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Data Reduction Case #5

Beam Switched Dual Beam Data

In GBT parlance, "beam switching" refers to the cycling of an electronic beam switch in the frontend. Beam switching is used with multibeam receivers. Although there may be applications for beam switching in the future, it is not a recommended observing mode for most programs, at this time.

Data reduction of the beam switched mode is similar to Case #1, see HowToReducePSwitchSingleBeamData. However, whereas the user has one set of sig/ref data in a total power position switched observations, there are four sets of sig/ref data for each integration of a beam switched, dual beam observation. Data are typically taken while nodding the telescope, and so two scans comprise the observing procedure. In the first scan the source is centered on beam 1, and in the second scan the source is centered in beam 2. For each scan, the beam switch is in either the "cross" or the "thru" position. The following table represents the data organization:

           Cross Position                  Thru Position


           Beam 1    Beam 2             Beam 1    Beam 2
        ----------------------       ----------------------
Scan 1  |   Sig1   |  Ref2   |       |   Sig3   |  Ref4   |
        ----------------------       ----------------------
Scan 2  |   Ref1   |  Sig2   |       |   Ref3   |  Sig4   |
        ----------------------       ----------------------

Each entry in the table includes a register for calon and caloff data, and usually there are two polarizations for each entry as well.

Calibration of beam switched data is complex. There are several "correct" ways to calibrate beam switched data, and the user must decide which is best for his program. In this example, we will discuss only one possibility: data from each of the corresponding sig/ref pairs in the above table can be calibrated as if the data were taken in Total Power Single Beam mode. That is, Sig1 is matched with Ref1 for the purpose of calibration, Sig2 is matched with Ref2, and so on. Hence the two scans result in four independent spectra which are then averaged (with weighting) to produce the final result. See HowToReducePSwitchSingleBeamData for the details on handling each of the Sig/Ref pairs.

Afterwards, calibration, reduction and analysis proceed as described in HowToReducePSwitchSingleBeamData.

-- JimBraatz - 27 Jul 2004