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 . 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 on the VLA archive tape. 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 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. This is accomplished through two tasks. 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.

4.8.1 Reading solar data from a VLA archive tape

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

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

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 2 of each using CLCAL.

4.8.2 Using SNPLT and LISTR to assess the nominal sensitivities

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:

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):

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 to apply the system-temperature correction

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. If you are using a version of  older than 15JUL94, you must copy CL table version 1 into CL table version 2 before running SOLCL (which does the copy for you in later releases). Then you apply the system-temperature correction to version 2 and correct mistakes by deleting and recreating version 2. To run SOLCL:

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