Element Ordering¶

The element axis of kotekan data arrays (visibilities, gains, flags, baseband, …) indexes the telescope’s correlator inputs. Different telescopes, and different parts of the pipeline for the same telescope, arrange the same physical inputs in memory in different orders, so an element index is only meaningful together with an element ordering. The ElementOrder enum (lib/utils/Telescope.hpp) names the defined conventions, and the Telescope class converts between element indices, canonical input identities, and physical geometry.

Defined orderings¶

Configuration options and file attributes use the enum names as strings.

Name	Arrangement	Description
`CHIMECorrelator`	table-defined	Order as received by the CHIME X-engine; slightly scrambled, defined via the input-reorder table.
`CHIMECylinder`	`[C][P][CF]`	Cylinders west to east, then polarization, then feeds within a cylinder south to north.
`CHIMEBeamformer`	`[P][D]`	Polarization slowest, then feeds ordered as in `CHIMECylinder`.
`CHORDEarly`	`[D][P]`	Pre-pathfinder CHORD: dish-major with polarization fastest — element \(2d\) is polarization 1 and element \(2d+1\) is polarization 2 of dish \(d\). Dish ordering is defined by the telescope’s `dish_inputs` table.
`CHORDBeamformer`	`[P][D]`	Production CHORD: polarization slowest — element \(p N_d + d\) is polarization \(p\) of dish \(d\). Dish ordering is defined by the table.

Station IDs and geometry¶

Internally each physical input has an ordering-independent station ID (station_id_t). Conversions always go through it:

Telescope::element_index_to_station_id(el_idx, order) and the inverse station_id_to_element_index(st_id, order) translate between an element index in a given ordering and the canonical input. For CHORD the station ID encodes (dish, polarization) as \(d + p N_d\); for CHIME it encodes (cylinder, polarization, feed-in-cylinder).
station_id_to_main_array_grid_indices(st_id) returns the input’s integer (x, y) position in the main-array grid — x counting east, y counting north, from the southwest corner — or (-1, -1) for inputs that are not part of the main array (RFI antennas, unpopulated inputs, …).
station_id_to_feed_position_m(st_id) returns the input’s 3D position in the grid frame, in metres, including any per-feed displacement from the fiducial station position.

The convenience methods get_main_array_grid_indices(num_elements, order) and get_feed_positions_m(num_elements, order) return these for a whole element axis at once, in the requested ordering.

In configuration¶

Stages that interpret (rather than just relay) element-ordered data take an input_order option, set to one of the enum names above. These currently include N2Accumulate, N2TimeDownsample, calcBBPhase, hdf5N2Write, hdf5FileWrite, and the CUDA FRB beamformer kernels. The CHORD pathfinder pipeline uses CHORDEarly throughout.

In data files¶

The HDF5 writers record the ordering so files are self-describing:

N2 visibility files (hdf5N2Write) and Per-frame HDF5 dumps (hdf5FileWrite) files carry an input_order root/dataset attribute, plus per-element main_array_grid_indices (int64, \(N_e \times 2\)) and feed_positions_m (float64, \(N_e \times 3\)) attributes computed in that same ordering.
All element-indexed quantities in those files, i.e. the element axes of datasets, and the input_a / input_b indices in /index_map/prod, follow the recorded input_order.