{{unreferenced, date=February 2015
Star hopping is a technique that
amateur astronomers
Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur astronomers ...
often use to locate
astronomical object
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
s in the
night sky
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
Natural light sources in a night sky include ...
. It can be used instead of or in addition to
setting circles
Setting circles are used on telescopes equipped with an equatorial mount to find astronomical objects in the sky by their equatorial coordinates often used in star charts or ephemerides.
Description
Setting circles consist of two graduated disks ...
or go-to/push-to systems.
The problem
Many
celestial objects
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
of interest are too faint to be visible to the unaided eye.
Telescopes
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
or
binoculars
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
collect much more light, making faint objects visible, but have a smaller field of view, thus complicating orientation on the sky.
The field of view of binoculars is rarely more than eight degrees, while that of typical amateur telescopes may be substantially less than one degree, depending on the magnification used. Many objects are best observed using higher magnifications, which inevitably go along with narrow fields of view.
The technique
Star hopping uses bright
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s as a guide to finding fainter objects. A knowledge of the relative positions of bright stars and target objects is essential. After planning the star hop with the aid of a
star chart
A star chart is a celestial map of the night sky with astronomical objects laid out on a grid system. They are used to identify and locate constellations, stars, nebulae, galaxies, and planets. They have been used for human navigation since ...
, the observer first locates one or more bright stars in a
finderscope
A finderscope is an accessory sighting device used in astronomy and stargazing, typically a small auxiliary refracting telescope/monocular mounted parallelly on a larger astronomical telescope along the same line of sight. The finderscope usuall ...
,
reflex sight
A reflector sight or reflex sight is an optical sight that allows the user to look through a partially reflecting glass element and see an illuminated projection of an aiming point or some other image superimposed on the field of view. These sigh ...
, or, at a low magnification, with the instrument to be used for observation. The instrument is then moved by one or more increments, possibly using a
reticle
A reticle, or reticule also known as a graticule, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the screen of an oscilloscope, ...
to identify specific
angular distance
Angular distance \theta (also known as angular separation, apparent distance, or apparent separation) is the angle between the two sightlines, or between two point objects as viewed from an observer.
Angular distance appears in mathematics (in pa ...
s, to follow identified patterns of stars in the sky, until the target object is reached.
Using a telescope equipped with a properly aligned
equatorial mount
An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis, the polar axis, parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras. The ...
, the observer may also follow the
equatorial coordinate system
The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fund ...
on a star map to "hop" or "slide" along the lines of
right ascension
Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the earth.
When paired w ...
or
declination
In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
from a well known object to find a target. This can be assisted using setting circles.
Once an instrument is centered on the target object, higher magnifications may be used for observation.
Example
A simple example of star hopping would be finding
Messier 13
Messier 13 or M13, also designated NGC 6205 and sometimes called the Great Globular Cluster in Hercules or the Hercules Globular Cluster, is a globular cluster of several hundred thousand stars in the constellation of Hercules.
Discovery and vis ...
, a
globular cluster
A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...
in the constellation
Hercules
Hercules (, ) is the Roman equivalent of the Greek divine hero Heracles, son of Jupiter and the mortal Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures.
The Romans adapted the Gr ...
, which is too faint to be seen by the unaided eye under most conditions. As shown on the star chart, M 13 lies on a line connecting the stars ζ Herculis and η Herculis. Using star hopping techniques an observer would first identify these two by the naked eye and then point an instrument (binocular or telescope) two thirds of the way up from ζ, one third down from η to see M 13. An observer using a telescope equipped with an equatorial mount would pan down from η in declination to site M 13.