Appendix E
Special Considerations for EVLA Data Calibration and Imaging in AIPS

The old VLA with its once state of the art, but now dated, correlator and electronics has been turned off. The new electronics and correlator of the EVLA1 have been turned on and made available to users. Observing is now on the customary terms although a “Resident shared risk” program is still available for certain esoteric, not fully commissioned, capabilities. Data may have many thousands of spectral channels and up to 8 GHz of bandwidth per polarization. AIPS software is capable of handling these data.

The EVLA has already produced amazing scientific results although the large data volume has created difficulties. Delays are sometimes not set accurately, causing the data analysis to begin with the “VLBI” task FRING needed to correct large slopes in phase across the bandpass. The flagging information known to the on-line system (telescope off source and the like) is now available in the data format and so can reach AIPS either as an initial flag table or as already flagged data. Substantial flagging effort may therefore still be required, particularly to deal with RFI which inevitably occurs in the wide bandwidths. System temperatures and gains are now transferred, but should be applied with caution to scale the visibilities and to compute data weights. The data weights without this adjustment reflect only integration time. The weather table is available with the data so that reasonable opacities may be determined. However, the “over-the-top”, table which is used in determining antenna positions, the frequency offset table, used in managing Doppler tracking, and the CQ table, used to correct amplitudes for spectral averaging in the presence of non-zero delays, are not available if the data come to AIPS via CASA. The OT table is now provided when the data are read using the OBIT package, but astrometric data used to compute accurate projected spacings are just now becoming available.

The following guide will not assume that all issues have been fixed. Steps that can be omitted or simplified when they are will be described. This appendix is written with the assumption that the reader is moderately familiar with AIPS as described in the preceding chapters. It is also written with the assumption that you are using current versions of the 31DEC11 or later releases of the software.

 E.1 Getting your data into AIPS
 E.2 Initial calibration — VLANT, FRING
 E.3 Initial editing
 E.4 Basic calibration
 E.5 Detailed flagging
 E.6 Calibration with the SysPower table
 E.7 Polarization calibration — RLDLY, PCAL, and RLDIF
 E.8 Target source data — edit and SPLIT
 E.9 Spectral-line imaging hints
 E.10 Continuum imaging hints
 E.11 Additional recipes
 E.12 Concluding remarks, early science