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zenith
zenith angle
zero-spacing flux density
zip
zoom

zenith

The point on the celestial sphere that is directly overhead, i.e. the projection onto the celestial sphere of the local vertical.

zenith angle

The angle between the direction to the zenith (local vertical) and the direction to a celestial object, i.e., 90°-(elevation ). Also known as the zenith distance.

zero-spacing flux density

An interferometer composed of finite-sized elements cannot measure the visibility V(u,v) (^ denotes Fourier transform) of the apparent sky brightness distribution f(x,y) in a neighborhood around u = v = 0. Allowing that f contains the weighting of the true sky brightness distribution by the primary beam pattern, the so-called zero-spacing flux density V(0,0) is equal to the (beam-weighted) integrated flux density of the sources in the field of view---i.e., it is given by

When the hole in the u-v coverage near the origin is fairly large, image reconstruction methods, such as the Högbom CLEAN algorithm, may interpolate the measured data poorly within this central region. This difficulty frequently is manifested by the appearance of a negative bowl artifact---a negative `baseline' beneath the reconstruction of f---owing to the reconstruction method having underestimated the zero-spacing flux density. The algorithm may therefore benefit if a datum at u = v = 0 is included when the dirty image is constructed.

For a well-isolated sources, the zero-spacing flux density estimate can be derived from single-dish measurements. Providing a proper estimate in the general case is difficult, however, because measurements with different primary beam patterns may be contaminated by differently by "confusing" sources. The best estimate corresponds to the flux density that would be measured by a telescope with the same primary beam response as the array elements. Also, it is not just a single datum V(0,0) which is missing, but rather the samples from a region---so determining the proper weighting for the zero-spacing information is tricky.

zip

1. Verb used to describe creation of a compressed MS-DOS archive (zipfile) from one or more files using PKWare's sliding-window PKZIP, or a compatible file-compression routine.

2. The name of a Unix utility that is compatible with PKZIP.

zoom

A procedure for changing the effective magnification of an image display by averaging, interpolating, or simply replicating, the intensities of adjacent pixels.

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