AIPS HELP file for LTESS in 31DEC24
As of Thu Apr 25 20:43:26 2024
LTESS: Task do make linear combination mosaics
INPUTS
Input image
INNAME Image name (name)
INCLASS Image name (class)
INSEQ 0.0 9999.0 Image name (seq. #)
INDISK Image disk drive #
Output image
OUTNAME Image name (name)
OUTCLASS Image name (class)
OUTSEQ -1.0 9999.0 Image name (seq. #)
OUTDISK Image disk drive #
NMAPS 1.0 4087.0 Number of maps to analyse
NOISE * 0.0 Image noise, units are
* (Jy/BEAM)
BLC 0.0 4096 Bottom left corner of LTESS
TRC 0.0 4096 Top right corner of LTESS
PBSIZE PB FWHM in arcseconds:
< 0 - no PB correction
= 0 - use PBPARM
> 0 - use Gaussian
PBPARM Beam parameters:
(1) Cutoff 0 -> 0.07
(2) > 0 -> Use (3)-(7)
(3)-(7) Beam shape
BADDISK Disks to avoid for scratch.
HELP SECTION
LTESS
Type: Task
Use: LTESS is used to make a linear combination mosaic by correcting
for the individual primary beam patterns and optimizing signal
to noise. The individual images must all be already placed on
the desired output projective geometry including image size and
reference pixel.
NOTE: FLATN can also do this tesselation among other things and
it takes care of the image geometry for you.
Adverbs:
INNAME......The input image name. Standard defaults.
INCLASS.....The input image class. Standard defaults.
INSEQ.......The input image seq. #. 0 => highest.
If NMAPS > 1 then images having sequence
numbers INSEQ,INSEQ+1,...,INSEQ+NMAPS-1 are
operated on.
INDISK......The input image disk drive #. 0 => any.
OUTNAME.....The LTESS image name. Standard defaults.
OUTCLASS....The LTESS image class. Standard behavior with
default = 'xLTESS' if INCLASS = 'xMAP', where x
is any character
'LTESS' if INCLASS = anything else
OUTSEQ......The LTESS image seq. #. 0 => highest unique.
If >0; image will be created if new,
overwritten if image name exists.
OUTDISK.....The LTESS disk drive no. 0 => highest with space
NMAPS.......Number of maps to be combined. Must be in
sequence starting at INSEQ.
NOISE.......The estimated R.M.S. error for each image is
NOISE (Jy/beam).
NOISE(64) is used for fields > 64.
BLC.........Bottom left corner of LTESS image, BLC(3) gives
the channel number to combine.
TRC.........Top right corner of image; both BLC and TRC
default do that the inner quarter is chosen.
PBSIZE......Size of primary beam in arcsec, FWHM of Gaussian model.
One number per field.
If = 0, use PBPARM beam with defaults suitable to the
VLA.
If < 0, do no primary beam correction, e.g. for fields
that are not interferometer data.
If > 0, use a Gaussian of FWHM of PBSIZE(I).
PBSIZE(64) is used for fields > 64.
PBPARM......Primary beam parameters:
(1) Lowest beam value to believe: 0 -> 0.07 Sources
outside this range are ignored.
(2) > 0 => Use beam parameters from PBPARM(3)-PBPARM(7)
Otherwise use default parameters for the VLA (or
ATCA where appropriate)
(3-7)..For all wavelengths, the beam is described by the
function:
1.0 + X*PBPARM(3)/(10**3) + X*X*PBPARM(4)/(10**7) +
X*X*X*PBPARM(5)/(10**10) + X*X*X*X*PBPARM(6)/(10**13)
X*X*X*X*X*PBPARM(7)/(10**16)
where X is (distance from the pointing position in arc
minutes times the frequency in GHz)**2.
See explain for details
BADDISK.....This array contains the numbers of disks on which
it is desired that scratch files not be located.
BADDISK has no effect on input and output maps.
EXPLAIN SECTION
LTESS : Task which makes linear combination mosaics
DOCUMENTOR: R.Braun NRAO/VLA
DATE OF DOCUMENTATION: 17 Dec. 1987
RELATED PROGRAMS: VTESS, STESS
VERSION: 15JAN88
PURPOSE
LTESS is used to make linear combinations of up to 4087 input
images which must already reside on the same grid with the same
projection. This can be arranged with HGEOM for example,
remembering to
LTESS was intended primarily to work on images produced by MEM-based
tasks VTESS and UTESS. The references below apply to those tasks.
