When using atlod
to read in spectral line data, there are a few
extra parameters that should be considered
Spectral Line Processing and RPFITS files
- The ATCA allows you to observe two frequency setups
ifsel allows you to load just one of them by
ifsel=2. The default is to load both
- 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
There are a few options in Miriad's atlod
interest to spectral line observers:
- 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
(for visibility datasets) or velsw
- The ATCA suffers self-interference at frequencies
which are multiples of 128 MHz (e.g. 1408 MHz). For spectral line
birdie option flags any channels that are affected
by the self-interference. This is strongly recommended.
- This option performs Hanning
smoothing of the
spectra, and discards every second channel.
- This causes the data to be written in a scaled 16-bit format,
thus reducing disk usage by a factor of two.
- If you are loading the ATCA's two frequency setups
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
to store the different frequency setups as