uv header

A uv table header contains all the informations of a GDF header but some of these informations have a special meaning in this context. Command HEADER is the standard way inside GILDAS to display in a human readable way the header of GDF file. For instance, the command
IMAGER> header gag_demo:demo-line.uvt
would display

 1  W-GDF,  UNKNOWN Velocity type defaulted to LSR
 2  File : /Users/guilloteau/gildas/gildas-exe-dev/demo/demo-line.uvt  REAL*4
 3  Size        Reference Pixel           Value                  Increment       
 4       103   16.0000000000000       220398.688000000     -0.183792725205421
 5      9146   0.00000000000000       0.00000000000000       1.00000000000000
 6  Blanking value and tolerance      1.23455997E+34   0.0000000
 7  Source name         GG_TAU
 8  Map unit            Jy
 9  Axis type           UV-DATA      RANDOM
10  Coordinate system   EQUATORIAL          Velocity    LSR
11  Right Ascension   04:32:30.34200        Declination       17:31:40.5230
12  Lii        0.000000000000000            Bii       0.000000000000000
13  Equinox            2000.0000
14  Projection type     AZIMUTHAL           Angle     0.000000000000000
15  Axis 0     A0     04:32:30.34200        Axis 0     D0     17:31:40.5230
16  Baselines               0.0       0.0
17  Axis 1 Line Name    13CO(21)            Rest Frequency   220398.6880000000
18  Resolution in Velocity   0.25000000     in Frequency        -0.18379273
19  Offset in Velocity        6.3000002     Doppler Velocity     -40.755900
20  Beam                   0.00                0.00                 0.00
21  NO Noise level
22  NO Proper motion
23  NO Telescope section
24  UV Data    Channels:     32, Stokes: 1 None        Visibilities:        9146
25  Column            1 (Size 1) contains U           
26  Column            2 (Size 1) contains V           
27  Column            4 (Size 1) contains DATE        
28  Column            5 (Size 1) contains TIME        
29  Column            6 (Size 1) contains IANT        
30  Column            7 (Size 1) contains JANT        
31  Column            3 (Size 1) contains SCAN

Comments:

Line 1

Indicates the velocity frame. If not present in the table (as here), it is assumed to be LSR.

Line 2

Indicates the filename associated to the currently displayed header.

Lines 3-5

Display the dimensions of the associated array. Here it is a rank 2 array of dimension 103 columns times 9146 lines, i.e. 9146 visibility spectra of 32 frequency channels. Line 4 describes the frequency axis of the visibility spectra stored in the uv table. Be careful that this is a convention, i.e. it must be decoded using the particular form of the table. In our case, each spectra has 32 frequency channels of width -183.8 kHz, the frequency of the reference pixel 16.0 corresponding to 220398.688 MHz. This last frequency is the frequency delivered by the correlator, i.e. seen by the observatory. In particular, this is the frequency that must be used to compute the primary beam of the interferometer.

Line 8

Indicates the unit of the real and imaginary parts of the visibilities, normally the Jansky (Jy).

Line 9

Indicates that this is uv table (UV-DATA and RANDOM).

Lines 10-13

Describe the coordinate system.

Lines 14-15

Describe the projection system. In the uv table format, A0 and D0 indicate the phase center while Right Ascension and Declination indicate where the antenna pointed when acquiring the signal. These informations are in general identical for single field imaging and different for mosaicing.

Lines 16

Indicates the baseline range in meters (m).

Lines 17-19

Describe additional information about the frequency axis of the visibility spectra. In particular, the rest frequency (here 220398.688 MHz, that of the 13CO J=2-1 line) corresponding to a velocity of 6.3 km/s in the velocity frame indicated at line 1 (in general LSR). Frequencies are always in MHz, and velocities always in km/s.

Line 20

Indicates the primary beam size of the interferometer in radian. This is an obsolescent way to pass the size of the interferometer antennas.

Line 21

The noise section has no meaning for the UV table.

Line 22

If present, proper motions are given in mas/yr. The epoch is used as the time origin.

Line 23

If the TELESCOPE section is present, this line would indicate telescope name, its geographic coordinates and the antenna diameter (in m). This section contains also the information to compute the primary beam.

Line 24

UV data section: number of channels, number of Stokes parameters and number of visibilities.

Line 25 to end

Special columns description, including the 7 first ones and the ntrail trailing ones.

In particular, Mosaic uv tables contain two trailing columns named L_PHASE_OFF, M_PHASE_OFF for the so-called ”Phase Offset Mosaics”, or X_POINT_OFF, Y_POINT_OFF for the ”Pointing Offset Mosaics”, which contains the angular offsets of the field centers with respect to the Phase reference.