The problem is the CCD sensor is cooled too fast. The cold finger has not catch all and CCD sensor get ice .Once the ccd sensor get ice , it is not easy to be disappear.
The best way is let the cold fnger freeze at first, then the CCD sensor will not freeze any more. Normally speaking, once ccd sensor not get ice at starting, it will not get ice in future after cooling down.
You try this way:
1. Clean the cold finger at first.(Remove any oil on it. it is more easy to catch the water.
2. Slow down the cooling speed.