System Temperature Corrections

The raw VLA correlator products are expressed in units of the system temperature, T$_{sys}$, which is measured and recorded over both a 50 MHz wide passband (the ``front-end'' T$_{sys}$), and over the narrower baseband defined by the spectral channels (the ``back-end'' T$_{sys}$, e.g., over 3.125 MHz for BW Code 4 in Appendix A.2). The T$_{sys}$ averaged over the front-end is used by default, since it has a higher S/N. Recall the system temperature can be written,

$$T_{sys} = T_{bg} + T_{atm} + T_{rec} + ... + T_{RFI} + T_{spill} + T_{line}$$ (3.6)

with the various terms representing contributions from radio background sources, the atmosphere, receiver electronics, etc. The last three terms represent the contributions from radio interference, ground emission (``spillover'') coming into feed and/or antenna sidelobes, and the emission line itself. When RFI is so strong that it affects the system temperature, one probably has to discard the data. For observations of weak lines (e.g., extragalactic HI at reasonable elevations) when RFI is not a problem, the last two terms can be ignored. However, there may be times when one of these is large enough to affect T$_{sys}$, and possibly affect the quality of your data or calibration.