Image Quality of SOLIS/VSM in Helium vs. Nitrogen
  The National Solar Observatory (NSO) Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector SpectroMagnetograph (VSM) is sealed and was designed to be filled with helium at slightly above ambient pressure. After 11 years of operation filled with helium, an acute shortage of helium prompted a test using nitrogen as the fill gas. Four months of nitrogen-filled observations in 2014 are compared the same months in 2013 with helium fill. On average, the image sharpness is slightly degraded when using nitrogen. 
J. W. Harvey
astro-ph.IM astro-ph.SR