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# RA, DEC vs Azimuth, Altitude difference.

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Hi, I'm trying to figure out what is the difference between RA, DEC vs Azimuth, Altitude difference. I watched dozens of videos and read a lot of material and I think I got it all mixed now.:)
From what I understand, if I want to find a star using let's say a telescope than I would need to know RA and DEC which I can get from an app or a chart. (Is there a professional way?)
But if I want to point at a star as an observer than I would use an Azimuth and Altitude?

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Not quite.  The practical difference is simply to do with the telescope mount that you are using.  If it’s an Alt/Az mount, with one vertical and one horizontal axis, then you need azimuth and elevation.  If you tip that mount so that the vertical now points towards the celestial pole, using a wedge, then your mount effectively becomes an equatorial, and you need the RA and declination.

The advantage of a wedge, or a proper equatorial mount, is that only one axis needs to move to follow a fixed point in the sky, and the image does not rotate as you do so... perfect for astrophotography!

Edited by AKB

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First get the principles clear in your mind. Imagine that you're standing on the north pole. Make an imaginary set of grid lines radiating out from Polaris above your head like the spokes of an umbrella held vertically. Then add a series of concentric rings starting with the horizon and getting smaller and higher till the last one is a little ring centered around Polaris. This umbrella-like grid, at the north pole, will work as a template both for the RA-Dec system and the Alt-Az system. But now take your imaginary skymap-umbrella with you to Europe, say Lat 45N, and hold it vertically. It no longer matches its position at the north pole because it's canted over at 45 degrees. To make it agree with how it was you need to tip it to 45 degrees and point it at Polaris.* Now it's the same as it was at the north pole.

The umbrella held at 45 degrees represents the RA and Dec system. The umbrella when held vertically represents the Alt-Az system.

In the RA and Dec system the stars follow the concentric rings around Polaris, never moving up or down the spokes. In the Alt Az system, once you leave the poles, the stars move realitive both to the spokes and the concentric rings.

Olly

*More strictly at the N celestial pole, but they are close enough for government work!

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What an eloquent description Olly.

Carole

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Thank you all for the information, it is very useful! I recently dived into Astronomy and there is so much to learn. This part was the most confusing.

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11 hours ago, Point38 said:

This part was the most confusing.

It gets worse!

Olly

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1 hour ago, ollypenrice said:

It gets worse!

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1 hour ago, AKB said:

Perfect example! When they don't fit you can't use them and when they do they stick inseparably together...

Olly

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If you want to see the difference in the RA/Dec and Alt/Az grids made in the sky at your location - download Stellarium and set your location. You can then switch the two different grids on and off using the tool menu bottom left of the screen.

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Brill @ollypenrice

Who'd'av thunk it, to use an umbrella as an essential piece of astro kit to hand  , , , oh, hang on though , , ,

hehee

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...which is why I use one of these for teaching...(mine is currently in my freezing loft, hence the picture of someone else's)

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Re reading my post I see that I missed a bit out  " ,, kit to hand in the UK "  but I guess you all were with me there !  -Just for clarification for chums who are fortunate to live in warm, dry, sunny climes. -

Welcome aboard @Point38

Edited by SilverAstro

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20 hours ago, BinocularSky said:

...which is why I use one of these for teaching...(mine is currently in my freezing loft, hence the picture of someone else's)

Lol, I need one like this, even though I'm in California.

On 2/4/2018 at 09:49, brantuk said:

If you want to see the difference in the RA/Dec and Alt/Az grids made in the sky at your location - download Stellarium and set your location. You can then switch the two different grids on and off using the tool menu bottom left of the screen.

Great software, thanks!

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On ‎2‎/‎3‎/‎2018 at 00:06, Point38 said:

Hi, I'm trying to figure out what is the difference between RA, DEC vs Azimuth, Altitude difference. I watched dozens of videos and read a lot of material and I think I got it all mixed now.:)
From what I understand, if I want to find a star using let's say a telescope than I would need to know RA and DEC which I can get from an app or a chart. (Is there a professional way?)
But if I want to point at a star as an observer than I would use an Azimuth and Altitude?

Think of altitude-azimuth coordinates as the location of your object relative to where you are standing on earth. No matter where on the planet you're standing, the coordinate's zero point is based on YOU and the direction you're pointed. Altitude will be the height of an object above the horizon in the direction you're looking, and azimuth will be the off-angle to the right or left of a vertical altitude line drawn from the horizon in the direction you're pointed. In degrees, where a specific object will be as measured in height above the horizon (alt) and offset (azimuth) will change with your position on Earth. A celestial coordinate does not change, no matter where you are standing.

