beamTab.html

Table 1:

BEAM: 2 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	s		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	rad		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. 2)
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 2:

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

APS_FORM	NUM_COEFF	Analytical expression
GAUSS	1	exp
POLY	n+1	_i=0ⁿ²ⁱ
COS	2	^a₁