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Telescope Beginners Guide: Alt-Az mounts January 26, 2010

Posted by aquillam in Astronomy, MichiganAstro, telescope beginners guide.
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Alt-Az Mounts

The altitude-azimuth system

Alt-Az mounts work in the altitude-azimuth coordinate system.  Although the words sound big, this is the one you usually use when you’re just talking to somebody. For example,a friend says “I saw this really bright light just above the horizon in the east this morning. Do you know what it was?” You reply “Of course! Venus is very bright and low in the east in the morning right now.” In this case, “low” is the altitude, and “in the east” is the azimuth.

Altitude

Altitude is in degrees above the horizon.  It is generally assumed that the zenith is at 90º, and the horizon  at 0º.  Of course what you can actually see can be affected by your location: a lot of trees may limit you to within a few degrees of the zenith. Colloquially, we may also use “high” or “low” for the altitude, but since different people have different ideas about where high stops and low begins, that’s not as helpful as actual degrees.

Azimuth

Azimuth is the direction along the horizon, such as east, or southwest. For more precise mounts, it is measured in degrees away from due north going clockwise. That means due east is actually 90º, and an azimuth of 200º would be SSE.

Altitude and Azimuth

Altitude and azimuth are shown by the blue arrows. Altitude is measured in degrees from the horizon, azimuth from due north.

Examples of mounts

A telescope on an alt-az tripod.

This is one of the easiest mounts to make. The most common design is a turntable with an axis for altitude adjustment.   Since azimuth is along the horizon, it is important that the mount be horizontal, or level. The simplest version, or at least the cheapest, is probably cardboard box mount. You turn the box to rotate in azimuth, and rotate the telescope to rotate in altitude.

The most common place you are likely to encounter an alt-az mount is a tripod for cameras or small ‘scope. a standard tripod has a mount that rotates in azimuth and has a handle for making quick adjustments in altitude.

A tabletop telescope, on a Dobsonian style mount with a single arm support.

A very popular mount, especially for do-it-yourselfers is the Dobsonian. A quick search for “home built dobsonian” turns up dozens of examples.  The body of a Dobsonian mount is actually just a box on a turntable.  Usually, a pivot attaches to the OTA and rests in a cradle in the box.  A carefully made Dobsonian may be accurate enough for setting circles and even a drive mechanism, which helps mitigate some of the disadvantages of alt-az mounts. One thing to note though is that these mounts really only work with Newtonian style telescope, where the eyepiece comes out the side of the tube near the top. If it’s a very big telescope, that may mean you’ll need a ladder to observe.

Most “tabletop” scopes are alt-az mounts. Dobsonians are common because the turntable can sit on the table without adding much of any height to the telescope. The OTAs are usually light enough that only one support arm is needed.

Advantages and Disadvantages

Advantages

  • Easy set-up. Alt-az mounts are the easiest mount to set up. Dobsonian types you just plop down on a level surface, pop in an eyepiece, and you’re observing.  If it’s a tripod, you’ll have to extend the legs, but you can make adjustments if the ground isn’t level.
  • Easy to operate. They work the same way you usually think about looking for things: up and over, so beginners usually find alt-az mounts much easier to aim.
  • Cost. As with anything, you can get some very fancy, expensive alt-az mounts.  But you can’t beat a cardboard box for price.
  • Easy to make. If you like building things yourself, a Dobsonian is the way to go.  You just have to make sure everything is square and you’ll have a working mount.

Disadvantages

  • star trails

    Star trails over the Gemini observatory. Note that the length of the trail depends on the part of the sky the star is in. Longer trails indicate a bigger change in alt-az coordinates.

    Tracking is hard. Unless you are observing the pole, any target you set the ‘scope on will move. Unless you are observing at the equator, the target will move at an angle, so both the altitude and azimuth will change. Even harder, the rate they change at depends on where the target is in the sky. A star close to the pole may only change 5 degrees in altitude over the course of the night, but a star that rises in the northeast may cover almost 90º over the course of the night. Images of star trails illustrate this. Near the pole, stars don’t move much over the time it takes to capture the image, so the trails are short, but farther from the pole the stars move more, and the trials are longer.  Modern telescope mounts overcome this problem by having a computer control the tracking, but that means if you want an alt-az mount that will track, you’ll have to spend the money on a computerized mount.

  • Coordinates are location dependant. The north celestial pole is at about 45º altitude, 0º azimuth for people  along most of the US Canadian border, but only about 30º altitude for people on the US Mexico border. And of course, most objects change in azimuth as the night goes on.  This means if you want to get the coordinates of an object like a comet, you’ll have to know your latitude and what time you’ll be looking for it. Again, the computer controlled mounts can fix some of that.  The computer can calculate where it should point as long as it knows where you are and what time it is.
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