Copying, Reordering and Regridding Images

Task regrid is smart enough to correctly handle different projection geometries, conversion between equatorial and galactic coordinates, equinox conversion between B1950 and J2000, as well as conversions between different velocity systems (radio vs optical, LSR vs barycentric).

If the template does not have quite the geometric grid that you want, then regrid provides a number of options to fiddle what it treats as the template coordinate system before it does the regridding. Option galeqsw causes the template to be switched from galactic to equatorial, or visa versa before the regridding operation, whereas the option equisw switches the equinox from B1950 to J2000 (or visa versa). The project keyword can be used to reset the projection geometry of the template. The noscale option causes the template coordinate system's cell size not to scale with frequency (the images made by invert are such that the cell size in inverse proportional to frequency).

Normally regrid makes the output exactly overlay the template or the axis descriptors. However, if you really only want them in the same coordinate system, with potentially different image sizes and offset perhaps by an integral number of pixels, then use options=offset.

Task regrid can be used to resample an image on its own. You would use this to set a different projection geometry, to switch between galactic and equatorial coordinates, or to change equinoxes. If both the template and axis descriptor are unset, regrid uses the main input as the template. To do this use options=offset along with the other options or parameters to change the template coordinate system.

There is a potential trap when regridding ATCA images to equatorial coordinates: ATCA images produced by invert are in the so-called ``NCP'' projection. As this projection has a singularity at the equator, it is generally disastrous to regrid an ATCA image which is near or on the galactic equator without also changing the projection. The ``SIN'' (sine) projection is a reasonable choice for this.

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