What is a German Equatorial Mount?
Most small, store bought, equatorially mounted telescopes have what is called a German Equatorial Mount. This is a three axis mount, with two of the three axis forming a “T”. The telescope on one side of the “T” is balanced by a counterweight on the other side. The two axis that form the “T” are called the Right Ascension axis and Declination axis. These are in turn mounted on a third axis which allows the user to account for the latitude location of the telescope on the curvature of the earth.
Why do I need to polar align my equatorial mount?
Because the earth’s surface is curved not everyone views the celestial pole from the same vantage point. Polar aligning your equatorial mount brings the telescope into alignment with earth’s rotational axis so that you can track a celestial object properly on a single axis, the right ascension axis, no matter where you are on the globe.
The Three Axis of a German Equatorial Mount.
LATITUDE AXIS - This axis allows you to account for your location in latitude on the curvature of the earth. Doing this adjusts the right ascension axis so that it is parallel to the earth’s rotational axis. The latitude axis also supports the entire telescope and the rest of the mount assembly on the tripod.
RIGHT ASCENSION AXIS - Once the telescope is properly aligned, this axis moves the scope in an east-west direction across the sky. This is also the tracking axis. It runs perpendicular to the declination axis and parallel to the earth’s rotational axis. It also supports the declination axle with its telescope andcounterweight.
DECLINATION AXIS - Once the telescope is properly aligned, this axis moves the scope in a north-south direction across the sky. If you have a good alignment, once an object is found you will not need to move the telescope in declination until you change targets. This axis in turn runs perpendicular to the right ascension axis and perpendicular to the earth’s rotational axis. The declination axle also supports the counterweight and telescope at opposite ends.
The Basic Alignment Procedure Your German Equatorial
STEP ONE: Setting your latitude.
When should I do this?
This can be done at any time during the day or night, but keep in mind you will most likely have to make minor adjustments as you proceed with alignment when you go to observe.
A) Find your observing location’s latitude on the globe.
This can be done by either using a map, GPS unit, or an online service like Google Earth. Latitude coordinates are usually given in the following format: 00° 00’ 00” N or S (degrees, minutes, seconds, north, or south). We are only concerned with the degrees latitude. For a basic alignment the minutes and seconds are not necessary. For example, the latitude location given by Google Earth of Meteor Crater in the state of Arizona, USA is 35° 01’ 41.05” N. If we were setting our telescope up at this location all we would need to know is that it is located at 35°.
B) Set the telescope in the home position.
Move the right ascension and declination axis until the counterweight is pointed at or near the ground and the open end of the telescope aperture is pointed at or toward the sky.
C) Adjust your latitude setting on the mount.
Loosen the locking mechanism on thelatitude axis and use the latitude adjust mechanism to move the entire telescope assembly above it until the pointer on the latitude settingindicates the latitude of your observing location. Tighten the locking mechanism again to preserve your setting. The mechanism by which you set and lock the latitude axis may vary by manufacturer but they provide essentially the same function.
STEP TWO: A Basic Polar Alignment.
When should I do this?
This must be done out under a clear, dark sky before you observe each night.
Aligning the telescope.
A) Set the telescope in the home position on a level surface, with the open end of the scope pointing at the celestial pole.
B) Lock the Declination axis and using the lowest magnification eyepiece possible (25mm or greater), rotate your tripod’s ground position* to the left or right and fine adjust the latitude axis north or south, until the celestial pole appears in your field of view in the eyepiece.
C) Once you have the celestial pole centered in the field of view you should be able to watch it spin while you rotate the telescope in Right Ascension. For northern hemisphere observers Polaris should pretty much stay in the field of view.
Don’t worry if its not perfect, its not the easiest thing in the world to manually polar align an equatorial mount. However, a good basic alignment will serve you well enough to track an object for 20 minutes or before the it drifts enough to make it necessary to adjust the Declination. You’ll know how good you were by the amount of time it takes before you have to adjust it.
There are methods to make your alignment more accurate but these are beyond the scope of the basic information covered on this web site. ( In other words, True Martian hasn’t figured them out for himself yet well enough to teach you. ;-D )
* Some tripods may allow you to rotate the latitude axis left or right. Latitude axis mechanisms vary.
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