polarization, ca, cb, ... Select a polarization and ranges of spectral-line channels to be processed. EXAMPLES -------- 1. Select visibilities of polarization I over two inclusive spectral-line channel ranges. 0>select I, 20, 25, 47, 65 Selecting polarization: I, channels: 20..25 47..65 Reading IF 1 channels: 20..25 47..65 0> 2. Select continuum channels either side of a spectral-line feature that occupies the range 21 to 60: 0>select I, 1, 20, 60, nif*nchan Selecting polarization: I, channels: 1..20 60..112 Reading IF 1 channels: 1..20 60..112 0> 3. Select channel ranges that span more than one IF. The following example is based on an observation that has three IFs, each of which has 64 channels. 0>select i,50,70 Selecting polarization: I, channels: 50..70 Reading IF 1 channels: 50..64 Reading IF 2 channels: 65..70 Reading IF 3 channels: (none) 0> 4. Channel ranges can be given in any order and can overlap. The following example shows that the select command re-organizes ranges into non-overlapping ranges. 0>select I, 23,45, 60,70, 25,30, 51,53, 40,50 Selecting polarization: I, channels: 23..53 60..70 Reading IF 1 channels: 23..53 60..70 0> 5. If the last range given is unpaired, then this denotes a range consisting of a single channel. 0>select I,10 Selecting polarization: I, channels: 10..10 Reading IF 1 channels: 10..10 0> ARGUMENTS --------- polarization - (Default = The polarization last selected, or on the first call, either I if available, or the first recorded polarization. The name of a polarization that was recorded in the UV FITS file, or one that can be derived from other recorded polarizations. Use the 'header' command to see which polarizations were recorded. Further details are provided below. ca, cb, ... - Default = The ranges of a previous selection, or all channels on the first call. One or more ranges of spectral-line channels from one or more IFs are specified by giving the indexes of the first and last channels of each range, using channel indexes taken from the set [1..nif*nchan]. If an odd number of channel indexes is specified, the last index is taken to represent a range that contains a single channel. The select command sorts channel ranges into ascending order and coalesces overlapping and contiguous ranges. CONTEXT ------- In Difmap, one combination of stokes parameters and spectral-line channels is mapped at a time. This combination is known as the "current processing stream". The 'select' command allows you to specify the scope of this stream in terms of the polarization to be processed and one or more ranges of spectral-line channels. Once such a selection has been made, most difmap commands operate on this data, rather than the raw data from the UV FITS file. However edits, and 'selfcal' and 'resoff' calibrations are not restricted to the selected data, and are applied both to the raw data set and to all future selections. For example, to map a spectral-line data-set, one would typically select one or more channels containing strong line-emmission from a single IF. This would then be mapped, and in the course of doing so, self-calibration would be applied. The corrections accumulated for the channels of the selected IF would thereafter be applied to all other channel selections from that IF, unless the corrections were cleared with the 'uncal' command. Furthermore, future corrections, based on other spectral-line channels of the same IF, would build upon the existing corrections. This ensures that the relative positions of features in different channels are preserved. Note that the 'specplot' command is independent of the current processing stream selection. This makes it a convenient way to see what channels to select before using the 'select' command. See also the spectral_line help topic. POLARIZATION SELECTION ---------------------- The polarizations recorded in the originating UV FITS file can be displayed with the 'header' command. The polarization argument may either be one of these polarizations, or be the name of a polarization that can be derived from them. The names of recognised polarizations are: I Q U V RR LL RL LR XX YY XY YX The following is a list of polarizations that may be contructed from other polarizations: Name Derived from: Derived as: I RR and LL (RR + LL)/2 Q RL and LR (RL + LR)/2 U RL and LR (LR + i.RL)/2 V RR and LL (RR - LL)/2 CHANNEL RANGES (and IF ranges) ------------------------------ The channel numbers that the 'select' command understands start at 1, and end with the total number of channels in the observation. When there are multiple IFs, the latter is the sum of the number of channels in all IFs. For specification purposes, channel numbers increase seamlessly from the first IF to the last IF, regardless of the relationship between neighboring IFs and the sign of their inter-channel frequency increments. This is the same convention used by the 'specplot' command, so if you are confused by this, use the 'specplot' command to look at the assignments of channels to IFs. For example, consider an observation that has two IFs, each with 64 channels. In the example, the 'header' command shows the following assignment of frequencies to the IFs: There are 2 IFs, and a total of 128 channels: IF Channel Frequency Freq offset Number of Overall IF origin at origin per channel channels bandwidth ------------------------------------------------------------- (Hz) 01 1 1.53575e+10 125000 64 8e+06 02 65 1.53655e+10 125000 64 8e+06 In this example channel number 1 refers to the first channel of IF 1 and has a center frequency of 1.53655e+10 Hz, whereas channel number 65 refers to the first channel of IF 2 and has a center frequency of 1.53575e+10 Hz. Note that when no channels are selected from a given IF, that IF is said to be unsampled, and it will not be processed, apart from the application of global editing. This is essential for spectral-line observations, but can also be used in continuum observations if different IFs must be mapped separately. Note that continuum observations are treated as spectral-line observations with only one channel per IF. In such cases, the channel numbers are equivalent to IF indexes, and one can then select ranges of IF indexes directly. IMPLEMENTATION DETAILS ---------------------- The 'select' command extracts the requested data from a scratch file called uvdata.scr that contains all the raw UV data from the last UV FITS file that was read. For each baseline of each integration, 'select' derives a single visibility per IF. This is formed by vector averaging visibilities of the given polarization, over the given range of spectral-line channels. These visibilities are then stored in another scratch file called ifdata.scr, from which one IF at a time is temporarily paged into memory for display or other processing. RELATED HELP TOPICS ------------------- spectral_line - An overview of spectral-line mapping in Difmap. multi_IF - An overview of multi-IF mapping in Difmap. RELATED COMMANDS ---------------- observe - Read a random-groups UV FITS file. header - Use this to see what polarizations, IFs and spectral-line channels are available. specplot - This plots visibility spectra, and provides a convenient way to see what channel ranges to select, based upon the location of spectral features.