| EXTRACT-QUADRANT | Form a new spectrum from nominated quadrant/sector |
| MERGE-QUADRANTS | Merge sectors/quadrants to form single spectrum |
| SET-QUADRANT-DISPLAY | Nominate which sectors/quadrants are to be displayed |
| SLIDE-QUADRANT | Average two halves of one quadrant (for 12 m data) |
| CONCATENATE-SPECTRA | Make X and Y spectra into separate quadrants in X. |
As mentioned previously, SPECX contains facilities for handling several spectra under one header. This is useful for example when observing with an autocorrelation receiver which typically has several `quadrants' or `sectors' centred on different frequencies, when observing in two polarizations at once, or when observing simultaneously with two or more different spectral resolutions. Different observatories use different words to describe this system -- at NRAO Kitt Peak they refer to different `Receivers', while historically the different channels of an autocorrelation backend have generally been referred to as `Quadrants' or `Sectors'. We use `quadrants' here to describe the general case.
SET-QUADRANT-DISPLAY allows you to control which quadrants are displayed by commands such as LIST-SPECTRUM, PLOT etc.-- it also controls to some degree which quadrants are affected by various reduction operations such as baseline fitting. Enter a 0 in a particular position to turn that quadrant off -- otherwise a 1 to leave it activated for processing. The current quadrant masking function can be determined by typing SET-QUADRANT-DISPLAY, and then accepting the default again.
A single quadrant may be extracted from a multi-quadrant spectrum by the command EXTRACT-QUADRANT. This replaces the multi-quadrant spectrum by a single-quadrant spectrum in the current array.
For some special operations, such as baseline removal, SPECX requires that only a single quadrant be reduced at a time. The program attempts to deduce which quadrant is required from the masking function and the information in the spectrum header, according to the following rules.
For data such as that obtained from the autocorrelation backend it is often desirable to be able to produce a single spectrum from a number of overlapping spectra, which have identical parameters except for their centre frequency. The function MERGE-QUADRANTS is supplied to knit the individual spectra into a single spectrum, using a chi-square fitting procedure and interpolation to the sampling of the `centre' quadrant as defined by the header variable IQCEN.
From time to time it may be useful to take two separate spectra, possibly with different but overlapping frequency coverage, and incorporate them into a single spectrum. CONCATENATE-SPECTRA takes the separate quadrants/sectors of the X spectrum and adds them as high-order sectors in the current Y-spectrum. The command fails if the number of sectors or total data length exceed those in force at the time. Spectra concatenated in this way can then be MERGE-d to form a single spectrum, provided that they have the same (telluric) frequency increment: if not one can be REGRIDded before concatenating.
The final command, SLIDE-QUADRANT, does not quite belong here, but it doesn't belong anywhere else either. This is designed explicitly for operating on dual-polarization data from Kitt Peak, which comes in the form of two lines in one receiver passband. SLIDE overlaps the left half of the receiver on the right half, and edits the centre velocity to make it right, thus averaging the two polarizations.