With the camera mounted on the telescope, the filling inlet of the dewar
will be pointing downwards. An inner tube must be mounted in the
cryostat - otherwise no liquid Nitrogen can be stored.
Pointing the telescope at Zenith will allow complete filling with a
holding time of about 30 hours. Observing close to the horizon
will cause up to half of the Nitrogen to spill out, implying a
proportional reduction in holding time. The LN
temperature
displayed in the BIAS CCD status window is a good indicator of the
amount of Nitrogen in the cryostat. A completely full tank will make a
temperature of slightly below -198^\;
C being displayed.
As the temperature sensor is placed in the upper end of the tank,
this end will become warmer when the tank is only partially full.
A temperature higher than -195^\;
C means that the tank is nearly empty.
The pressure in the dewar is measured by a Pirani gauge and
the output is interpreted and displayed by a auxiliary electronics box.
The gauge and display are calibrated for each other - changing one of
the components can result in very misleading output.
In spite of the calibration, the display of pressures below 
mBar
are somewhat deviating. The Pirani output has been compared to the
outputs from Penning sensors of two different brands, which are more
reliable in this pressure range.
The relation between Penning and Pirani pressure is shown in figure
12.
The repeatability of the measurements are apparently not very good, but
when the display shows less than 
mBar, it seems safe to
consider the displayed pressure an upper limit to the actual pressure.
Beware that at pressures near atmospheric pressure, the display may
show a too small pressure by a factor of around three. This should
especially be considered when filling the dewar with dry air from a
pressurized bottle.
Figure 12:
BroCamII Pirani sensor pressure versus Penning sensor pressure.
Measurements at to different times are shown - the repeatability is apparently
not that good.
The dewar is equipped with a activated charcoal sieve, which will help
to keep the vacuum when cold.
The operating pressure should be below 
mBar. If the pressure
goes above this while the camera is cold, the dewar should be pumped
for a day at room temperature to evacuate the sieve.
If the CCD temperature goes above -50^\;
C, the
Oxygen soaking must be repeated.