Polar vs. Alt/Az

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You may have asked yourself “Why would I want or need to mount my scope in Polar Mode?”

First let’s take a look at the two modes and how they work.

Altitude/Azimuth or Alt/Az mode is probably the most common way a telescope is mounted. The Scope, pointed at any point above the horizon (Altitude) remains at that Altitude as it rotates around on it’s base. This rotation is call Azimuth. It’s is the way we view our surroundings. We rotate around on our feet and lift our heads up and down. Because of this it is easy for us to comprehend the use of a telescope mounted in this fashion.

Polar Mode is the method that mounts your scope at the angle to the ground that is equal to Latitude of your location. Huh? Let’s take a look at what’s going on here. If you were to take a very very very… long pole and shove it thru the Earth from the North Pole to the South Pole and then standing anywhere on the surface of the Earth hold a stick parallel to that very very very… long pole and measure the angle to the level ground it would be exactly the same as your Latitude.

Now lets take a look at what happens with your scope in both modes.

In Alt/Az mode if you point your non tracking scope (not powered by motors)  at a star with a low power eyepiece you will notice that to recenter the star you will need to move your scope both in Altitude and Azimuth.

Doing the same test in Polar mode to recenter the star you will only need to move the scope Right and Left (rotating on it’s base which is mounted at an angle) This rotation is called Right Ascension or RA. You will not need to move the scope Up or Down which is called Declination or Dec.

So why is this important?

If you have looked at any Star Charts or observing lists you may have noticed that all locations are given in RA and Dec. This is because these locations never change. (well, not completely true. Look up procession). The Alt/Az location of the object is always changing. So that means a goto scope mounted in Alt/Az mode must make a major calculation based on month/day/year and time of day to find the object and then make those calculations continuously in order to track the object. A Scope mounted in Polar mode must make only a minor calculation to fine the object and then rotate the scope to the right, around the base, at one standard rate. No more calculations are necessary.

So how should you mount your scope?

If you want to do wide field or deep sky astrophotography the answer is easy. Polar mode. There are 2 reasons for this. First you will only need to guide to fix gear imperfections and poor alignment. (see my page ‘Scope Align’). If you take the long exposers necessary for deep sky objects and wide field in Alt/Az mode the stars will make circular streaks in your pictures. If all you plan to do is visual work then Alt/Az will work just fine.

Some more thoughts… Some scope types don’t give you the option of using the Polar mounting method. The Dobsonian or Dob type scopes do not give you this option and so if you ever plan to do photography you may want to choose a different scope. (Unless of course you’re made of money and can afford 2 scopes). Also there are 2 types of Polar mounts. The wedge is used for fork mounted OTA’s and German Equatorial mounts OTA’s that are not fork mounted.

I hope this page is helpful and I welcome all comments, criticisms and questions. Just use the  comment form below

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Welcome , today is Wednesday, March 12, 2014