To grid the data with MOPSI
1,
one first needs to subtract baselines of all
spectra in CLASS and possibly correct for ripples etc.
The reduced spectra then need to be written into Gildas Table Format (gtf)
using the ANALYSE
GRID command in CLASS (see previous section for
details; you can omit the /IMAGE option since only the ``gtf'' files are
needed for MOPSI).
The ``GTF'' files are read in MOPSI with the command ``READ <Filename>.gtf''. It will try to read all channels simultaneously. If you run into trouble with memory, you can also specify just a subset of channels using the ``PLANE p1 p2'' option (i.e. the command is ``READ <Filename>.gtf PLANE p1 p2'').
The gridding command in MOPSI is called RANDOM. In its simplest form it accepts two parameters to define the pixel size of the gridded map and another two parameters to define the convolution radii in x and y. A typical command for 345 GHz data would be
RANDOM 2 2/NEI 11 11
which defines a pixel size of 2''
2'' and a convolution radius
of 11'' (about half the beam size) in each direction.
After gridding the data, the PLOT command can display the channel maps or integrated map (use DEVICE IMAGE BLACK or WHITE to get a display window). Note that currently the x axis runs from the left to the right. This is probably a bug in MOPSI and may be corrected later. You can reverse the axis in GREG or GRAPHIC using
LIMITS/RG/REV X.
To go on with the data reduction you can use MOPSI if you are familiar with it (it also contains GREG) or you can write the data to a Gildas Data File (GDF) using the MOPSI command
WRITE <Filename>.gdf
(the ``.gdf'' extension is necessary to tell MOPSI to write GDF format). The resulting GDF file can then be read e.g. in GRAPHIC for further plotting etc. WARNING: The current version of MOPSI (10/1998) has a bug that causes a velocity shift of a few km/s when writing data cubes to the GDF format. We therefore recommend using the GRID command in CLASS to reduce spectral line OTF maps (see previous section).