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TELESCOPE MOUNT COMPARISON
ALTITUDE/AZIMUTH vs EQUATORIAL
Updated December 18, 2014 to include more information and mount options
LEFT: Synta AZ3 mount. RIGHT: Synta EQ2 mount.
There are 2 basic types of mounts available for telescopes: Altitude/Azimuth (also called Altazimuth and Alt/Az) and Equatorial. Altazimuth mounts are designed for simple left/right, and up/down operation. Equatorial mounts operate in a different manner that better corresponds to the individual observers location and the rotation of the Earth as related to the sky. Each type of mount has its advantages and drawbacks. For this comparison I have chosen 2 common entry level mounts that I own myself and am quite familiar with. They are/were supplied as standard equipment on telescopes sold under brand names such as Skywatcher, Celestron and Orion to name a few. These mounts share the same basic features as other mounts of the same type (Altazimuth or Equatorial) sold by other companies.
Synta AZ3 mount on standard aluminum tripod
The AZ3 mount shown in this photo came with my 4" Skywatcher shorttube refractor. The telescope just balances like a rotatable teeter-totter on top of a set of adjustable legs. There are no hand knobs to loosen and tighten as you aim the telescope. Once it is balanced on the mount you just push the tube to where you want to point it. There are 2 manual fine tuning controls on long flexible sticks you can then turn to get the aim just right. These mounts are perfect for daytime land viewing using refractor telescopes. With the use of a mirror diagonal you get views that are right side up, and using an erecting prism they are oriented correctly like in a pair of binoculars.
To set up an altazimuth mount, you simply spread the tripod legs and adjust them to a convenient height with tripod level to the ground. You can rotate the telescope to point east, west, etc. You can also raise and lower the aim closer and farther away from the horizon.
4" shorttube refractor on AZ3 mount
These mounts are great for scanning the horizon or observing wildlife on the ground, but they are sometimes not ideal for astronomical purposes. If you examine the photos above, you might notice that the tube ring towards the viewing end of the telescope has been modified where it joins the mount. I have moved it closer to the center of the tripod so I can slide the optical tube farther forward and make it a little top-heavy. This is necessary when aiming the telescope high to keep it from falling backwards.
If you look at the way the telescope is balanced on the mount, you will notice that as you aim the telescope higher, the weight of it shifts behind its center of gravity. If you go high enough, the weight of the tube makes it fall back and point straight up. The only way to counteract this would be to tighten the main bolt at that pivot point, but that would make the telescope difficult to move. Even with the tube slid as far forward as it will go on my modified mount, the weight of a 27 Panoptic is pushing the limit when it comes to pointing very high in the sky without falling back.
Vixen Optics Portamount
There are some altazimuth mounts available that can aim at any height above the horizon without falling back. This is done by mounting the telescope on a U-shaped bracket (cradle) that places the telescope in the center of the axis it pivots up and down on. I have seen them available for larger telescopes, and they are much more expensive than any of the models I have shown above. Vixen Optics makes a mount called the Vixen Portamount that will handle smaller telescopes and point anywhere above the horizon without falling back, and includes the manual fine tuning adjustments. It is more expensive than an AZ3 mount. Explore Scientific also makes a mount called the Twilight Mount which is similar and less expensive. The models I have seen for larger telescopes did not have fine-tuning adjustments, which means that you must push the telescope slightly with your hand to adjust the aim rather than turn a knob. At higher magnifications this is difficult to do precisely. New products are always coming out, so it is possible some company now makes an altazimuth cradle mount for larger telescopes with manual fine aiming adjustments today.
Synta EQ2 mount on standard aluminum tripod
The Equatorial mount is geared specifically towards astronomical use. They use the gallactic poles of the Earth's sky. These are the points directly above the Earth's spinning axis that never change their position from the viewer's location, and which the entire night sky seems to rotate around as the hours pass by. In the northern hemisphere this point is very close to Polaris (the north star). It is the end star on the handle of the "Little Dipper" in Ursa Minor. At my location Polaris is towards the north roughly 44 degrees above the horizon. In the southern hemisphere the gallactic pole is in the constellation Octans, and is not close to any bright star.
