What is the single greatest adversary to high astrometric precision for ground-based telescopes?
Answer
The Earth's atmosphere causing refractive effects known as "seeing"
For telescopes situated on the ground, the movement and ever-shifting layers of the Earth's atmosphere introduce refractive effects that cause the apparent position of starlight to wander. This phenomenon is universally referred to as "seeing." This atmospheric turbulence effectively smears and blurs the true position of a star on the detector, introducing positional errors that are typically much larger than the subtle angular shifts astronomers are attempting to measure, especially when looking for the tiny shifts associated with nearby stars.
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