KVN and VERA Array (KaVA)

B New observation mode from 2018A

Two new observations modes are available from the 2018A semester; C2 mode and Wide-field imaging mode. Details are summarized in the following subsections.

B.1 C2 mode

KaVA has provided two correlation modes with 8IFs X 32 MHz and 16 IFs X 16 MHz, called as the C4 and C5 modes, respectively. To obtain the accurate amplitude values across the all frequency channels, however, the number of basebands (or IFs in AIPS data handling) yielded by the digital filter unit (DFU) is better to be reduced because the amplitude losses mainly occur at the edges of each baseband. This reduction is especially helpful to observe continuum sources, such as active galactic nuclei (AGN). The C2 mode, which has 2 IFs 2 IFs X 128 MHz, therefore, is opened but at only Q-band in 2018A. Note the following two matters: there are some amplitude slopes mainly at VERA stations (Figures 12 and 13 in the VERA status report: http://veraserver.mtk.nao.ac.jp/restricted/CFP2018A/status18A.pdf), which must be corrected by all the gain calibration procedures (AIPS tasks ACCOR, BPASS, and APCAL); multiply the scaling factor to recover the quantization loss (1.3 for Daejeon correlator) [8].

B.2 Wide-field imaging mode

This mode is required to fully image 44 GHz methanol maser emissions associated with star-forming regions, which are generally distributed on the angular scale over 10 arcsec. The wide-field imaging (WFI) mode is achieved with an accumulation period shorter than the usual one of 1.6384 sec in Daejeon correlator at KJCC. Theoretically, the field of view (FoV) within an amplitude loss of 1%, 5%, and 10% is estimated on the basis of the time-average smearing effect due to a finite accumulation period [13]. The FoVs calculated for accumulation periods of 0.2048, 1.6384, and 3.2768 sec are summarized in Table 12, in the case of the highest angular resolution at Q-band of 0.6 mas with KaVA.

Table 12 : FoV within a given amplitude loss in each accumulation period$^{\ast}$.
Accumulation Amplitude loss
period 1.0% 5.0% 10.0%
(sec) (arcsec) (arcsec) (arcsec)
0.2048 8.6 19.4 27.4
1.6384 1.1 2.4 3.4
3.2768 0.5 1.2 1.7
to 0pt$^{\ast}$ Under an assumption of the highest angular resolution at Q-band of 0.6 mas with KaVA.

In the current available specification of Daejeon correlator, there is a trade-off between a shorter accumulation period and a larger number of IF channels to yield higher spectral resolution. The most highly recommended setup is the combination of C2 mode and an accumulation period of 0.2048 sec, in which both a sufficiently high velocity resolution (0.11 km s$^{-1}$ for 44 GHz methanol masers) and a sufficiently wide FoV (10 arcsec or more) can be obtained. Thus the recommended set-up for WFI mode is summarized in Table 13.

Table 13 : Recommended set-up for WFI mode in the current situation.
Correlation Sampling Bandwidth Accumulation Spectral
mode rate /IF period channels/IF
C2 1024 Mbps 128 MHz 0.2048 sec 8,192

The evaluation for the WFI tests was done by the following two ways: comparing the data of an accumulation period of 0.1 sec produced in DiFX to those of 0.2048 sec in Daejeon correlator, and comparing the latter data to the same data but with averaging in 3.2768 sec. These ways provide us a chance to estimate whether such an isolated maser can be detected or not and how much rate of the amplitude loss occurs. The evaluation might be updated on the basis of a comparison between a short-accumulation period data and a multi-tracking center data in the near future.

If you would like to require this WFI mode, please describe your requests in the following two items:

  • Requested setting parameters for WFI in the proposal cover sheet
  • Reasons for requiring WFI mode in the scientific justification

Finally, note that the file size of correlated data for WFI is as huge as ~600GByte. We therefore recommend to check and improve the performance of your internet environment and personal computer as high as possible for comfortable data downloading and data processing, respectively. Please refer an example parameters in Table 14:

Table 14 : Required performances of internet and personal computer.
Forward speed $\geq$ 10 MByte s$^{-1}$
HDD/SSD volume $\geq$ 1.5 TByte
RAM $\geq$ 16 GByte

Here, the experiment to verify the time-average smearing effect due to a finite accumulation period has been done, however we will also verify the bandwidth smearing effect to KaVA observations in the near future.

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KaVA ( KVN and VERA Array )