David’s review https://britastro.org/node/25816 is very helpful and explains the physics well – thankyou. The position of the dew heater band is important for best effect. I discovered this for myself as i was somewhat mislead by some commercial products that stitch a dew band into the dew shield. In this design its easy to just heat up a column of air above the optical surface. The warm air escapes and does nothing much to prevent dew. The band should be over the OTA near the optical surface so that the glass itself is warmed that fraction of a degree.
I thought i would just mention this story since it took a winter and some loss of observing time to “discover” what i was doing wrong, or maybe i just didn’t position the shield correctly ? All’s well now.
Hi Alex, Thank you for your reply. My solution was to set up a similar arrangement with the dew band around the C11 OTA, and a Celestron dew shield over the top. The Celestron dew shield is quite heavy, so your foam mat is a better idea. Im now using my Newtonian in a Pulsar 2.7m so dew is much less of a problem.