Glossary

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 better).

(large) Picture of focal plane;

Comparison 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.

Making a tile: Ysteps;

Making a tile: Xsteps;

Resulting Exposure 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 twice.