In very humid conditions it is possible for moisture to condense on the OUTSIDE of a TEC camera optical window even though the window has a special warmer. Usually the moisture forms a round “spot” in the centre. This is because the TEC cooling can sometimes be too powerful for the front optical window heaters to catch up, and unequal temperature combined with high humidity can cause condensation.
Here’s an example of what this looks like in an astro image:
1) Slower or less aggressive TEC cooling can allow the small built-in heaters more time to keep up with the TEC cooling, by allowing more time for the heat to propagate to the centre of the glass. Please note that Dewing is less likely if you cool the sensor less. In fact, with most CMOS sensors, cooling to below -10C does not really give any benefit in noise reduction after stacking – it just increases the risk of dew and more power being used, with more stress on the components. In most situations -5C is more than adequate for most modern low noise CMOS sensors. Note: In extremely humid conditions, we recommend only cooling down to zero C if you are still experiencing dew build up on the front glass.
2) Use an Altair camera & filter wheel dew heater to GENTLY heat the front face of the camera and this usually clears it. Note: There is no need for excessive heat, but you can clear the optical window by applying more power initially. We do not recommend using the Altair DewZapper industrial dew controller on full power for this application, because the heater can end up fighting the camera cooling resulting in a waste of power. Gently does it…