... AAS link ] . ... Our internal perspective makes it difficult to determine the spiral structure of our own galaxy. ... H I absorption, on the other hand, traces colder, denser gas that should be more confined in discrete clouds and perhaps discrete spiral arms. In addition, H I self-absorption (HISA) requires background H I emission at the same velocity as the foreground absorbing gas, thus introducing a much-needed constraint on the large-scale sightline geometry and kinematics. ...
... 2.3 Data acquisition Spectra generated by the Multibeam correlator contain the usual structure seen in spectra from other H i instruments: a bandpass spectrum which is the sum of the sky , ground and receiver temperatures multiplied by the product of the filters in the receiver chain; superimposed on this is a noise ... Note also that sources extended much beyond ,40 arcmin can be imaged poorly in HIPASS images because of the bandpass sidelobe effect described in Section 3.3. ...