FEED Feed charactersitics.

Name

Format

Units

Measure

Comments

Columns

Key

ANTENNA_ID

Int Antenna identifier

FEED_ID

Int Feed identifier

SPECTRAL_WINDOW_ID

Int Spectral window identifier

TIME

Double s EPOCH Interval mid-point

INTERVAL

Double s Time interval for validity of the parameters

Data description

NUM_RECEPTORS

Int Number of receptors Nr on this feed

Data

FEED_NUM

Int Feed number

RECEIVER_ID  

Int( Nr) Receiver properties.

BEAM_ID

Int( Nr) 2D antenna beam response.

BEAM_OFFSET

Double(2, Nr) rad DIRECTION Beam position offset (on sky but in antenna reference frame)

(FOCUS_REFERENCE)

Double(3, Nr) m Focus reference position

(ILLUM_OFFSET)

Float m Illumination offset

(ILLUM_OFFSET_PA)

Float rad Illumination offset position angle

POLARIZATION_TYPE

String( Nr) Type of polarization to which a given RECEPTOR responds

POL_RESPONSE

Complex( Nr, Nr) Feed polarization response

POSITION

Double(3) POSITION Offset of the feed relative to the feed reference

RECEPTOR_ANGLE

Double( Nr) rad The reference angle for polarization
Notes: A feed is a collecting element on an antenna, such as a single horn, that shares joint physical properties and makes sense to calibrate as a single entity. It is an abstraction of a generic antenna feed and is considered to have one or more recptors that respond to different polarization states. A feed may have a time-variable beam and polarization response. Feeds are numbered from 0 on each separate antenna and for each SPECTRAL_WINDOW_ID. Consequently, FEED_NUM should be non-zero only in the case of feed arrays, i.e. multiple, simultaneous beam on the sky at the same frequency and polarization.
ANTENNA_ID
Antenna identifier, as indexed from an element in the ANTENNA_ARRAY collection in the CONFIG_DESCRIPTION table.
FEED_ID
Feed identifier, as indexed from an element in the FEED_LIST collection in the the CONFIG_DESCRIPTION table.
Note that FEED_ID is zero-based relative to each combination of the 5 other keys.
SPECTRAL_WINDOW_ID
Spectral window identifier. A value of -1 indicates the row is valid for all spectral windows. If a specific spectral window has its own feed characteristics these have to be defined separately using a value > 0, to override the global specifications for that specific window. 
TIME
Mid-point of time interval for which the feed parameters in the row are valid. The same reference used for the TIME column in the MAIN must be used.
INTERVAL
Time interval.
NUM_RECEPTORS
Number of receptors Nr on this feed. See POLARIZATION_TYPE for further information.  
FEED_NUM
Feed number, 0 based. For ALMA it is always 0.
RECEIVER_ID
Receiver identifier(s) for each receptor.
N.B.: in MSv2.0  RECEIVER_ID is indexed in the SPECTRAL_WINDOW table.  
BEAM_ID
Beam identifier. Points to a BEAM sub-table defining the far field voltage pattern and the polarization response for this FEED. TBD
BEAM_OFFSET
Beam position offset, as defined on the sky but in the antenna reference frame.
FOCUS_REFERENCE
Focus reference position. Mandatory for ALMA. Notice that receptor independent part in the focus position is specified in the FOCUS sub-table.  
ILLUM_OFFSET
Illumination offset
ILLUM_OFFSET_PA
Illumination offset position angle
POLARIZATION_TYPE
Polarization type to which each receptor responds (e.g. “R”, “L”, “X”, “Y”). This is the receptor polarization type as recorded in the final correlated data (eg “RR); i.e. as measured after all polarization combiners.  
POL_RESPONSE
Polarization response at the center of the beam for this feed. Expressed in a linearly polarized basis using the IEEE convention.
POSITION
Offset of the feed relative to the feed reference position for this antenna (see ANTENNA sub-table).
RECEPTOR_ANGLE
Polarization reference angle. Converts into parallactic angle in the sky domain.