Download Chapter 1 : Installation

Appendix E: Maintaining High-performance
Liquid Nitrogen Dewars.
Re-Pumping the Dewar
The symptoms that indicate the dewar needs to be re-pumped are:
•
•
•
The liquid nitrogen temperature indicated by the software as being the SINK temperature
should normally be approximately -190C. The CCD temperature is usually much higher,
approximately -120C. If the vacuum starts to degrade then the CCD temperature will be
seen no longer to reach as low a level as it normally does because of the greater losses
associated with the poor vacuum.
The length of time for which a single fill of liquid nitrogen lasts before the dewar needs to
be re-filled will be noticed to progressively decrease.
Condensation forms on the outside of the dewar or the dewar feels cold and clammy to the
touch. Should condensation occur, and it can occur particularly on the front window of the
dewar, then this may be an indication that the vacuum needs to be improved. Remember,
however, that no matter how good a liquid nitrogen dewar is, under conditions of high
humidity, condensation will occur. It is inevitable that one of the coldest parts of any liquid
nitrogen dewar will be the window that protects the CCD from the outside world. As a
result of high humidity conditions obtaining then it is quite likely that problems will arise
because of condensation on the window. If this is a serious problem for example because
the unit is being used on a telescope in a cold, damp, climate then it is sometimes helpful to
arrange that dry nitrogen gas (possibly from the boil-off from the nitrogen inside the liquid
nitrogen dewar) is directed with a small tube onto the front window of the dewar.
In practice most dewars will need to be re-pumped occasionally, depending on usage.
Equipment Required
A vacuum pumping system such as a turbo molecular pumping system or a vapour diffusion
pumping system. Each should be able to achieve a vacuum of 2x10 -7 mbars or better. You really
need to have a vacuum pumping system with associated vacuum gauges and gauge heads. Suitable
vacuum piping and vacuum connectors must also be provided.
Warning: never use an ion pump or a sputter-ion pump as they can cause
irreparable damage to static sensitive CCDs.
The dewar contains a sorb pump whose function is to remove the remaining gas molecules once the
dewar has been pumped and the valve closed. It operates by adbsorbing free gas molecules when
cold. If the dewar is cold when being pumped with an external vacuum pump, air in the space
above the diffusion pump and oil from the diffusion pump may be drawn into the dewar. This oil
will contaminate the sorb pump and the face of the CCD and will be almost impossible to remove,
leaving the dewar and the CCD chip within it irreparably damaged. The liquid nitrogen filled
dewar is a much more powerful pump than the diffusion pump so pumping the dewar when cold
may actually be worse than not pumping the dewar at all.
Page 69