A class of methods in which continuum estimation and subtraction is done before convolution. Dirty map subtraction: subtract a pseudo-continuum dirty map and beam (made by combining line-free channels with SQASH and COMB) from the channel dirty maps. Then use APCLN with the original dirty beam.
The UVLIN technique is similar to UVBAS, but more flexible and with a better fitting routine. There are two versions of this algorithm. UVLIN performs fitting in the uv plane; IMLIN performs it in the image plane. Unlike UVBAS, phases are not fit. IMLIN and UVLIN should give identical results. Both can be improved by using UVSUB to pre-subtract the brightest sources and increase the field of view. Use MX with NITER 0 and IMLIN with TRANS TRANSCOD '312'.
UVBAS (visibility-based subtraction) averages uv data in two ranges, then subtracts this pseudo-continuum from each channel in uv space. It performs a fit of the amplitude and phases. Unfortunately, these are nonlinear operations which couple errors on all sources and produce an amplitude bias at low SNR. Tim Cornwell developed UVLIN and UVBAS to provide a better fitting routine. UVBAS can only work if the uv coverage is the same for all spectral line channels, but it can deal with frequency-dependent flagging. It is much faster than CLEANing a dirty pseudo-continuum, then using UVSUB. Finally, UVBAS will not work on a field dominated by discrete sources.
References
AIPS software package HELP files for UVBAS and UVLIN.
Cornwell, T. J.; Uson, J. M.; and Haddad, N. "Radio-Interferometric Imaging of Spectral Lines: The Problem of Continuum Subtraction." Astron. Astrophys. 258, 583-590, 1992.
van Langevelde, H. J. and Cotton, W. D. "Visibility-Based Continuum Subtraction in Spectral Line Observations with Radio Synthesis Telescopes." Astron. Astrophys. 239, L5, 1990.
