UVData Parameters

These are the standard attributes of UVData objects.

Under the hood they are actually properties based on UVParameter objects.

Angle type attributes also have convenience properties named the same thing with ‘_degrees’ appended through which you can get or set the value in degrees.

Similarly location type attributes (which are given in topocentric xyz coordinates) have convenience properties named the same thing with ‘_lat_lon_alt’ and ‘_lat_lon_alt_degrees’ appended through which you can get or set the values using latitude, longitude and altitude values in radians or degrees and meters.

Required

These parameters are required to have a sensible UVData object and are required for most kinds of uv data files.

Nants_data

Number of antennas with data present (i.e. number of unique entries in ant_1_array and ant_2_array). May be smaller than the number of antennas in the array

Nants_telescope

Number of antennas in the array. May be larger than the number of antennas with data

Nbls

Number of baselines

Nblts

Number of baseline-times (i.e. number of spectra). Not necessarily equal to Nbls * Ntimes

Nfreqs

Number of frequency channels

Npols

Number of polarizations

Nspws

Number of spectral windows (ie non-contiguous spectral chunks).

Ntimes

Number of times

ant_1_array

Array of numbers for the first antenna, which is matched to that in the antenna_numbers attribute. Shape (Nblts), type = int.

ant_2_array

Array of numbers for the second antenna, which is matched to that in the antenna_numbers attribute. Shape (Nblts), type = int.

antenna_names

List of antenna names, shape (Nants_telescope), with numbers given by antenna_numbers (which can be matched to ant_1_array and ant_2_array). There must be one entry here for each unique entry in ant_1_array and ant_2_array, but there may be extras as well.

antenna_numbers

List of integer antenna numbers corresponding to antenna_names, shape (Nants_telescope). There must be one entry here for each unique entry in ant_1_array and ant_2_array, but there may be extras as well.Note that these are not indices – they do not need to start at zero or be continuous.

antenna_positions

Array giving coordinates of antennas relative to telescope_location (ITRF frame), shape (Nants_telescope, 3), units meters. See the tutorial page in the documentation for an example of how to convert this to topocentric frame.

baseline_array

Array of baseline numbers, shape (Nblts), type = int; baseline = 2048 * (ant1+1) + (ant2+1) + 2^16

channel_width

Width of frequency channels (Hz). If flex_spw = False and future_array_shapes=False, then it is a single value of type = float, otherwise it is an array of shape (Nfreqs), type = float.

flex_spw

Option to construct a “flexible spectral window”, which storesall spectral channels across the frequency axis of data_array. Allows for spectral windows of variable sizes, and channels of varying widths.

freq_array

Array of frequencies, center of the channel, shape (1, Nfreqs) or (Nfreqs,) if future_array_shapes=True, units Hz

future_array_shapes

Flag indicating that this object is using the future array shapes.

history

String of history, units English

instrument

Receiver or backend. Sometimes identical to telescope_name

integration_time

Length of the integration in seconds, shape (Nblts). The product of the integration_time and the nsample_array value for a visibility reflects the total amount of time that went into the visibility. Best practice is for the integration_time to reflect the length of time a visibility was integrated over (so it should vary in the case of baseline-dependent averaging and be a way to do selections for differently integrated baselines).Note that many files do not follow this convention, but it is safe to assume that the product of the integration_time and the nsample_array is the total amount of time included in a visibility.

lst_array

Array of local apparent sidereal times (LAST) at the center of integration, shape (Nblts), units radians.

multi_phase_center

Only relevant if phase_type = “phased”. Specifies the that the data set contains multiple sources within it.

object_name

Name(s) of source(s) or field(s) observed, type string. If multi_phase_center = True, set to “multi”.

phase_type

String indicating phasing type. Allowed values are “drift” and “phased” (n.b., “drift” is not the same as cat_type=”driftscan”, the latter of which _is_ phased to a fixed az-el position).

polarization_array

Array of polarization integers, shape (Npols). AIPS Memo 117 says: pseudo-stokes 1:4 (pI, pQ, pU, pV); circular -1:-4 (RR, LL, RL, LR); linear -5:-8 (XX, YY, XY, YX). NOTE: AIPS Memo 117 actually calls the pseudo-Stokes polarizations “Stokes”, but this is inaccurate as visibilities cannot be in true Stokes polarizations for physical antennas. We adopt the term pseudo-Stokes to refer to linear combinations of instrumental visibility polarizations (e.g. pI = xx + yy).

spw_array

Array of spectral window numbers, shape (Nspws)

telescope_location

Telescope location: xyz in ITRF (earth-centered frame). Can also be accessed using telescope_location_lat_lon_alt or telescope_location_lat_lon_alt_degrees properties

telescope_name

Name of telescope (string)

time_array

Array of times, center of integration, shape (Nblts), units Julian Date

uvw_array

Projected baseline vectors relative to phase center, shape (Nblts, 3), units meters. Convention is: uvw = xyz(ant2) - xyz(ant1).Note that this is the Miriad convention but it is different from the AIPS/FITS convention (where uvw = xyz(ant1) - xyz(ant2)).

vis_units

Visibility units, options are: “uncalib”, “Jy” or “K str”

Optional

These parameters are defined by one or more file standard but are not always required. Some of them are required depending on the phase_type (as noted below).

