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I have a Celestron Astromaster 130 EQ. After aligning my telescope's polar axis with Polaris, in order to track stars I need information about the coordinates in terms of right ascension (RA) and declination.

But in all the astronomy apps I have found that they have fixed the RA and declination coordinates and are tracking stars with respect to azimuth and altitude.

How am I supposed to use this data? Please help!

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The really short answer is ... you don't (use Alt/Az data). You own a German Equatorial Mount that uses RA/Dec values and have no need of the Alt/Az coordinate system.

A slightly expanded answer is that use of coordinates is meant to help you find objects. Realistically it would be challenging to do with your telescope mount because the RA/Dec setting circles on the mount are not highly accurate (and sometimes move when they aren't supposed to move). More on this later.

Right Ascension (& Declination)

The Right Ascension and Declination system for objects in space is somewhat like Longitude and Latitude coordinates on Earth. But since Earth spins, we can't use actual Longitude / Latitude values.

A object directly above Earth's North Pole has a Declination of +90°. An object above Earth's South Pole has a declination of -90°. A declination of 0° means the object lies along a path somewhere directly above Earth's equator.

Right-Ascension works like a clock (which is why it is expressed in hours and minutes) based on 24 hour day. If you look at the setting circles for your telescope, you'll probably notice that the Right Ascension setting circles have two sets of values... one counting up, the other counting down. Those are actually to allow for use in the Northern vs. Southern Hemispheres (a telescope in the Southern Hemisphere is aligned to the Southern celestial pole, but the motor drive needs to spin the telescope in the opposite directly as compared to the Northern Hemisphere.

There is a 00 hour 00 minute position in the sky. It's "name" is the "First point of Aries" (a confusing name because the point is not in Aries ... for reasons I'll get to in a moment).

The point is defined based on the position of the Sun (relative to the background stars ... which of course we cannot see during the daytime) at the Vernal equinox (when the Sun crosses the equator from the Southern Hemisphere into the Northern Hemisphere on or around March 20 each year).

But since Earth's axis precesses very slowly (a cycle that takes nearly 26,000 years), the point moves. When the point was originally defined, the Sun was in Aries... but a couple of thousand years later, it has migrated to Pisces ... and is actually very close to Aquarius.

So imagine if you could point your telescope at the First Point of Aries (the 00 hour 00 minute 00 second point) and then ... switch off the motor and just watch the sky drift along from East to West. How long would you have to wait before the imaginary "longitude" value of the object you want to view in the night sky is in front of your telescope? That time duration... is how the RA coordinate gets it's value.

With the exception of solar system objects (planets, comets, asteroids, etc.) objects in the universe seem as if they don't move. They do move... but are so incredibly far away that it requires very precise measurements to detect any movement at all. This means that, for the most part, we can assign them fixed coordinates that don't change (these coordinates are occasionally updated). On the timescale of a single human lifetime, we don't notice the movement.

So this means, conveniently enough, that we can mostly get away with a simple look-up table to find objects and ignore their proper motion.

This is why the RA/Dec values do not change when you look up objects in those apps... because they don't actually change from hour to hour or day to day. Many years are required to notice even very subtle changes ... and using very accurate instruments to take those measurements.

Also, because of the way this coordinate system works... the RA/Dec coordinates for these non-solar-system objects are the same for all observers, everywhere on Earth, regardless of date or time (well... regardless of date or time excluding very large amounts of time... such as thousands of years ... at which point their movements would change enough to notice.)

One little tiny nit... Polaris is not located at Earth's celestial North Pole ... but it is close. It's roughly 2/3rds of a degree away from the pole. But keep in mind that the angular width of the Moon (as observed from Earth) is just about 1/2° wide in the sky. This means there is enough angular width between Polaris and the true Celestial North Pole... that you could fit the Moon in that space with a little room to spare.

For visual astronomy ... pointing your mount at Polaris is "close enough". A computer driven telescope would miss it's "go to" targets ... but probably get close enough that those objects would appear in the Finderscope (and just need to be re-centered). Not a big deal. (For astro-imaging using long camera exposures ... a more accurate alignment is needed.)

Altitude Azimuth

This coordinate system is much simpler to understand. But it has a major drawback.

The azimuth is simply the compass heading... in which compass heading do you need to be facing in order to view the object, and... how high above the horizon line do you need to look? The compass heading is the Azimuth. The angle above the horizon is the Altitude. Simple enough.

But suppose you are collaborating with someone about an object you are observing ... so you want to share coordinates with your collaborator. But your collaborator is not nearby ... they live on another continent. The Earth is spherical and the curvature of the Earth means that the Altitude and Azimuth you use ... isn't the same as the Altitude and Azimuth your colleague will need to use. Even if they live along the same latitude and are simply in an adjacent time zone... their coordinates will be different enough to completely miss finding your object.

This is why Altitude/Azimuth values aren't useful in catalogs. They change ... a lot.

The German Equatorial Mount

The benefit of this mount is that once you orient the mount so that it's major axis (the Right Ascension) is parallel to Earth's axis, then as the Earth rotates from West to East ... the telescope mount can be moved East to West. If this mount were equipped with a motor and if that motor spins the mount at the same rate that Earth spins... then the telescope will remain fixed on the same position in the sky.

Also, adjusting the mount in the RA direction causes the telescope to move in a perfect east-west direction. Adjusting the mount in Declination moves the telescope in perfect north-south direction.

This simplifies the process of tracking objects in the sky because once you target and center on an object, only the Right Ascension motor needs to operate to track an object. With an Alt/Az mounted motorized scope, a computer needs to interpolate the movement of both axes in order to track an object. The exception is... if that telescope happens to be located directly at Earth's North Pole or South Pole ... then an alt/az telescope works just as well as a German Equatorial Mount (RA/Dec).

Converting from one system to the other requires a bit of math -- but not particularly complicated.

Using RA/Dec to find objects

I previously mentioned RA/Dec (for non-solar-system objects) is mostly just a lookup-table of values that don't change in a very noticeable way within the span of a single human lifetime.

Computer-controlled "go to" telescope mounts have to be "aligned". The mount is polar aligned (physically). The computer will typically want to know your geographic coordinates as well as the date & time to have a good idea of what objects should be visible in the sky. Most "go to" mounts will then ask that you accurately center the telescope on a couple of bright stars ... and use this information to create a "fix" on the entire celestial sphere. From this ... the computer can now automate moving the telescope to any object you want to observe.

As your telescope mount is not not motorized and controlled by a computer, what point is there to have setting circles with the RA/Dec values?

It turns out you can (theoretically) use the setting circles as a kind of analog computer.

Suppose you want to locate some hard-to-find object.

Start ... by pointing the telescope to a known easy-to-find object. A really bright star such as Sirius... or Vega... etc.

Look up the RA/Dec values for that really bright star. Now adjust the setting circle values on your mount to match the values in the table.

Next, find the RA/Dec values for your hard-to-find object. Then re-point your telescope'S Declination axis to the declination of the hard-to-find object. Now do the same for the RA value.

Theoretically your telescope is now pointed at the hard-to-find object. I say "theoretically" because large professional telescope used many years ago to find these objects (before telescopes were controlled by computers) had very large settings circles that were much easier to use. Consumer telescopes usually have tiny setting circles that are sometimes rather frustrating to use (My personal experience with my own budget mounts were that the bits that were meant to be adjusted to match an object and then remain "fixed" as you move the mount ... often slipped and moved while adjusting the mount. This made accurate pointing a fairly frustrating experience.)

This means "star hopping" is probably the preferred method to find objects (find the nearest easy-to-find object .. then move the mount only slightly to locate your hard-to-find object).

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  • $\begingroup$ Thank you so much mr Tim for such a vivid explanation....much appreciated . $\endgroup$ – Pritam Saikia Sep 13 at 13:40
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The coordinates of the object are not needed in order to track with a German equatorial mount. All objects move at a rate of 1 revolution in 24 hours (approx). You just point the scope at the object and turn on the drive.

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  • $\begingroup$ I've slightly edited the question for grammar, but I'm pretty sure they want to know how use RA and Dec in order to point the telescope at an object by using published coordinates, so "You just point the scope at the object and turn on the drive." might miss the "point" of the question. :-) $\endgroup$ – uhoh Sep 13 at 0:37

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