Fabry-Pérot section:

• designed to perform high resolution observations:
  ­ increases spectral resolution by a factor > 100
  ­ resolution R = 20,000 - 35,000   (~ 10 km/s)

• two FP channels with very similar optical design:
  ­ FP1:   11.4 - 26. µm
  ­ FP2:   26. - 44.5 µm
  

• used for a larger range as originally anticipated:
  ­ FP1:   11.4 - 15. µm at reduced resolution
  ­ FP2:   35. - 44.5 µm at reduced sensitivity

  ->   Fabry-Pérot diagram

Fabry-Pérot optics:

• FP input:
  ­ LW grating section acts as order selector for the two FPs
  ­ fed into via extra exit slits at the LW spectral image plane
  ­ flat folding mirrors direct beams towards the collimators

• FP collimator:
  ­ convex torroidal mirror
  ­ off-axis paraboloidal mirror
  => parallel beam with increased diameter

• FP scanning device:
  ­ transmitting tunable FP-interference filters

• FP re-imaging optics:
  ­ paraboloidal mirror
  ­ hyperaboloidal mirror
  ­ flat folding mirror
  => directs beam onto FP detectors

• FP detector block:
  ­ one pair of detectors for each Fabry-Pérot
  ­ only one of them is actually used (det 49 or 51)

Fabry-Pérot mechanism:

• FP meshes mounted on a single pair of plates
• separation varied by changing current in three pull coils
• wavelength scanning performed by increasing gap width
• gap width 12.5 - 13.5 mm at a tolerance of 0.135 mm
• only up scans due to hysteresis of FP drive:
  up scan in FP position is down scan in wavelength
• each FP has its own calibrated scan current table
  => only one of the FPs can be operated at a time
• FPs act as tuneable narrow band filter
• LW grating sections act as order selectors

  ->   Fabry-Pérot filters

Instrumental/environmental effects:

• glitches / glitch tails
• memory effects (esp. band 5, less band 6)
• grating scanner tracking noise