Observatorio ARVAL

Latitude 10° 30' N, Longitude 66° 50' W (UT - 4hrs.)

Polar Alignment (with Polaris) for Equatorial Telescopes


The purpose of Polar Alignment is to point the Right Ascension axis (Polar Axis) of the telescope's equatorial mount to a Celestial Pole, enabling it to compensate for the rotation of the Earth and follow a fixed celestial object without adjusting the Declination.

This page offers information on how to accomplish the Polar Alignment of an equatorial telescope with the help of the Polar Star.


Polar Alignment of the equatorial mount of a telescope:

- It is required that Polaris is visible from the observing site.
(This is frequently not the case from Caracas, use Polar Alignment (without Polaris) for Equatorial Telescopes)

- It is not required to know the latitude of the observing site. (10° 30' North for the valley of Caracas)

- It is recommended to have an equatorial mount equipped with declination circles (with a precision of 1°) and Right Ascension circles.

- It is recommended to use a compass to determine references for geographical North, South, East and West, considering the local magnetic variation.

- It is required to use a bubble level to level the base of the mount.


Simple Adjustment:

1) Adjusting the base of the equatorial mount:
The base of the mount must be oriented in the North-South direction so the Polar Axis of the telescope is within the plane of the local meridian.

Use a good compass for this alignment, taking care that it is not affected by the metallic masses near to it, like the telescope's mount and its base.

Consider the variation of the magnetic North from the geographic at the observing site (in 2003, some 11° West for Caracas, 8° West for Mérida, 8° for Maracaibo, 9° for Barquisimeto, 10° for Valencia, 12° for Margarita, Pto. La Cruz and Pto. Ordáz). The compass will indicate the Azimuth of the Celestial North Pole (and the geographic North).
See A Brief Introduction to Geomagnetism (U.S. Geological Survey) and Magnetic Declination Calculator (Canadian Geomagnetic Reference Field).

Use it to determine on the horizon (or, if not available at your observing level, on the object that impedes the view to the horizon) a reference for the direction North or geographic South, from the exact site where the base of the mount will be placed.

Mark a North-South line, on the surface were you will place the telescope, which passes through the exact site where the base of the mount will be vertically placed.
Mark another perpendicular line, in the East-West direction, that intersects the North-South line in the place where the mount will be placed.

Determine on the horizon (or at your observing level) references for East, South and West; Check the marked lines and place the mount over them.

The precision of the geographic references can be increased later, using the procedure for refined adjustment described in paragraph 5).

The base of the mount must be horizontal both in the East-West and the North-South directions. This is accomplished by carefully adjusting the length of the legs of the tripod or the base, placing the bubble level on the base. (If the length of the legs is not adjustable, use supplements under them)

Check these levels with the completely assembled telescope (it is convenient to make a preliminary adjustment on the elevation of the Polar Axis) to eliminate any effect of the weight on the base and legs. Check these levels every time you move the base.

2) Approximated adjustment of the angle of Elevation and the Azimuth of the Polar Axis:
Adjust the telescope for a declination approximately equal to 90° N and center Polaris in the finder telescope using the Elevation and Azimuth adjustments. The latitude of the observing site will indicate the Elevation of Polaris.
Adjust the finder telescope so it points to Polaris while the star is centered in the main telescope at high power.

Finely adjust the Declination, Azimuth and Elevation so that you can move the telescope in Right Ascension while Polaris remains centered in the field of view of the finder telescope. Refine by using the main telescope, starting with low power and then increasing it. Mark this exact 90° N position in your mount's Declination circles.

This approximated adjustment of the angle of Elevation of the Polar Axis will not have to be modified, if the latitude of the observing site does not change considerably (more than 1°, or more than 110 Km North or South from the original site).
This adjustment is affected by the North-South horizontality of the mount.

This approximated adjustment of the angle of Elevation and the Azimuth of the Polar Axis is not precise because Polaris is not exactly at the North Celestial Pole. Polaris is at Right Ascension 2h 32m, and Declination 89° 16' N (Epoch 2000), 44' from the North Celestial Pole.

[The Earth's axis wobbles like that of a top (precession), describing a 23.5° circle in the sky every 25,800 years. Polaris is now getting closer to the North Celestial Pole, but by the year 2100 it will start to move away.]

3) Testing:
In this condition, the telescope, with a low power eyepiece, should be able to track for tens of minutes, with little adjustment in Declination, any fixed celestial object, if we act on the Right Ascension adjustment mechanism. (Or we turn ON the Right Ascension tracking mechanism)

With middle or low power eyepieces it should be able to track for a few minutes, with little adjustment in Declination, any fixed celestial object.

This approximate alignment should be refined, reiterating this procedure until you obtain the described precision. But for photographic purposes it is required to perform the refined adjustment procedures described below.

Mark the positions of the legs of the tripod or base, to facilitate repositioning the telescope without losing the approximate alignment (always check the level of the base).


Locating Objects:
In this condition, the telescope, with the lowest power eyepiece, should be able to locate, with little error in Declination, any fixed celestial object, if we act on the Right Ascension mechanism.
After locating the first celestial object, we should adjust the Right Ascension circle to indicate correctly for the object. Then we turn ON the Right Ascension tracking mechanism (if available). The Right Ascension marker will indicate the Local Sidereal Time.

If a motorized Right Ascension tracking mechanism is not available we must readjust the Right Ascension circle from time to time, for the Local Sidereal Time. Because every minute that passes since the adjustment is a minute of error in Right Ascension
Readjust the Right Ascension circle before leaving one object for another.

You may use the ARVAL Catalog of Guide Stars to obtain the names and positions of the brightest stars. And for other classes of objects, the other sections of the ARVAL Catalogue of Bright Objects.



Refined Adjustment:

- The refined adjustment procedures are generally only required for photographic purposes requiring a mechanism for continuous Right Ascension tracking in the telescope's mount. But they can be used for perfecting an approximated alignment and refining the geographical reference points.

- The refined adjustment procedures depend on the horizontality of the mount for adjustments in Elevation not to affect Azimuth and vice versa.

- The Refined Adjustment procedures are indicated for observing sites with Northern latitude; Inverting the North and South directions of the drifts they can be used for Southern latitudes.

5) Refined adjustment of the Azimuth of the Polar Axis:
Point the telescope with a high power eyepiece (preferably with an illuminated reticule) to a star with Declination near 0° (within ±5° if possible) and near the Meridian (within ±0.5 hrs if possible).

Follow this star during a few minutes noting the direction of its Declination drift (correct and disregard any Right Ascension drift).

- If the star drifts to the South, the Polar Axis of the mount points too far East.

- If the star drifts to the North, the Polar Axis of the mount points too far West.

Correct the Azimuth of the Polar Axis to stop the Declination drift.

6) Refined Adjustment of the Angle of Elevation of the Polar Axis:
Point the telescope with a high power eyepiece (preferably with an illuminated reticule), to a star with Declination near 0° (within ±5° if possible) and near the Eastern Horizon (with an Elevation somewhat greater than 20° if possible).

Follow this star during a few minutes noting the direction of its Declination drift (correct and disregard any Right Ascension drift).

- If the star drifts to the South, the Polar Axis of the mount points too low.

- If the star drifts to the North, the Polar Axis of the mount points too high.

Correct the Elevation of the Polar Axis to stop the Declination drift.

7) Testing:
In this condition, the telescope, with a high power eyepiece, should be able to track for tens of minutes, with no adjustment in Declination, any fixed celestial object, if we correct with the Right Ascension tracking mechanism.

This alignment should be refined by reiterating this procedure until you get the required precision for the planned photographic purposes (in general, no adjustments in Declination during an exposure).

Determine on the horizon (or your observation level) the references for North, South, East and West. You will use them in the future to align the telescope.



This page was updated in: June 12 '07, June 13 '02

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Spanish: Alineación Polar (con Polaris) para Telescopios con Montura Ecuatorial

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