Setting circles
Encyclopedia
Equatorial mount
An equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...
to find astronomical objects in the sky by their equatorial coordinates
Equatorial coordinate system
The equatorial coordinate system is a widely-used method of mapping celestial objects. It functions by projecting the Earth's geographic poles and equator onto the celestial sphere. The projection of the Earth's equator onto the celestial sphere is called the celestial equator...
often used in star chart
Star chart
A star chart is a map of the night sky. Astronomers divide these into grids to use them more easily. They are used to identify and locate astronomical objects such as stars, constellations and galaxies. They have been used for human navigation since time immemorial...
s or ephemeris
Ephemeris
An ephemeris is a table of values that gives the positions of astronomical objects in the sky at a given time or times. Different kinds of ephemerides are used for astronomy and astrology...
.
Description
Setting circles consist of two graduated disks attached to the right ascensionRight ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
(RA) and declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
(DEC) axis of an equatorial mount
Equatorial mount
An equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...
. The right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
disk is graduated into hours, minutes, and seconds. The declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
disk is graduated into degrees, minutes, and seconds. Since right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
coordinates are fixed to the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
the RA disk is usually driven by a clock mechanism in sync with sidereal time
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...
. Locating an object on the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
with settings circles is similar to finding a location on a terrestrial map using latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
and longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
. Sometimes the right ascension setting circle has two scales on it: one for the Northern and one for the southern hemisphere.
Research telescopes
Historically setting circles have rivaled the telescopes optics as far as difficulty in construction. Making a set of setting circles required a lot of precision crafting on a dividing engineDividing engine
A dividing engine is a device specifically employed to mark graduations on measuring instruments.-History:There has always been a need for accurate measuring instruments...
. Setting circles usually had a large diameter and when combined with a vernier scale
Vernier scale
A vernier scale is an additional scale which allows a distance or angle measurement to be read more precisely than directly reading a uniformly-divided straight or circular measurement scale...
could point a telescope to nearly an arc minute of accuracy. In the 20th century setting circles were replaced with electronic encoders
Rotary encoder
A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed...
on most research telescopes.
Portable telescopes
In amateur astronomyAmateur astronomy
Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...
, setting up a portable telescope equipped with setting circles requires:
- Polar alignmentPolar AlignmentPolar alignment is the act of aligning the rotational axis of a telescope's equatorial mount in parallel with that of the Earth. There are various ways to achieve this.- Alignment Methods :...
- The telescope must be aligned with either the north celestial poleCelestial poleThe north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...
or the south celestial pole. PolarisPolarisPolaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....
is roughly at the north pole, while Sigma OctantisSigma OctantisSigma Octantis is a magnitude 5.6 star in the constellation Octans most notable for being the current South Star. Sigma Octantis is approximately 270 light years from Earth, and is classified as a giant, with a spectral type of F0 III...
is roughly at the south pole. - Setting Right Ascension - After polar alignment, the observer uses a calculator or a known star to synchronize the right ascension circle with Sidereal Time.
Accuracy of pointing the telescope can be hard to achieve. Some sources of error are:
- Less-than-perfect polar alignment
- The optical tube not being perpendicular to the declination axis
- The declination and right ascension axis not being perpendicular
- Errors in rotating the setting circles when setting up
- Errors in reading the setting circles
- Confusion between Northern and Southern hour angles (Right Ascension)
It is common to blame an unlevel tripod as a source of error, however when a proper polar alignment is performed, any induced error is factored out.
These sources of error add up and cause the telescope to point far from the desired object. They are also hard to control; for example, Polaris is often used as the celestial north pole for alignment purposes, but it is over half a degree away from the true pole. Also, even the finest graduations on setting circles are usually more than a degree apart, which makes them difficult to read accurately, especially in the dark. Nothing can be done if the optical tube is not perpendicular to the declination axis or if the R.A. and Dec axes are not perpendicular, because these problems are next to impossible to fix.
In the southern hemisphere the Right Ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
scale operates in reverse from in the Northern Hemisphere. The term Right Ascension took its name from early northern hemisphere observers for whom "ascending stars" were on the east or right hand side. In the southern hemisphere the east is on the left when an equatorial mount
Equatorial mount
An equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...
is aligned on the south pole. Many Right Ascension setting circles therefore carry two sets of numbers, one showing the value if the telescope is aligned in the northern hemisphere, the other for the southern.
Even with some inaccuracies in polar alignment or the perpendicularity of the mount, setting circles can be used to roughly get to a desired object's coordinates, where a star chart can be used to apply the necessary correction. Alternatively, it is possible to point to a bright star very close to the object, rotate the circles to match the star's coordinates, and then point to the desired object's coordinates. Setting circles are also used in a modified version of star hopping
Star hopping
Star hopping is a technique that is often used by amateur astronomers to locate objects in the night sky. It can be used in place of or in conjunction with setting circles.-The problem:...
where the observer points the telescope at a known object and then moves it a set distance in RA or declination to the location of a desired object.
Digital setting circles
Digital setting circles (DSC) consist of two rotary encoderRotary encoder
A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed...
s on both axis of the telescope mount and a digital readout. They give a highly accurate readout of where the telescope is pointed and their lit display makes them easier to read in the dark. They have also been combined with microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
s to give the observer a large database of celestial objects and even guide the observer in correctly pointing their telescope.
In contrast to a GOTO
GoTo (telescopes)
In amateur astronomy, "GoTo" refers to a type of telescope mount and related software which can automatically point a telescope to astronomical objects that the user selects...
telescope mount, a mount equipped with DSC alone is sometimes called a "PUSH TO" mount.