[daip] issues with task CONVL

[Eric Greisen]

Sayali Avachat wrote:
> Hello, 
> 
> I am trying to convolve radio maps at different frequencies to the 
> resolution of a map at one particular frequency (15GHz resolution at B 
> config). 
> What I observed is, the task CONVL can convolve the beam parameters, viz 
> bmaj, bmin and bpa, fine, but I don't know why it affects the flux 
> minimum and maximum values in the header of original file. I don't think 
> CONVL is suppose to change anything but beam parameters of the specified 
> image.
> Below are the headers of original file (before CONVL) and convolved file 
> (after CONVL).
> Can you please help me understand what is going on here?
> 
> *Original map header (before using CONVL)*
> *
> *
> AIPS 2: Got(1)   disk= 1  user=1306   type=MA   M87_AX.ICL001.1
>  >imh
> AIPS 2: Image=M87       (MA)         Filename=M87_AX      .ICL001.   1
> AIPS 2: Telescope=VLA                Receiver=VLA
> AIPS 2: Observer=AH822               User #= 1306
> AIPS 2: Observ. date=02-JUN-2003     Map date=20-SEP-2013
> AIPS 2: Minimum=-2.67477240E-03      Maximum= 4.22793436E+00 JY/BEAM
> AIPS 2: ----------------------------------------------------------------
> AIPS 2: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
> AIPS 2: RA---SIN  2048    12 30 49.000     776.19      -0.025000    0.00
> AIPS 2: DEC--SIN  1024    12 23 30.000     434.84       0.025000    0.00
> AIPS 2: FREQ         1   8.4601120E+09       1.00  1.0000000E+08    0.00
> AIPS 2: STOKES       1   1.0000000E+00       1.00  1.0000000E+00    0.00
> AIPS 2: ----------------------------------------------------------------
> AIPS 2: Coordinate equinox 2000.00
> AIPS 2: Map type=NORMAL              Number of iterations=   28669
> AIPS 2: Conv size= 0.22688 X 0.22085   Position angle=  -1.17
> AIPS 2: Rest freq      0.000         Vel type: OPTICAL wrt YOU
> AIPS 2: Alt ref. value  0.00000E+00  wrt pixel    1.00
> AIPS 2: Maximum version number of extension files of type CC is   1
> AIPS 2: Maximum version number of extension files of type CG is   1
> AIPS 2: Maximum version number of extension files of type HI is   1
> AIPS 2: Maximum version number of extension files of type ST is   1
> AIPS 2: Maximum version number of extension files of type MF is   1
> AIPS 2: Maximum version number of extension files of type PL is   1
> AIPS 2: Keyword = 'CCFLUX  '  value =  1.197020E+01
> AIPS 2: Keyword = 'CCTOTAL '  value =  1.197020E+01
> AIPS 2: Keyword = 'CORRMODE'  value = '        '
> AIPS 2: Keyword = 'VLAIFS  '  value = 'ABCD    '
> AIPS 2: Keyword = 'CORRCOEF'  value =           -1
> 
> *Convolved map header (after using CONVL)*
> *
> *
> AIPS 1: Got(1)   disk= 1  user=1306   type=MA   TEST-M87AX.CONVL.1
>  >imh
> AIPS 1: Image=M87       (MA)         Filename=TEST-M87AX  .CONVL .   1
> AIPS 1: Telescope=VLA                Receiver=VLA
> AIPS 1: Observer=AH822               User #= 1306
> AIPS 1: Observ. date=02-JUN-2003     Map date=03-OCT-2013
> AIPS 1: Minimum=-6.78434409E-03      Maximum= 4.42203140E+00 JY/BEAM
> AIPS 1: ----------------------------------------------------------------
> AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
> AIPS 1: RA---SIN  2048    12 30 49.000     776.19      -0.025000    0.00
> AIPS 1: DEC--SIN  1024    12 23 30.000     434.84       0.025000    0.00
> AIPS 1: FREQ         1   8.4601120E+09       1.00  1.0000000E+08    0.00
> AIPS 1: STOKES       1   1.0000000E+00       1.00  1.0000000E+00    0.00
> AIPS 1: ----------------------------------------------------------------
> AIPS 1: Coordinate equinox 2000.00
> AIPS 1: Map type=NORMAL              Number of iterations=   28669
> AIPS 1: Conv size= 0.41771 X 0.39941   Position angle=  49.54
> AIPS 1: Rest freq      0.000         Vel type: OPTICAL wrt YOU
> AIPS 1: Alt ref. value  0.00000E+00  wrt pixel    1.00
> AIPS 1: Maximum version number of extension files of type HI is   1
> AIPS 1: Maximum version number of extension files of type CC is   1
> AIPS 1: Maximum version number of extension files of type ST is   1
> AIPS 1: Maximum version number of extension files of type MF is   1
> AIPS 1: Keyword = 'CCFLUX  '  value =  1.197020E+01
> AIPS 1: Keyword = 'CCTOTAL '  value =  1.197020E+01
> AIPS 1: Keyword = 'CORRMODE'  value = '        '
> AIPS 1: Keyword = 'VLAIFS  '  value = 'ABCD    '
> AIPS 1: Keyword = 'CORRCOEF'  value =           -1
>  >

CONVL performs a correct convolution not just of 3 header parameters but 
of the image itself.  This will change ALL image values including the 
maximum and minimum and even the units since Jy/beam depends on the beam 
area so "beam" changes also.  Imagine, for simplicity, convolving with a 
very big beam - the image will become essentially 1 value representing 
some average of all the values with a conversion for the units.

The units are handled very carefully by CONVL - see information on 
spectral cubes having initially different resolutions in each channel.

Eric Greisen