[daip] UVMOD/UVFIX question
Hanno Spreeuw
J.N.Spreeuw at uva.nl
Fri Jun 13 06:08:13 EDT 2008
Thank you for clearing this issue.
So I guess it is a known fact among the experts that the accuracy of
astrometry on the brightest sources is limited by the angular distance
from the tangent point in the sense that beamsize/(s/n) accuracy cannot
be attained. To get the optimum accuracy one has to move the tangent
point close to the bright source.
I was not aware of that.
I thought that any source measurement in a facet with a size recommended
by SETFC would be ok.
If I understand correctly, distortions grow quadratically with distance
from the tangent point, so with FPOS=0 250, the position error will be
at least 3.36"/16=0.21".
Perhaps it is simplest for the purpose of testing our source extraction
code to run UVMOD on data from a linear array.
Hanno.
PS: Is it not recommended to do astrometry on images from FLATN? Does
FLATN cause an extra error?
Eric Greisen wrote:
> Hanno Spreeuw wrote:
>
>> Hello Eric, I have posted a UVMOD test on my homepage, please have a
>> look.
>> http://staff.science.uva.nl/~hspreeuw/
>> Cheers,
>> Hanno.
>>
>> Eric Greisen wrote:
>>
>>> Hanno Spreeuw wrote:
>>>
>>>> Thank you,
>>>>
>>>> It seems that inserted sources with FPOS<>0 do not appear in the
>>>> images exactly where they should. The errors increase away from the
>>>> tangent point. This confirms earlier results by James Miller-Jones
>>>> (CC). However, with FPOS=0, the source always appears at the tangent
>>>> point.
>>>> We are testing new source extraction code. In order to check our
>>>> deblending algorithm, we are inserting many sources in a small patch
>>>> of the sky. With these artificial sources one always knows the true
>>>> flux and position, so that is good for testing.
>>>> I am considering the workaround with UVFIX for these tests.
>>>
>>>
>>> If what you say is true, then it is unlikely for any of AIPS to work
>>> correctly. UVMOD uses the same phasing routines used elsewhere in
>>> AIPS including but not limited to UVFIX and IMAGR. What telescope
>>> has produced your data and what are you doing?
>
>
>
> I have loaded your data and - sort of - followed your procedure. I
> worried that the shift of 1000 arcsec might be rather large. SETFC does
> say that one can make this image - but I am surprised that it does.
> Anyway - what I did was to do 2 fields with RASH=0; DECSH=0,1000 and
> with DO3D 1; OVERL 2. In the first run, I restricted the clean box to
> the second shifted field. Afterwards, IMFITs gave the desired answers
> to great accuracy in each field. Then I reran IMAGR, restricting the
> Cleaning to field one. Afterwards, IMFITs gave very similar and shifted
> results. This is due to the geometry and various 3D effects which are
> much worse at low elevations, esp low declinations. This is illustrated
> in some of the lectures in the NRAO Synthesis Workshop volume(s). One
> should also note that, compressing the CCs in the first case led to only
> 3 pixels with CCs - a nice clean source although my window allowed for
> some negatives etc to appear. The second clean compressed down to 24
> CCs - a lot of smeared flux. In fact the flux will be smeared along an
> ellipse which will have a maximum where the traverse of the ellipse is
> slowest (at the bottom) and then curves upwards on both sides. You can
> see it in the images. I attach the IMFIT print outs.
>
> Thus, the problem lies in your use of AIPS IMAGR, not in UVMOD.
>
> Thanks you for pursuing this so that the correct answer could be found.
