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: http://www.atnf.csiro.au/computing/software/miriad/userguide/node94.html
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Before you calibrate, you must prepare your data. This consists of loading, flagging, perhaps converting to ``channel-0'' datasets, and then splitting.
Things to look out for while flagging are:
select=shadow(22.5)
in your uvflag
command.
Note that tracking errors should be distinguished from pointing errors. Tracking errors are the inability of the antenna drive system to follow the requested path. Pointing errors correspond to the difference between the position that the astronomer requests and the actual position. Tracking errors are caused by flaws in the drive servo system, whereas pointing errors are a sum of tracking errors and errors in the antenna pointing model.
Since October 2003, tracking errors have been saved in the Miriad dataset.
The uv variables corresponding to the rms and
maximum tracking
error in a cycle are axisrms
and axismax
.
Both the maximum and rms give two tracking error values for each antenna, nominally corresponding to the tracking error in the azimuth and elevation axes. However the ATCA on-line system produces a single composite value, which is replicated for the two axes in the dataset.
When the tracking error is less than , the ATCA on-line system believes that this error is sufficiently small, and thus considers the antenna to be tracking. However this tolerance is not generally good enough for 3-mm observations. To flag based on this criterion, use the pointing parameter that the select keyword accepts.
UVSPLIT | |
vis=multi.uvxy | The input dataset. |
select | Extra selection. |
For CABB data, or large spectral line data-sets, if disk space is low, it may be useful not to split off the program source. Rather you can split off the calibrators, determine the calibration tables from them, copy the calibration back to the multi-source file, and then image directly from the multi-source file. This way you avoid making a second copy of your program source data. For example, to avoid the source ``vela'' from being split off, use
UVSPLIT | |
vis=multi.uv | The input dataset. |
select=-source(vela) | Do not select vela data. |
There are a few things to look out for when using uvsplit. If you don't care about the files already extant in the current directory, you may use options=clobber to make uvsplit overwrite any files it wants to create.
If the parent file contains more than one frequency band at the same frequency, then uvsplit will output two datasets in the normal way, but these will have .1 and .2 etc. appended to the end of their filenames, indicating which frequency band the data has come from.
Further to that, sometimes it can be handy to operate on only one IF as an individual dataset, and this can be achieved by using options=nosource. This creates a dataset for each IF in the parent file, but will not split out the individual sources. A similar thing can be done to keep all IFs in each source file (options=nofreq).
UVSPLIT | |
vis=multi.uv | The input dataset. |
maxwidth=0.128 | The maximum bandwidth for each output dataset, in GHz. |
Splitting the data into smaller bandwidth chunks in order to more accurately calibrate it has been dubbed "divide and conquer".
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