Task uvsector will flag a sector of visibility data with a given u-v position angle (or hour angle). While you can give the position angle of the sector to be flagged directly (via the angle parameter), it is more convenient to indicate the position angle indirectly via the stripe direction in an image. How do you specify a stripe direction? You do this by specifying a long, thin region in a image. Normally you will generate the region by using cgcurs (see Section 17.3 for more information on cgcurs). With cgcurs, you display a greyscale of the image, and then use a cursor to selection a long, thin, region containing the crest of a single stripe. The selected region is then written into a text file, cgcurs.region. While this may be a rather odd way to define a direction, it allows uvsector to use the interactive facilities of cgcurs.
Typical inputs to use cgcurs are
|Input image (probably CLEANed).
|Set the range to highlight the stripes.
|PGPLOT device - Xwindows here.
|Define a region of interest.
The width of the sector that uvsector flags is given as a angle (in degrees) via the width parameter. Remember that 1 degree is equivalent to 4 minutes of observing time, so do not set it to any value larger than you need. Normally you would set this to a few degrees, based on your confidence in the accuracy of the stripe direction. The default is 5 degrees.
As uvsector flags a sector, if your input visibility data-set contains multiple configurations, data within that sector for all configurations will be flagged by default. Generally this is not what you are likely to want. If you know which configuration contains the bad data, you can use the select keyword to ensure just that configuration is affected. Selecting by time is probably the easiest.
The breadth of a stripe also contains useful information - a glitch in just
the short baselines will cause a broad stripe, whereas a glitch in just the
long baselines will cause a narrow stripe separation. A glitch in just one
baseline will give a two dimensional sinusoid.
Given the separation
between stripes, you could make an approximate calculation of the corresponding
u-v radius of the bad data.
For a stripe separation (crest to crest) of x arcseconds, the corresponding
u-v radius is
|Visibility data to be flagged.
|Leave unset if defining position
|angle by a stripe direction.
|The image containing stripes,
|used only for its coordinate information.
|A long, thin, region along the crest
|of a stripe.
|Flag data over 3 degrees (12 minutes of time)
|Leave unset to flag all data in the sector,
|Flag data within the sector and between 8 to 12