WFCAM Pipeline Design

Jim Lewis, Simon Hodgkin, Dafydd Evans, & Mike Irwin

23rd October 2002

1  Glossary of Terms

channel - data channel from each 2kx2k detector through to the backend of the pipeline
pointing - a telescope slew to position with new guide star acquisition. Note that microstep sequences and dither sequences are done within one pointing.
read - a read is the act of physically reading a detector. This generally means the act of digitising the data, and can be either destructive (i.e. followed by a reset) or non-destructive
exposure - an exposure is a sequence of one or more reads, that are used to produce a single output image
integration - an integration is the result of a sequence of one or more exposures. Whereas the exposure is the result of a sequence of reads, the integration is a summation (or similar) of a sequence of exposures
darks - reference exposures of the same duration as the images in an exposure sequence used for correcting detector artefacts
MSB - a Minimum Schedulable Block is a sequence of integrations that makes up a largely self-calibrating and useful scientific observation. The component parts of an MSB are defined below and in turn are constrained by the scientific requirements on flatfielding accuracy, astrometric and photometric calibration and sky removal.
mesostep sequence - a type of MSB being a sequence of exposures designed to completely sample across the face of a detector to monitor residual systematics in the photometry
focus run - a type of MSB with a series of exposures for use in providing an estimate of best focus
microstep sequence - a sequence of 1x1, 2x2 or 3x3 integrations taken with accurate sub-pixel (including small integral) shifts combined by interleaving the pixels
offset - a macrostep to another region of sky within an MSB sequence which may be either another target region or an offset region used for sky estimation
observation - an observation is a sequence of integrations that makes up a usable scientific observation. An observation is the level at which calibration frames (flats, skies, darks etc.) are collected
reduced frame - result of reducing an integration
superframe - the result from combining (interleaving) the reduced frames from a microstep sequence. The superframe can be 2kx2k, 4kx4k or 6kx6k depending on microstep mode
loops - a contiguous sequence of multiple exposures in one place to increase integration time
dithers - sequence of integrations offset in "spiral" pattern for use in bad pixel rejection, deep stacking etc.
tile - the four pointings and observing sequences necessary such that a combination of them covers a contiguous area of sky involving all four detectors
coadd - image resulting from combining exposures taken sequentially during an integration
stack - image resulting from combining frames from a single detector, including combining superframes
mosaic tile - complete contiguous image combined from using exposures from all detectors
confidence map - a normalised inverse variance weight map defining the "confidence" associated with the intensity value in each pixel, this also encodes for hot pixels, bad pixels, dead pixels and so on. The raw confidence maps for each frame output by the Data Acquisition System (DAS) will be derived from regular (frequency TBD) analysis of the calibration flats and darks. All output processed frames (stacked, tiled, mosaiced etc.) will also have an associated derived confidence map.

File translated from TEX by TTH, version 3.30.
On 24 Jan 2003, 15:49.