AIPS HELP file for SPIXR in 31DEC24
As of Fri Apr 26 14:48:04 2024
SPIXR: Finds the spectral index from each row of an image
INPUTS
INNAME Input image name (name)
INCLASS Input image name (class)
INSEQ 0.0 9999.0 Input image name (seq. #)
INDISK 0.0 9.0 Input image disk unit #
OUTNAME Output image name (name)
OUTSEQ -1.0 9999.0 Output image name (seq. #)
OUTDISK 0.0 9.0 Output image disk unit #.
BLC Bottom left corner of input
TRC Top right corner of input
FLUX Use only data > FLUX
REFREQ Reference frequency in GHz
< 0 -> header, 0 -> 1 GHz
OPTYPE 'CURV' fit incl curvature
PBPARM Beam parameters:
(1) Cutoff: 0 -> no PB corr
(2) > 0 -> Use (3)-(7)
(3)-(7) Beam shape
DPARM Flag if bad answer levels:
(1) N < (2) Sp < (3) Sp >
(4) T/min < (5) T/max >
(6) T < (7) T > (8) c <
(9) c >
CPARM Flag if uncertainty >
(1) Brightness
(2) Spectral index
(3) Curvature
BADDISK Disk to avoid for scratch
HELP SECTION
SPIXR
Task: Fits each row of an image cube to make an image of spectral
index (intensity = frequency to the power spectral index).
It may be instructed to make images of the uncertainties and
the curvature. SPIXR actually fits the function
log(T(x,y,f)) = log(T(x,y,f0)) + S(x,y) log(f/f0) +
C(x,y) (log(f/f0))^2
and writes images of T(x,y,f0), S(x,y), and C(x,y). Note that
the logarithms are base 10 and f0 is 1 GHz. These affect the
values in the curvature image. That is a change done
2010-07-13.
It is an error to feed this task any data cube not having the
first axis as FREQ, FQID, or SEQ.NUM. One may build a data
cube for use in this task. First one must make the image at
each frequency lie on the same coordinate grid (OHGEO) and have
similar spatial resolution (CONVL). If it might matter, it
would be best to correct for the single-dish primary beam
pattern (PBCOR). Then build the cube with MCUBE using
DOCONCAT=TRUE. Finally run TRANS to transpose the cube, making
the frequency (FQID usually or SEQ.NUM.) axis first. If the
axis is SEQ.NUM. then, in TRANS, do not reverse the order of
the frequency pixels since SPIXR must read the history file to
find the true frequencies for each pixel number. If the axis
is FQID, this caution does not apply.
SPIXR provides only very elementary blanking capabilities.
Normally, users will wish to do more elaborate blanking with
BLANK before running SPIXR. The data cube must have the same
spatial resolution and units in all spectral channels. If not,
the resolutiona and units issues alone will create erroneous
spectral indices. Running CONVL specifying BMAJ, BMIN, BPA
after running IMAGR with BMAJ=0 is recommended.
Adverbs:
INNAME.....Input image name (name). Standard defaults.
INCLASS....Input image name (class). Standard defaults.
INSEQ......Input image name (seq. #). 0 => highest.
INDISK.....Disk drive # of input image. 0 => any.
OUTNAME....Output image name (name). Standard defaults.
The OUTCLASSes are 'B TEMP', 'D BT', 'SP CNT', 'SPIX',
'D SPIX', 'SPCU', 'D SPCU' for brightness, brightness
uncertainty, count, spectral index, spectral index
uncertainty, curvature, and curvature uncertainty.
OUTSEQ.....Output image name (seq. #). 0 => highest unique.
OUTDISK....Disk drive # of output image. 0 => highest
number with sufficient space.
BLC........Bottom right corner in input image of desired
subimage. Default is entire image.
TRC........Top right corner in input image of desired
subimage. Default is entire image.
FLUX.......A flux cutoff in the same units as the input image (i.e.
Jy/beam). Data values below FLUX are ignored in the
spectral index computation. NOTE that 0.0 is not a null
value. Instead, it means ignore all negative
brightnesses (which it has to do anyway).
REFREQ.....Compute total flux, spectra index and curvature with
reference frequency REFREQ in GHz
0 => use 1 GHz; < 0 => use header frequency
Output imges will be assigned this as the frequency.
OPTYPE.....'CURV' => fit curvature as well as spectral index
anything else => fit only spectral index.
PBPARM.....Primary beam parameters: Adjust the cutoff levels to
account for the primary beam.
(1) Lowest beam value to believe: 0 -> do not do a
primary beam correction. The maximum correction is
a factor of 100.
(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
DPARM......The images can be dominated by bad answers, making it
hard to see the good parts. Therefore flag the soluion
for a pixel if
(1) Number samples > FLUX is < DPARM(1) 0 -> 0
NOTE: solutions will be found for only 2 or 3 (CURV)
samples - so this is a serious thing to test.
(2) Spectral index < DPARM(2) 0 -> -large
(3) Spectral index > DPARM(3) 0 -> +large
(4) Brightness/(min brightness) < DPARM(4) 0 -> 0
(5) Brightness/(max brightness) > DPARM(5) 0 -> +large
(6) Brightness < DPARM(6) 0 -> 0
(7) Brightness > DPARM(7) 0 -> +large
(8) Curvature < DPARM(8) 0 -> -large
(9) Curvature > DPARM(9) 0 -> +large
You should invoke some of these - 2,3 and 8,9 at least.
CPARM......The image can also be messed up by fits that are very
uncertain. Therefore, flag the solution for a pixel if
(1) Uncertainty in brightness > CPARM(1) 0 -> large
(2) Uncertainty in spectral index > CPARM(2) 0 -> large
(3) Uncertainty in curvature > CPARM(3) 0 -> large
BADDISK....Disk drives to avoid for scratch files.
EXPLAIN SECTION
SPIXR has the option of scaling the cutoff values on a
pixel-by-pixel basis to "correct" for the primary beam. Thus, as the
beam value goes down the cutoff value goes up. This allows SPIXR to be
run on data cubes after the application of PBCOR. Since the primary
beam is a function of frequency, the spectral moments are affected by
the primary beam correction. Unfortunately this correction also
raises the noise, making the option to raise the cutoff useful.
SPIXR 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.