What causes dew on a telescope?

When the surface of your telescope objective or camera lens drops below the ambient temperature of the air outside, moisture begins to condense. On a night with high levels of moisture in the air, dew forms even more aggressively. A telescope that has been “dewed-up” no longer captures clear images of your deep-sky target, and instead captures a washed-out featureless image that resembles a flat frame.
To be a little more technical, the heat from the warmer telescope transfers to the colder outside temperature via radiation. When the telescope radiates its heat to the sky, it quickly cools down and drops below the dew point temperature. This chain of events causes condensation to form on your telescope or camera lens.

Dew will ruin your imaging process

Using dew heaters with your astrophotography rig can be the difference between capturing hours worth of exposure time on a target, and coming up empty-handed.
Dew prevention is an essential precaution needed for long exposure imaging sessions and is something you will inevitably need to address at one point or another.