E.3 Initial editing

You should use the tools below to flag out obviously bad data. The tasks which automatically flag data for you, however, depend on meaningful amplitudes and flat spectral shapes. Therefore, flagging and calibration are an iterative process. Do the obvious flagging without spending a lot of time on it. Then do an initial calibration of bandpass, amplitude, and phase. Use that calibration to run RFLAG and/or other auto-flagging tasks. Then throw away all CL tables following VLANT and begin again with FRING, BPASS, CALIB, etc.

There will be data validity information prepared both by the on-line control software and by the WIDAR correlator and most of this information is available as an initial flag table. The tasks above will have applied this table for you by default since FLAGVER 0. On-line flags may already have caused data to be flagged within your data set (but not deleted) by CASA. Unfortunately, a flag table is present only via the OBIT route. UVFLG will be needed to add flags for shadowing (APARM(5)=25 or so) to a flag table. We still need to look at the data to flag out whatever remains of the time off source not flagged using on-line flagging information. There have also been drop outs in which the visibility is pure zero, typically for all channels and IFs and a single integration. The drop outs should now be handled by OBIT, UVLOD, and FITLD. Note, however, that CASA and FITLD pass along all data samples, including those that are fully flagged. This makes the data set rather larger than one might wish. Use UVCOP (or TYAPL — see E.6) to remove all fully flagged data samples. Before doing this, use TVFLG (4.4.3) to look for any more data samples that might need to be flagged fully. Check especially samples at the beginnings and ends of scans. Try


to reset all adverbs and choose the task.

> INDI n; GETN m  C R

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

> DOCAL 1 ; DOBAND -1  C R

to apply the delay calibration. If a bandpass has been determined, use DOBAND 3 or 1 to apply it.

> BCHAN c1 ; ECHAN c2  C R

to average across a range of channels — not as flexible as ICHANSEL but probably okay here.


to average all the channels into one number.

> BIF j ; EIF BIF  C R

to edit one IF only, which will suffice for problems that are not IF dependent, such as drop outs, antenna not on source, etc. Choose an IF that is reasonably free of RFI.

> CALCODE ’*’  C R

to do just calibrators for the moment.

> DPARM(6) Δ t  C R

to do no time averaging in the work file set Δ t to the data interval in seconds.

> GO  C R

to start the task.

The default smoothing time shown in the display will probably be some multiple of Δ t. Change the smoothing time to one times the basic interval in order to edit in detail; TVFLG will break up your data into overlapping “groups” which you may edit is sequence. Remember to change the initial setup so that the flags apply to all channels and all IFs. See 4.4.3 for more information. The EVLA has shown a tendency to produce periods of data which are too low in amplitude to be normal noise, but which are not zero. Use TVFIDDLE or TVTRANSF functions to enhance the brightness of the amplitudes to make sure that apparently black regions really are black (flagged already).

We note here that some users feel that the data need to be inspected more carefully than with just an average of most of the channels. POSSM (below) may be of use to find RFI. Avoiding the worst of that, you may still wish to run TVFLG to look at the average of a few channels at a time. Use NCHAV and CHINC appropriately. Task SPFLG (10.2.2) is the ultimate weapon when looking for channel-dependent difficulties and is even capable of normalizing by source flux including spectral index. However, it is onerous when there are many baselines. FTFLG is similar to SPFLG but displays all baselines together. It would be a quick way to look for wide-spread RFI. These more onerous tools should probably not be used at this preliminary stage; use them after some of the auto-flagging tasks have been run.