3 Add Liquid Nitrogen to the BIG anti-contaminator dewar

LN2 in the cold trap dewar is necessary for vacuum recovery. It is very useful, especially when you need to change grids and/or do a cryo session.

Note: If you want to use 651N anti-contaminator, make sure it is in good vacuum condition by pumping it with heat before use. If you are unsure, but want to proceed in using the scope, you should not use the 651N small dewar.