Nphase

Required if multi_phase_center = True. Specifies the number of sources contained within the data set.

antenna_diameters

Array of antenna diameters in meters. Used by CASA to construct a default beam if no beam is supplied.

blt_order

Ordering of the data array along the blt axis. A tuple with the major and minor order (minor order is omitted if order is “bda”). The allowed values are: time ,baseline ,ant1 ,ant2 ,bda

data_array

Array of the visibility data, shape: (Nblts, 1, Nfreqs, Npols) or (Nblts, Nfreqs, Npols) if future_array_shapes=True, type = complex float, in units of self.vis_units

dut1

DUT1 (google it) AIPS 117 calls it UT1UTC

earth_omega

Earth’s rotation rate in degrees per day

eq_coeffs

Per-antenna and per-frequency equalization coefficients

eq_coeffs_convention

Convention for how to remove eq_coeffs from data

extra_keywords

Any user supplied extra keywords, type=dict. Keys should be 8 character or less strings if writing to uvfits or miriad files. Use the special key “comment” for long multi-line string comments.

filename

List containing the unique names of input files

flag_array

Boolean flag, True is flagged, same shape as data_array.

flex_spw_id_array

Required if flex_spw = True. Maps individual channels along the frequency axis to individual spectral windows, as listed in the spw_array. Shape (Nfreqs), type = int.

gst0

Greenwich sidereal time at midnight on reference date

nsample_array

Number of data points averaged into each data element, NOT required to be an integer, type = float, same shape as data_array.The product of the integration_time and the nsample_array value for a visibility reflects the total amount of time that went into the visibility. Best practice is for the nsample_array to be used to track flagging within an integration_time (leading to a decrease of the nsample array value below 1) and LST averaging (leading to an increase in the nsample array value). So datasets that have not been LST averaged should have nsample array values less than or equal to 1.Note that many files do not follow this convention, but it is safe to assume that the product of the integration_time and the nsample_array is the total amount of time included in a visibility.

phase_center_app_dec

Required if phase_type = “phased”. Declination of phase center in the topocentric frame of the observatory, units radians. Shape (Nblts,), type = float.

phase_center_app_ra

Required if phase_type = “phased”. Apparent right ascension of phase center in the topocentric frame of the observatory, units radians.Shape (Nblts,), type = float.

phase_center_catalog

Only relevant if multi_phase_center = True. Dictionary that acts as a catalog, containing information on individual phase centers. Keys are the names of the different phase centers in the UVData object. At a minimum, each dictionary must contain the key “cat_type”, which can be either “sidereal” (fixed position in RA/Dec), “ephem” (position in RA/Dec whichmoves with time), “driftscan” (fixed postion in Az/El, NOT the same as phase_type`=”drift”) and “unphased” (baseline coordinates in ENU, but data are not phased, similar to `phase_type`=”drift”). Other typical keyworks include “cat_lon” (longitude coord, e.g. RA), “cat_lat” (latitude coord, e.g. Dec.), “cat_frame” (coordinate frame, e.g. icrs), “cat_epoch” (epoch and equinox of the coordinate frame), “cat_times” (times for the coordinates, only used for “ephem” types), “cat_pm_ra” (proper motion in RA), “cat_pm_dec” (proper motion in Dec), “cat_dist” (physical distance), “cat_vrad” (rest frame velocity), “info_source” (describes where catalog info came from), and “cat_id” (matched to the parameter `phase_center_id_array. See the documentation of the phase method for more details.

phase_center_dec

Required if phase_type = “phased”. Declination of phase center (see uvw_array), units radians. Can also be accessed using phase_center_dec_degrees.

phase_center_epoch

Required if phase_type = “phased”. Epoch year of the phase applied to the data (eg 2000.)

phase_center_frame

Only relevant if phase_type = “phased”. Specifies the frame the data and uvw_array are phased to. Options are “icrs”, “gcrs”, and “fk5”; default is “icrs”

phase_center_frame_pa

Required if phase_type = “phased”. Position angle between the hour circle (which is a great circle that goes through the target postion and both poles) in the apparent/topocentric frame, and the frame given in the phase_center_frame attribute.Shape (Nblts,), type = float.

phase_center_id_array

Required if multi_phase_center = True. Maps individual indices along the Nblt axis to an entry in phase_center_catalog, with the ID number of individual entries stored as cat_id, along with other metadata. Shape (Nblts), type = int.

phase_center_ra

Required if phase_type = ‘phased’. Right ascension of phase center (see uvw_array), units radians. Can also be accessed using phase_center_ra_degrees.

rdate

Date for which the GST0 or whatever… applies

timesys

We only support UTC

uvplane_reference_time

FHD thing we do not understand, something about the time at which the phase center is normal to the chosen UV plane for phasing

x_orientation

Orientation of the physical dipole corresponding to what is labelled as the x polarization. Options are ‘east’ (indicating east/west orientation) and ‘north (indicating north/south orientation)

last updated: 2021-10-21