Consider the Altazimuth mount mentioned above, and consider the point straight up in the sky to be its "neutral" position. There are 2 degrees of motion for an Altazimuth mount. The user can adjust how far from straight up the telescope points on one axis, and the other axis basically rotates in a circle around that "neutral" point to choose which direction (North, South, etc.) the telescope faces.
An Equatorial mount is similar, but its "neutral" position is not straight up: Its "neutral" position is the gallactic pole. One axis adjusts how far from the gallactic pole the telescope is aimed (declination), and the other axis aims the telescope in a circle around the gallactic pole (right ascention). Equatorial mounts work great for observing the sky at night. If an object moves as the Earth rotates, you can re-center it by adjusting the fine tuning for the right ascention only, as its declination hasn't changed. The motor for a mount that tracks objects on an auto-driven mount is connected to the right ascention drive, and simply turns it slow enough to keep an object centered as the earth rotates on its axis. With an Altazimuth mount, you usually have to adjust both fine tuners to re-center an object that has moved out of view. Furthermore, unlike most Altazimuth mounts the telescope and mount can be adjusted so that it will be balanced no matter where it is pointed. This is done by positioning the tube on the mount to balance it on one axis, and by moving the counterweight to balance it on the other axis.
LEFT: Synta EQ2 mount with 70mm longtube refractor.
RIGHT: Close-up of head angled for 44 degrees latitude.
By looking at the Equatorial mount shown above, you can immediately notice 2 things that are different that an Altazimuth mount. For one thing, the mount seems angled as compared to the ground. That is because it's operation centers around the gallactic pole, and the 2 axis of motion must be lined up accordingly for the observer's location in order to work properly. To do this you must level the tripod, and adjust the angle of the mount to match your latitude. In my case it is 44 degrees, so the mount must be angled upwards of the level horizon to match that amount. Since I am in the northern hemisphere, I aim towards the north gallactic pole (near polaris), and the mount is now polar aligned. That is the basic concept, but there is a lot more to it when it comes to polar aligning accurately enough to use co-ordinates as a means of locating an object, or to aim a mount for the purpose of motorized tracking during long photographic exposures. You cannot just place the mount on the ground and expect it to track-it must be placed facing the right direction and set to the correct latitude. Equatorial mounts generally come with instructions on how to do this.
You may notice that there is also a counterweight on the mount. This is due to the nature of the mount, and the fact that the telescope will sometimes be beside the mount as it relates to the horizon in order to aim in certain directions. The counterweight is necessary to keep the tube from falling downwards as it would when aiming high with an Altazimuth mount. There are also locking screws on an Equatorial mount that will keep the telescope aimed. The tube should be balanced enough to remain stationary when you let go of the tube, but you must tighten these knobs to ensure the operation of the fine tuning adjustments for the aim, as well as any motorized drives for tracking the mount might have.
Equatorial mounts are not very useful for daytime viewing on land though. It's hard to describe the way a telescope moves on an Equatorial mount as you try to aim it. It can turn almost upside-down and could be in any position in relation to the tripod depending on where you're aiming. At times you must rotate, flip, and spin the telescope to aim from one part of the sky to another. I like to call this series of motions "the Equatorial Tango". Let's pretend you had a telescope set up on a lakeshore and wanted to scan the other side of the lake. Using an Equatorial mount, you aim straight across the lake and adjust your diagonal to place the eyepiece in a convenient location for viewing. As you try to scan along the opposite shoreline you must rotate the scope on both axis', and the orientation of the view changes. If you scan one way the view tilts farther to the right, and as you scan the other it tilts farther to the left. This can be corrected by rotating the diagonal and the tube on the rings, but it is a pain in the butt and can turn a relaxing nature viewing session into a frustrating exercise in patience. Altazimuth mounts are better suited for viewing in this case.
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