This is the distance (usually in mm.) from the lens (or primary mirror) to the point where the telescope is in focus (focal point).

就是从透镜(或者主反射镜)到焦点的距离，通常单位是毫米(mm)。

youdao

This is the distance (usually in mm.) from the lens (or primary mirror) to the point where the telescope is in focus (focal point).

就是从透镜(或者主反射镜)到焦点的距离，通常单位是毫米(mm)。

youdao