next up previous contents index
Next: Parameter Files Up: Basic pointing parameters Previous: Basic pointing parameters   Contents   Index

Parameter List

Let dX and dY be the offsets on the sky parallel to the Azimuth axis and Elevation axis respectively, produced by a pointing error. The standard pointing model for the Plateau de Bure antennas has the following parameters:

IAZ      azimuth encoder zero  
         dX = IAZ*cos(El)               dY = 0
IEL      elevation encoder zero 
         dX = 0                         dY = IEL
COH      telescope azimuth collimation
         dX = cos(El)*asin(COH/cos(El)) dY = -asin(sin(El)/sqrt(1-COH**2))
         for COH << cos(El), equivalent to
         dX = COH                       dY = 0, 
COV      telescope vertical collimation
         dX = 0                         dY = COV
MVE      Azimuth axis tilt towards East
         dX =  MVE*cos(Az)*sin(El)      dY = -MVE*sin(Az)
MVN      Azimuth axis tilt towards North
         dX = -MVN*sin(Az)*sin(El)      dY = -MVN*cos(Az)
NPE      Elevation axis tilt (axis non perpendicularity)
         dX = -NPE*sin(El)              dY = 0
         (assuming small NPE and COH in practice.)
REF0     First order refraction coefficient
         dX = 0                         dY = -REF0/tan(El)
REF1     Second order refraction coefficient
         dX = 0                         dY = -REF1/tan(El)**3
ELES     gravity+eccentricity of Elevation encoder 
         dX = 0                         dY =  ELES*sin(El)
ELEC     gravity+eccentricity of Elevation encoder
         dX = 0                         dY =  ELEC*cos(El)
AZES     eccentricity of Azimuth encoder
         dX = AZES*sin(Az)*cos(El)      dY = 0
AZEC     eccentricity of Azimuth encoder
         dX = AZEC*cos(Az)*cos(El)      dY = 0
HEL      Homology elevation bending (cos(El))
         dX = 0                         dY = -HEL*cos(El)
The total pointing correction is thus, in the small COH approximation
    dX = IAZ.cos(El) + COH + sin(El) * (MVE*cos(Az)-MVN*sin(Az)-NPE)
          + cos(El) * (AZES*sin(Az)+AZEC*cos(Az))
    dY = IEL+COV - (MVE*sin(Az)+MVN*cos(Az))
          + (ELES*sin(El)+(ELEC-HEL)*cos(El))
          - REF0/tan(El) - REF1/tan(El)**3 - REF2/tan(El)**5

Since each antenna is equipped with an off-axis optical telescope, there are two sets of (COH,COV) values, one for the radio axis and one for the optical axis. As only IEL+COV is meaningful, the radio COV is set to zero, fixing the IEL value.

Five terms dominate the pointing model : IAZ, IEL+COV, COH, MVE and MVN. In principle COH, IEL, and COV depend only on the antenna, MVE and MVN depend on the station and antenna (azimuth bearing), and IAZ depends on the antenna (encoder), the station (footpad positions) and the positioning of the antenna on the station (repeatability 1 mm).

The refraction coefficients are weather dependant. Accordingly, the parameters are computed in real time, using temperature, pressure and humidity. REF0 and REF1 are thus fixed to zero in the pointing fit.

Among the other terms, all of which depend on the antenna, just two have been found to be of some importance for some antennas: ELEC which enters only in combination with HEL in radio (small values on all antennas), and possibly NPE (Antenna 3). These are marginally different from zero, however, and confirmation will be required. The other parameters can be set to zero.


next up previous contents index
Next: Parameter Files Up: Basic pointing parameters Previous: Basic pointing parameters   Contents   Index
Gildas manager 2018-12-15