Ebert.len - A simple grating monochromator

Ebert.len - A simple grating monochromator

The Ebert monochromator, also known as the Fastie-Ebert monochromator, since it was developed for practical use by Fastie, is a very simple design that uses a plane grating at or near the focal point of a spherical mirror. The design included in OSLO puts the grating exactly at the focal point, which makes the system telecentric on both object and image sides. The focal points of the overall system are also at infinity, so the system is afocal. Notwithstanding this, the object and image are both at finite distance, and the system is set up as a focal system.

The grating is the aperture stop. This means that the chief ray enters the system parallel to the axis. To accommodate this, the tele general operating condition in OSLO must be turned on. An additional concern caused by the grating being the stop comes from the fact that it is square (the grating used is 25mm square with 600 grooves/mm). This means that the input numerical aperture must be large enough so that the grating is filled, which means that the reported paraxial data does not report the actual performance (see the discussion of apertures).

*LENS DATA
Ebert Grating Monochromator
SRF      RADIUS      THICKNESS   APERTURE RADIUS       GLASS SPE   NOTE
  0        --        125.000000    1.0000e-06             AIR   *

  1   -250.000000   -125.000000     50.000000      REFL_HATCH

  2        --        125.000000 P   12.500000 AX   REFL_HATCH   *

  3   -250.000000 P -125.000000     50.000000 P       REFLECT

  4        --            --         25.000000

The system setup is somewhat unorthodox. The field of view is very narrow, and the object surface is decentered to move the entrance slit off the axis of the spherical mirror. The mirror is tilted using a return_coordinates (rco) command in which the coordinates are returned to the current surface. This has the effect of providing a local coordinate system in which the tilt of the current surface is removed prior to moving to the next surface (cf. the rco commands used in anaprism.len).

*TILT/DECENTER DATA
  0     DT    1         DCX       --       DCY   -25.000000   DCZ       --
                        TLA       --       TLB       --       TLC       --
  2     RCO   2
        DT    1         DCX       --       DCY       --       DCZ       --
                        TLA    10.000000   TLB       --       TLC       --

*SURFACE TAG DATA
  2     GOR  -1          GSP     0.001667

*APERTURES
SRF   TYPE APERTURE RADIUS
  0     SPC   1.0000e-06
  1     SPC    50.000000
  2     SPC    12.500000

     Special Aperture Group 0:
     A  ATP    Rectangle  AAC     Transmit  AAN       --
        AX1   -12.500000  AX2    12.500000  AY1   -12.500000  AY2    12.500000

  3     PKP    50.000000
  4     SPC    25.000000

Another interesting aspect of the Ebert monochromator setup is the default drawing rays, which must be set to fractional coordinates that account for the oversized pupil, and to wavelengths that show the grating dispersion.

*OPERATING CONDITIONS: LENS DRAWING
   Initial distance:        125.000000    Final distance:          125.000000
   Horizontal view angle:          240    Vertical view angle:             30
   First surface to draw:            0    Last surface to draw:             0
   X shift of drawing:          --        Y shift of drawing:          --
   Drawn apertures (solid):   Quadrant    Image space rays:        Final dist
   Rings in aperture (solid):        3    Spokes in aperture (solid):       4
   Number of field points (rays):    3    DXF/IGES file view:     Unconverted
   Draw aperture stop location:    Off    Hatch back side of reflectors:   On
   Fpt  Frac Y Obj  Frac X Obj Rays Min Pupil   Max Pupil     Offset  Fan Wvn
    1      --          --        3  -0.650000    0.650000      --      Y   1
    2      --          --        3  -0.650000    0.650000      --      Y   2
    3      --          --        3  -0.650000    0.650000      --      Y   3

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