Multibeam Geometry

In order to define the position of the feeds of the multibeam at a particular time and sky location you need to know a few angles and a few equations:

  • Feed position angle (FA): relative to the line connecting beam1 to beam3, angles are positive increasing towards beam 10 and negative decreasing towards beam 9.
  • Telescope feed angle (TA): this is the angle that you enter into TCS as fdang.
  • Parallactic angle (PA): The usual angle. On showtel, it is defined to be negative before transit (not sure if this is true for North sources).
  • The projected position angle of the feed on the sky is:

PPA = FA + TA + PA

  • The RA and DEC of the feeds can be calculated using the RA and DEC of the centre beam (RA1 and DEC1) , the declination of the source, and radial offsets (ROFF) of the feeds, which 29.1 arcminutes for the inner 6 feeds and 50.8 arcminutes for the outer feeds.

RA = RA1 + ROFF*sin(PPA)/cos(DEC)