>
> Eric Greisen
>
>
> ------------------------------------------------------------------------
>
> Field 2, cleaning done in field 2
> Task IMFIT (release of 31DEC08) begins
> Fit to histogram gives RMS = 3.332E-02
> Model fit to Test UVMOD .ICL002. 1
> Source= SNAKEP1 in units JY/BEAM
> X-ref pix= 512.0 RA 17 42 15.000 Pix sep (asec) -4.0000
> Y-ref pix= 513.0 DEC -29 39 20.00 Pix sep (asec) 4.0000
> Axes 3-7 pixels = 1 1 1 1 1
> Axis 3 : IPOL
> Axis 4 : 324.385 MHZ
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> ******** Input Model ****************************************
> Component= 1 Gaussian
> Peak= 9.9815E+00 JY/BEAM
> Xpos= 512.00 Ypos= 513.00 pixels
> Size= 13.15 ** x 10.44 ** Pixels at pa -80.52 ** deg
> Component= 2 Baseline
> Zero lev= 0.0000E+00 JY/BEAM
> ** means parameter is held fixed
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Beam= 13.15 X 10.44 Pixels: in pa-80.52 Deg
>
> Initial guess RMS 2.8936E-02 in 3551 usable pixels
> IMPROPER INPUT PARAMETERS. NITER= 2
> ****** RMS of fit= 1.4773E+00 per point ******
> Begin tweaking each parameter
> ****** New RMS of fit= 2.8483E-02 per point ******
> Writing MF file version 1 attached to input image
>
> ********* Solution from IMFIT *********************************
> Error estimate based on rms 3.332E-02 JY/BEAM
>
> Component 1 - Gaussian
> Peak intensity = 9.9865E+00 +/- 3.33E-02 JY/BEAM
> Integral intensity= 9.9865E+00 +/- 3.33E-02 JANSKYS
> X-position = 512.011 +/- 0.0185 pixels
> Y-position = 513.000 +/- 0.0149 pixels
> RA 17 42 14.996 +/- 0.006
> DEC -29 39 20.00 +/- 0.06
> Major axis = 13.149 +/- 0.0000 pixels
> Minor axis = 10.441 +/- 0.0000 pixels
> Position angle = 99.476 +/- 0.000 degrees
> Major axis = 52.598 +/- 0.000 asec
> Minor axis = 41.766 +/- 0.000 asec
> Position angle = 99.476 +/- 0.000 degrees
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Deconvolution of component in pixels
> Nominal Minimum Maximum
> Major ax 0.000 0.000 0.000
> Minor ax 0.000 0.000 0.000
> Pos ang 0.000 0.000 0.000
> Deconvolution of component in asec
> Nominal Minimum Maximum
> Major ax 0.000000 0.000000 0.000000
> Minor ax 0.000000 0.000000 0.000000
> Pos ang 0.000000 0.000000 0.000000
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Component 2 - Baseline
> Zero level= 1.9598E-03( 8.7193E-03) Ampl. units
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> returns adverbs to AIPS
> Appears to have ended successfully
> primate 31DEC08 TST: Cpu= 0.1 Real= 0 IO= 9
>
> ******************************************************************
> Field 1 cleaning done in field 2
> Task IMFIT (release of 31DEC08) begins
> Fit to histogram gives RMS = 3.217E-02
> Model fit to Test UVMOD .ICL001. 1
> Source= SNAKEP1 in units JY/BEAM
> X-ref pix= 512.0 RA 17 42 15.000 Pix sep (asec) -4.0000
> Y-ref pix= 513.0 DEC -29 55 60.00 Pix sep (asec) 4.0000
> Axes 3-7 pixels = 1 1 1 1 1
> Axis 3 : IPOL
> Axis 4 : 324.385 MHZ
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> ******** Input Model ****************************************
> Component= 1 Gaussian
> Peak= 1.0067E+01 JY/BEAM
> Xpos= 512.00 Ypos= 763.00 pixels
> Size= 13.15 ** x 10.53 ** Pixels at pa -80.11 ** deg
> Component= 2 Baseline
> Zero lev= 0.0000E+00 JY/BEAM
> ** means parameter is held fixed
