V.4 Improving antenna surfaces: holography

In “holography” data from the EVLA, reference antennas are pointed towards the nominal source position, as normal. All other antennas are pointed to an offset position controlled by the observer. The angular offset from the nominal position, for each antenna, is written to data sets in the same variables that are normally used for u,v,w. In this mode, only data on a baseline between a reference antenna and an offset (non-reference) is considered useful. The correlations observed on these baselines are stored in the multi-source file in the normal way. Data are calibrated as usual and then extracted with UVHOL for holographic analysis. This analysis includes determination of the shapes of the primary beams with PBEAM, Fourier analysis with HOLOG to make images of the dish surface including the deviations from the desired shape, and finally PANEL to prepare detailed prescriptions for adjusting the antenna panels. Verb TVLAYOUT plots an antenna panel layout on top of a TV holography image and has the layouts for the VLA and VLBA built in.

Holography data are taken by driving the moving antennas to a desired offset and then dwelling for a period of time, accumulating some number of samples. Then the moving antennas are driven to the next desired offset, usually horizontally or vertically, and the process is repeated. Holography samples are recorded with the u random parameter as the azimuth offset direction cosine, the v random parameter as the elevation direction cosine, and the w random parameter as a counter of the dwell interval. In the old VLA, the w had an additional 40000.0 added as a holography indicator. With the EVLA, the source name is the sole method of discriminating between regular visibility data taken for calibration and the holography data.

UVHOL has three main modes of operation: printing the data much like UVPRT, averaging and writing the data to text files for analysis by HOLOG, and plotting the data in AIPS plot files or on the TV display. UVHOL has all of the usual data selection and calibration adverbs. In addition it has a number of basic holography-specific adverbs. ANTENNAS is used to list the reference antennas and BASELINE is used to list the moving antennas. Since it takes some time for the antennas to settle down after each move, adverbs NPOINTS or APARM(1) through APARM(3) control which samples within each pointing are retained. APARM also controls whether the data are averaged over all moving antennas and/or all reference antennas and/or over time. Normally, holography data should not have the parallactic angle correction applied on DOPOL, but it can be done if desired. DPARM is used to advise the print routines on scaling of data and weights and also to invoke special plotting options (use db for amplitudes, plot phase or imaginary rather than amplitude/real). There are a number of the usual plot options, such as SYMBOL, DO3COL, LTYPE, and the like. DOCRT controls where the print out is sent, either to the terminal or to OUTPRINT.

On OPTYPE=’HOLGmode, the holography data are written to text files, a number of files for each pair of antennas. Each line of these files, after a number of header lines, contains l, m, amplitude or real, phase or imaginary, error in amplitude or real, and error in phase or imaginary. In this mode, OUTPRINT specifies the logical directory name of the output files which are then named with an additional HOLOnn-mmppii where nn is the reference antenna (BASELINE(j) or 0 if multiple), mm is the antenna (or 0 if multiple), pp are 2 letters giving the polarization, and ii is the IF. These files are then given to HOLOG.

An example of plotting one short raster at 4.8 GHz has inputs

> DEFAULT UVHOL  C R

to initialize the task name and all adverbs.

> INDISK n ; GETN m  C R

to specify the holography data set.

> SOURCES ’HOLORASTER’ ’  C R

to specify the standard holography source name. Required with EVLA data. Note the extra quote mark at the end which makes all the other values of SOURCES blank.

> STOKES ’HALF”  C R

to plot RR and LL.

> ANTENNAS 9,0  C R

to specify one reference antenna.

> BASELINE 8,0  C R

to specify one moving antenna.

> ICHANSEL 5, 60  C R

to omit outer channels from the average.

> DOCALIB 1 ; DOBAND 1  C R

to apply the calibration.

> APARM 1, 3, 2  C R

to include all but the first 3 samples and last 2 in each pointing with no averaging.

> DPARM 0 0 0 0 0 1 0  C R

to plot amplitudes in db.

> DO3COLOR 1  C R

to plot the two polarizations in different colors.

> OPTYPE PLOT  C R

to make plots rather than output text.

> INP  C R

to check the inputs.

> GO  C R

to make the plot file shown in Figure V.2.


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Figure V.2: Example of the amplitudes measured in a single holography scan as plotted by UVHOL. Antenna 8 moved in the L coordinate while antenna 9 remained pointed directly at the source. Both polarizations are plotted with multiple samples at each pointing (not averaged).


Now, to continue the holography analysis, we write out the data in appropriate text files. An example, would be

> TGET UVHOL  C R

to get the previous parameters for UVHOL.

> OPTYPE HOLG  C R

to set the holography mode.

> OUTPRINT ’MYAREA:’  C R

to set the logical directory for the output text files.

> ANTENNAS 1, 5, 9, 17  C R

to specify more of the reference antennas.

> APARM 1, 3, 2, 0, 1, 1  C R

to include all but the first 3 samples and last 2 in each pointing averaging over time within the pointing and over all reference antennas.

> INP  C R

to double check the inputs.

> GO  C R

to write out the data for HOLOG.

