[daip] CMETHOD in IMAGR

[Lynn D. Matthews]

Thanks for tracking down the filtering problem in IMAGR; will report back
tomorrow if there are any further difficulties.

Meanwhile, my colleague Ciriaco Goddi has been imaging large fields (8192)
using the old (2005) version of AIPS. In the course of investigating why
certain channels of the data cube had such hideous sidelobes, we
discovered that forcing IMAGR to use either CMETHOD='DFT' or 'GRID' both
produced good images (DFT somewhat better than GRID). However, letting
IMAGR to its own devices (i.e., CMETHOD='') resulted in ugly sidelobes and
much higher noise. A sample comparison of the results of IMSTAT for a box
near a bright source are given below.

Since both NEW and TST aips now force a DFT for large images, this may now
be a non-issue. However, I don't understand things well enough to know
whether for smaller images with sources near the map edge, the "hybrid"
approach that IMAGR defaults to in NEW and TST might still in some cases
produce inferior cleaning. I'm also wondering if it's worth re-imaging
some of the large cubes I produced earlier last year, before the "Forced
DFT" was implemented.

Lynn

CMETHOD=''

AIPS 1: Mean=-6.9370E-04 rms= 4.7006E-02 JY/BEAM   over  403840. pixels
AIPS 1: Maximum= 2.5575E-01 at  4049  1920    1    1    1    1    1
AIPS 1: Skypos: RA 05 35 14.50782210  DEC -05 22 30.659257
AIPS 1: Skypos: IPOL  43115.939 MHZ
AIPS 1: Minimum=-2.0024E-01 at 3890 2425    1    1    1    1    1
AIPS 1: Skypos: RA 05 35 14.50835444  DEC -05 22 30.634007
AIPS 1: Skypos: IPOL  43115.939 MHZ
AIPS 1: Flux density = -5.9147E+00 Jy.   Beam area =  47.36 pixels

CMETHOD='GRID'

AIPS 1: Mean= 3.8284E-04 rms= 2.1941E-02 JY/BEAM   over  403840. pixels
AIPS 1: Maximum= 9.7174E-02 at  4023  1868    1    1    1    1    1
AIPS 1: Skypos: RA 05 35 14.50790915  DEC -05 22 30.661857
AIPS 1: Skypos: IPOL  43115.939 MHZ
AIPS 1: Minimum=-1.2267E-01 at 4016 1941    1    1    1    1    1
AIPS 1: Skypos: RA 05 35 14.50793258  DEC -05 22 30.658207
AIPS 1: Skypos: IPOL  43115.939 MHZ
AIPS 1: Flux density =  3.2643E+00 Jy.   Beam area =  47.36 pixels

CMETHOD='DFT'

AIPS 1: Mean= 1.0113E-03 rms= 2.4188E-02 JY/BEAM   over  403840. pixels
AIPS 1: Maximum= 1.0001E-01 at  4123  2191    1    1    1    1    1
AIPS 1: Skypos: RA 05 35 14.50757434  DEC -05 22 30.645707
AIPS 1: Skypos: IPOL  43115.939 MHZ
AIPS 1: Minimum=-1.0683E-01 at 4016 1941    1    1    1    1    1
AIPS 1: Skypos: RA 05 35 14.50793258  DEC -05 22 30.658207
AIPS 1: Skypos: IPOL  43115.939 MHZ
AIPS 1: Flux density =  8.6223E+00 Jy.   Beam area =  47.36 pixels