From: boyd@apt0.sao.arizona.edu   
      
   laser wrote:   
   > All the wavelength will be measured in air.   
   > Do you know the reason that there is an option to change wavelength from air   
   to vacuum?   
      
   The spectroscopes here at Fairborn Observatory  (and many others) are   
   operated in a > 10-6 Torr vacuum. We're measuring stellar radial   
   velocities for planet searchs to less than 1 meter/second. The   
   spectroscopes are also cooled and temperature stabilized to better than   
   .01 degree kelvin.   Having the variable index of refraction of the air   
   inside  the spectroscope to deal with would be an unacceptable and   
   unnecessary source of error.  It's hard enough to deal with the the   
   constantly changing velocity vector of the Earth.   
      
   Your statement that "All the wavelength sill be measured in air" is   
   simply false.   
      
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