This will write a file named MYAREA:HOLO00-08RR01 and one named MYAREA:HOLO00-08LL01. Note that we did not specify the one IF to be used, so IF number one was what we got.

HOLOG reads the visibility data from the text file produced by UVHOL. It then Fourier transforms the data to produce the grading in the aperture plane of the antenna. Images of the re-gridded observed amplitude and phase (A_AMP, A_PHA), of the weights in the re-gridding (WGT), of the derived amplitude and phase of the illumination in the antenna aperture (V_AMP, V_PHA), of the amplitude and phase of the point-spread function (P_AMP, P_PHA), of the phase corrections removed by the focus model (MODEL), of the surface deviations of the antenna (V_DEV) in meters, and of the interpolated antenna power pattern (A_PWR) may be produced under control of adverb DPARM. Task PANEL requires V_AMP (DPARM(4)) for the mask and V_DEV (DPARM(9) for the deviations. An example of the images produced by HOLOG is shown in Figure V.3.


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Figure V.3: Example of the mask (V_AMP) and deviation (V_DEV) images produced by HOLOG. Task GREYS can do both the pseudo-coloring with OFMFILE = ’RYPHLAME’ and the panel layout with INFILE = ’VLA.


Many of the control adverbs must be set to appropriate values. A sample set of inputs is

> DEFAULT HOLOG  C R

to initialize everything.

> INFILE ’MYAREA:HOLO00-28RR01  C R

to point at the text file, 28 is the moving antenna with several reference antennas.

> OUTNAME  ’28-Q2-40935E’  C R

to name the output images.

> OPTYPE ’SUBR’  C R

to specify the type of antenna, sub-reflector, sub-reflector with reference pointing, or primary focus.

> FACTOR 8.8  C R

to set magnification factor for sub-reflector antennas; default 10.0.

> APARM 0, -1, 25, 3.4, 9  C R

to set antenna outer diameter, inner diameter, and focal length in meters.

> BPARM 30, 256, 0, 1, 1, -2, -2.5, -12, 100, -1  C R

to set data reduction parameters.

> DPARM 0 0 0 1 0 1 0 1 1 0  C R

to request V_AMP, P_AMP , MODEL, and V_DEV images.

> INP  C R

to double check everything.

> GO  C R

to make the images.

BPARM specifies the required image size in meters, the image size in pixels, minimum and maximum antenna scan angle, amplitude scaling factor, Fourier transform control (-2 says use DFT with VLA sign convention), minimum and maximum coordinate used in correcting for pointing, focus, and feed offset (negative says in the radial coordinate), a control parameter (100 inhibits local phase ambiguity correction in the V_PHA plane), and type of data (-1 for VLA). CPARM all zero says to use the default spheroidal function in gridding on both L and M axes; highly recommended, but not relevant when using DFT imaging. Note that HOLOG can also generate images from a feed model using VPARM.

The last task in the process of measuring corrections to the antenna surface is PANEL

> DEFAULT PANEL  C R

to initialize everything.

> INDISK n ; GETN m  C R

to specify the V_DEV image.

> IN2DI i ; GET2N j  C R

to specify the V_AMP image used as a mask.

> OUTFILE ’MYAREA:PANEL.out  C R

to specify the text file to receive the panel adjustment instructions.

> DOPLOT 7  C R

to request plots of the input deviations, the panel adjustments, and the residuals.

> APARM 2, 1, 0, 0, 42  C R

To set the panel model, the clip level in the mask image, and the number of pointings. The map size and frequency should be available from the image header.

> INLIST    C R

to use the default VLA panel layout.

> INP  C R

to review the inputs.

> GO  C R

to fit the data, write the adjustment instructions, and make plots.

Example plots of the panel adjustments and residual are shown in Figure V.4.


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Figure V.4: Example of the panel adjustment image and the residual image computed and plotted by PANEL.


With appropriate holography data, the output of UVHOL may also be used to fit an analytic function to the primary beam. The task that does this is called PBEAM and it fits a polynomial function and makes contour plots of the input data, the fit model, the residual, and IRING-like plots comparing data and model in two colors, and then IRING-like plots of the residual image and residual image divided by the model.

Sample inputs could contain

> DEFAULT PBEAM  C R

to initialize everything.

> OUTNAME PBEAM Cband  C R

to make output images of the data, model, and residual. Required if DOTV is false and DOPLOT is > 0.

> INFILE ’MYWORK:C-6052I_1’  C R

to provide the text file from UVHOL.

> OUTFILE ’MYWORK:CbandTxt  C R

to save the messages including answers to a text file.

> VPARM 0  C R

to use all defaults in the fitting.

> DOPLOT 7  C R

to get all possible plots.

> OPTYPE ’ELLI’  C R

to fit eccentricity and position angle.

> INP  C R

to double check everything.

> GO  C R

to do the fit and write out images and plots.

Sample plots are shown in Figure V.5.


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Figure V.5: Example of the plots from PBEAM: contour plot of the residual image and IRING-like plot of the residual image divided by the model.