Specify the inclination between gravity and the geocentric vertical.

Arguments:

Double north

The northerly inclination of gravity to the vertical implied by the latitude and longitude of the telescope (degrees).

Double east

The easterly inclination of gravity to the vertical implied by the latitude and longitude of the telescope (degrees).

Examples:

The following example says that the local gravitational vertical is tilted 0.0072 degrees northwards and 0.00325 degrees eastwards of the geocentric zenith indicated by the longitude and latitude of the telescope.

  grav_tilt 0.0072, 0.00325

The tilt of the local vertical.

When one converts from geocentric coordinates to topocentric coordinates, one converts from a coordinate system around the rotational axis of the earth, to a coordinate system around the radial line that is implied by the specified latitude and longitude of the telescope. In general the local gravitational vertical does not lie precisely along this line, and the azimuth axis, in turn, does not lie precisely along the gravitational vertical. Thus to compute where the telescope should actually be pointed, to make it point at the intended topocentric coordinates, one must first rotate these coordinates onto the local gravitational vertical, and then rotate the result onto the vertical that corresponds to the direction of the azimuth axis. The north and east arguments of the grav_tilt command, allow one to tell the control system the northerly and easterly components of the angle of the local direction of gravity, relative to the ideal vertical dictated by the telescope latitude and longitude. The remaining tilt between this local vertical and the direction of the azimuth axis, which can vary with time, is measured using tilt meters on the azimuth platform. Note that if the tilt meters are turned off, then the north and east arguments must be changed to contain the total tilt between the azimuth axis and the ideal vertical. This isn't desirable, since the azimuth axis tilt can change with time, as the telescope distorts and its foundations settle.

Determining the tilt values.

The values of the arguments of the grav_tilt command must be determined empirically, by fitting a pointing model that includes these grav_tilt, to the measured positions of the peak fluxes of a number of known point sources.