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 ephemeris. An equatorial mount is a mount that has one rotational axis parallel to the Earth's axis of rotation A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. An equatorial mount is a mount that has one rotational axis parallel to the Earth's axis of rotation The equatorial coordinate system is probably the most widely used Celestial coordinate system, whose equatorial coordinates are Declination (\delta A star chart is a map of the Night sky. Astronomers divide these into grids to easily use them An ephemeris (plural ephemerides; from the Greek word ἐφήμερος ephemeros "daily" is a table of values that gives the positions of
Contents |
Description
Setting circles consist of two graduated disks attached to the right ascension (RA) and declination (DEC) axis of an equatorial mount. Right ascension (abbrev RA; symbol α) is the Astronomical term for one of the two Coordinates of a point on the Celestial sphere In Astronomy, declination (abbrev dec or δ) is one of the two coordinates of the Equatorial coordinate system, the other being either An equatorial mount is a mount that has one rotational axis parallel to the Earth's axis of rotation The right ascension disk is graduated into hours, minutes, and seconds. Right ascension (abbrev RA; symbol α) is the Astronomical term for one of the two Coordinates of a point on the Celestial sphere The declination disk is graduated into degrees, minutes, and seconds. In Astronomy, declination (abbrev dec or δ) is one of the two coordinates of the Equatorial coordinate system, the other being either 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. Right ascension (abbrev RA; symbol α) is the Astronomical term for one of the two Coordinates of a point on the Celestial sphere In Astronomy and Navigation, the celestial sphere is an imaginary rotating Sphere of "gigantic Radius " Sidereal time is a measure of the position of the Earth in its rotation around its axis or time measured by the apparent Diurnal motion of the Vernal equinox Locating an object on the celestial sphere with settings circles is similar to finding a location on a terrestrial map using latitude and longitude. In Astronomy and Navigation, the celestial sphere is an imaginary rotating Sphere of "gigantic Radius " Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the Longitude (ˈlɒndʒɪˌtjuːd or ˈlɒŋgɪˌtjuːd symbolized by the Greek character Lambda (λ is the east-west Geographic coordinate measurement Sometimes the right ascension setting circle has two scales on it: one for the Northern and one for the southern hemisphere.
Application
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 engine. A dividing engine is a device specifically employed to mark graduations on measurement instruments 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. 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 A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. In the 20th century setting circles were replaced with electronic encoders on most research telescopes. An encoder is a device used to change a signal (such as a Bitstream) or Data into a Code.
Portable 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. Amateur astronomy, a subset of Astronomy, is a Hobby whose participants enjoy studying and observing celestial objects The north and south celestial poles are the two imaginary points in the sky where the Earth's Axis of rotation, "infinitely extended" intersects the Polaris is roughly at the north pole, while Sigma Octantis is roughly at the south pole. Polaris (α UMi / α Ursae Minoris / Alpha Ursae Minoris commonly North(ern Star or Pole Star, and sometimes Lodestar Sigma Octantis (σ Oct / σ Octantis is a magnitude 56 star in the constellation Octans most notable for being the current South Star.
- 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
- An unlevel tripod
- 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
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.
Despite these inaccuracies 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. Star hopping is a technique that is often used by amateur astronomers to locate objects in the night sky
Digital setting circles
Digital setting circles (DSC) consist of two rotary encoders on both axis of the telescope mount and a digital readout. A rotary encoder, also called a shaft encoder, is an Electro-mechanical device used to convert the angular position of a shaft or axle to an analog 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. microcomputer is a Computer with a Microprocessor as its Central processing unit.
In contrast to a GOTO telescope mount, a mount equipped with DSC alone is sometimes called a "PUSH TO" mount. In amateur astronomy, " GoTo " refers to a type of telescope mount and related Software which can automatically point a telescope to
References
External links
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic