Losmandy Telescope Mounts

The Celestial Coordinate System

In order to help find objects in the sky, astronomers use a celestial coordinate system which is similar to our geographical coordinate system here on Earth. This system has poles, lines of longitude and latitude, and an equator. For the most part, these remain fixed against the background stars.

The celestial equator runs 360° around the Earth and separates the northern celestial hemisphere from the southern. Like the Earth’s equator, it bears a reading of zero degrees. On Earth this would be latitude. However, in the sky this is referred to as declination, or DEC for short. Lines of declination are named for their angular distance above and below the celestial equator. The lines are broken down into degrees, minutes, and seconds of arc. Declinations south of the equator carry a minus sign (-) in front of the coordinate and those north of the celestial equator are either blank (no designation) or preceded by a plus sign (+).

The celestial equivalent of longitude is called Right Ascension, or R.A. for short. Like the Earth’s lines of longitude, they run from pole to pole and are evenly spaced 15° apart. Although the longitude lines are separated by an angular distance, they are also a measure of time, with each line of longitude being one hour apart from the next. Since the Earth rotates once every 24 hours, there are 24 lines total. As a result, the R.A. coordinates are marked off in units of time. It begins with an arbitrary point in the constellation of Pisces designated as 0 hours, 0 minutes, 0 seconds. All other points are designated by how far (how long) they lag behind this coordinate after it passes overhead moving towards the west.

Your Losmandy telescope Mount comes equipped with setting circles that translate the celestial coordinates into a precise location for the telescope to point. The setting circles will not work properly until you have polar aligned the telescope and aligned the R.A. setting circle.

The Wheel of Stars

The daily motion of the Sun across the sky is familiar to even the most casual observer. This daily trek is, of course, not the Sun moving as early astronomers thought, but the result of the Earth’s rotation. This rotation also causes the stars to do the same, scribing out large circles as the Earth completes one rotation. The size of the circular path a star follows depends on where it is in the sky. Stars near the celestial equator form the largest circles rising in the east and setting in the west. Moving toward the North Celestial Pole, the point around which the stars in the northern hemisphere appear to rotate, these circles become smaller. Stars in the mid-celestial latitudes rise in the northeast and set in the northwest. Stars at high celestial latitudes are always above the horizon, and are said to be circumpolar because they never rise and never set. You will never see stars complete one circle because the sunlight during the day will wash out the starlight, unless you are within a Polar Circle during a day when the Sun does not rise. This circular motion of stars in the sky can be seen by setting up a camera on a tripod and opening the shutter for a couple of hours. The processed film will reveal semicircles that arch around the pole. This description of stellar motions also applies to the southern hemisphere except all stars south of the celestial equator move around the south celestial pole.

Polar aligning your Losmandy Mount

In order for the telescope to track the stars, you must meet two criteria, First, you need a drive motor that moves at the same rate as the stars. A polar axis finder is offered as an optional accessory. The second thing you need is to set the telescope’s axis of rotation so that it tracks in the right direction. Since the motion of the stars across the sky is caused by the Earth’s rotation about its axis, the telescope’s axis must be made parallel to the Earth’s. The polar axis is the axis around which the telescope rotates when moved in right ascension. This axis points in the same direction even when the telescope moves in right ascension.

Polar alignment is the process by which the telescope’s axis of rotation (called the polar axis) is made parallel with the Earth’s axis of rotation. Once aligned, a telescope with a clock drive will track the stars as they move across the sky. The result is that objects observed through the telescope appear stationary. They will not drift out of the field of view because the motors and gears exactly compensate for the motion caused by the Earth’s rotation. Even if you are not using the clock drive, polar alignment is still desirable since it will reduce the number of corrections needed to follow an object and limit all corrections to R.A. axis. There are several methods of polar alignment, all of which work on a similar principle, but perform somewhat differently. Each method will be considered separately, beginning with the easier methods and working to the more difficult.

Although there are several methods mentioned here, you will never use all of them during one particular observing session. Instead, you may use only one if it is a casual observing session. Or, you may use two methods, one for rough alignment followed by a more accurate method if you plan on doing astro-photography.

Where are the Poles?

In each hemisphere, there is a point in the sky around which all the other stars appear to rotate. These points are called the celestial poles and are named for the hemisphere in which they reside. For example, in the northern hemisphere all stars move around the North Celestial Pole. When a telescope’s polar axis is pointed at the celestial pole, it is parallel to the Earth’s rotational axis. The North Celestial Pole is the point in the northern hemisphere around which all stars appear to rotate. The counterpart in the southern hemisphere is referred to as the South Celestial Pole.

