Dipole delays

A tile's dipole delays control where it is "pointing". Delays are provided as numbers, and this controls how long a dipole's response is delayed before its response correlated with other dipoles. This effectively allows the MWA to be more sensitive in a particular direction without any physical movement.

e.g. This set of dipole delays

 6  4  2  0
 8  6  4  2
10  8  6  4
12 10  8  6

has the North-East-most (top-right) dipole not being delayed, whereas all others are delayed by some amount. See this page for more info on dipole ordering.

Dipole delays are usually provided by metafits files, but can also be supplied by command line arguments, e.g.

--delays 6 4 2 0 8 6 4 2 10 8 6 4 12 10 8 6

would correspond to the example above. Note that these user-supplied delays will override delays that are otherwise provided.

Dipoles cannot be delayed by more than "31". "32" is code for "dead dipole", which means that these dipoles should not be used when modelling a tile's response.

Ideal dipole delays

Most (all?) MWA observations use a single set of delays for all tiles. Dipole delays are listed in two ways in a metafits file:

  • In the DELAYS key in HDU 1; and
  • For each tile in HDU 2.

The delays in HDU 1 are referred to as "ideal" dipole delays. A set of delays are not ideal if any are "32" (i.e. dead).

However, the HDU 1 delays may all be "32". This is an indication from the observatory that this observation is "bad" and should not be used. hyperdrive will proceed with such observations but issue a warning. In this case, the ideal delays are obtained by iterating over all tile delays until each delay is not 32.