MODS Liquid Nitrogen Fill Procedure

Each MODS has two CCD cameras with 7-liter LN2 reservoirs. Experience since Sept 2010 shows that properly filled LN2 reservoirs will hold cryogen for 24 to 30 hours before running dry.

As such, the MODS dewars need to be filled daily.


  1. Telescope must be at Zenith
  2. The rotator need not be in any particular position, unless masks are to be loaded in which case the rotator should be set to 105 degrees
  3. Lock out telescope elevation and LDG rotator.
  4. Attach the fill line from 120 liter storage dewar to the MODS LN2 fill port.

MODS with the LN2 line connected to the fill valves:

NOTE: The supply dewar should have 20 psi of pressure. If not, attach external pressure line (set to 20 psi) to fitting on VENT side of dewar and open the valve.


  1. Open both of the RED and BLUE valves on MODS.
  2. Open the LIQUID valve on dewar.
  3. Watch the vent lines on upper part of MODS during fill. Blue side will generally fill first, when liquid comes pouring (not just dribbling) out of the vent line, shut off the BLUE valve.
  4. When Red fills (a few minutes after Blue - it too will pour liquid out), shut off the LIQUID valve on dewar and allow the pressure in the fill line to be relieved.

MODS1 blue CCD dewar LN2 blow-off gas vent line:


  1. Close RED valve on MODS.
  2. Detach fill line from MODS.
  3. If and external pressure line was used, disconnect it from storage dewar.
  4. Move the storage dewar clear of MODS and the telescope.
  5. Unlock the LDG rotator and the telescope elevation.

-- RichardPogge - 03 Nov 2010
Topic revision: r6 - 20 Aug 2012, JohnMorris
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