Pointing Model

  The pointing model for the INT has 17 terms, comprising the standard errors of an equatorially-mounted telescope (offsets in HA and Dec, collimation error in HA, misalignment of the polar axis and non-perpendicularity of axes), together with an empirical (Fourier series and polynomial) model of flexure in the telescope structure and large-scale errors in the Ferranti encoder readings. The (software) zero point of the pointing model is determined with the ``SNAFU'' procedure (section ). If pointing tests are made sufficiently frequently in order to update the pointing model, the rms error in the absolute pointing of the INT should always be better than 5 arcsec: values between 2.8 and 4.5 arcsec have been measured. Short-term tracking errors are <0.25 arcsec (there is a small residual oscillation at about 1 Hz in HA). Longer-term drifts should be removed by engaging the autoguiders.