Many of the methods of polar alignment require that you know how to find the celestial pole by identifying stars in the area. For those in the northern hemisphere, finding the celestial pole is not too difficult. Fortunately, we have a naked eye star less than a degree away. This star, Polaris, is the end star in the handle of the Little Dipper. Since the Little Dipper (technically called Ursa Minor) is not one of the brightest constellations in the sky, it may be difficult to locate from urban areas. If this is the case, use the two end stars in the bowl of the Big Dipper (the pointer stars). Draw an imaginary line (away from the “pan”) through them toward the Little Dipper. They point almost directly to Polaris. Since the position of the Big Dipper rotates throughout the night as well as during the year it may be difficult to locate, or even perhaps be below the horizon

Observers in the southern hemisphere are not as fortunate as those in the northern hemisphere. The stars around the south celestial pole are not nearly as bright as those around the North Celestial Pole. The closest star that is relatively bright is Sigma Octantis. This star is just within the naked eye limit (magnitude E.5) and lies about 59 arc minutes from the pole. For more information about stars around the south celestial pole, please consult a star atlas.

Latitude Scales

The easiest way to polar align a telescope is with a latitude scale. Unlike other methods that require you to find the celestial pole by identifying certain stars near it, this method works off of a known constant to determine how high the polar axis should be pointed. The Losmandy G-11 mount can be adjusted from 14° to 64° .

The constant, mentioned above, is a relationship between your latitude and the angular distance the celestial pole is above the northern (or southern) horizon. The angular distance from the northern horizon to the North Celestial Pole is always equal to your latitude. To illustrate this, imagine that you are standing on the north pole, latitude +90°. The North Celestial Pole, which has a declination of +90°, would be directly overhead (90° above the horizon). Now, let’s say that you move 1° south. Your latitude is now +89° and the celestial pole is no longer directly overhead. It has moved 1° closer toward the northern horizon. This means the pole is now 89° above the northern horizon. If you move 1° further south, the same thing happens again. As you can see from this example, the distance from the northern horizon to the celestial pole is always equal to your latitude.

If you are observing from Los Angeles, which has a latitude of 34°, then the celestial pole is 34° above the northern horizon. All a latitude scale does then is to point the polar axis of the telescope at the right elevation above the northern (or southern) horizon. To polar align your telescope:
  1. Make sure the polar axis of the mount is pointing due north. Use a landmark that you know faces north.
  2. Level the tripod. There is a bubble level built into the mount for this purpose. Please note that leveling the tripod is only necessary if using this method of polar alignment. Perfect polar alignment is still possible using other methods described later without leveling the tripod.
  3. Adjust the mount in altitude until the latitude indicator points to your latitude. Moving the mount affects the angle the polar axis is pointing. For specific information on adjusting the equatorial mount, please see the section

Pointing at Polaris

This method utilizes Polaris to polar align your mount. Since Polaris is less than 1° from the North Celestial Pole, you can simply point the polar axis of your telescope at Polaris. Although this is by no means perfect alignment, it does get you within 1°. Unlike the previous method, this must be done in the dark when Polaris is visible.
  1. Set the telescope up so that the polar axis is pointing north.
  2. Loosen the DEC clutch knob and move the telescope so that the tube is parallel to the polar axis. When this is done, the declination setting circle will read +90°. If the declination setting circle is not aligned, move the telescope so that the tube is parallel to the polar axis.
  3. Adjust the mount in altitude and/or azimuth until Polaris is in the field of view of the finder.
  4. Centre Polaris in the field of the telescope using the fine adjustment controls on the wedge.
Remember, while polar aligning, do NOT move the telescope in R.A. or DEC. You do not want to move the telescope itself, but the polar axis. The telescope is used simply to see where the polar axis is pointing.

Like the previous method, this gets you close to the pole but not directly on it. The following methods help improve your accuracy for more serious observations and photography.

The Polar Axis Finder

The Polar Axis Finder is designed to minimize polar alignment set-up time while maintaining maximum accuracy. The installation of this optional accessory is described in the section on Installing the Polar Axis Finder. Here’s how to use it:
  1. Turn the Polar Axis Finder illuminator on.
  2. Place Polaris in the field of the polar axis finder by adjusting the mount in altitude and azimuth.
  3. Rotate the polar scope until the orientation of the stars on the reticle matches the star pattern in the sky (as seen with the naked eye).
  4. Adjust the mount in altitude and azimuth until Polaris is in the small space on the line between Eta h Ursa Major (Alkaid - at the end of the handle of the Big Dipper) and Epsilon e Cassiopeia (Segin - the beginning of the W).
  5. Note the second brightest star in the field.
  6. Place this star in space on the line between Cassiopeia and the bowl of the Big Dipper. If you can not get Polaris and this second star in their respective places, rotate the polar axis finder until you can.
When finished, the mount is accurately polar aligned.

