How do I keep my finder from dewing over when I observe?
Your finder should have a light shield (“dewcap”), which you can cover between uses. Or you can purchase an anti-dewing device. This need not be expensive. You probably have a box full of them at home: A simple tissue stuffed into the light shield is an effective deterrent.
First, it keeps the finder’s lens from seeing the sky. An object facing the night sky radiates its heat to the cold of space and can actually become colder than the surrounding air. When its temperature
drops below the dew point of the air, moisture will condense on it.
Second, a tissue absorbs moisture. If your finder already has some light dew, a tissue in the light shield can absorb some of that moisture and clear the lens for you. If the anti-dewing device gets
soggy, toss it; you have a hundred more in the box.
— Sue French