[daip] TVSTAT: Understanding outputs - # of pixels

[Eduardo Ibar]

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)

Thank you very much,

edo


--------------------------
Eduardo Ibar Plasser

Institute for Astronomy
University of Edinburgh
The Royal Observatory
Blackford Hill
Edinburgh
EH9 3HJ
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