Polar alignment

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Polar alignment is the act of aligning the rotational axis of a telescope's equatorial mount or a sundial's gnomon with a celestial pole to parallel Earth's axis.

Contents

Alignment methods

The method to use differs depending on whether the alignment is taking place in daylight or in night. Furthermore, the method differs if the alignment is done in the Northern Hemisphere or Southern Hemisphere. The purpose of the alignment also must be considered; for example, the value of accuracy is much more significant in astrophotography than in casual stargazing.

Aiming at the pole stars

In the Northern hemisphere, sighting Polaris the North Star is the usual procedure for aligning a telescope mount's polar axis parallel to the Earth's axis. [1] Polaris is approximately three quarters of a degree from the North Celestial Pole, and is easily seen by the naked eye.

σ Octantis, sometimes known as the South Star, can be sighted in the Southern hemisphere to perform polar alignment. At magnitude +5.6, it is difficult for inexperienced observers to locate in the sky. Its declination of -88° 57′ 23″ places it 1° 2′ 37" from the South Celestial Pole. An even closer star BQ Octantis of magnitude +6.9 lies 10' from the South Pole as of 2016. Although not visible to the naked eye, it is easily visible in most polar 'scopes. (It will lie its closest to the South Pole, namely 9', in the year 2027.

Location of s Octantis in the southern sky, with dashed lines as guides to the approximate location. Pole01-eng.jpg
Location of σ Octantis in the southern sky, with dashed lines as guides to the approximate location.
Celestial South Pole around 2016 and bright stars around it. Right ascension 0h is up and each circle is 1 degree of declination from the pole. The trapezoid top right is just visible to the naked eye. Celestial Southpole 2016.png
Celestial South Pole around 2016 and bright stars around it. Right ascension 0h is up and each circle is 1 degree of declination from the pole. The trapezoid top right is just visible to the naked eye.

Rough alignment method

In the Northern hemisphere, rough alignment can be done by visually aligning the axis of the telescope mount with Polaris. In the Southern hemisphere or places where Polaris is not visible, a rough alignment can be performed by ensuring the mount is level, adjusting the latitude adjustment pointer to match the observer's latitude, and aligning the axis of the mount with true south or north by means of a magnetic compass. (This requires taking the local magnetic declination into account). This method can sometimes be adequate for general observing through the eyepiece or for very wide angle astro-imaging with a tripod-mounted camera; it is often used, with an equatorially-mounted telescope, as a starting point in amateur astronomy.

There are ways to improve the accuracy of this method. For example, instead of reading the latitude scale directly, a calibrated precision inclinometer can be used to measure the altitude of the polar axis of the mount. If the setting circles of the mount are then used to find a bright object of known coordinates, the object should mismatch only as to azimuth, so that centring the object by adjusting the azimuth of the mount should complete the polar alignment process. Typically, this provides enough accuracy to allow tracked (i.e. motorized) telephoto images of the sky.

For astro-imaging through a lens or telescope of significant magnification, a more accurate alignment method is necessary to refine the rough alignment, using one of the three following approaches.

Polarscope method

An alignment suitable for visual observation and short exposure imaging (up to a few minutes) can be achieved with a polarscope. This is a low magnification telescope mounted co-axially with the mount (and adjusted to maximise the accuracy of this alignment). A special reticle is used to align the mount with Polaris (or in the southern hemisphere a group of stars near the polar region). While primitive polarscopes originally needed the careful adjustment of the mount to match the time of year and day, this process can be simplified using computer apps which calculate the correct position of the reticle. A new-style northern-hemisphere reticle uses a 'clock-face' style with 72 divisions (representing 20 minute intervals) and circles to compensate for the drift of polaris over around thirty years. Use of this reticle can allow alignment to within an arc minute or two. [2]

Drift alignment method

Drift alignment is a method to refine the polar alignment after a rough alignment is done. The method is based on attempting to track stars in the sky using the clock drive; any error in the polar alignment will show up as the drift of the stars in the eyepiece/sensor. Adjustments are then made to reduce the drift, and the process is repeated until the tracking is satisfactory. For the polar axis altitude adjustment, one can attempt to track a star low in the east or west. For the azimuth adjustment, one typically attempts to track a star close to the meridian, with declination about 20° from the equator, in the hemisphere opposite of the observing location.

Plate solving

For telescopes combined with an imaging camera connected to a computer it is possible to achieve very accurate polar alignment (within 0.1 minute of arc) by approximately aligning it, then identifying the exact field of view when aimed at stars near the pole - 'plate solving'. The telescope is then rotated ninety degrees around its right ascension axis and a new 'plate solve' carried out. The error in the point around which the images rotate compared to the true pole is calculated automatically and the operator can be given simple instructions to adjust the mount for close polar alignment. [3]

Equipment

Crosshair eyepiece

A crosshair eyepiece is an ordinary ocular with the only difference that it has a crosshair for aiming and measurement of the angular distance. This is useful in any type of polar alignment, but especially in drift.

Auto guiding systems

Dedicated polar scope

A small telescope usually with an etched reticle that is inserted into the rotational axis of the mount.

See also

Related Research Articles

Right ascension Astronomical equivalent of longitude

Right ascension is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the point in question above the earth. When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system.

Celestial sphere imaginary sphere of arbitrarily large radius, concentric with the observer

In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location.

Celestial pole Two imaginary points in the sky where the Earths axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere

The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at the Earth's North Pole and South Pole, respectively. As the Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other points appear to rotate around them, completing one circuit per day.

