Help with understanding humidity and dew point

Hi guys.
I have some questions concerning humidity (absolute and relative), dew
point, and air density.

Does it make sense for the weather channel to report a dew point below
freezing?
Is this a result of a calculation from measured relative humidity, air
temperature, and barometric pressure? Or can you actually condense
water out of the air below freezing?

Is there a source of standard functions relating relative humidity,
absolute humidity, pressure and temperature?

If I want a more accurate method of measuring absolute humidity, can I
cool a plate and record the temperature when moisture condenses on the
plate and use that as dew point to calculate relative and absolute
humidity? (This is how they determine humidity in the chamber used to
house the Declaration of Independence without opening the chamber.)

Thanks in advance for any help.
.

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