General discussion of editing in difmap. EDITING VISIBILITY DATA ----------------------- It is rare to find an interferometer data set that is not contaminated with interference, correlator errors or other unwanted observational artifacts. So it is important to be able to flag selected parts of the data to be ignored by future processing, and to be able to undo such flags if it becomes apparent that prior editing was unwarranted. In Difmap there are currently three types of flagging. Deleted data. ------------ Visibilities that have been given zero weight, and visibilities which do not appear in the input UV FITS file are regarded as "deleted". These visibilities are not displayed in any plots. None of the commands in Difmap change good or flagged data to deleted data, and such data can only come from input UV FITS files. Similarly, there is no way to undelete such data. Visibility flags. ---------------- Individual visibilities can be interactively flagged or unflagged in a number of difmap plot commands, such as vplot, tplot, radplot, projplot and uvplot. When flagged visibilities are displayed, they are plotted in red. When unflagged (good) visibilities are plotted, they are plotted in green. Correction flags. ---------------- Telescope based self-calibration corrections consist of three parts. A phase correction, an amplitude correction and a correction flag. Initially the phase correction is set to zero, the amplitude correction to unity and the flag is not set. To fill in ampitude and phase corrections the 'selfcal' command must be used. This determines new incremental telescope corrections based on the input visibilities and a model. Where there is insufficient information to determine a correction, the correction-flag member is set. When self-calibration corrections are applied, if a correction is flagged, the flag is also applied to all affected visibilities. These correction flags are recorded in addition to other visibility flags and can be removed without affecting normal visibility flags by running the 'uncalib' command, or by interactively toggling them in the 'corplot' command. In plot commands, visibilities that are flagged due to self-calibration are displayed in blue, to differentiate them from good visibilities plotted in green and otherwise flagged visibilities that are plotted in red. Other commands don't differentiate between the flag types, and thus any visibilities that are flagged for any reason are ignored. TELESCOPE VERSUS BASELINE EDITING. --------------------------------- Telescope based editing. ----------------------- Many observational artifacts are telescope based. For instance loss of cooling and other receiver problems are always telescope based, whereas external interference will appear as telescope based problems for VLBI and VLBA work, but as array-wide problems for compact arrays such as the VLA. The 'vplot' command groups baseline plots by telescope. This gives one the opportunity to determine whether a problem is telescope based, and to home in on the bad telescope. Telescope highlighting in the 'radplot' command can also be used to search out bad telescope based artifacts. When such problems are identified, one should display all baselines of the bad telescope in 'vplot' and select the default telescope-based editing mode before editing. Baseline editing. ---------------- Fortunately, very few observational artifacts appear as baseline based errors. This is fortunate because self-calibration is unable to remove such problems, and correcting them with resoff is a bad idea. Known ways in which baseline errors can be introduced, are correlator problems (particularly analogue correlators), telescope-shadowing and poorly matched receiver bandpasses. If you intend to use self-calibration then it is always best to use telescope based editing where pertinent. This is because self-calibration can get confused by incomplete sampling of a given telescope. The 'selfcal' command attempts to detect such cases and flag them rather than make a correction, but there are some cases that it is not able to detect. One such case is where a telescope is sampled by two effectively disconnected arrays, because visibilities that would have connected the two arrays have been flagged. In such cases there are two possible telescope correction solutions. Multi-IF editing scope ---------------------- In plot commands that individually show data of all IFs on a single plot, edits are applied only to the source IFs of the selected points. These include the following commands: radplot - Plot visibility amplitudes and/or phases versus UV radius. uvplot - Plot observed UV points on the UV plane. In plot commands that by their nature can only plot data from one IF at time, edits are by default extended over all IFs (except when restricted by channel-range selections as described below). To optionally constrain edits to only be applied to the displayed IF, all of these commands provide a toggleable option, bound to the 'I' key. The relevant commands are: cpplot - Interactive display and editing of observed and model closure phases. projplot - Plot visibility amplitudes and/or phases versus projected UV distance. radplot - Plot visibility amplitudes and/or phases versus UV radius. tplot - Plot time sampling for each telescope of an observation. vplot - Plot visibility amplitudes and phases versus time. Polarization editing. -------------------- In Difmap all edits applied to one polarization are always applied to all other recorded polarizations. However, be warned that unflagging a visibility which has one or more polarizations deleted, can not unflag deleted visibilities, and will thus result in different UV plane sampling in different polarizations. For the most part this is irrelevant, since the select command will not attempt to derive polarizations from combined stokes parameters if one of the stokes parameter visibilities is flagged or deleted, but if you are making maps of the individual stokes parameters to later be turned into percentage polarization maps, the results might be adversely effected. Multi-channel editing scope --------------------------- By default visibility edits are applied to all spectral line channels, but all plot commands that allow editing, allow one to constrain edits to the ranges of channels selected with the 'select' command. This option is toggled with the 'W' key. Self-calibration correction flag scope. -------------------------------------- A single self-calibration flag is always effectively applied to all spectral-line channels and polarizations of a single IF, so the 'corplot' command does not have either the 'I' or 'W' options provided for normal visibility editting.