Keyhole problem

The keyhole problem, in the context of astronomy, refers to the difficulty that azimuth-elevation type telescopes or antenna gimbal systems encounter in crossing the zenith.

To visualize this shape, imagine drawing a quarter circle spanning from the horizon to directly above you and revolving it around the vertical axis.

If, on the other hand, the gimbal has a range from 0 to slightly less than 90 degrees elevation, the telescope is unable to see a region of sky.

This will pose practical issues for a physical system with a motor than can only move the azimuthal axis at a limited rate and acceleration.

And so the keyhole problem is a fundamental issue for any real azimuth-elevation type tracking system, even if designed to reach the whole range of half-hemispherical angles.