REFERENCES
Cornwell T.J., and Evans K.F., "A simple Maximum Entropy
deconvolution algorithm", Astronomy and Astrophysics, (1985)
Burch,S.F, Gull,S.F., and Skilling,J., "Image restoration by a
powerful Maximum Entropy method", Computer Vision, Graphics and
Image processing, 23, 113-128 (1983).
--------------------------------
Primary beam correction
FACES corrects an image for the primary beam attenuation of
the antennas. The function used to model the primary beam for normal
VLA frequencies
F(x) = 1.0
+ parm(3) * 10E-3 * x
+ parm(4) * 10E-7 * x*x
+ parm(5) * 10E-10 * x*x*x
+ parm(6) * 10E-13 * x*x*x*x
+ parm(7) * 10E-16 * x*x*x*x*x
where x is proportional to the square of the distance from the
pointing position in units of [arcmin * freq (GHz)]**2, and F(x)
is the multiplicative factor to divide into the image intensity at the
distance parameter x. For other antennas, the user may read
in appropraite constants in PBPARM(3) through PBPARM(7). The
flag, PBPARM(2) must be set to a positive number to invoke this
option and PBPARM(3) must not be zero.
This correction scales with frequency and has a cutoff
beyond which the map values are set to an undefined pixel value GIVEN
in PBPARM(1). At the VLA frequencies the default cutoff is
1.485 GHz 29.8 arcmin
4.885 GHz 9.13 arcmin
15 GHz 2.95 arcmin
22.5 GHz 1.97 arcmin
and occurs at a primary beam sensitivity of 2.3 percent of the value at
the beam center. Corrections factors < 1 are forced to be 1.
The estimated error of the algorithm is about 0.02 in (1/F(x))
and thus leads to very large errors for x>1500, or at areas
outside of the primary response of 20 percent. The cutoff level
may be specified with DPARM(1).
Default values of PBPARM for the VLA are given by Perley's fits:
0.0738 GHz -0.897 2.71 -0.242
0.3275 -0.935 3.23 -0.378
1.465 -1.343 6.579 -1.186
4.885 -1.372 6.940 -1.309
8.435 -1.306 6.253 -1.100
14.965 -1.305 6.155 -1.030
22.485 -1.417 7.332 -1.352
43.315 -1.321 6.185 -0.983
For the ATCA, these are by default:
1.5 GHz -1.049 4.238 -0.8473 0.09073 -5.004E-3
2.35 -0.9942 3.932 -0.7772 0.08239 -4.429E-3
5.5 -1.075 4.651 -1.035 0.12274 -6.125E-3
8.6 -0.9778 3.875 -0.8068 0.09414 -5.841E-3
20.5 -0.9579 3.228 -0.3807 0.0 0.0
For the Karl G Jansky VLA ("EVLA"), the defaults are frequency
dependent. If the observing frequency is between two tabulated
frequencies, then the beam is computed for each of the tabulated
frequencies and then interpolated to the observing frequency. The
values used are far too numerous to give here, see EVLA Memo 195,
"Jansky Very Large Array Primary Beam Characteristics" by Rick Perley,
revision dated June 2016. Obtain it from
http://library.nrao.edu/evla.shtml
RICK PERLEY'S (OLD) REPORT
Polynomial Coefficients from LSq Fit to VLA Primary
Beam raster scans.
Functional form fitted:
1 + G1.X^2 + G2.X^4 + G3.X^6
where X = r.F,
and r = radius in arcminutes
F = frequency in GHz.
Fits were made to 3 percent cutoff in power for 24 antennas.
Poor fits, and discrepant fits were discarded, and the most
consistent subset of antennas had their fitted coefficients
averaged to produce the following 'best' coefficients.
Freq. G1 G2 G3
1.285 -1.329E-3 6.445E-7 -1.146E-10 *
1.465 -1.343 6.579 -1.186 "
4.885 -1.372 6.940 -1.309
8.435 -1.306 6.253 -1.100
14.965 -1.305 6.155 -1.030
22.485 (old) -1.350 6.526 -1.090 *
22.485 (new) -1.417 7.332 -1.352
43.315 -1.321 6.185 -0.983
The estimated errors (from the scatter in the fitted
coefficients) are generally very small:
G1: .003 at all bands except Q (.014)
G2: .03 to .07 at all bands except Q (.15)
G3: .01 to .02 at all bands except Q (.04)
R. Perley 21/Nov/00
* The 1.285 and 22.485 old feed values are not used.