4.8 Solar data calibration

The calibration of solar uv data differs from normal continuum and spectral-line calibration in one critical respect: the system temperature correction to the visibility data is applied by the observer in AIPS. See Lecture 21 in Synthesis Imaging in Radio Astronomy for a discussion of the system temperature correction as it applies to VLA solar visibility data. The system temperature correction is embodied in a quantity referred to as the “nominal sensitivity,” an antenna-based numerical factor normally applied in real time to the scaled correlation coefficients before they are written to the VLA archive. With the exception of X and L band, only a handful of VLA antennas are equipped with so-called “solar CALs.” The nominal sensitivity is only computed for those antennas so-equipped, namely (for the old VLA) antennas 5, 11, 12, and 18 (at K, U,and C bands) and antennas 7, 12, 21, and 27 (at P band). The system-temperature correction for those antennas without solar CALs must, therefore, be bootstrapped from those antennas which do have solar CALS. This is accomplished through two tasks for the old VLA and two other tasks for the new vLA.. Fpr the old VLA, FILLM fills the uncalibrated visibility data to disk and places the nominal sensitivities in a TY extension table. Then, SOLCL applies the nominal sensitivities to calibration parameters in the CL table. For the new VLA, BDF2AIPS reads the data in, writing a SysPower (SY) and CalDevice (CD) table. Then SYSOL applies the gain and weight corrections. See E.1 and E.6.

4.8.1 Reading solar data from the VLA archive

New VLA data are loaded from the archive following the instructions in E.1 with no special considerations for Solar observing except that the versions of OBIT and AIPS must be after mid-June 2017.

To load a solar uv-data file to disk from an old VLA archive data set follow the general instructions given above (4.1.1 and 4.7.1) with the following additions:

> VLAMODE ’S ’  C R

to indicate solar mode observing.

> CPARM(2) 16  C R

to indicate that moving sources are allowed without renaming.

If your experiment involved observing active solar phenomena, (e.g., flares), you may wish to update the system-temperature correction every integration time. For example, if you observed a flare with an integration time τ = 1.67 seconds, choose

> CPARM(8) 1.67 / 60  C R

for 1.67 sec CL and TY table intervals.

Loading an entire solar uv-data set to disk with the minimum integration time results in very large disk files which make all subsequent programs take a long time to run. A useful strategy is to load the data with relatively low time resolution (20–30 seconds for observations of active solar phenomena) and to proceed with the usual continuum data calibration, deferring the system temperature correction. When a satisfactory calibration is obtained, the relevant SN table may be saved using TASAV. (Note that you must save the SN table, before running CLCAL rather than the final CL table.) Then run CLCAL and inspect the data for interesting periods of activity — try UVPLT with BPARM = 11, 1 for plots of amplitude versus time or TVFLG, displaying amplitudes as a function of baseline length and time. Use FILLM to load the relevant time ranges of solar uv data to disk with no averaging. The saved SN table is then copied to each high-time resolution data set. Assess, and possibly edit, the nominal sensitivities (4.8.2) and then apply the system-temperature corrections (4.8.3). Finally, apply the saved/copied SN table to the CL table of each using CLCAL.

4.8.2 Using SNPLT and LISTR to assess the nominal sensitivities

For new VLA data sets, use SNPLT, LISTR, and EDITA or SNEDT on the SY table (E.6). The following applies to old VLA data primarily.

When solar uv data are written to disk, FILLM writes the nominal sensitivities of those antennas equipped with solar CALs into the TY table. Before bootstrapping the system temperature correction for antennas without solar CALs from those which do, it is always wise to examine the nominal sensitivity for each of the solar CAL antennas for each of the IFs. The tools available for this purpose include: SNPLT, which plots the nominal sensitivities in graphical form, LISTR or PRTAB, which allow one to inspect the values directly, and EDITA, which provides an interactive display of the TY data and allows you to edit the data. To make plots:

> TASK SNPLT’ ; INP  C R

to review the inputs needed.

> IND m ; GETN n  C R

to specify the input uv file.

> INEXT ’TY’  C R

to plot data from TY extension table.

> INVERS 0  C R

to use the highest version number.

> SOURCES ’SUN’ , ’ ’  C R

to plot solar source only.

> TIMERANG 0  C R

to select all times.

> ANTENNAS 5 11 12 18  C R

to select only CAL-equipped antennas; this sample list for K, U, or C band.

> PIXRANGE 0  C R

to self-scale each plot.

> NPLOTS 4  C R

to do 4 plots on a page.

> FACTOR 2 ; SYMBOL 5  C R

to use triangles to mark the data and enlarge them by a factor of 2. The symbols may even be connected by lines.

> XINC 1  C R

to plot every XINCth point.

> OPTYPE ’TSYS’  C R

to plot nominal sensitivities.

> INP  C R

to review the inputs.

> GO  C R

to run the program when you’re satisfied with inputs.

SNPLT produces a PL extension file which may be plotted using LWPLA, TKPL, or TVPL — or you could set DOTV TRUE in SNPLT and get the display directly (and temporarily) on the TV. Then to inspect the values over some limited time range in detail, run LISTR (assuming the adverbs set above and):

> TASK LISTR’ ; INP  C R

to review the inputs needed.

> OPTYPE GAIN  C R

to list quantities in a calibration file.

> INEXT ’TY’  C R

to select the sensitivities.

> TIMER d1 h1 m1 s1 d2 h2 m2 s2  C R

to select by suspect time range.

> DOCRT -1  C R

to route output to the printer.

> DPARM 10 0  C R

to list nominal sensitivities.

> INP  C R

to review the inputs.

> GO  C R

to run the program when you’re satisfied with inputs.

Task SNIFS is similar to SNPLT except that it plots IF on the x axis to compare solutions across them. It has numerous binning options to control the otherwise excessive plotting.

The use of EDITA with TY tables is described extensively in 4.4.2 and need not be described further here.

4.8.3 Using SOLCL or SYSOL to apply the system-temperature correction

For the new VLA, task SYSOL is provided with both non-Solar and Solar Tcals in the CD table and with measured switched power values in the SY table. The latter also tells AIPS tasks which Tcals apply to which times. Therefore, the inputs to SYSOL are simpler than those for the old VLA.

For the old VLA, once you have identified the appropriate subset of reference solar CAL antennas for each source and IF, you are ready to bootstrap the system-temperature correction of the remaining antennas. It is recommended that you run SOLCL before applying any other calibration to the CL table. In this way, you can easily verify that the appropriate corrections have been made to each antenna. Then you apply the system-temperature correction to version 2 and correct mistakes by deleting and recreating version 2. To run SOLCL:

> TASK SOLCL’ ; INP  C R

to review the inputs needed.

> SOURCES ’*’  C R

to correct all sources.

> STOKES  ’  C R

to correct both polarizations.

> TIMERANG 0  C R

to correct all times.

> ANTENNAS 5 11 12 18  C R

to use the listed antennas as references.

> SUBARRAY 1  C R

to modify sub-array 1.

> GAINVER 2  C R

to write corrected entries to CL table version 2.

> INP  C R

to review the inputs.

> GO  C R

to run the program when you’re satisfied with inputs.

After applying the system temperature correction, you may proceed with the usual AIPS data calibration procedures outlined in previous sections, including the special solar tactics described in 4.8.1.