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Beam= 13.15 X 10.53 Pixels: in pa-80.11 Deg
>
> Initial guess RMS 3.1117E-02 in 2820 usable pixels
> Solution unchanging. niter= 18
> ****** RMS of fit= 2.9932E-02 per point ******
> Begin tweaking each parameter
> ****** New RMS of fit= 2.9948E-02 per point ******
> Writing MF file version 1 attached to input image
>
> ********* Solution from IMFIT *********************************
> Error estimate based on rms 3.217E-02 JY/BEAM
>
> Component 1 - Gaussian
> Peak intensity = 1.0028E+01 +/- 3.22E-02 JY/BEAM
> Integral intensity= 1.0028E+01 +/- 3.22E-02 JANSKYS
> X-position = 512.023 +/- 0.0178 pixels
> Y-position = 763.009 +/- 0.0145 pixels
> RA 17 42 14.993 +/- 0.005
> DEC -29 39 19.96 +/- 0.06
> Major axis = 13.146 +/- 0.0000 pixels
> Minor axis = 10.534 +/- 0.0000 pixels
> Position angle = 99.888 +/- 0.000 degrees
> Major axis = 52.583 +/- 0.000 asec
> Minor axis = 42.138 +/- 0.000 asec
> Position angle = 99.888 +/- 0.000 degrees
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Deconvolution of component in pixels
> Nominal Minimum Maximum
> Major ax 0.000 0.000 0.000
> Minor ax 0.000 0.000 0.000
> Pos ang 0.000 0.000 0.000
> Deconvolution of component in asec
> Nominal Minimum Maximum
> Major ax 0.000000 0.000000 0.000000
> Minor ax 0.000000 0.000000 0.000000
> Pos ang 0.000000 0.000000 0.000000
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Component 2 - Baseline
> Zero level= 1.4000E-03( 2.1904E-03) Ampl. units
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> returns adverbs to AIPS
> Appears to have ended successfully
> primate 31DEC08 TST: Cpu= 0.2 Real= 1 IO= 9
>
> *******************************************************************
> Field 1 cleaning done in field 1
> Task IMFIT (release of 31DEC08) begins
> Fit to histogram gives RMS = 3.206E-02
> Model fit to Test UVMOD .ICL001. 2
> Source= SNAKEP1 in units JY/BEAM
> X-ref pix= 512.0 RA 17 42 15.000 Pix sep (asec) -4.0000
> Y-ref pix= 513.0 DEC -29 55 60.00 Pix sep (asec) 4.0000
> Axes 3-7 pixels = 1 1 1 1 1
> Axis 3 : IPOL
> Axis 4 : 324.385 MHZ
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> ******** Input Model ****************************************
> Component= 1 Gaussian
> Peak= 9.9417E+00 JY/BEAM
> Xpos= 512.00 Ypos= 764.00 pixels
> Size= 13.15 ** x 10.53 ** Pixels at pa -80.11 ** deg
> Component= 2 Baseline
> Zero lev= 0.0000E+00 JY/BEAM
> ** means parameter is held fixed
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Beam= 13.15 X 10.53 Pixels: in pa-80.11 Deg
>
> Initial guess RMS 9.1427E-02 in 2820 usable pixels
> Solution unchanging. niter= 18
> ****** RMS of fit= 8.5448E-02 per point ******
> Begin tweaking each parameter
> ****** New RMS of fit= 8.5454E-02 per point ******
> Writing MF file version 1 attached to input image
>
> ********* Solution from IMFIT *********************************
> Error estimate based on rms 3.206E-02 JY/BEAM
>
> Component 1 - Gaussian
> Peak intensity = 9.9595E+00 +/- 3.21E-02 JY/BEAM
> Integral intensity= 9.9595E+00 +/- 3.21E-02 JANSKYS
> X-position = 512.076 +/- 0.0179 pixels
> Y-position = 764.113 +/- 0.0145 pixels
> RA 17 42 14.977 +/- 0.005
> DEC -29 39 15.54 +/- 0.06
> Major axis = 13.146 +/- 0.0000 pixels
> Minor axis = 10.534 +/- 0.0000 pixels
> Position angle = 99.888 +/- 0.000 degrees
