When using a single spectral window, and observing spectral lines, the
bandwidth of the ATCA can be problematically narrow, or the channel
widths can be too coarse. This is most likely to be an issue when observing
spectral lines of extragalactic sources at high frequencies.
A partial work around for this situation is to use the two ATCA
frequency bands to observe in adjacent somewhat overlapping windows, and using the
same bandwidths and channel counts for the two windows. With the ATCA this
is possible with some correlator configurations when observing with
bandwidths of 16, 64 or 128 MHz. Miriad allows you to stitch these
two spectral windows together in a reasonably straightforward fashion (although
some quirks do become apparent). In
so doing, it handles any overlap region between the two windows in a sensible
fashion. The steps to achieve this are as follows:
- Make sure you set a rest frequency in atlod
(or during the
subsequent processing). If you really want to view the spectrum in
frequency (rather than velocity), then you might set a dummy rest frequency
as the average of the centre frequencies of the two spectral windows.
- During the flagging process, flag any channels in the overlap region
between the two spectral windows which you do not wish to contribute
to the final output stitched spectrum. These would be channels
where the bandpass gain is quite low in one, but reasonable in
another band. Task uvflag's
parameter is the easiest way to flag these channels.
- Perform the calibration as described above, keeping both bands
within the one file.
- Subtract any continuum, using task uvlin
as normal. Task
handles multiple spectral windows within a single dataset.
processes each frequency band separately.
You will want to give some thought when setting the range of
channels to use in the fitting of the continuum.
spectral of interest in near the edge of the individual spectra, you will want
to set the channels to use in uvlin's fitting process carefully. You
may wish to use a zeroth order fit, and just fit using channels at the
edges away from the overlap region. Alternatively you may wish to run
after the two spectra are stitched together instead. Some
experimentation (and certainly thought) will be required.
when handling multiple bands, both Miriad's
line and uvlin's
chans parmaters number channels consecutively from 1 to the total
number of simultaneous channels. The distinction between simultaneous bands is
ignored in this numbering. But when fitting to the continuum, the
boundaries between bands act as a break point.
- It is nearly time to stitch the two spectral windows together.
To do this you will need to use the ``velocity linetype''
One of Miriad's quirks is that you cannot apply on-the-fly
and use a ``velocity linetype'' within the same task. You will need
to make a copy of the dataset with the bandpass (and presumably other
calibration) applied directly to the data. If you have used
uvlin, you will have done this already. If not, create a copy
of your dataset using uvaver.
- With a dataset with calibration applied, you can now stitch the two
spectral windows together.
See Sections 5.4 and 16.8 for information on
using velocity linetypes.
Do the copying and linetype conversion with uvaver
(or possible uvlin
if you have not already used it) to generate a new stitched
copy of the data.
By using a velocity linetype, when creating an output spectrum,
Miriad copies channels by their velocity. Any channels that
overlap between the two spectral windows are averaged together.