Basic ideas

• determine effective FP gap - wavelength relation
• LW grating section acts as order sorting filter
• wavelength calibration of LW section is critical for FP operation,
  but does not affect the FP wavelength calibration

Calibration sources

• one for each grating:
  ­ positioned next to the entrance apertures
  ­ black body plus fixed FP
  ­ producing a series of fringes (SW:250, LW:60)
  ­ calibrated by H2O, NH3 and HCl vapour emission lines
• one for FP:
  ­ fixed, stable FP
  ­ predispersed by LW section ­ providing a series of very narrow transmission peaks
  ­ spectrum known to a high accuracy from fourier transform spectroscopy
• correction for off-aperture position of the calibrators

    Pre-launch measurements:

• Grating:
  ­ vapour absorption lines of H2O, NH3, HCl
  ­ and laser lines at 4, 10, 16 and 28 µm
    used as wavelength references
• Fabry-Pérot:
  ­ H2O and NH3 vapour absorption lines used for determination of the
    effective FP gap - wavelength relation

    In-orbit calibration:

• Grating:
  ­ H recombination lines and fine structure lines of planetary nebulae,
    like NGC 6543, NGC 7027, IC 2501, ...
• Fabry-Pérot:
  ­ H recombination lines in the very brightest planetary nebulae and
    water emission lines from envelopes of oxygen-rich late-type stars

Accuracy

• wavelength calibration in gerneral:
  ­ very accurate
  ­ rather stable

• Grating:
  ­ wavelength calibration accurate to: w/dw ~ 10,000
    or 1/5 of a grating resolution element
  ­ limited by uncertainties of position of the source in the slit
  ­ spectral resolving power consistent with theoretical model

• Grating spectral profile:
  ­ very well represented by a Gaussian shape ­ width consistent with theoretical model

• Fabry-Pérot:
  ­ wavelength calibration accurate to: w/dw ~ 100,000
    or 1/3 of a FP resolution element ­ calibration of FWHM of the instrumental profile to 10%

  => New wavelength determinations of mid-IR FS-lines by ISO-SWS