[evlatests] A remarkable result at L-band

[Bob Sault]

Rick,

The mirror sources could be Gibbs-like phenomena
(Bos, Indirect Imaging Workshop proceedings, 1984).
WIDAR could certainly be prone to these sorts of effects.
Changing the lag weighting can be used to eliminate Gibbs
like phenomena in continuum imaging. It's plausible that
the pseudo-continuum also averages out the problem.

Best regards
Bob

--
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Phone: +61-3-83447074
Email:   rsault at unimelb.edu.au
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> -----Original Message-----
> From: evlatests-bounces at donar.cv.nrao.edu [mailto:evlatests-bounces at donar.cv.nrao.edu] On Behalf Of
> Rick Perley
> Sent: Thursday, 30 July 2009 8:59 AM
> To: evlatests at aoc.nrao.edu
> Subject: [evlatests] A remarkable result at L-band
> 
>     Michael observed the standard calibrator 3C286 at L-band, using 11
> EVLA antennas with WIDAR.  The four subbands, each of 128 MHz BW were
> adjacent, the lower edge of subband 1 was set to 1300 MHz.  3C286 is an
> excellent test source -- 14 Jy unresolved, with 500 mJy in background
> objects easily visible within the primary beam.
> 
>     Subbands 1 (1300 -- 1428) and 4 (1684 -- 1812 MHz) are nearly free
> of interference, so I concentrated on these.    The calibration method
> was as follows:
> 
>     1) Basic editing to remove data taken while slewing, and antennas
> which mispointed due to power glitches, etc.
>     2) FRING was run on a short piece of data to determine the basic
> delay errors (typically 20 nsec).
>     3) CLCOR is used to apply these.
>     4) A basic calibration was done, using a subset of channels which
> are free of RFI in all four subbands.   This was done to set the
> intensity level for the clipping stage.
>     5) SPFLG was used to view the spectra (an amazing sight, to wondrous
> to describe briefly here, due to the beating between the RFI and the
> source).   Roaming about the image permitted a clip level to be set.
>     6) CLIP is run to remove strong RFI -- the upper level was set to 25
> Jy, the lower level 0.001 (this needed because about 0.1% of the
> visibilities are perfectly zero).
>     7) BPASS was used to determine the mean bandpass.
>     8) Sub-bands 1 and 4 were then SPLITted out, averaging in 10 MHz
> 'chunks' (20 channels), with 12 'chunks' per sub-band.  This is needed
> to remove chromatic aberration in the image.
>     9) A point-source self-cal was run on these databases -- averaging
> the 12 'chunks' in each to obtain the solution.
> 
>     The resulting image shows the primary background sources quite well
> -- very encouraging.  But there's lots of rumble lying about, looking
> like the result of variable bandpasses.
>     I then tried using BPASS to remove variable bandpasses -- this
> provided a solution for each of the 12 'chuncks', using a solution time
> of 1 minute.   The program BPLOT shows small (few percent, few degrees)
> variations in these gains.
> 
>     The resulting 'final' image is very lovely, with many background
> sources and nice clean noise.
> 
>     HOWEVER:
> 
>     It is easily seen that every strong background source *** has a
> mirror image of 1/10 the strength *** present, exactly on the opposite
> side.  How can this be?  I can't see these mirrored images on the map
> made prior to the BPASS gain solution, but perhaps the sensitivity is
> poorer due to the errors.  Certainly these mirrored objects were not
> present in an image made with a pseudo-continuum database with 100 MHz
> effective bandwidth -- but on these, the background sources are quite
> attenuated due to chromatic aberration.
> 
> 
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