KVN and VERA Array (KaVA)

C New observation mode from 2019A

New observation mode, phase referencing (relative astrometry) mode, is available from the 2019A semester. Details are summarized in the following subsection.

C.1 Phase referencing mode

KaVA phase-referencing mode is opened as a risk sharing. We have confirmed that relative astrometric observation with KaVA can potentially achieve positional accuracies of ∼20 microarcseconds (µas) in right ascension and ∼40 µas in declination, where K-band (22 GHz) data was used for the evaluation (see Fig. 12 and Table 15)1.

Figure 12: (Top row) Phase-referenced images of 0556+238, relative to the phase reference source 0601+245. Only VERA data was used for making the images. The dates of the observations are September 28th, October 30th and November 8th in 2017 from left to right images. (Bottom row) Same as the top rows, but for KaVA data, used for making the images.

C.1.1 Combination of dual-beam and fast switching observations

In an astrometric observation, the observation using dual-beam for VERA and singlebeam for KVN is recommended. Since KVN should be operated by fast-switching mode, antenna scan difference between KVN and VERA should be cared in data reduction. For the data reduction, a flag file is generated based on an antenna log and is provided to an observation PI. Also, it is strongly recommended to observe pair sources with small separation angle (e.g., less than 1 degree) at high elevation (see Table 16).

This will reduce the position errors caused by the residuals in atmospheric zenith delay. Note that Table 16 could be used to estimate accuracies of relative astrometry for VERA and KaVA, since the maximum baselines of both arrays are consistent with each other (i.e. positional accuracy of relative astrometry can be given by ∼ θsep c∆τ |B| where θsep is the separation angle between target and phase reference sources, c is the speed of light, |B| is the baseline length and ∆τ is the uncertainty in a delay measurement, see Reid & Honma 2014).

C.1.2 Digital filter mode for relative astrometric observation

In the relative astrometric observation with KaVA, digital filter mode of “VERA1” (see Table 7) can be used in the case of pair observation of continuum sources (same as Fig. 12). Digital filter mode of “VERA7” or “VERA7MM” (see Table 7) could be used in the case of pair observation of line (e.g. H2O maser) and continuum sources, although the setup has been under evaluation for KaVA array. 2

C.1.3 Data reduction

Generally, users are encouraged to carefully carry out data reduction in consultation with a contact person in the KaVA project group. Procedure of astrometric data reduction for VERA data has been summarized in previous papers (e.g. Fig. 11 of Kurayama et al. 2011; Fig. 5 of Imai et al. 2012). Basically, the procedure of data reduction for KaVA data is consistent with that for VERA data, expect for some points. For instance, parallactic angle should be corrected for KVN array (e.g. with the NRAO AIPS task ”CLCOR” combined with the OPCODE = "PANG"), while no need to calibrate the parallactic angle for VERA array (field rotator is equipped in the VERA array).

C.1.4 Note

After the huge earthquake on 11 March 2011, the crustal motion at Mizusawa station is still unstable and may not be simply approximated by linear motion, and this could potentially degrade the astrometric accuracy.

1 We have not finished for evaluation of parallax accuracy with KaVA

2 The digital filter modes, “VERA7” and “VERA7MM”, have been used regularly for VERA astrometric observations.

KASI logo image

KaVA ( KVN and VERA Array )