This tutorial follows on from Tutorial 8, and uses the same dataset. We will be improving upon our preliminary images of 1848+283 and J1849+3024 by using self-calibration loops. We will then go back and calibrate and image some of the other sources in the file.
For those that want them, the final calibration tables from T8 may be found here. The file is ~10MB. Load the data into AIPS using FITLD (make sure you don't use a close quote in datain) and use TACOP to copy over any tables you may require.
Before we start AIPS let's set up the environment variable MYDIR again. In the directory where your data are type
(for tcsh)setenv MYDIR `pwd`
(for bash)export MYDIR=`pwd`
Start AIPS:
aips tv=local
When AIPS asks for a userid, put in the same userid that you used for T8.
We can see what data are loaded into AIPS by typing
pcat
>pca AIPS 1: Catalog on disk 1 AIPS 1: Cat Usid Mapname Class Seq Pt Last access Stat AIPS 1: 1 6 DATA .SPLAT . 1 UV 07-SEP-2015 01:21:16 AIPS 1: 2 6 J1849+3024 .SPLIT . 1 UV 07-SEP-2015 18:37:58 AIPS 1: 3 6 1848+283 .SPLIT . 1 UV 07-SEP-2015 01:25:30 AIPS 1: 4 6 TAR .IBM001. 1 MA 07-SEP-2015 18:38:05 AIPS 1: 5 6 TAR .ICL001. 1 MA 07-SEP-2015 18:38:15
Recall that when we split the data, we applied all the calibration. That is, the "split" data files do not contain calibration tables. E.g. For the target J1849+30:
>getn 2; imhead AIPS 1: Got(1) disk= 1 user= 7 type=UV J1849+3024.SPLIT.1 AIPS 1: Image=J1849+30 (UV) Filename=J1849+3024 .SPLIT . 1 AIPS 1: Telescope=EVN Receiver=VLBA AIPS 1: Observer=N14C3 User #= 7 AIPS 1: Observ. date=22-OCT-2014 Map date=08-SEP-2015 AIPS 1: # visibilities 11865 Sort order TB AIPS 1: Rand axes: UU-L-SIN VV-L-SIN WW-L-SIN TIME1 SUBARRAY AIPS 1: INTTIM CORR-ID ANTENNA1 ANTENNA2 AIPS 1: ---------------------------------------------------------------- AIPS 1: Type Pixels Coord value at Pixel Coord incr Rotat AIPS 1: COMPLEX 3 0.0000000E+00 1.00 1.0000000E+00 0.00 AIPS 1: STOKES 4 -1.0000000E+00 1.00 -1.0000000E+00 0.00 AIPS 1: FREQ 1 4.9347400E+09 1.00 1.6000000E+07 0.00 AIPS 1: IF 8 1.0000000E+00 1.00 1.0000000E+00 0.00 AIPS 1: RA 1 18 49 20.103 1.00 3600.000 0.00 AIPS 1: DEC 1 30 24 14.237 1.00 3600.000 0.00 AIPS 1: ---------------------------------------------------------------- AIPS 1: Coordinate equinox 2000.00 AIPS 1: Rest freq 0.000 Vel type: OPTICAL wrt YOU AIPS 1: Alt ref. value 0.00000E+00 wrt pixel 0.00 AIPS 1: Maximum version number of extension files of type NX is 1 AIPS 1: Maximum version number of extension files of type FQ is 1 AIPS 1: Maximum version number of extension files of type HI is 1 AIPS 1: Maximum version number of extension files of type AN is 1 AIPS 1: Keyword = 'OLDRFQ ' value = 4.92699000D+09
| Table | Content |
| empty... |
We made a first-pass image of J1849+30.
What can you see from the noise in this image? Are there phase errors? Are there amplitude errors? Hint: error recognition lecture!
Below are some tasks to play with.
reca | Renumber the catalogue files. Not super important. |
getn n | n is the catalogue number of your clean map (ICL001) |
tvall | Show the image on the tv |
tvlab | Overplot labels |
tvbox | Draw a box around the source. |
tvmax | Determine the max value in the vicinity of the mouse. |
imstat | Image statistics. Turn inver on and off to measure the peak/noise. |
We wish to improve our first-pass image using self-calibration. It is useful to measure certain properties of our image so we can quantify any improvements. For point sources such as this one, I like to take note of the peak flux and the rms. I use imstat for this.
default calib | Task to determine calibration |
getn 2 | the split-off J1849+30 catalogue |
docal -1 | Do not calibrate (although no cal tables anyway yet) |
get2n 5 | Get the first-pass image. This is the model we are using |
refant 1 | Still EF... |
solint XXX | Want to maximise SNR and time-sampling |
solty 'l1r' | A robust solution type. |
solmo 'P' | Phase-only |
aparm(1) 3 | Min. #Ant |
ncomp 100 0 | Number of clean components to use (Look at CC table) |
doapply -1 | This prevents a cal file from being made |
inp | |
go |
| Table | Content |
| SN 1 | Phase-only self-cal - 1st iteration |
Have a look at the resulting SN plot. What should you plot on the y-axis?
