davebl I want to keep the camera in the house during the day and install it when I;m ready to image to see if that helps.
Probably not.
This is caused by moisture that has leaked into and trapped inside the camera chamber. As you cool the sensor down, the sensor surface temperature reaches below the dew point that corresponds to the amount of moisture that is present inside the chamber. And what you see are teeny tiny ice crystals, if you cool it below 0ºC.
Quite often, it appears in the form of a disk that is centered on the sensor since that is coolest part of the sensor.
Just replace the desiccant in the camera chamber with new tablets (or recharge them in a microwave oven). And never allow the camera flange to become loose and admit moisture into the sensor.
The hardest part is to avoid dust on the sensor when you seal the chamber back, and you may have to repeat a few iterations of opening the chamber repeatedly to remove dust using a dust bulb that has a bulit-in HEPA filter (lots of those for sale since all serious photographers have them -- I actually have a small motorized hand-held blower with a HEPA filter).
Whatever you do, do not blow at the sensor with your mouth, or you may need to remove the residue (that dust really sticks to) eventually with a sensor cleaning solvent. Just like filters, if the sensor surface is clean, dust bulbs are good enough to blow the dust away. Best also to open the camera chamber up on a dry day so that you don't end up with much moisture to begin with.
With some of ZWO's cameras, you also need to be careful that the protection window does not fall off when you open the chamber, although the larger cooled cameras appear to have the windows bolted in place.
Happens to everyone sooner or later when enough moisture seeps in and saturates the desiccant.
Chen