My astronomy project:
Making of a heating band or dew heater

Content:

1. Introduction, attach a heater to the dew shield
2. Heater band for 5" refractor, field test outdoors
3. Making a USB powered dew heater
4. Lens USB dew heater
5. Dew USB heater narrow band model with regulator

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!

4: Lens USB dew heater

When I use my my dew heater on my Sigma 150mm f/2.8 lens I have noticed sometimes that it's almost impossible to get it in perfect focus. I have an idea that it's arise from the dew heater. My special built dew heater is very thin and just touch the narrow space at the front lens. Maybe that's the problem, uneven heating of the lens. I have now purchased a much bigger dew heater band that cover the whole lens to doing test with.

Looks very good and with straps to hold it in place.

This is how my lens look, now I have to attach this big dew heater all over the lens barrel. Then I also cover the focus ring. That maybe will be a problem, but if it solve my out of focus problem it's ok.

Here the lens with the dew heater mounted. I use the same battery as earlier, an USB power pack with 10 Amp/h capacity. Last for more then one night. The dew heater takes 0.8 Ampere, 4 Watt. I report back when I have tested it.

Update:
It works good but is too big and it's too difficult to focus the lens (manually, when in autofocus mode the barrel does not rotate). It also need a lot of power when it heat the whole lens.