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Spectral Line Processing and RPFITS files

When using atlod to read in spectral line data, there are a few extra parameters that should be considered

ifsel:

The ATCA allows you to observe two frequency setups simultaneouslyifsel allows you to load just one of them by setting ifsel=1 or ifsel=2. The default is to load both of them.

restfreq:

The RPFITS file does not contain the rest frequency of the spectral line observed, so you need to give this at some stage during data reduction - the earlier the better. The rest frequency can be entered (via restfreq, in GHz) in atlod.

There are a few options in Miriad's atlod of particular interest to spectral line observers:

bary:

By default, task atlod uses the LSR as the rest frame when computing velocity information. However you can change this to the barycentre by using options=bary. Note that LSR velocities are the standard for Galactic astronomy, whereas barycentric are more commonly used by extragalactic astronomers. If you load with the wrong velocity reference frame, you can change this later with uvredo (for visibility datasets) or velsw (for images).

birdie:

The ATCA suffers self-interference at frequencies which are multiples of 128 MHz (e.g. 1408 MHz). For spectral line modes, the birdie option flags any channels that are affected by the self-interference. This is strongly recommended.

hanning:

This option performs Hanning smoothing of the spectra, and discards every second channel.

compress:

This causes the data to be written in a scaled 16-bit format, thus reducing disk usage by a factor of two.

noif:

If you are loading the ATCA's two frequency setups simultaneously, atlod normally tries to stack the two together as ``spectral windows'' (IFs in AIPS terminology). However this will not be possible if the two setups sample different sets of polarisation parameters. In this case, you will need to use options=noif to get atlod to store the different frequency setups as different visibilities.