Observing Strategies

Observations with the Low Band Antennas (LBA): 

Single observations that are continuous in time/Hour Angle: 

- Half of the available bandwidth (BW) on the target field (BW<=48 MHz, <=244 subbands) and half on a calibrator (same frequencies as the target, BW<=48 MHz, <=244 subbands). 

- Observations in the band of 10-90 MHz (either 10 MHz or 30 MHz filters) are possible.

- Suggested range of correlator subbands is 114-357.  

- Processing performed with the Pre-processing Pipeline. 

- Suggested averaging factors are 8 channels in frequency and 2 seconds in time. More details are given in the sections below. 

 

Observations with the High Band Antennas (HBA): 

Observations can be specified in 4 different schemes, covering one of the 3 HBA bands: 110-190 MHz (with sampling clock 200 MHz), 170-230 MHz (with sampling clock 160 MHz) or 210-250 MHz (with sampling clock 200 MHz). 

 

i) Continuous in time/Hour Angle observation of the target bracketed by short calibrator runs. Alternatively, one could adopt the LBA strategy (Half of the available bandwidth on the target field and half on a calibrator) when a bright calibrator is present within the analogue beam of the HBA tiles (up to ~10 degrees from the target). 

-Processing performed with the Pre-processing Pipeline.

 

ii) Two scans, one on the calibrator (5-10 min) and a long continuous run on the target. 

- Up to the full available bandwidth (BW < 80 MHz).

- Processing performed with the Pre-processing pipeline. 

- This is the optimal strategy to use if advanced faceted imaging needs to be used in the calibration process.  

 

iii) Interleaved calibrator observations (eg. 5 min) with target field (eg. ~30 min), quasi-continuous in HBA. Note that the amount of data products generated increases proportionally to the number of repeated blocks of scans, so users are advised to consider case ii above as first option.

- Up to the full available bandwidth (BW < 80 MHz).

- Observations in one of three HBA bands: 110-190 MHz, 170-230 MHz or 210-250 MHz 

- Processing performed with the Pre-processing pipeline.  

 

iv) If the user has a good initial model of the target field at his/her disposal, observations could be performed using the full bandwidth on the target. 

- Processing performed with the Pre-processing Pipeline. 

 

Suggested ranges of correlator subbands for observations in band 110-190 MHz are: 51-442 (i.e. 110-186 MHz) for continuous bandwidth, or the ranges 77-356 (i.e. 115-169 MHz), 358-396 (i.e. 170-177 MHz), 407-456 (i.e. 179-189 MHz) which exclude the known RFI bands. Suited ranges of correlator subbands for observations in band 170-230 MHz will be advertised soon (commissioning ongoing). 

For radio continuum studies not including the international baselines, suggested averaging factors are 4 channels in frequency and 2 seconds in time. 

More details about the processing are given in the processing and characterization pages.    

 

Calibrators. 

3C196 or 3C295 are strongly recommended as flux calibrators, either by including them on a long run or by observing them for 5 min at the beginning and at the end of the observation. In the HBA, 3C147 and 3C48 may also be used. For calibrating European baselines, the proposer is adviced to use 3C196, 3C147, or 3C48. Note that for accurate gain calibration at LOFAR-NL or European scales, these sources need resolved models. In Table A we summarize the relevant parameters of the flux calibrators.

    A. Flux calibrators 

    Source Kind
    Band
    RA (h m s) DEC (o ‘ ")  I at 150 MHz   spectral index
    3C196 Seyfert 1 Galaxy LBA+HBA 08 13 36.07 +48 13 02.58  83.084  -0.699, -0.110
    3C295 Seyfert 2 Galaxy LBA+HBA 14 11 20.52 +52 12 09.86  97.763  -0.582,-0.298, 0.583,-0.363
    3C147 Seyfert 1 Galaxy LBA+HBA 05 42 36.26 +49 51 07.08  66.738 -0.022,-1.012,0.549 
    3C48 Quasar LBA+HBA 01 37 41.30 +33 09 35.12  64.768 -0.387,-0.420,0.181 
    3C286 Quasar  LBA+HBA 13 31 08.3  +30 30 33   27.477 -0.158,0.032,-0.180 
    3C287 Quasar LBA+HBA 13 30 37.7 +25 09 11   16.367  -0.364
    3C380 Quasar LBA 18 29 31.8  +48 44 46  77.352  -0.767

    Table A: calibrators to be used in LOFAR observations

     

    The list of polarized sources, required for monitoring ionospheric RM changes for accurate polarimetry, is given in Table B:

    B. Polarized sources

    Source Kind Band RA ( h m s) DEC (o ‘ ")
    Western hotspot DA240 Radio galaxy hotspot HBA 07 49 48.02 +55 54 22.1
    PSR B0834+06 Bright pulsar HBA 08 37 05.64 +06 10 14.56
    PSR B1642-03 Bright pulsar LBA+HBA 16 45 02.04 -03 17 58.32
    PSR B1919+21 Bright pulsar LBA+HBA 19 21 44.82 +21 53 02.25
    PSR B1937+21 Bright pulsar HBA 19 39 38.56 +21 34 59.14
    PSR B2217+47 Bright pulsar LBA+HBA 22 19 48.14 +47 54 53.93

    Table B: polarized sources for LOFAR observations

Day/Night observations

It is common sense to perform during the night low frequency observations that require high S/N. This is usually done to avoid human hand RFIs. However changes in tickness of the TEC layer and magnitude of the electron density fluctuations need to be considered when planning observations with LOFAR in LBA or HBA, as described below. The level of accuracy achieved in the editing and calibrating LOFAR data opened up the possibility to observe during the day while obtaining high level scientific results. 

HBA

For the HBA band this has been demonstrated by the Survey Key Science project (the LoTSS survey),  which makes use of both day/night-time observations, avoiding sunrise and sunset. The plots in figure 1 and 2 show how the rms of the images varies versus the fraction of the observations during daytime, for data treated with direction independent and direction dependent calibration respectively. These plots show how the level of the rms achieved for observations performed during daytime is comparable to the one obtained for observations performed during the night. This means that the level of accuracy in RFI editing and TEC calibration is good enough to achieve rms of the order of 0.1-0.06 mJy/beam at 150 MHz with the Dutch array at any time of the day except during dawn and dusk.

Figure 1: RMS versus fraction of observations during daytime for direction independent
calibrated HBA images. 
 
 
Figure 2: RMS versus fraction of observations during daytime for direction dependent
       calibrated HBA images

         LBA 

The noise statistics for the LBA band are much lower, but some information can be inferred from the   project LC8_031 which is surveying the LBA sky.
Although the absolute TEC level is higher during the day, the variations in the ionosphere (dTEC) measured during daytime result to have lower magnitude (e.g. <+/-0.3 between core and RS509) when compared to night time (usually dTEC ~+/-0.5).

The ionosphere seems to have day-time oscillations typically faster than night-time ones of about a factor of 2, nevertheless:

- day-time oscillations seems to be more structured, as they are waves instead of instabilities (see figure 3 where multiple waves seems to overlap)

- amplitude scintillations appear to be suppressed during daytime

If oscillations are structured (e.g. long TIDs), these are easier to solve and scintillation plays less of a problem. Because of the above evidences, we advise PI's to perform LBA observations during day-time avoding dawn and dusk.

Figure 3: TEC solutions of an LBA observation during daytime.


Design: Kuenst.    Development: Dripl.    © 2020 ASTRON