[daip] flux not conserved in CONVL task

[Eric Greisen]

Sayali Avachat wrote:
> To Whomsoever it may concern,
> 
> I am having some issues with the task CONVL.
> I am trying to convolve optical maps (take from HST observations and 
> obtained using HST data reduction processes) to the resolution of VLA 
> maps at 15GHz.
> It looks like CONVL does the job of converting the beam size and PA to 
> that of radio, but at the same time it does change the flux values of 
> optical image.
> I tried using factor = (new beam)/(old beam), leaving opcode blank and 
> using radio map in In2name.
> 
> I am not sure what is happening here. I have started to get the feeling 
> that essentially being two different observations from two different 
> telescopes, AIPS is doing some weird things and it cannot read certain 
> header information in order to retain the flux of the original optical 
> image.
> 
> I manually PUTHEADed the beam size (0.06675x0.06675) in the map header 
> before I ran CONVL.
> 
> Below are the headers of optical image, before and after convolution.
> BEFORE -
> AIPS 1: Image=NONE      (MA)         Filename=M87F606W0505.IFLUX .   1
> AIPS 1: Telescope=HST                Receiver=ACS
> AIPS 1: Observer=                    User #= 1306
> AIPS 1: Observ. date=09-MAY-2005     Map date=03-OCT-2013
> AIPS 1: Minimum=-1.59414474E+02      Maximum= 3.76874359E+02 uJy
> AIPS 1: ----------------------------------------------------------------
> AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
> AIPS 1: RA---TAN  2496    12 30 49.503    1248.00      -0.025000    0.00
> AIPS 1: DEC--TAN  2464    12 23 29.124    1232.00       0.025000    0.00
> AIPS 1: ----------------------------------------------------------------
> AIPS 1: Coordinate equinox 2000.00
> AIPS 1: Map type=DIRTY               Number of iterations=       0
> AIPS 1: Conv size= 0.06675 X 0.06675   Position angle=   0.00
> AIPS 1: Maximum version number of extension files of type HI 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   2
> 
> AFTER -
> AIPS 1: Image=NONE      (MA)         Filename=M87F606W0505.ICONV .   1
> AIPS 1: Telescope=HST                Receiver=ACS
> AIPS 1: Observer=                    User #= 1306
> AIPS 1: Observ. date=09-MAY-2005     Map date=10-OCT-2013
> AIPS 1: Minimum=-5.96672314E+03      Maximum= 5.02040039E+02 uJy
> AIPS 1: ----------------------------------------------------------------
> AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
> AIPS 1: RA---TAN  2496    12 30 49.503     776.19      -0.025000    0.00
> AIPS 1: DEC--TAN  2464    12 23 29.124     434.84       0.025000    0.00
> AIPS 1: ----------------------------------------------------------------
> AIPS 1: Coordinate equinox 2000.00
> AIPS 1: Map type=DIRTY               Number of iterations=       0
> AIPS 1: Conv size= 0.41771 X 0.39941   Position angle=  49.54
> AIPS 1: Maximum version number of extension files of type HI is   1
> 
> Can someone please help me?

The units listed 'uJy' are not the units of a sky image.  I assume the u 
is mucro and the Jy is units of flux (integral of brightness over an 
area).  Proper brightness units which AIPS will handle are JY/BEAM (flux 
per area) or other things like Kelvin, Mag/pixel, etc about which AIPS 
will not be smart.  Further we do not bury scaling into units normally 
and will not recognize it.  When you say "does not conserve flux" you 
previously meant does not conserve brightness (image values) - to 
determine a flux you need to integrate the brightness over an area with 
e.g. TVSTAT, IMSTAT, IMEAN.  AIPS will conserve flux in that case in 
CONVL when it recognizes the image units - which it will not in this case.

Eric Greisen