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Ebert is a simple grating monochromator that uses a plane grating. It should be studied to understand telecentric systems, rectangular special apertures, off-axis field points as base operating conditions, and the use of diffraction gratings. 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 alsin OSLO puts the grating exactly at the focal point,o 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

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 surfathe coordinates are returned to thece is removed prior to moving to the next surface (cf. the rco commands used in anaprism.len).


0 DT 1 DCX -- DCY -25.000000 DCZ --
      TLA -- TLB -- TLC --
2 RCO 2            
  DT 1 DCX -- DCY -- DCZ --
      TLA 10.000000 TLB -- TLC --


2 GOR -1 GSP 0.001667


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.


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 Y 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