East Asian VLBI Network (EAVN)

Gain Calibration

VERA, KVN, and NRO45 have the chopper wheel of the hot load (black body at the room temperature), and the system noise temperature can be obtained by measuring the ratio of the sky power to the hot load power (so-called R-Sky method). Thus, the measured system noise temperature is a sum of the receiver noise temperature, spillover temperature, and contribution of the atmosphere (i.e. so-called T* sys corrected for atmospheric opacity). The hot load measurement can be made before/after any scan. Also, the sky power is continuously monitored during scans, so that one can trace the variation of the system noise temperature. The system noise temperature value can be converted to SEFD (System Equivalent Flux Density) by dividing by the antenna gain in K/Jy, which is derived from the aperture efficiency and diameter of each telescope.

T* sys data (TY table) and antenna gain information (GC table) are provided with the ANTAB-readable format in AIPS. The TY and GC tables can be loaded by the AIPS task ANTAB, and these tables are converted to the SN table by the AIPS task APCAL.

On the other hand, T *sys measurement provided by Tianma65 contains atmospheric opacity effects, thus the opacity correction should be applied to Tianma's data in the course of data reduction

Alternatively, one can calibrate the visibility amplitude by the template spectrum method, in which auto-correlation spectra of a maser source is used as the ux calibrator. This calibration procedure is made by the AIPS task ACFIT (see AIPS HELP for ACFIT for more detail). For an EAVN observation including Tianma65 and/or NRO45, we strongly recommend users to observe a maser source or a compact continuum gain calibrator for every ≤1 hr. This offers an additional cross-check of the amplitude calibration for Tianma65/NRO45. Along with this, these two telescopes will do regular antenna pointing scans for every ≤ 1 - 2 hr.

As for Tianma65, moreover, frequent pointing check is necessary for observations at bot K- and Q-bands. The pointing check is done semi-automatically with a continuum back-end system and the quality of pointing check is judged by on-site operators, who decide whether or not to apply the correction. We strongly recommend to keep at least 3 minutes for the pointing check itself with additional slewing time between target and pointing sources. For example, it is preferable to secure 5-min gap in total for the pointing check toward a pointing source with the angular separation of ~ 15◦ from the target.

Further correction is made for VLBI observations taken with 2-bit (4-level) sampling, for the systematic effects of non-optimal setting of the quantizer voltage thresholds. This is done by the AIPS task ACCOR. Another correction should be applied to recover the amplitude loss, which are attributed to the combination of two steps of 2-bit quantization in the digital filtering at the backend system and characteristics of Daejeon correlator. This is done by multiplying the scaling factor of 1.3 (the best current estimation)[4] in the AIPS task APCAL (adverbs APARM(1) = 1.3, OPCODE = '', and DOFIT = 1) or SNCOR (adverbs OPCODE = 'MULA', and SNCORPRM(1) = 1.3). Note that this correction should be applied to all EAVN telescopes. The amplitude calibrations with EAVN are accurate to 15% or better at both K- and Q-bands.




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EAVN ( East Asia VLBI Network )