Pointing Model Parameters

The dependence of the pointing residuals (daz,del) upon the various misalignments of the antenna structures, encoder zero-points, etc, are described for the old TCS in MTUN005 (by Kenderdine), and in the TPOINT document (by Pat Wallace), and are summarised below.

Click on the 1st column to see the primary effects of changing this parameter by +5".
Click on the 2nd column to see the primary effects of changing this parameter by +10".
Click on the 3rd column to see the geometry involved.

where A = azimuth, E = elevation, Z = zenith distance, L = observer's latitude

Notes

• ** The effect in the azimuth direction on the sky as measured in arc seconds, for instance, requires an additional multiplicative factor of cos(E)

• Diurnal local gravity changes due to Earth-tidal tilts upon Hawaii are expected to be small (<0.1arcsec).

• The MTUN005 description of (old TCS) parameter #2 includes cos(L) multiplying factors in each case which have been omitted here because they are not used in the (FIT9) algorithm. The comparison of the functional forms of these parameters with those used by TPOINT is then exact. A comparison of the two parameterizations is given in a new TCS document .

• Clock errors also generate recognizable error patterns, particularly in elevation . They may be most easily determined by including the TPOINT parameter IH with the normal alt-azimuth terms in the fitting procedure.
  • Check the ASM clock.

• Additional terms such as the misalignments, in 2 axes of the tertiary mirror, and misalignments of the nasmyth focus, NRX and NRY, also show unique error patterns. They may make for a more complicated model, but they may be incorporated fairly naturally into the current code. Such terms are described both in TPOINT and in MTUN062.

The current values for these parameters, stored in /jac_sw/itsroot/src/tcs/thi/THI.tide, may be adjusted following a pointing run that produces systematic variations of daz or del with any of the above forms. The TPOINT procedure is essentially a least-squares determination of the errors in the current values of the (7) parameters used in the fit (usually IE, CA, IA, NPAE, AW, AN, TF - in that order of importance). The execution of this program, as well as the display of tabular and graphical representations of pointing data, is described in the TPOINT document.

HARP pointing is complicated by the intervention of the field rotator (K-mirror), whose description as a set of misalignments may need a further 4 or 6 extra model parameters . . .