Integration - a simple snapshot,
within the Data Acquisition System (DAS), of a specified elapsed
time. This elapsed time is known as the Detector Integration Time
- DIT secs.
Exposure - the stored product of
many individual integrations, that have been co-added in the DAS.
Each exposure is associated with an exposure time.
Microstep (pattern) - a pattern
of exposures at positions each shifted by a very small movement
(<3 arcsec) from the reference position. Unlike a jitter the
non-integral part of the shifts are specified as 0.5 of a pixel,
which allows the pixels in the series to be interleaved in an effort
to increase resolution. A microstep pattern can be contained within
each position of a jitter pattern.
Jitter (pattern) - a pattern of exposures
at positions each shifted by a small movement (<30 arcsec) from
the reference position. Unlike a microstep the non-integral part
of the shifts is any fractional number of pixels. Each position
of a jitter pattern can contain a microstep pattern.
Pawprint - the 16 non-contiguous images
of the sky produced by the VISTA IR camera, with its 16 non-contiguous
chips. The name is from the similarity to the prints made by the
padded paw of an animal (the analogy suits earlier 4-chip cameras
of focal plane;
with other Cameras
Tile - a filled area of sky fully sampled
(filling in the gaps in a pawprint) by combining multiple pawprints.
Because of the detector spacing the minimum number of pointed observations
(with fixed offsets) required for reasonably uniform coverage is
6, which would expose each piece of sky, away from the edges of
the tile, to at least 2 camera pixels.
a tile: Ysteps;
a tile: Xsteps;
time map for a filled tile of 6 pawprints (no jittering). Dark
green = 1, light green = 2, magenta = 3, red = 4, yellow = 6, in
units of the single-pawprint exposure time. For larger surveys the
dark green areas at top and bottom will be overlapped by corresponding
areas from other tiles and the whole area wil be covered at least