| The naming of cats is a delicate matter... |
T.S.Elliot |
The naming of 2dF targets is complicated by the fact that the tiling strategy means that each object may occur on several fields. This problem is illustrated by the following diagram:
Targets are tagged by the colour of the field to which they are assigned, and are designated by a `T' within that field. Any target which occurs serendipitously on a second field is labelled with the name given to the target on its assigned field, but with the `T' changed to an `X' to denote the fact that the present field is NOT the field to which this target is assigned.
The second letter of each name refers to a `G'alaxy or `M'erger target for the galaxy survey, or a `Q'uasar target for the QSO survey. Mergers are targets that were flagged as having a high probabilty (k >1.4) of being a merged object, and so were tweaked by GBDs algorithm.
The third letter denotes `S'GP or `N'GP to avoid confusion, and the two numbers separated by a Z (Matthew didn't like underscores!) give the number of the target's assigned field and the number of the target on that field -- Quasar numbering begins after galaxy numbering, so don't expect to see a Quasar with 001 anywhere in there!
Any object can therefore be referred to as Gh???Z??? or Qh???Z??? where h is the hemisphere, and G may be replaced by M if the object is a merger.
OK, that was the easy bit...
The next point is that a target may be observed on one of the fields to which it is assigned, depending on how the configuration constraints work out, and it may even be observed on more than one field if there are free fibres left somewhere that can't be assigned to anything more useful.
All of this begins to break down a little with changing parameters of the system, as targets are allocated to a given field to be observed by GBDs code, but sometimes can't actually be observed on that field if the desired fibre is missing (for example). In this event the import routine tries to make the best it can of a bad job by first trying to recover the target that was lost, but with a different fibre, or by trying to recover other targets that were designated as 'unassignable', given that there's now a gap somewhere because a fibre hasn't been placed where it should have been.
I hope that makes sense.. please get back to me with any queries!