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). More than one spectral window is not currently supported.
- Ntimes
Number of times
- ant_1_array
Array of first antenna indices, shape (Nblts), type = int, 0 indexed
- ant_2_array
Array of second antenna indices, shape (Nblts), type = int, 0 indexed
- 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.
- 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)
- freq_array
Array of frequencies, center of the channel, shape (Nspws, Nfreqs), units Hz
- 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 lsts, center of integration, shape (Nblts), units radians
- object_name
Source or field observed (string)
- phase_type
String indicating phasing type. Allowed values are “drift”, “phased” and “unknown”
- 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).
- 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, Nspws, Nfreqs, Npols), 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.
- flag_array
Boolean flag, True is flagged, same shape as data_array.
- 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_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 “gcrs” and “icrs”, default is “icrs”
- 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: 2020-07-08