[daip] Conv size vs. Coord incr

[Andrzej Marecki]

Dear all,

This is a bit strange to me but maybe it's elementary...

I tried different CELLSIZEs in IMAGR/SCMAP and it turned out that the shape 
of the calculated beam depends on the CELLSIZE. These is a MERLIN L-band 
observation with a pretty good u-v coverage - see the attached file
uvcoverage.ps.gz. The resolution attained by MERLIN at that wavelength
should be better than 0.2 arcsec. Therefore the CELLSIZE amounting to
(0.045, 0.045) seems to be perfectly OK. Nevertheless the beam is highly 
elliptical in that case. However, when I go down to CELLSIZE (0.02, 0.02) 
the beam is almost circular - see the IHMEAD output below. In neither case 
IMAGR complains about too large/small CELLSIZE. Is that right? Nb. we get 
the same result regardless of whether IMSIZE is (512, 512) or (1024, 1024).

--
Andrzej

---------------------------

AIPS 1: Image=1236+327  (MA)         Filename=1236+327    .ICL001.   4
AIPS 1: Minimum=  -3.6668            Maximum=  91.9683 MilliJY/BEAM
AIPS 1: --------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value  at Pixel    Coord incr   Rotat
AIPS 1: RA---SIN   512    12 39 09.090     256     -0.020000    0.00
AIPS 1: DEC--SIN   512    32 30 27.430     256*     0.020000    0.00
AIPS 1: FREQ         1    1.658500E+09       1    1.4000E+07    0.00
AIPS 1: STOKES       1    IPOL               1        1.0000    0.00
AIPS 1: --------------------------------------------------------------
AIPS 1: Map type=NORMAL              Number of iterations=    5000
AIPS 1: Conv size= 0.19459 X 0.18302   Position angle=  86.52
AIPS 1: Max ext file #: CC   1    HI   1
AIPS 1: * => NOT the formal reference pixel on this axis

---------------------------

AIPS 1: Image=1236+327  (MA)         Filename=1236+327    .ICL001.   6
AIPS 1: Minimum=  -2.7366            Maximum= 123.4603 MilliJY/BEAM
AIPS 1: --------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value  at Pixel    Coord incr   Rotat
AIPS 1: RA---SIN   512    12 39 09.090     256     -0.045000    0.00
AIPS 1: DEC--SIN   512    32 30 27.405     256*     0.045000    0.00
AIPS 1: FREQ         1    1.658500E+09       1    1.4000E+07    0.00
AIPS 1: STOKES       1    IPOL               1        1.0000    0.00
AIPS 1: --------------------------------------------------------------
AIPS 1: Map type=NORMAL              Number of iterations=    5000
AIPS 1: Conv size= 0.32922 X 0.16850   Position angle= -36.56
AIPS 1: Max ext file #: CC   2    HI   1
AIPS 1: * => NOT the formal reference pixel on this axis

---------------------------
-- 
-----------------------------------------------------------------------------  
Andrzej Marecki                | 
Torun Centre for Astronomy     |   e-mail: amr at astro.uni.torun.pl
N. Copernicus University       |   WWW:    http://www.astro.uni.torun.pl
ul. Gagarina 11                |   tel: +48 56 6113032
PL-87-100 Torun, POLAND        |   fax: +48 56 6113009
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