Techniques of Observational Astronomy
AST3722C

Basic Telescopes

• Telescope Mounts
  the telescope mount allows the telescope to point anywhere in the sky and to track stars as they rise and set
• Balancing
  Proper balance is important if the telescope is to track the stars in all parts of the sky without drifting
• Polar alignment
  It is necessary that an equatorial mount be closely aligned on the pole to ensure accurate tracking

Mounts

• Equatorial Mount
  two axes: a polar axis is aligned with the Earth's axis, allowing diurnal tracking; a perpendicular declination axis
  • advantage: one axis diurnal tracking
  • disadvantage: asymetry to gravity



The bending of the declination axis of a German Equatorial mount will change depending on the part of the sky being observed, causing tracking and pointing errors (a similar problem occurs with the forks of a Fork Mount). While the altitude axis of an altazimuth mount may bend, the bending is constant and can be compensated by realignment of the tube or optics.

• Altazimuth Mount
  two axes: an azimuth axis aligned with the zenith; a perpendicular altitude axis
• advantage: symetrical to gravity
• disadvantage: 2 axis tracking
  • "dead" zone at zenith
  • field rotation

An animation of the altazimth mounted GTC

Tracking and Pointing

A modern telescope has motors on both axes to point the telescope and to track the rising and setting of objects (diurnal motion). Usually these motors are computer controlled and the position of each axis is read out to the computer by angle encoders. Tracking can be accomplished by motion around the polar axis alone with an equatorial mount but requires motion of both axes with an altazimuth mount. Tracking near the zenith requires a very rapid motion in azimuth, approaching infinity in the limit of a star passing exactly throughthe zenith. The result is a "dead zone" near the zenith for altazimuth mounts. Observations are stopped a few minutes before zenith passage and the telescope is pointed at the position on the other side of the zenith where the object will be reaquired. (A similar problem with movement can occur for a equatorial mount near the pole but it affects only pointing and not tracking.)

Rotation of Field

With an equatorial mount, the field seen in an eyepiece or camera is fixed in orientation (e.g. perhaps west to the right, north up, etc.). This is not the case with an altazimuth mount. Consider the case of tracking a star that passes near the zenith: as the zenith is passed the telescope must rotate nearly 180° in azimuth which means that a camera mounted rigidly to the telescope tube would rotate relative to the objects being imaged. Thus instruments on an altazimuth mounted telescope must be mounted on a bearing which must be rotated at the correct rate to compensate for field rotation.

Equatorial Mounts

• German Mount: Most frequently used for refracting telescopes. Must be "reversed" to observe on both sides of the meridian.
• Modified German Mount: By extending the polar axis (and proving support at the far end) the tube can clear the pier without the need to reverse.
• English Mount: Must be "reversed" to observe on both sides of the meridian. Full support for both ends of the polar axis may restrict use near the pole.

• Fork Mount: most frequently used for reflecting telescopes. To reach pole, tube must clear fork.
• Yoke Mount: Added support forpolar axis. Cannot reach pole.
• Horseshoe Yoke Mount: Allows reaching pole while providing support at both ends of polar axis.

Examples

In spite of the great difficulty in working with this telescope, the 0.91m Crossley telescope at the Lick Observatory produced many of the most important spectroscopic results. This was probably due to the great effort expended by its users and the almost perfect match of its prime focus spectrograph to its optical system.

Balancing an Equatorial Telescope

In order to move a telescope smoothly it must remain in balance in all positions in the sky. This can only be the case if the orthogonal axes intersect and the center of mass lies at the intersection point.

• (A) Dec axis: Balance the tube in the z direction. Usually there are sliding weights available for this task.
• (B) Dec axis: Balance the tube in the y direction. This balance is frequently negelected. You may have to mount fixed weights.
• (C) Polar axis: Balance with counter weight at end of Dec axis.
• (D) Polar axis: This balance is frequently negelected. You may have to mount fixed weights.

General notes:

• To test balance, you must be able to let the scope move freely about the relevant axis. This may require releasing the motor.
• In principle, balance can be calculated in advance from the moments and this is what is done for large scopes.
• The basic scope and permanent equipment can be carefully balanced using permanent counter weights. Changes in symetrical equipment can then usually be balanced by sliding weights in positions (A) and (C).

Polar Axis Alignment

For an equatorial telescope to track accurately, its polar axis must be aligned with the pole. Misalignment will cause a star to drift in both declination and right ascension.

Drift Alignment

This method can be done with either an eyepiece or with CCD imaging. Note that the stars will be moving due to the diurnal motion of the Earth as it rotates around the true pole.

• Point the telescope at a star at about declination 0° near the East horizon (or west horizon with appropriate sign changes).
• If the telescope polar axis is tilted towards the south from the true pole, the telescope will be following a path taking it south of the equator (and the star), thus the star will appear to drift north.

• Point the star towards the south at a star at about 0° declination.
• If the telescope polar axis is tilted towards the east from the true pole, the telescope will be following a path taking it north of the equator (and the star), thus the star will appear to drift south.

Analysis

• At the celestial equator (0° declination) the x motion is 15° per hour or 15' per minute; the drift at any given declination is then:

• The y drift is then given by

• These equations can be solved for the polar tilt angles alpha and beta.

Once the tilt angles have been determined, the telescope axis can be (carefully) realigned. Some telescopes will have adjustment screws for this purpose. Others will require shims (thin pieces of metal) to be inserted between the telescope and its base or between the base and the pier.

This page was last edited September 30, 2003 10:32 AM