National Radio
Astronomy Observatory

5.3.4.3 Sub-parameter ftmachine

This sub-parameter is activated for imagermode=’mosaic’. Note: The actual “ftmachine” used may be overridden by choices made to other parameters, such as gridmode.

The ftmachine parameter controls the gridding method and kernel to be used to make the image. A string value type is expected. Choices are: ’ft’, ’sd’, ’both’, or ’mosaic’ (the default).

The ’ft’ option uses the standard gridding kernel (as used in clean).

The ’sd’ option forces gridding as in single-dish data.

For combining single-dish and interferometer MS in the imaging, the ’both’ option will allow clean to choose the ‘ft’ or ’sd’ machines as appropriate for the data.

The ’mosaic’ option (the default) uses the Fourier transform of the primary beam (the aperture cross-correlation function in the uv-plane) as the gridding kernel. This allows the data from the multiple fields to be gridded down to a single uv-plane, with a significant speed-up in performance in most (non-memory limited) cases. The effect of this extra convolution is an additional multiplication (apodization) by the primary beam in the image plane. This can be corrected for, but does result in an image with optimal signal to noise ratio across it.

The primary beams used in CASA are described in § 5.2.13.

ALERT: Note that making a non-square image (e.g. using unequal values in imsize) for ftmachine=’mosaic’ will grid the data into a uv-plane with correspondingly non-square cells. This has not been extensively tested, and may results in undesired image artifacts. We recommend that the user make square mosaic images when possible, but in principle non-square images should work.

More information about CASA may be found at the CASA web page

Copyright © 2010 Associated Universities Inc., Washington, D.C.

This code is available under the terms of the GNU General Public Lincense