E.2 Initial calibration — VLANT, FRING

As with the VLA, NRAO maintains text files describing any changes which are made to our estimate of the antenna locations. Users may wish to apply these changes if their data were taken between the time when antennas were moved to their current stations and the time that the corrections were entered into the on-line control data base. Task VLANT works for both VLA and EVLA data, reading these text files and performing the needed changes to the CL table, writing a new one. Thus

> DEFAULT VLANT ; INP  C R

to initialize the VLANT inputs and review them.

> INDI n; GETN m  C R

to select the data set on disk n and catalog number m.

> GO  C R

to run the task, writing CL table 2.

We have had difficulty setting all of the delays in the EVLA to values which are sufficiently accurate. If the delay is not set correctly, the interferometer phase will vary linearly with frequency, potentially wrapping through several turns of phase within a single spectral window (“IF band”). We hope that bad delays will not arise in future, allowing you to skip this section, but use POSSM to check for phase slopes. However, even very modest delay errors may matter over the very wide bandwidths of the EVLA. Note that FRING computes its values around the center of each spectral window but then writes a phase correction appropriate to the frequency reference pixel. Use IMHEADER to see what this is in your data set. Traditionally it has been 1.0, but Nchan2 + 1 is a better choice. New task CENTR, or numerous other tasks with adverb FQCENTER, will convert your data set to this reference. Note that this conversion requires re-scaling the u,v,w values and adjusting some FQ table values; it is not just a simple change in the header.

Delay errors are problem familiar to VLBI users and AIPS has a well-tested method to correct the problem. Using your LISTR output, select a time range of about one minute toward the end of a scan on a strong point-source calibrator, usually your bandpass calibrator. Then

> DEFAULT FRING ; INP  C R

to initialize the FRING inputs and review them.

> INDI n; GETN m  C R

to select the data set on disk n and catalog number m.

> SOLINT 1.05 * x  C R

to set the averaging interval in minutes slightly longer than the data interval (x) selected.

> TIMERANG db , hb , mb , sb , de, he , me , se  C R

 

to specify the beginning day, hour, minute, and second and ending day, hour, minute, and second (wrt REFDATE) of the data to be included. Too much data will cause trouble.

> DPARM(9) = 1  C R

to fit only delay, not rate. This is very important.

> DPARM(4) = t  C R

to help the task out by telling it the integration time t in seconds. Oddities in data sample times may cause FRING to get a very wrong integration time otherwise.

> INP  C R

to check the voluminous inputs.

> GO

to run the task, writing SN table 1 with delays for each antenna, IF, and polarization.

The different IFs in current EVLA data sets may come from different basebands and therefore have different residual delays. The option APARM(5) = 3 to force the first Nif2 IFs to have one delay solution while the second half of the IFs has another is strongly recommended, but only when the first half all come from one of the “AC” or “BD” basebands (hardware IFs) and the second half come from the other. The 3-bit data path of the EVLA actually has four hardware IFs, so APARM(5) = 4 produces four delay solutions, dividing the IFs in quarters. Note that, at low frequencies, the phases may also be affected by dispersion (phase differences proportional to wavelength). FRING now offers APARM(10) to enable solving for a single delay plus dispersion from the fitted single-IF delays. This SN table will need to be applied to the main CL table created by INDXR or OBIT.

> TASK CLCAL’ ; INP  C R

to look at the necessary inputs.

> TIMERANG 0  C R

to reset the time range.

> GAINUSE 0 ; GAINVER 0  C R

to select the highest CL table as input and write one higher as output (version 2 and 3, resp. in this case).

> SNVER 1 ; INVER 1  C R

to use only the SN table just created.

> INP  C R

to review the inputs.

> GO  C R

to make an updated calibration table.

Be sure to apply this (or higher) CL table with DOCALIB 1 in all later steps.