ZWO Cooled Camera Condensation

If like me you use a cooled ZWO dedicated CMOS astronomy camera, you may have noticed some strange blobs or markings on your images. I have sought answers from the web, retailers and ZWO themselves and am happy to share the solutions I found.

Noticing an issue…

I first noticed an issue back in April 2020 when a large shape was visible on my Flats during an imaging session. I use the ASI294MC Pro controller by the ASIAIR. The blob shape was changing over time so it clearly wasn’t a fixed object like a bit of dust on the sensor. These following images show how the overall shape and density but left a persistent and strong mark. The fact that the mark changed shape meant that removing it by subtraction was not possible and the processed light frames retained a remnant that was impossible to remove totally in post processing. The worst area was at the bottom of the frame - was I prepared to sacrifice this portion in cropping or try and make use of the full sensor?

Remedies

I inspected the camera and all elements on the image train including filters and focal reducer but saw no objects or obvious marks that matched the shape on the images. I assumed it must be moisture inside the camera so I followed online instructions to remove the front of the camera and ‘recharged’ the desiccant tablets which sit inside the sealed unit. The purpose of these is to control moisture behind the protective window over the sensor. This was easy but should be done with care to not introduce any foreign objects.

After doing this, there was no change so I bought new desiccant tablets and replaced the old ones. But…again no improvement.

Ask the experts

I contacted the retailer I bought the camera from as well as trying to contact ZWO via email or Twitter. I got the usual dummy questions; have I checked the sensor etc? Stumped, they promised to pass it up the chain. Then nothing until many weeks later I got a reply from ZWO via the retailer.

Condensation

I didn’t necessarily agree with point 1, as my experience was once it appeared, it stayed around for the evening. However for point 2, their answer was that dew or condensation was forming on the sensor caused by cooling the camera too quickly. This made sense as my routine was to plug in the camera to the ASIAIR, power up both and immediately send the Asi294MC Pro down to -20deg. In Spring and Autumn you could expect the condensation issue to be worst as the warm days clashed with cooler evenings.

I followed the advice by first cooling to 0 deg for a few minutes then down to -20deg. It produced excellent results and I have done this ever since and not had any problems.

So if you see similar stubborn marks appearing on your images then I suggest you try this process too. Clear skies!