What's the difference between Antenna1 and Antenna2? Axis definitions in plotms

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plotms is a powerful tool for visualizing your measurement set, and provides you with lots of choices for X- and Y- Axes. It's not always clear what the choices mean, so below are some explanations.

Scan

The scan number, as listed by listobs.

Field

The field number, as listed by listobs. Typically, each source/pointing should have a different field number.

Time

The time at which the visibility was observed, given in terms of calendar year (yyyy/mm/dd/hh:mm:ss.ss)

Time_interval

The integration time in seconds.

Spw

The spectral window number (analogous to what VLA users know as an IF). The characteristics of each spectral window are listed in listobs.

Channel

The frequency channel number.

Frequency

Frequency in units of GHz. The frame for the frequency (e.g., topocentric, barycentric, LSRK) can be set in the Trans tab of plotms.

Velocity

Velocity in units of km/s. Velocity is defined by parameters in the Trans tab of plotms, namely:

  • the reference frame (barycentric, topocentric, etc.)
  • the velocity definition (radio, optical, true, etc.)
  • the rest frequency of the spectral line.

Corr

Correlations which have been assigned integer IDs:

  • 5: RR
  • 6: RL
  • 7: LR
  • 8: LL

Antenna1, Antenna2

Antenna1 is the first antenna in a baseline pair; for example, for baseline 2-4, Antenna1 = 2. Antenna2 is the second antenna in a baseline pair; so for baseline 2-4, Antenna2 = 4. Antennae are numbered according to the antenna IDs listed in listobs.

Antenna

For plotting antenna-based quantities like elevation, use just plain Antenna. Antennae are numbered according to the antenna IDs listed in listobs.

Baseline

The baseline number.

UVDist, UVDist_L

Projected baseline separations. UVDist is in units of meters, while UVDist_L is in units of the observing wavelength (lambda, not kilolambda). Note that UVDist_L is a function of frequency, while UVDist is not. Therefore, when plotting UVDist_L, there will be a different data point for each frequency channel.

U, V, W

u, v, and w in units of meters.

Beta Alert: Some day soon there will be U_L, V_L, and W_L, which will be in units of observing wavelength. Like UVDist_L, these quantities are functions of frequency and will change across your bandwidth.

Amp

Data amplitudes in units which are proportional to Jansky (for data which are fully calibrated, the units should be Jy).

Phase

Data phases in units of degrees.

Real, Imag

The real and imaginary parts of the visibility in units which are proportional to Jansky (for data which are fully calibrated, the units should be Jy).

Flag

Data which are flagged have Flag = 1, whereas unflagged data are set to Flag = 0. Note that, to display flagged data, you will have to click on the Display tab and choose a 'Flagged Points Symbol'.

Azimuth, Ant-Azimuth

Azimuth in units of degrees. 'Azimuth' plots a fiducial value for the entire array, while 'Ant-Azimuth' plots the azimuth for each individual antenna (their azimuths will differ by small amounts, because each antenna is located at a slightly different longitude, latitude, and elevation).

Elevation, Ant-Elevation

Elevation in units of degrees. 'Elevation' is a representative value for the entire array, while 'Ant-Elevation' is the elevation for each individual antenna (their elevations will differ by small amounts, because each antenna is located at a slightly different longitude, latitude, and elevation).

HourAngle

Hour Angle in units of hours. This is a fiducial value for the entire array.

Beta Alert: Some day soon, Ant-HourAngle will be added.

ParAngle, Ant-ParAng

Parallactic angle in units of degrees.'ParAngle' is the fiducial parallactic angle for all antennae in the array, while 'Ant-ParAng' plots the parallactic ange for each individual antenna (their parallactic angles will differ by small amounts, because each antenna is located at a slightly different longitude, latitude, and elevation).

Row

Data row number. A row number corresponds to a unique time, baseline, and spectral window in the measurement set.

FlagRow

In some tasks, if a whole data row is flagged, then FlagRow will be set to 1 for that row. Unflagged rows have FlagRow = 0. However, note that some tasks (like plotms) may flag a row, but not set FlagRow = 1. It's probably better to plot Flag than FlagRow for most applications.

Data Column

For relevant data axes like Amp and Phase, you will have the option to plot the raw data or the calibrated data. This can be selected via a drop-down menu called 'Data Column', located directly under the drop-down menu for X or Y Axis selection. To plot raw data, select 'data'; to plot calibrated data, select 'corrected'. Note that this choice will only have an impact on your plot if you have applied a calibration table.

If you have applied a data model, you can also plot this up by selecting 'model' from the 'Data Column' menu. Finally, you can plot the differences between the calibrated data and the model by selecting 'residual' from 'Data Column'.


Last checked in CASA Version 3.0.1 (r10380).

--Laura Chomiuk 19:51, 19 February 2010 (UTC)

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