feedTab.html

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 N_r on this feed

Data
FEED_NUM	Int			Feed number
RECEIVER_ID	Int( N_r)			Receiver properties.
BEAM_ID	Int( N_r)			2D antenna beam response.
BEAM_OFFSET	Double(2, N_r)	rad	DIRECTION	Beam position offset (on sky but in antenna reference frame)
FOCUS_REFERENCE	Double(3, N_r)	m		Focus reference position
ILLUM_OFFSET	Float	m		Illumination offset
ILLUM_OFFSET_PA	Float	rad		Illumination offset position angle
POLARIZATION_TYPE	String( N_r)			Type of polarization to which a given RECEPTOR responds
POL_RESPONSE	Complex( N_r, N_r)			Feed polarization response
POSITION	Double(3)		POSITION	Offset of the feed relative to the feed reference
RECEPTOR_ANGLE	Double( N_r)	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 N_r 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.