A zenith camera is an astrogeodetic telescope used today primarily for the local

survey Survey may refer to: Statistics and human research * Statistical survey, a method for collecting quantitative information about items in a population * Survey (human research), including opinion polls Spatial measurement * Surveying, the techniq ...

s of Earth's gravity field. Zenith cameras are designed as transportable field instruments for the direct observation of the

plumb line A plumb bob, plumb bob level, or plummet, is a weight, usually with a pointed tip on the bottom, suspended from a string and used as a vertical reference line, or plumb-line. It is a precursor to the spirit level and used to establish a vertic ...

(

astronomical latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

and

longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...

) and

vertical deflection The vertical deflection (VD) or deflection of the vertical (DoV), also known as deflection of the plumb line and astro-geodetic deflection, is a measure of how far the gravity direction at a given point of interest is rotated by local mass anom ...

s.Hirt., C; Bürki, B.; Somieski, A.; Seeber, G. ''Modern Determination of vertical deflections using digital zenith cameras''. Journal Surveying Engineering 136(1), Feb 2010, 1-12.
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Instrument

A zenith camera combines an optical lens (about 10–20 cm aperture) with a digital image sensor ( CCD) in order to image stars near the zenith. Electronic levels (tilt sensors) serve as a means to point the lens towards zenith. Zenith cameras are generally mounted on a turnable platform to allow star images to be taken in two camera directions (two-face-measurement). Because zenith cameras are usually designed as non-tracking and non-scanning instruments, exposure times are kept short, at the order of few 0.1 s, yielding rather circular star images. Exposure epochs are mostly recorded by means of the timing-capability of

GPS The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...

-receivers (time-tagging).

Data processing

Depending on the CCD sensor - lens combination used, few tens to hundreds of stars are captured with a single digital zenith image. The positions of imaged stars are measured by means of digital image processing algorithms, such as image moment analysis or point spread functions to fit the star images. Star catalogues, such as Tycho-2 or

UCAC Undeb Cenedlaethol Athrawon Cymru ( Welsh, the sole official name, meaning 'National Union of Teachers of Wales') commonly referred to as UCAC, was formed in Wales from a split from the NUT in the late 1940s over the issue of the Welsh language ...

-3 are used as celestial reference to reduce the star images. The zenith point is interpolated into the field of imaged stars, and corrected for the exposure time and (small) tilt of the telescope axis to yield the direction of the plumb line.

Accuracy and applications

If the

geodetic coordinates Geodetic coordinates are a type of curvilinear orthogonal coordinate system used in geodesy based on a ''reference ellipsoid''. They include geodetic latitude (north/south) , ''longitude'' (east/west) , and ellipsoidal height (also known as geod ...

of the zenith camera is known or measured, e.g., with a

-receiver, vertical deflections are obtained as result of the zenith camera measurement. Zenith cameras deliver astronomical coordinates and vertical deflections accurate to about 0.1 seconds of arc. Zenith cameras with CCD image sensors are efficient to collect vertical deflections at about 10 field stations per night. Zenith cameras have been used for accurate local surveys of Earth's gravity field (

geoid The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...

, quasigeoid).

Instrument

Data processing

Accuracy and applications

See also

References

Further reading