- Setting circles
Setting circles consist of two graduated disks attached to the right ascension (RA) and declination (DEC) axis of an equatorial mount. The right ascension disk is graduated into hours, minutes, and seconds. The declination disk is graduated into degrees, minutes, and seconds. Since right ascension coordinates are fixed to the celestial sphere the RA disk is usually driven by a clock mechanism in sync with sidereal time. Locating an object on the celestial sphere with settings circles is similar to finding a location on a terrestrial map using latitude and longitude. Sometimes the right ascension setting circle has two scales on it: one for the Northern and one for the southern hemisphere.
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 engine. Setting circles usually had a large diameter and when combined with a vernier scale could point a telescope to nearly an arc minute of accuracy. In the 20th century setting circles were replaced with electronic encoders on most research telescopes.
In amateur astronomy, setting up a portable telescope equipped with setting circles requires:
- Polar alignment - The telescope must be aligned with either the north celestial pole or the south celestial pole. Polaris is roughly at the north pole, while Sigma Octantis 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 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 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 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 encoders 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 microcomputers 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 telescope mount, a mount equipped with DSC alone is sometimes called a "PUSH TO" mount.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Circles (Berio) — Circles is a composition for female voice, harp and two percussionists by the Italian composer Luciano Berio. Written in 1960 Circles is a setting of three poems by E. E. Cummings. Contents 1 Context of the composition 2 Musical style and form … Wikipedia
Literature Circles — are a student s equivalent in the classroom of an adult book club. The aim is to encourage student choice and a love of reading in young people. The true intent of Literature Circles is to allow students to practice and develop the skills and… … Wikipedia
Villarceau circles — In geometry, Villarceau circles (pronEng|viːlɑrˈsoʊ) are a pair of circles produced by cutting a torus diagonally through the center at the correct angle. Given an arbitrary point on a torus, four circles can be drawn through it. One is in the… … Wikipedia
Three circles — Three circlesThree concentric circles exercise where three circles are drawn, one inside the other (like a bull s eye). The addict then places his or her bottom line behaviors (behaviors he or she is avoiding) in the bottom circle (bottom right… … Wikipedia
Amateur astronomy — Amateur astronomy, a subset of astronomy, is a hobby whose participants enjoy studying and observing celestial objects. Overview The typical amateur astronomer is one who does not depend on the field of astronomy as a primary source of income or… … Wikipedia
Dobsonian telescope — A Dobsonian telescope on display at Stellafane in the early 1980s A Dobsonian telescope is an alt azimuth mounted newtonian telescope design popularized by the amateur astronomer John Dobson starting in the 1960s. Dobson s telescopes featured a… … Wikipedia
List of circle topics — This list of circle topics includes things related to the geometric shape, either abstractly, as in idealizations studied by geometers, or concretely in physical space. It does not include metaphors like inner circle or circular reasoning in… … Wikipedia
Right ascension — (abbrev. RA; symbol α) 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.ExplanationRA is the celestial equivalent of… … Wikipedia
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 problemMany celestial objects of interest are too faint to be visible to the… … Wikipedia
Equatorial mount — An equatorial mount is a mount that has one rotational axis parallel to the Earth s axis of rotation [cite web | url = http://www.astrogea.org/soldevilla/las monturas.htm | title = LAS MONTURAS | publisher = Observatorio J. A. Soldevilla] [cite… … Wikipedia