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  1. #1
    Carl &/or Amy's Avatar
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    Default Alt/az-vs-RA/dec



    What are the differences btwn the Altitude/azimuth system and the Right Ascension/declination system? Why two systems?

  2. #2
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    Default

    there are a couple of differences between the two.
    firstly alt/az is easier to use than ra/dec.
    the latter can have you in some weird positions
    at the ep.
    also,ra/dec is the best for astro photography,as this follows
    the movement of the earth,allowing for better tracking if you
    have a motorised/goto mount.
    you can do ap with an alt/az mount,but you are limited in
    exposure times,because of issues with field rotation,
    causing star trails.
    hope this helps.
    clear skies,

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  4. #3
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    Why two systems? --------- Ha! why is water wet! because it is wet. Really no good answer-- one Altitude Azimuth is the same as you would see on a compass-- the other RA & DEc are the "addresses" of objects in the night sky and is used to both locate them--- and to TRACK them accurately when doing astro-photography (RA & DEC).

    Altitudeand Azimuth can also be used to locate an object and track it using motor drives-- or slow motion controls BUT the back ground stars will ROTATE around the object that is being tracked. (field rotation) this is fine for visual because the motion is very slow-- but not for long term image exposure.

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  6. #4
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    The two coordinate systems are both useful because they are measuring different things. And telescope mounts based on those two systems are useful to different people for different reasons.

    Alt-azimuth is easy because it is intuitive: up/down, left/right. You can set up the mount without much thought or planning, and can use it without training. For those reasons, it appeals to a lot of people. The coordinate system based on those motions is useful to others who are in the same location as you: people at a star party, for example. You could tell someone to look to the south-west at 30 degrees above the horizon, for example, and they would know where to look.

    However, that would not work if you got on the phone to someone a long way away and wanted to tell them where to look. A star that is in your southwest might be in his southeast. So, for telling people unambiguously where objects are in the sky, you need a coordinate system that is fixed to the sky, so that the coordinates will be the same for everyone. That is what RA and Declination are for.

    Mounts based on the RA/Dec system (called equatorial mounts) have the advantage that they can track the motion of the sky with a single movement. You can set an objects coordinates on its setting circles and know that the scope will be pointing in the right direction.

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    To add to Keith's excellent explanation.

    Until the recent development of computerized mounts, the only way to have a telescope that could automatically track a star or other object in the sky was to use an equatorial mount. A polar aligned equatorial mount can easily be motorized and track objects in the sky in one movement that is constant in speed. This made the equatorial mount ideal for photography and for automatically tracking objects in the sky. The fact that the equatorial mount is aligned with the celestial pole and not the earth's pole makes its movements awkward ... some practice is required to learn its usage. It also has to be accurately aligned with the celestial pole.

    An azimuth mount requires two intuitive movements, one in azimuth (north, south, east, west) and one in attitude (up and down). Tacking an object in the sky needs two movements. The speed of the movements varies dependent upon the location of the object in the sky ... far too complicated for a motorized tracking system until the introduction of computers. This made the azimuth mount unsuitable for photography and for automatically tracking objects in the sky. However, the movement of an azimuth mount is intitutive ... just point the scope where you want to see and no alignments are needed.

    Today, the azimuth mount is a very popular mount in both its non-computerized manual versions and computerized versions. Dobson Telescopes are azimuth mounted telescopes and are very popular among astronomers who only visually view the sky. Computerized versions are popular as they automatically find and track objects in the sky. These computerized azimuth mounts can even be used for astrophotography but are not as versatile as equatorial mounts for this purpose.

    Which one is best? Neither; it just depends upon the needs of the observer. For photography, the equatorial mount has several advantages ... primarily longer exposure times and access to the entire night sky. For observing, the azimuth mount has several advantages ... easy of use, no polar alignment, and portability.
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  10. #6
    Carl &/or Amy's Avatar
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    Thank you all, now are altitude and declination two different names for exactly the same thing? Both 90 degrees at zenith and then on down? Or is zero degrees altitude considered the local horizon, whereas zero degrees declination is the celestial equator?

    Thanks for the excellent answers folks.

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    Quote Originally Posted by Carl &/or Amy View Post
    Or is zero degrees altitude considered the local horizon, whereas zero degrees declination is the celestial equator?
    ^^This.

    Here are a couple of images to illustrate the difference - this is the view from Phoenix, AZ:


    0 degrees declination is the blue line passing south of Jupiter, while 90 degrees altitude (the zenith) is where all of the orange lines intersect. From my particular location objects on the celestial equator reach a maximum altitude when they are due south of roughly 57 degrees, but if I lived on the Earth's equator then objects with 0 degrees declination would reach a maximum altitude of 90 degrees when they passed directly overhead.

    Now if you look east you can see that the celestial equator reaches an altitude of 0 degrees right at due east on the horizon:


    Hopefully this helps explain things.
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    Quote Originally Posted by Carl &/or Amy View Post
    Thank you all, now are altitude and declination two different names for exactly the same thing? Both 90 degrees at zenith and then on down? Or is zero degrees altitude considered the local horizon, whereas zero degrees declination is the celestial equator?

    Thanks for the excellent answers folks.
    No. An object's altitude will continually change throughout the night, it's declination will not. Declination is measured from the celestial equator (0) to the celestial poles (90/-90). Altitude is measured from the horizon (0) to zenith (90).

    The difference is more apparent with the following example. If you live at 50N latitude, the altitude of your zenith is 90 (same at any latitude). The declination of your zenith, from that same location, is actually 50.

    Here's a link describing the two systems. It's not the greatest, but the best that I could find quickly. If you do a Google search on celestial spere or celestial coordinate system, you may have better luck.

    Introduction to Astronomical Coordinate Systems
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    ALT-AZ and RA-DEC are the result of different philosophy of explaining the sky and positions in objects in it. Starry sky moves relatively to horizon - it's what we see. The stars don't move in relation to each other (for practical purpose).
    First way to explain star position is to point where it is over the horizon - easy to point, easy to find. It's ALT-AZ system. The only problem is - stars move at an angle to horizon, as horizon rotates together with the Earth. It's what Keith explained so well. The RA-DEC system takes as a start point that stars don't move relatively to each other, rather seem to be rotating all together around some point in the sky called Celestial Pole. Celestial pole has DEC of 90 degrees and is close to Earth's Pole in AZ it's ALT is the lattitude of the point on Earth you are standing at. That's the point all stars rotate around. More about the subject here: Declination - Wikipedia, the free encyclopedia

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    Download Stellarium and switch back and forth between the two systems, the differences will be apparent
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