> Major axis = 52.583 +/- 0.000 asec
> Minor axis = 42.138 +/- 0.000 asec
> Position angle = 99.888 +/- 0.000 degrees
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Deconvolution of component in pixels
> Nominal Minimum Maximum
> Major ax 0.000 0.000 0.000
> Minor ax 0.000 0.000 0.000
> Pos ang 0.000 0.000 0.000
> Deconvolution of component in asec
> Nominal Minimum Maximum
> Major ax 0.000000 0.000000 0.000000
> Minor ax 0.000000 0.000000 0.000000
> Pos ang 0.000000 0.000000 0.000000
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Component 2 - Baseline
> Zero level= 5.6694E-03( 6.2508E-03) Ampl. units
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> returns adverbs to AIPS
> Appears to have ended successfully
> primate 31DEC08 TST: Cpu= 0.2 Real= 0 IO= 9
>
> ********************************************************************
> Field 2 - but cleaning done in field 1
> Task IMFIT (release of 31DEC08) begins
> Fit to histogram gives RMS = 3.265E-02
> Model fit to Test UVMOD .ICL002. 2
> Source= SNAKEP1 in units JY/BEAM
> X-ref pix= 512.0 RA 17 42 15.000 Pix sep (asec) -4.0000
> Y-ref pix= 513.0 DEC -29 39 20.00 Pix sep (asec) 4.0000
> Axes 3-7 pixels = 1 1 1 1 1
> Axis 3 : IPOL
> Axis 4 : 324.385 MHZ
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> ******** Input Model ****************************************
> Component= 1 Gaussian
> Peak= 9.8903E+00 JY/BEAM
> Xpos= 512.00 Ypos= 514.00 pixels
> Size= 13.15 ** x 10.44 ** Pixels at pa -80.52 ** deg
> Component= 2 Baseline
> Zero lev= 0.0000E+00 JY/BEAM
> ** means parameter is held fixed
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Beam= 13.15 X 10.44 Pixels: in pa-80.52 Deg
>
> Initial guess RMS 8.3278E-02 in 3584 usable pixels
> IMPROPER INPUT PARAMETERS. NITER= 2
> ****** RMS of fit= 1.4571E+00 per point ******
> Begin tweaking each parameter
> ****** New RMS of fit= 7.9094E-02 per point ******
> Writing MF file version 1 attached to input image
>
> ********* Solution from IMFIT *********************************
> Error estimate based on rms 3.265E-02 JY/BEAM
>
> Component 1 - Gaussian
> Peak intensity = 9.9301E+00 +/- 3.26E-02 JY/BEAM
> Integral intensity= 9.9301E+00 +/- 3.26E-02 JANSKYS
> X-position = 512.042 +/- 0.0183 pixels
> Y-position = 514.058 +/- 0.0147 pixels
> RA 17 42 14.987 +/- 0.006
> DEC -29 39 15.77 +/- 0.06
> Major axis = 13.149 +/- 0.0000 pixels
> Minor axis = 10.441 +/- 0.0000 pixels
> Position angle = 99.476 +/- 0.000 degrees
> Major axis = 52.598 +/- 0.000 asec
> Minor axis = 41.766 +/- 0.000 asec
> Position angle = 99.476 +/- 0.000 degrees
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Deconvolution of component in pixels
> Nominal Minimum Maximum
> Major ax 0.000 0.000 0.000
> Minor ax 0.000 0.000 0.000
> Pos ang 0.000 0.000 0.000
> Deconvolution of component in asec
> Nominal Minimum Maximum
> Major ax 0.000000 0.000000 0.000000
> Minor ax 0.000000 0.000000 0.000000
> Pos ang 0.000000 0.000000 0.000000
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Component 2 - Baseline
> Zero level= 3.7940E-03( 1.4590E-02) Ampl. units
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> returns adverbs to AIPS
> Appears to have ended successfully
> primate 31DEC08 TST: Cpu= 0.1 Real= 0 IO= 9
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