Declination Drift

This method of polar alignment allows you to get the most accurate alignment on the celestial pole and is required if you want to do long exposure deep-sky astro-photography through the telescope. The declination drift method requires that you monitor the drift of selected guide stars. The drift of each guide star tells you how far away the polar axis is pointing from the true celestial pole and in what direction. Although declination drift is quite simple and straightforward, it requires a great deal of time and patience to complete when first attempted. The declination drift method should be done after any one of the previously mentioned methods has been completed.

To perform the declination drift method you need to choose two bright stars. One should be near the eastern horizon and one due south near the meridian. Both stars should be near the celestial equator (0° declination). You will monitor the drift of each star one at a time and in declination only. While monitoring a star on the meridian, any misalignment in the east-west direction will be revealed. While monitoring a star near the east/west horizon, any misalignment in the north-south direction will be revealed. As for hardware, you will need an illuminated reticle ocular to help you recognize any drift. For very close alignment, a Barlow lens is also recommended since it increases the magnification and reveals any drift faster.

When looking due south with the scope on the side of the mount, insert the diagonal so it points straight up. Insert the cross hair ocular and align cross hairs to be parallel to declination and right ascension motion Use ±16x guide setting to check parallel alignment.

First choose your star near where the celestial equator and the meridian meet. The star should be approximately ± 1/2 hour of the meridian and ±5° of the celestial equator. Centre the star in the field of your telescope and monitor the drift in declination.
  • If the star drifts south, the polar axis is too far east.
  • If the star drifts north, the polar axis is too far west.
Make the appropriate adjustments to the polar axis to eliminate any drift. One you have managed to eliminate all drift, move to the star near the east horizon. The star should be 20° above the horizon and ± 5° of the celestial equator.
  • If the star drifts south, the polar axis is too low
  • If the star drifts north, the polar axis is too high.
Once again, make the appropriate adjustments to the polar axis to eliminate any drift. Unfortunately, the latter adjustments interact with adjustments ever so slightly. Therefore, repeat the process again to improve the accuracy checking both axes for minimal drift. Once the drift has been eliminated, the telescope is very accurately aligned. You will be able to do prime focus deep-sky astro-photography for long periods.

NOTE: If the eastern horizon is blocked, you may choose a star near the western horizon however, you will have to reverse the polar high/low error directions. If using this method in the southern hemisphere, the procedure is the same as described above. However, the direction of drift is reversed.

Setting Circles

Before you can use the setting circles to find objects in the sky, you need to align both the R.A. and DEC setting circles. You will need to know the names of a few of the brightest stars in the sky. If you do not, you will need to consult a current astronomy magazine or chart. To align the R.A. setting circle:
  1. Locate a bright star near the celestial equator. The farther you are from the celestial pole, the better will be your reading of the R.A. setting circle. The star you choose to align the setting circle should be a bright one whose coordinates are known and easy to look up. (For a list of bright stars to align the R.A. setting circle see the list at the back of this manual.)
  2. Centre the star in the finder.
  3. Centre the star in the field of the telescope.
  4. Start the clock drive so that the mount tracks the star.
  5. Look up the coordinates of the star from a star catalog.
  6. The R.A. setting circle should rotate freely. Adjust it until the proper coordinates line up with the R.A. indicator. Every hour is labeled and each engraved line equals four minutes.
The R.A. setting circle is now aligned and ready to use. The R.A. setting circle is clutched to the R.A. gear rotation. As long as the R.A. drive is operating, the circle does not need to be reset once indexed to the correct coordinate (i.e., once aligned). If the drive is ever turned off, then the R.A. setting circle must be reset when re-activated. While the R.A. setting circle tracks with the drive motor, it does not move when slewing the telescope. After the R.A. setting circle has been aligned, you are ready to align the DEC setting circle. To align the DEC setting circle:
  1. Leave the star used to align the R.A. setting circle Centre in the field of the telescope.
  2. Look up the coordinates of the star from a star catalog.
  3. Locate the small Allen head screw on the DEC setting circle. It is generally located between the 0° and l0° marks.
  4. Loosen the allen head screw using the appropriate Allen wrench.
  5. Rotate the DEC circle until the proper coordinate lines up with the DEC indicator. The DEC setting circle is scaled in degrees with a marker every 1°.
  6. Tighten the allen screw to hold the DEC setting circle in place.
Once complete, the setting circle should not need to be aligned unless it comes loose. With both setting circles aligned, you are ready to use the setting circles to locate objects in the night sky.


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