The cryostat liquid Nitrogen tank is tilted relatively to the CCD to achieve an upright position when mounted on the ALFOSC with the telescope pointing towards Zenith. This is done in order to maximize holding time.
Spilling will occur if the dewar is tilted shortly after filling. Filling in the morning after observations will result in the least spilling during observations. In this way, one filling per day may be sufficient.
Filling the dewar by using a tube connected to a pressurized tank will result in overflow a bit before the tank is full due to gas in the tube. An hour after filling, a tilt of more than 45 degrees can be applied without any liquid Nitrogen spilling out. This means that most observations can be made shortly after filling without a significant reduction in holding time.
Filling by pouring liquid Nitrogen into the dewar using a funnel or lifting the mouth of the pressurized filling tube to flush with the opening of the cryostat allows the tank to become more full before spilling. This can add more than two hours of holding time compared to the pressurized filling method, but requires less tilting of the dewar after filling.
A 90^
tilt without spilling requires the tank to be less than half full.
The elapsed time after filling, where spilling is avoided has not been
determined precisely, but 15 hours after a pressurized refill, no spilling
occurred from a 90 degree tilt.
Holding time, with CCD at -100^
C: 32 hours till rise of cold finger
temperature, and at least one hour more till rise of CCD temperature. If the
scoop filling method is used, three hours can be added.
The CCD temperature remains at -100^
more than 1 hour after the temperature
of the cold finger begins to rise. During this period, the pressure
rises about a factor of 50.
The temperature of the cold finger is about -193^
C when there is
in the tank.
The required time for a room-temperature camera to reach a CCD operating
temperature of -100^
is approximately 3 hours. The relatively long
time is due to a thin cold finger, implying less Nitrogen consumption.
With the CCD temperature control set to a very low temperature, the
CCD temperature will not go lower than -115^
C.
With a full 
tank, the CCD temperature can be forced to about
ambient temperature by setting the CCD reference temperature to, say,
+30^
C. This is again due to the thin cold finger.
The pressure, with the CCD at -100^
C should not be much above
mBar.
After the 
is exhausted and the camera returns to room temperature,
the pressure will be about 
mBar. Evacuation before cooling
again is recommended.
The pressure sensor emits light. This has not been detected in previous designs, as the sensor was further away from the CCD. A baffle has been placed in front of the sensor, reducing the illumination. Less than one electron per pixel per hour originating from the sensor is detected on the CCD. The distribution of the light is uniform.