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PH512
Professor Michael Smith
MULTIMEDIA ASTRONOMY
School of Physical Sciences
Convenor Prof. Michael Smith
Taught in Spring Term
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PH512
ECTS Credits 7.5
Kent Credits 15 at Level H
Assignment 3: Astrometry
1. Introduction
Astrometry is the branch of astronomy that relates to precise
measurements and explanations of the positions and movements of
stars and other celestial bodies.
Although once thought of as an esoteric field with little useful
application for the future, the information obtained by astrometric
measurements is now very important in contemporary research into
the kinematics and physical origin of our Solar System and our
Galaxy, the Milky Way.
Astrometry: the branch of astronomy concerned with the
measurement of the positions of celestial bodies on the celestial
sphere, conditions such as precession, nutation, and proper
motion that cause the positions to change with time, and
corrections to the positions due to distortions in the optics,
atmosphere refraction, and aberration caused by the Earth’s
motion.
Astronomers use astrometric techniques for the tracking of nearEarth objects.
It has been also been used to detect extrasolar planets by
measuring the displacement they cause in their parent star's
apparent position on the sky, due to their mutual orbit around the
center of mass of the system.
NASA's planned Space Interferometry Mission (SIM PlanetQuest
will utilize astrometric techniques to detect terrestrial planets orbiting
200 or so of the nearest solar-type stars.
Gaia is a European Space Agency (ESA) astrometry space mission,
and a successor to the ESA Hipparcos mission. It was included
within the context of the ESA Horizon 2000 Plus long-term scientific
programme in 2000. It is expected to be launched by the ESA in the
second half of 2011, and will be operated in a Lissajous orbit around
the Sun-Earth L2 Lagrangian point.
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Professor Michael Smith
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Gaia will compile a catalogue of approximately one billion stars to
magnitude 20. Its objectives comprise:
 astrometric (or positional) measurements, determining the
positions, distances, and annual proper motions of stars with
an accuracy of about 20 µas (microarcsecond) at 15 mag, and
200 µas at 20 mag
2. Coordinates
Accurate astrometric measurements of the position of an object, have as a goal
the determination of the sky coordinates of that object.
Common Coordinate Systems Used in Astronomy
– Horizon (altitude, azimuth system)
– Equatorial
– Ecliptic
– Galactic
Typical sky coordinates are the equatorial coordinates "Right
Ascension" (RA) and "Declination" (Dec), analogous to longitude and
latitude on the Earth.
One must also specify the specific "epoch," or year, of the equatorial
coordinate system being used, since the coordinate grid is defined by the
Earth's orientation, and the Earth slowly "precesses".
Most commonly 1950.0 or by now 2000.0 (noting that there is a slight
difference between Besselian B1950 and Julian J2000 epochs, so these two
systems are not related solely by precession).
Related concepts are sidereal time (right ascension currently crossing the
observer's meridian) and hour angle (RA difference between an object and the
sidereal time).
Declination is defined purely by the Earth's equator and poles;
right ascension requires an arbitrary zero point.
Aberration Observations from a moving platform (all observations) suffer
aberration in the arrival direction of starlight, due to the finite speed of light
(a.k.a. the umbrella effect). To high accuracy, if we look at an angle θ to the
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Professor Michael Smith
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instantaneous motion with respect to some constant reference frame (say the
Sun's motion), the displacement is δ θ = v sin θ /c. The amplitude of this
annual aberration is 30 km/s × 206264.8 arcsec / c or 20 arcseconds in each
direction.
A given star then sweeps out an apparent ellipse of this semi-major axis each
year. There also exists diurnal aberration, caused by the Earth's rotation; its
amplitude is much smaller at 0.32 arcsecond. Differential aberration across the
field of view is actually an issue for HST observations; one doesn't want to pick
the wrong instrument as the primary for certain observations as that will induce
PSF blurring in one far from the optical axis.
The are several effects that cause the coordinates of a star to
deviate from those given in star catalogs.
– Precession
– Nutation
– Proper Motion
– Parallax
– Atmospheric Refraction
3. Narrow-field astrometry
Most high-precision astrometry uses differential measures across a
small field, using some set of local standard stars (an exception
is the Hipparcos global solution).
Here, we define some mapping from celestial to image coordinates,
and determine the constants of the mapping by using coordinates
of well-known stars in the same image. This determination as
known historically as a plate solution.
The reference stars must finally tie back into sets of fundamental
stars, measured using transit or zenith instruments fixed to the
Earth. Such sets include the FK3 and FK4, Perth-70, and at lower
accuracy but larger numbers, the SAO and HST-GSC catalogues.
The USNO catalogue is a significant improvement over the GSC.