter (EM50 to EM53)

In short:

# The main optical layout (F2 to SP2) is static and doesnot require changes for daily operation.
# Spectral confguration of the multiline is obtained using:  
   * both gratings rotation
   * masks plate tuning in translation 
   * manual setup of the spectral cameras at the SP2 output