Figure: The SWS input section

• four-position shutter mechanism
• dichroic beamsplitter
• spectrometer slits
• interference filters

dichroic beamsplitters:

• Reststrahlen crystal filters act as dichroic beamsplitters:

    ­ transmit short wavelength radiation
    ­ whilst reflecting longer wavelengths

• located behind the entrance aperture shutter

• three dichroic beamsplitters, one for each aperture

• shutter directs incoming radiation through one of them

• beamsplitters are made out of Al2O3, LiF and SrF2

• split input radiation into a short and long wavelength beam:

    ­ transmitted beam enters SW section
    ­ reflected beam enters LW section

spectrometer slits:

• follow the dichroic beamsplitters

• actual entrance slits of the SWS

• located in the focal plane of the ISO telescope

• slit width: 0.6 mm, except for SrF2, reflected: 0.9 mm

• determine the FOV in dispersion direction

• due to the different light path, the backprojection of the
  field-of-view onto the sky has a different size for the FP

• perpendicular to dispersion the slits are oversized (~2-3 mm)
  FOV in this direction is limited by the detector dimensions

interference filters:

• located behind the spectrometer entrance slits

• blocking filters used for spectral order separation

• followed by two independent sets of optics
  feeding the radiation into the SW/LW section

• the reflected beam enters the LW section after
  a second reflection against the beamsplitter material
  

  
  ->   SWS optical layout

  ->   SWS optical flow diagram