Dew on the optics

The same bright star in an early sub and a late one. In the late sub the halo is larger and the star softer.

1. 01

   Dew on the objective or corrector

   Once the glass cools below the dew point, a film of water forms on it. The guide scope is smaller and fogs first.

   FixTurn dew heaters on before the optics cool to the dew point. Heat clears dew slowly once it has formed, so the subs taken meanwhile are soft.

   Session Checklist

• Bloated, soft stars

  Focus drift softens stars without growing halos.

• Lost to cloud

  Cloud drops the star count and leaves star size alone. Dew softens stars and grows halos, gradually.

• Halos around bright stars

  A reflection halo is the same size from the first sub. A dew halo grows.

• High cloud

  High cloud comes and goes. Dew builds steadily, and a light on the glass shows it.

One frame shows soft stars and halos. That they are growing needs subs from across the night, or a look at the glass.

Halos the same size in the last sub as in the first, and dry glass at the end of the night.

No outside source is cited here. The signature, the causes and their order are the atlas’s own, from imaging experience. The order is a judgment of likelihood; no measured frequency is behind it.