AIPS HELP file for DCONV in 31DEC24
As of Thu Apr 25 5:44:51 2024
DCONV: Task to deconvolve an image, given a gaussian beam.
INPUTS
INNAME Image name (name)
INCLASS Image name (class)
INSEQ 0.0 9999.0 Image name (seq. #)
INDISK 0.0 9.0 Disk drive #
IN2NAME Second Image name (name)
IN2CLASS Second Image name (class)
IN2SEQ 0.0 9999.0 Second Image name (seq. #)
IN2DISK 0.0 9.0 Second Disk drive #
OUTNAME Output image name (name)
OUTCLASS Output image name (class)
OUTSEQ 0.0 9999.0 Output image name (seq. #)
OUTDISK 0.0 9.0 Output disk drive #
BLC 0.0 4096.0 Bottom left corner of image
0=>entire image
TRC 0.0 4096.0 Top right corner of image
0=>entire image
OPCODE 'IN2C' : use weighting map,
'UNWT' : no weighting map.
FACTOR 0.0 1.0 Points with less than FACTOR
times the maximum # points in
their vicinity are not cor-
rected (Isolation criterion).
PIXSTD 0.0 999999.9 Stop criterion, rms.
PIXAVG 0.0 999999.0 Stop criterion, correction.
NITER 0.0 999.0 Max. # iterations. 0=>1
BMAJ 0.0 9999.0 Beam major axis (arcsec)
BMIN 0.0 9999.0 Beam minor axis (arcsec)
0=> BMAJ
BPA -360.0 360.0 Beam position angle. (N->E)
APARM (1) Lower limit output, (2)
upper limit output, (1) and
(2) 0: use scratch files.
(3) type of gainfunction.
0: triangle, 1:rectangle.
BADDISK 0.0 15.0 Disks to avoid for scratch
HELP SECTION
DCONV
Type: Task
Parameters :
INNAME Image name (name)
INCLASS Image name (class)
INSEQ Image name (seq. #) 0=> lowest unique
INDISK Disk drive #.
IN2NAME Name second input image (name)
IN2CLASS Name second input image (class)
IN2SEQ Name second input image (seq. #) 0=> lowest unique
IN2DISK Disk drive second input image.
OUTNAME Name output image (name)
OUTCLASS Name output image (class)
OUTSEQ Name output image (seq. #) 0=> lowest unique
OUTDISK Disk drive output image.
BLC Bottom left corner of image. 0=> from catalog header.
TRC Top right corner of image. 0=> from catalog header.
OPCODE 'IN2C' : use weighting map; 'UNWT' : no weighting.
FACTOR Percentage of non-blanked pixels in neighbourhood
needed to call pixel 'not isolated'.
PIXSTD Stop iterations if rms difference between O and
convolved T drops below PIXSTD.
PIXAVG Stop iterations if increase in average correction per
pixel drops below PIXAVG.
NITER Maximum # iterations.
BMAJ Major axis beam (arcsec).
BMIN Minor axis beam (arcsec).
BPA Position angle major axis (degrees, N=>E).
APARM 1: lower pixel limit; 2: upper pixel limit. If 1 and
2 are zero, use scratch files. 3: shape of gain func-
tion, 0: triangle, 1: rectangle (see EXPLAIN file).
BADDISK Disk(s) to avoid using for the 2 scratch files.
EXPLAIN SECTION
DCONV :
DCONV sharpens an image which is blurred by a gaussian. I am
not quite sure of its usefulness in astronomical applications,
therefore I welcome suggestions. I see its use mainly in cases
where (of a spectral line cube) only the moment maps are readily
available.
Use : DCONV solves the following equation for T (by iteration) :
B o W . T
O = ------------
B o W
where O = O(x,y), the observed distribution; B = B(x,y), the
gaussian beam; W = W(x,y), an optional weighting map;
T = T(x,y), the unknown true distribution. The 'o' stands for
convolution. If no map for weighting is specified, uniform
weights are assumed, and the equation becomes, provided B is
normalized to unity :
O = B o T
The latter is solved by iterating :
T(k) = T(k-1) + G * (O - O(k)),
with O(k) = B o T(k-1),
T(0) = O,
G the gain function (see below),
and k the iteration number.
Without noise, the method works fine with G equal to unity.
Since usually noise will be present, G should assume values be-
tween 0 and 1, depending on the value of O(k) and the limits to
the values O(k) can assume. These limits are set by APARM(1)
(lower) and APARM(2) (upper).
APARM(3) = 0 => triangular gain function:
G = 0 at the user defined limits and beyond, and increases
linearly to 1 halfway between the limits.
APARM(3) = 1 => rectangular gain function:
G = 0 beyond the user defined limits, and G = 1 throughout
between the limits.
The previous case applies when there are global (preferrably
physical) limits which hold all over the map, e.g. all inten-
sities should be > 0.
In other cases the limits are local rather than global. In a
velocity field a value of 200 km/s might be well possible at
one position, but extremely unlikely at the opposite end of
the map. In such circumstances, put both APARM(1) and (2) to
zero. Then the limits are determined per pixel by analyzing
the statistics of the pixel values in each pixel's neighbour-
hood in the original map (O). To achieve this, the adverb FAC-
TOR is needed to determine whether a pixel is isolated or not.
Examples :
1) deconvolve a total HI map, or any intensity map.
Use uniform weighting (no second map), APARM(1) = 0 (don't
allow negative flux densities). Use a rectangular function
with very large APARM(2), or a triangular one with APARM(2)
in the order of the map maximum. Set PIXSTD to 2 or 3 ti-
mes the estimated rms noise in the map.
2) deconvolve a velocity field, or any first moment map.
Use a second (weighting) map, preferably the result of exa-
mple 1. Since there are no sensible fixed limits to the
velocities, set APARM(1) and APARM(2) to 0. This will cau-
se DCONV to fill scratch files with the statistics of the
input map, and to use these in determining the gain G. FAC-
TOR = 0.5 eliminates most edge problems. For PIXSTD try
the estimated error per pixel. (This is usually much smal-
ler than the velocity resolution).
General :
Do not iterate too long: after just a few iterations all time
is likely to be spent trying to make a solution to either
noise or a few badly specified points.