Question

why is it important for the clock dirve to turn exactly at this rate for astronomical observations , particularly longb exposure photographs

Answer 1

As the astronomical objects are relatively faint to capture and need a very long exposure, these need to be overcomed.The telescopes need to be robust to any exposures hence the telescope mount needs to be rotated at a constant rate by the clock drives so that they accurately aim at a fixed point for a very long period of time.

The clock drive functions by rotating the polar axis (This axis that is usually parallel to the polar axis of the Earth) of the telescope mount in the direction opposite to that of the rotation of the Earth (Usually 1 revolution at every 23 hours56 minutes). Hence, it cancels that motion. This helps the telescope to stay exactly fixed and it sticks to a particular point in the sky. This removes the overhead work of constantly re-aiming the telescope due to the Earth's rotation.