Equatorial coordinate system celestial coordinate system used to specify the positions of celestial objects

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 fundamental plane consisting of the projection of Earth's equator onto the celestial sphere, a primary direction towards the vernal equinox, and a right-handed convention.

Horizontal coordinate system type of celestial coordinate system that uses the observers local horizon as the fundamental plane

The horizontal coordinate system, also known as topocentric coordinate system, is a celestial coordinate system that uses the observer's local horizon as the fundamental plane. Coordinates of an object in the sky are expressed in terms of altitude angle and azimuth.

Sundial Device that tells the time of day by the apparent position of the Sun in the sky

A sundial is a device that tells the time of day when there is sunlight by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate and a gnomon, which casts a shadow onto the dial. As the Sun appears to move across the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude.

Diurnal motion is an astronomical term referring to the apparent motion of celestial objects around Earth, or more precisely around the two celestial poles, over the course of one day. It is caused by Earth's rotation around its axis, so almost every star appears to follow a circular arc path called the diurnal circle.

In astronomy, a planisphere is a star chart analog computing instrument in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date. It is an instrument to assist in learning how to recognize stars and constellations. The astrolabe, an instrument that has its origins in Hellenistic astronomy, is a predecessor of the modern planisphere. The term planisphere contrasts with armillary sphere, where the celestial sphere is represented by a three-dimensional framework of rings.

Telescope mount mechanical structure which supports a telescope

A telescope mount is a mechanical structure which supports a telescope. Telescope mounts are designed to support the mass of the telescope and allow for accurate pointing of the instrument. Many sorts of mounts have been developed over the years, with the majority of effort being put into systems that can track the motion of the fixed stars as the Earth rotates.

Sigma Octantis Star in the constellation Octans

Sigma Octantis, officially named Polaris Australis, is the current South Star. Its position near the southern celestial pole makes it the southern hemisphere's pole star. This is a solitary star in the southern circumpolar constellation of Octans. Located approximately 281 light-years from Earth, it is classified as a giant star with a spectral type of F0 III. Sigma Octantis is a Delta Scuti variable, with its average magnitude of 5.47 varying by about 0.03 magnitudes every 2.33 hours.

Pole star Visible star that is approximately aligned with the Earths axis of rotation

A pole star or polar star is a star, preferably bright, closely aligned to the axis of rotation of an astronomical object.

Reticle fine lines in the eyepiece of a sighting device

A reticle, or reticule, also known as a graticule, is a pattern of fine lines or markings built into the eyepiece of a sighting device, such as a telescopic sight in a telescope, a microscope, or the screen of an oscilloscope, to provide measurement references during visual examination. Today, engraved lines or embedded fibers may be replaced by a computer-generated image superimposed on a screen or eyepiece. Both terms may be used to describe any set of lines used for optical measurement, but in modern use reticle is most commonly used for gunsights and such, while graticule is more widely used for the oscilloscope display, microscope slides, and similar roles.

Altazimuth mount two-axis mount for supporting and rotating an instrument (e.g. telescope) about two perpendicular axes – one vertical and the other horizontal

An altazimuth or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two perpendicular axes – one vertical and the other horizontal. Rotation about the vertical axis varies the azimuth of the pointing direction of the instrument. Rotation about the horizontal axis varies the altitude of the pointing direction.

Equatorial mount

An equatorial mount is a mount for instruments that compensates for Earth's rotation 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 advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any celestial object with diurnal motion by driving one axis at a constant speed. Such an arrangement is called a sidereal or clock drive.

A polar mount is a movable mount for satellite dishes that allows the dish to be pointed at many geostationary satellites by slewing around one axis. It works by having its slewing axis parallel, or almost parallel, to the Earth's polar axis so that the attached dish can follow, approximately, the geostationary orbit, which lies in the plane of the Earth's equator.

Meridian circle instrument for timing of the passage of stars across the local meridian

The meridian circle is an instrument for timing of the passage of stars across the local meridian, an event known as a culmination, while at the same time measuring their angular distance from the nadir. These are special purpose telescopes mounted so as to allow pointing only in the meridian, the great circle through the north point of the horizon, the north celestial pole, the zenith, the south point of the horizon, the south celestial pole, and the nadir. Meridian telescopes rely on the rotation of the sky to bring objects into their field of view and are mounted on a fixed, horizontal, east–west axis.

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.

GoTo (telescopes) type of telescope mount and related software

In amateur astronomy, "GoTo" refers to a type of telescope mount and related software that can automatically point a telescope at astronomical objects that the user selects. Both axes of a GoTo mount are driven by a motor and controlled by a computer. It may be either a microprocessor-based integrated controller or an external personal computer. This differs from the single-axis semi-automated tracking of a traditional clock-drive equatorial mount.

Filar micrometer

A filar micrometer is a specialized eyepiece used in astronomical telescopes for astrometry measurements, in microscopes for specimen measurements, and in alignment and surveying telescopes for measuring angles and distances on nearby objects. The word filar derives from Latin filum, meaning 'a thread'. It refers to the fine threads or wires used in the device.

Autoguider

An autoguider is an automatic electronic guidance tool used in astronomy to keep a telescope pointed precisely at an object being observed. This prevents the object from drifting across the field of view during long-exposures which would create a blurred or elongated image.

References

  1. Michael A. Covington (1999). Astrophotography for the amateur. ISBN   978-0-521-62740-5.
  2. "New Style Reticle for PolarFinder". www.stubmandrel.co.uk.
  3. "Polar Alignment – SharpCap – Lunar, Planetary, Solar and Deep Sky Imaging. EAA and Live Stacking".