A versatile and reliably re-usable ultrahigh vacuum viewport
  We present a viewport for use in Ultra-high vacuum (UHV) based upon the preflattened solder seal design presented in earlier work, Cox et al. Rev. Sci. Inst. 74, 3185 (2003). The design features significant modifications to improve long term performance. The windows have been leak tested to less than 10^-10 atm cm^3/s . From atom number measurements in an optical dipole trap loaded from a vapor cell magneto-optical trap (MOT) inside a vacuum chamber accommodating these viewports, we measure a trap lifetime of 9.5s suggesting a pressure of around 10^-10 Torr limited by background Rubidium vapor pressure. We also present a simplified design where the UHV seal is made directly to a vacuum pipe 
K. J. Weatherill, J. D. Pritchard, C. S. Adams, P. F. Griffin, U.   Dammalapati and E. Riis