Artifacts in FTIR spectrometer.   
   (I' pretty sure artifacts is not the right name, but I don't   
   know what to call them.)   
   If any of you troll SED, this is part of my saga at UB.   
   I'm helping a grad student with an old Bomen FTIR spectrometer.   
   We were trying to measure the transmission spectra   
   of a piece of black poly (used in a low temperature probe   
   to block thermal radiation.)   
   Optical path is involved..   
   Source, source mirror, source aperture, then the scanning mirror spectrometer,   
   and steering mirrors for various ports on the spectrometer.   
   Then focusing mirrors for the sample, and another set to   
   Re-image the source onto the detector.   
   (So a focus at the sample and another sharper focus at the detector.   
      
   Typically there is another aperture at the sample... but we were   
   starved for light (after the black poly) so I suggested we get rid of it   
   and open everything up... No problems so far.   
      
   We then went to take the background spectrum of the light source.   
   Now there was too much light on the detector.   
   (Detector was HgCdTe photodiode... I do like photo diodes!)   
   Well not too much light, but too much light for the preamp   
   we had.. minimum gain of 100k ohm.)   
   So we reduced the source aperture to get the signal on scale.   
   (Still no sample aperture.)   
   When looking at the interferogram (the raw data) there was   
   a little "echo" of the zero path signal displaced from the zero   
   path.  And this showed up as interference type fringes in the FFT.   
   (like there was a thin slab of something in the beam path.)   
   But the "echo" was only on one side of the interferogram.   
   (A thin film would give symmetric bumps.)   
      
   By changing the source aperture side we could make the "echo"   
   move around in the time domain.   
      
   We finally broke vacuum and put a sample aperture   
   into the beam path and this fixed things...   
      
   (Wow sorry that was a long introduction.)   
      
   Anyway does anyone know what these "echoes" are called   
   and from whence they come..   
   (I mumbled something about non-axial rays.. without really knowing of what I   
   speak.)   
      
   TIA   
   George H.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)