CLASS OTF Data Gridding

In CLASS you need to select the source, line, backend and offsets to get a list of observations for the OTF map. Subtract baselines using the BASE command in a FOR NEXT loop and write the spectra to a new output file, e.g.:

FIND/ALL

FOR I 1 TO FOUND

GET NEXT

BASE 1

WRITE

NEXT

At the SMT site, you can access your data by logging into jever or hannen as user smtobs and using the ``spectra'' symbol (see note on the monitors in the control room).

After the baseline subtraction, the spectra need to be processed with the ANALYSE $\backslash$GRID command. It contains a gridding algorithm that can be accessed with the /IMAGE option. It is possible to write all individual channels to get a data cube. The command for this is

GRID <Filename>.gtf NEW /CHANNEL V v$\rm\sf _1$ v$\rm\sf _2$/IMAGE BEAM <beam>

where v$\rm\sf _1$ and v$\rm\sf _2$ define the velocity interval to be used and ``beam'' is the beam size in arc-seconds. The filename should have ``.gtf'' as extension to facilitate reading it in MOPSI (see next section). The above command produces two files: <Filename>.gtf and <Filename>.lmv. The ``gtf'' (Gildas Data Format) files contain the spectra as binary tables. The ``lmv'' files contain the gridded cube in Gildas Data Format which can be read with IMAGE in GRAPHIC. They can also be processed with the GRAPHIC imaging tasks (see EXPLAIN in GRAPHIC).

For a quick look at the data while mapping, one can use the ``TDV'' function which allows to integrate a part of the spectrum and write the area to the ``gtf'' and ``lmv'' files. The syntax for this is

GRID <Filename>.gtf NEW TDV(v$\rm\sf _1$,v$\rm\sf _2$)/IMAGE BEAM <beam>

These GRID commands produce data cubes or images with half beam sized pixels. This is quite coarse. One can tell GRID to resample to a smaller pixel size using the LAM and BET options. You need to know the size of the map to correctly specify the arguments. For example, if the map is 240''$\times$ 120 '' centered on (0'',0''), the GRID command to get e.g. 2'' sized pixels would be

GRID <Filename>.gtf NEW TDV(v$\rm\sf _1$,v$\rm\sf _2$)/IMAGE BEAM <beam> LAM 100 1 -120 2 BET 50 1 -60 2

which means that the final map is 100 $\times$ 50 pixels large. The second number is the reference pixel, the third is the offset at the reference pixel and the fourth number is the pixel size (all in arc-seconds). Please see HELP GRID in CLASS for a detailed description of these parameters.

The ``lmv'' data cubes or images can be displayed in GRAPHIC using the IMAGE command. A typical sequence would be

IMAGE <Filename>.lmv

LIMITS/RGD

PLOT

BOX/U S

WEDGE

You can also use the INPUT and GO BIT or GO MAP procedures to make channel maps of your data (see GREG and GILDAS manuals).