When installing the astronomical filter, it is usually coated on one side

Coated sheet, facing the direction of light coming in. On the other side are antireflective films to reduce reflection

How to distinguish the front and back of the filter.

It's mainly to see the reflection of the object. It's very practical and simple to see if there's any ghosting

The non ghosting is the coated surface, which is often called the front side

The one with double shadow is the antireflective surface, which is often called the back