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UVT versions

The analysis of $ (u,v)$ data needs to handle two frequency referentials: the topocentric frame that corresponds to the observed frequency and the source or rest frame. The former defines the angular resolution of the telescope, or the $ (u,v)$ coordinates that are expressed wavelength units, the latter is used for the identification of lines or in converting frequency shifts to velocity shifts. Both are linked through equation 40.

Historically (for GDF v1), the frequency axis was in the topocentric frame. The rest frequency was given separately. The spectral resolution did not distinguish between topocentric and source frame: it was only approximate

With GDF v2, UV table contains a UV Tables version stored in the table header. Version 2.0 corresponds to the historical GDF v1 implementation (see above). This version has both a lousy spectral axis definition and a lousy angular resolution. For narrow bands, it can still produce correct results, provided that the correct rest frequency is accounted for (option /FREQUENCY of command TABLE).

UV Table versions 2.1 and 2.2 provide a clear definition of the spectral axis: it is the rest frequency, in the source frame (at the source velocity in the selected system frame). The spectral resolution is in that same source frame. The doppler factor is provided to convert the source frame frequency to topocentric frame for computing the effective angular resolution from the u,v coordinates measured as a length in meters.

The difference between versions 2.1 and 2.2 is that in 2.1 the mean Doppler factor is provided where with version 2.2, a time dependent Doppler factor is stored as an extra column. Note that if the observatory section of the uv table is filled, the time dependent Doppler factors can be recomputed.


next up previous contents index
Next: Command TABLE Up: ( ) tables Previous: GDF v2   Contents   Index
Gildas manager 2022-01-17