[daip] TVSTAT: Understanding outputs - # of pixels

[Eric Greisen]

Eduardo Ibar wrote:
> Hello there,
> 
> In order to estimate the flux density from an extended source (for example from 
> a FR II AGN) I have been using TVSTAT. Now, I wander to know about the total 
> surface area of the source by looking at the number of pixels from the TVSTAT 
> output. However, I see that TVSTAT does not print out the number of pixels that 
> I expect.
> 
> This is an example using TVSTAT:
> 
> AIPS 2: Begin setting region number  1
> AIPS 2: Press button A to set intermediate vertex
> AIPS 2: Press buttons B, C, or D to set final vertex
> AIPS 2: C => then reset a vertex,  D => then exit
> AIPS 2: Mean= 1.2236E-04 rms= 2.2598E-04 JY/BEAM   over    2216. pixels
> AIPS 2: Maximum= 1.4469E-03 at  1784   780    1    1    1    1    1
> AIPS 2: Skypos: RA 10 51 59.448  DEC 57 00 13.74
> AIPS 2: Skypos: IPOL  602.188 MHZ
> AIPS 2: Minimum=-5.3311E-05 at 1776  795    1    1    1    1    1
> AIPS 2: Skypos: RA 10 52 00.673  DEC 57 00 32.49
> AIPS 2: Skypos: IPOL  602.188 MHZ
> AIPS 2: Flux density =  8.0894E-03 Jy.   Beam area =  33.52 pixels
> 
> First, I see that the Beam area printed out by TVSTAT = 33.52 pixels. However, 
> printing the header of the image ("imh") gives the
> synthesized beam in arcsec units:
> 
> AIPS 2: Pixel type: FLOATING         Magic value blanking
> AIPS 2: Minimum=-7.11229630E-04      Maximum= 3.21057998E-02 JY/BEAM
> AIPS 2: ----------------------------------------------------------------
> AIPS 2: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
> AIPS 2: RA---SIN  3600    10 51 57.000    1800.00         -1.250    0.00
> AIPS 2: DEC--SIN  3600    57 21 30.000    1801.00          1.250    0.00
> AIPS 2: FREQ         1   6.0218750E+08       1.00 -1.3125000E+07    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=   30467
> AIPS 2: Conv size=   7.10 X   6.51   Position angle=  75.00
> AIPS 2: Observed RA   10 51 02.002    DEC  57 13 50.39
> AIPS 2: Rest freq      0.000         Vel type: OPTICAL wrt YOU
> AIPS 2: Alt ref. value  0.00000E+00  wrt pixel    0.47
> 
> Bmaj =  7.10
> Bmin =  6.51
> cellsize = 1.25
> 
> which implies an elliptic area defined by:
> 
> => Area = pi*Bmaj/2*Bmin/2
>          = 36.30 arcsec^2
>          = 23.23 pixels^2 (using cellsize)
> 
> which contradicts the TVSTAT value!. There is a factor ~sqrt(2) in difference 
> between these two area values.
> 
> Please, can you explain me what is the correct number of pixels printed out by 
> TVSTAT? (beam area and TV selected area)

The Bmaj and Bmin are the full width to half maximum.  So the beam area 
formula is  1.1331 * CATR(KRBMJ) * CATR(KRBMN) rather than pi/4.  The 
pixels number above is the number of pixels over which the mean 
brightness is determined.  The beam area in pixels is then used to 
convert the sum of the brightness to Jy from Jy/beam.

Eric Greisen