Advanced calibration strategies

Over the next few months, the performance of the instrument will improve due to several factors that are currently under development.

Several analysis steps are planned to be included in the Standard Imaging Pipeline as part of the development towards the next LOFAR software version. These points are all being addressed by the Calibration and Imaging Tiger Team (CITT). Some of these are major new software elements while others are simple additions to existing procedures:

Direction-dependent calibration has shown to improve significantly the quality of the images. An improvement by a factor of 3-5 in the image noise has been obtained when applied to MSSS LBA observations (see B. Adebahr's report at: http://www.lofar.org/operations/lib/exe/fetch.php?media=msss:adebahr-wee...). In HBA observations, the thermal noise limited images have been obtained using SAGECal (see E. Orru' report at: http://www.lofar.org/operations/lib/exe/fetch.php?media=public:lsm_new:2012_09_12_orru_lsm.pdf), factor (https://github.com/lofar-astron/factor) and Kill-MS+DDFacet (https://github.com/saopicc/killMS).

Clock/TEC separation: Initial tests demonstrated that frequency dependent effect due to the clock delays between stations and delays due to TEC differences can in principle be addressed in order to separate instrumental effects from ionospheric direction-dependent effects (see M. Mevius report at: http://www.lofar.org/operations/lib/exe/fetch.php?media=commissioning:maaijke_report.pdf).

Ionospheric correction is crucial in order to reach the thermal noise and high quality images at LOFAR wavelengths. Two main approaches have been attempted and are currently under investigation in order to address the issue. One involves solving for many directions during the calibration phase (DDC) on short time scales so that the ionospheric effects are absorbed in the calibration solutions (see  e.g. Yatawatta et al. 2013, A&A, 550A, 136Y). The other involves fitting a phase screen to the directional TEC phase solutions and applying this during the imaging stage (method under commission inspired on SPAM algorithm (see Intema et al. 2009, A&A, 501, 1185I). 

More recently, the CITT, produced two semi-automatic pipelines called prefactor and factor, implementing the concept of the facet calibration described in van Weeren et al. 2016. Both pipeline run as option of the generic pipeline which is available in the LOFAR software. The prefactor pipeline is available. A stable version and some documentation is available at https://github.com/lofar-astron/prefactor . The factor pipeline is available at https://github.com/lofar-astron/factor and the documentation can be found in the cookbook. 




Advanced Offline Post Processing Reduction Strategies 

Expert users have adopted standard LOFAR software to produce images, exploring various analysis strategies, in some cases involving also self-calibration and direction dependent calibration. Their results are reported in Table 1 for both HBA and LBA observations.

 

1. Cycles and commissioning fields - HBA & LBA

 

Commisioner(s)	de Gasperin	Yatawatta	van Weeren	Retana Montenegro
Band	LBA 30-90 MHz	HBA 110-190 MHz	HBA 110-190 MHz	HBA 180-220 MHz
Target Field	RXSJ0603	NCP	Toothbrush	J1427385+331241
Total observing time (hrs)	6	260	8	8
Bandwidth	6	46	60	0.2
Resolution $(arcsec)	23x15	6 x 6	6 x 6	28 x 18
Imaged FOV (deg)	9	12	7	3.3
Final RMS Noise (mJy/beam)	8	0.03	0.093	5.0
Equivalent noise over  2 MHz bandwidth and  6 hours (mJy/beam)	14	0.8	0.8
Noise /thermal noise ratio	4.4	1.2	2	10
Calibration strategy	Transfer of amplitude solutions, FR calibration, clock/TEC separation, self-calibration. Use of Pill LBA pipeline	BBS intial  calibration with 2000 sources,  robust SAGECAL  with 20000 sources, excon imaging	Facet calibration extreme peeling. Similar results obstined using factor	LOFAR pipeline (prefactor) for continuum and custom scripts for line study

 

^$ The resolution depends on the stations used for the imaging

Table 1: Examples of sensitivities reached in Cycles and commissioning observations in HBA and LBA

In the LBA band, expert users have demonstrated that, using a simple analysis strategy that does not employ position dependent calibration or self-calibration, total intensity sensitivities of ~10 times the theoretical thermal noise in relatively long observations (6-10 hours) can be reached. Using more involved calibration techniques (as self-calibration or direction dependent calibration), sensitivities of 4-5 times the theoretical thermal noise have already been achieved.

For the HBA, direction independent calibration (i.e. prefactor) has proved to reach sensitivities of the order of the order of 5 times the theoretical thermal noise in images at a resolution of 20"-30". On the other hand, a noise of 1-2 times the thermal noise, a resolution of about 5"-10" and high fidelty images  can be achieved using direction dependent calibration techniques (e.g. factor, Sagecal or Kill-MS+DDFacet).