BEAM: Two dimensional antenna beam response

BEAM: Beam response

Name

Format

Units

Measure

Comments

Columns

Data

FEFF  

Float Forward efficiency

AEFF  

Float Aperture efficiency

(APS_FORM)   

String Form of the analytical model for the Antenna Power Spectrum

(NUM_COEFF)   

Int Number of coefficients in the analytical form of the model

(APS_COEFF)   

Double (NUM_COEFF) Hz rad-1 Coefficients

(APS_CUTOFF)   

Float Radial cutoff for the validity of the model

Notes: This table holds state information of known properties of the individual beams. The beam response is function of characteristics of the individual antennas, on the observing frequency and the feed number. The beam may suffer from an illumination offset. For this reason it is indexed in the Data section of the FEED  table.
FEFF
Fraction of the beam volume looking at the sky.  
AEFF
Aperture efficiency.  
APS_FORM
E.g. GAUSS, POLY, COS (see Tab. 1 )
NUM_COEFF
Number of coefficient in the analytic model. NB: for APS_FORM=1 NUM_COEFF=1.
APS_COEFF
Coefficients:
APS_CUTOFF

Comments:
The antenna beam also depends of parameters such as the elevation and the ambient temperature. Only simple examples of analytical expressions are given here; they are elevation and temperature invariant. More accurate analytic formula including such dependencies could be set in the future. If those are only valid over a time interval smaller than the interval of time to define the feed characteristics, then it may be necessary to reconsider how this BEAM  table be indexed.
The FEED  table provides the on-axis axis polarization response. The 2D response will need to be defined as needed for ALMA in this BEAM  table.


Table 1:

The ai are the Antenna Power Spectrum coefficients of the model, r the radial distance to the pointing center and the n observed frequency.

APS_FORMNUM_COEFFAnalytical expression
GAUSS 1 exp( r) - a0n
POLY n+1 sum i=0n( r) ain2i
COS 2 ( ( r)) cos a0na1