Let's use this newly created solution table to improve our image. Note that we can directly feed the SN table into gainu here, only because this is a single-source catalogue. Otherwise it is necessary to run CLCAL and create a CL table.
default imagr | |
getn 2 | |
imsiz 256 | No. of pixels across the image |
cell 0.0002 | Size per pixel in arcsec. |
outna 'TAR_P' | |
niter 1000 | |
robust 0 | Image weighting. 0 is somewhere between uniform and natural. |
dotv 1 | Clean interactively. |
docal 1; gainu 0 | Apply cal. |
inp | |
go |
Has the noise "pattern" in the image changed?
Feed the resulting image back into CALIB and repeat until your image ceases to improve.
Note that you can tget calib, and make sure you update get2n with the newest image model.
default calib | Task to determine calibration |
getn 2 | the split-off J1849+30 catalogue |
docal 1 | Calibrate using the final table from the phase-only |
gainu n | Note that this does NOT change per iteration |
get2n m | Get the previous image. This is the model we are using |
refant 1 | Still EF... |
solint XXX | Want to maximise SNR and time-sampling |
solty 'l1r' | |
solmo 'A&P' | Amplitude and phase |
aparm(1) 3 | Min. #Ant |
ncomp 100 0 | Number of clean components to use (Look at CC table) |
doapply -1 | This prevents a cal file from being made |
inp | |
go |
| Table | Content |
| SN 1 | Phase-only self-cal - 1st iteration |
| SN n | Phase-only self-cal - nth iteration |
| SN n+1 | Amp & phase self-cal - 1st iteration |
Have a look at the resulting SN plot.
Use this newly created solution table to improve our image.
default imagr | |
getn 2 | |
imsiz 256 | No. of pixels across the image |
cell 0.0002 | Size per pixel in arcsec. |
outna 'TAR_AP' | |
niter 1000 | |
robust 0 | Image weighting. 0 is somewhere between uniform and natural. |
dotv 1 | Clean interactively. |
docal 1; gainu 0 | Apply highest cal table |
inp | |
go |
As before, feed the resulting image back into CALIB and repeat until your image ceases to improve. Note that here the SN table you use does not change. Below is a gif showing a number of both phase-only and A&P iterations.
We can compare our final image with the EVN pipeline output. This source is also found in the VLBA calibrator catalogue.
You should image and self-cal 1848+283 yourself. I would usually have made a preliminary image of this source first to make sure all of the calibration performed in T8 worked.
1848+283 was used as the "phase-calibrator". Will phase-only self-cal improve the solution? What was the solution interval we used in FRING?
Here is the first-pass image
After one round of phase-only self-cal
After one round of phase-only self-cal and one round of amplitude and phase self-cal. Note that you might find multiple iterations yield a superior image.
You have no doubt taken very good notes on what is contained in each calibration table. CL3 contains the instrumental delay and was applied to all time, while CL4 is time-dependent and is applied only to the two sources that we imaged above.
| Table | Content |
| CL 1 | Pristine calibration table (always keep) |
| CL 2 | A-Priori calibration: amplitude, parallactic angle (CL1+SNx+SNy+..) |
| SN 1 | Fringe Finder: instrumental delay/clocks |
| CL 3 | CL2+SN1: Cumulative calibration |
| SN 2 | Fringe Fit/Rate (atmospheric phase/delay) |
| CL 4 | CL3+SN2: Total calibration |
| BP 1 | Bandpass calibration |
Task for students: Choose another calibrator and rerun fring (be careful with gainuse...) to determine the instrumental delay (=> SN3) and confirm that the delay should be very similar to CL3 (=> CL5). (HINT: 3C345, the brightest source in this observation, is observed by all antennas at ~1300.) It is very important that you are careful with your CL tables!
You now have a choice of which CL table to use. It shouldn't matter which you use because the delays should be very similar. Rerun FRING to determine your time-dependent calibration. Split off 3C345 and J1640+3946 and image these sources. Don't forget to apply all your calibration, including your BP table!