The celestial coordinate system (Declination and Right Ascension) is based on the planet and its position in space. Where alt-az is dependent on where you stand, Dec/RA doesn't change with your position. If you consider the lines of latitude and longitude as an earth coordinate system, the celestial coordinates are just the lines of latitude and longitude extended into space. Latitude is the same as Declination, Longitude is the same as Right Ascension. Where Latitude has the Equator, so does the celestial system, to delineate where North and South meet. Where Longitude has the Prime Meridian  to determine a zero point to number from, so does the celestial coordinates have the First Point of Aries (which oddly enough is in Pisces) to determine where Right Ascension is numbered from. The First Point of Ares is one of two points where the plane of the ecliptic crosses the celestial equator. Opposite this is the First Point of Libra, which is 180 degrees  away on the ecliptic and celestial equator.

Brantuk has the best way to see how they are alike/different. You can choose either or both systems to show on Stellarium.

@ Binocular Sky:  Love the umbrella. Is this a star chart for rainy nights?  Where can I find one of those?

Edited by Luna-tic

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Simple really. RA and Dec are a complete pain, often quite literally, whilst Alt/Az is easy and painless.

Unless you’re a masochist, or into astrophotography, stick to Alt/Az.

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Anyone into astro. photography would out of common sense, choose an equatorial mount for the job.
In my early days in AP, my Newton 12" f6 telescope I equipped with Setting circles, 10 inches in diameter, and carefully scribed with the Hours and Minute divisions, RA. and Degrees and Minutes Dec.
for  both Right Ascension, and Declination.
It was arranged so the RA circle could be slipped, independently but allowed to move with the driven  worm wheel at  Sidereal Rate.
A local meridian fixed pointer over the RA circle would indicate local sidereal, or star time. Locating any star of known Right Ascension would provide that time, also it's Declination.
Both circles would be set to those coordinate, and by leaving the Stepper Drive motor running, a  RA Circle pointer fixed to the polar shaft, would service the East side of the mount, and another the West side.
Once set on a star's known Declination, and Right Ascension, It was then simple to Index any object in the sky by finding it's RA. and Dec. from an atlas or Book.
These were the things you needed to do, to make object location easier way back in my early days as an Observer and erstwhile Imager (Not very good results unfortunately. Film was OK, but I was no David Malin)
Sometimes the sought after object would be in the FOV first time, but many times a little shunt back and forward with N S E W buttons was needed to bring it into view.
I certainly refute the claim Equatorial  Mounts are a pain, on the contrary, they are very desirable.  Up and Down, and left and right are fine for general observing , but unless an Alt Az. mount can be adapted for equatorial use, it is not ideal for long exposure Imaging.

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I saw one of those astro umbrellas while on a visit to Japan - very  neat and good looking.....

I haven't found a local supplier. Has anyone got a good lead to follow??

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Thanks mate..

I think the cost to ship to Oz may be excessive.

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I think of it like this. RA/Dec are fixed to all intents and purposes and unaffected by time and location. If you can get to the point where the RA / Dec coordinates say they are then it will be there no matter when or where you are.

To find an alt  az target using coordinates you need to consider when and where you are as the coordinates change constantly.

That said I'd tend to star hop not work on coordinates for altaz manual observing.

I prefer an equatorial mount when tracking solar system objects or sketching. When zipping about the sky just touring I prefer a manual altaz mount.

As a visual only observer I actually never use coordinates for either.

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1 hour ago, Moonshane said:

I prefer an equatorial mount when tracking solar system objects or sketching. When zipping about the sky just touring I prefer a manual altaz mount.

Ditto Shane. An EQ makes a lot of sense if tracking one target for a long time, either visually or for imaging. Alt az much easier for star hopping. I either eyeball a target directly to line it up or star hop, never bother with co-ordinates.

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On 3/3/2018 at 18:48, Donkeiller said:

Simple really. RA and Dec are a complete pain, often quite literally, whilst Alt/Az is easy and painless.

Unless you’re a masochist, or into astrophotography, stick to Alt/Az.

Are you talking about mounts or the system of co-ordinates?

Olly

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On ‎6‎/‎02‎/‎2018 at 16:02, Point38 said:

Lol, I need one like this, even though I'm in California.

Great software, thanks!

They come in both north and southern hemispheres

Been southern hemisphere, and approx. 28 degrees south of the equator, get a different view what above us

Even Orion is upside down as viewed from the northern hemisphere

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That's interesting....

Where did you see a Southern Hemisphere version???

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On ‎5‎/‎03‎/‎2018 at 13:35, Merlin66 said:

That's interesting....

Where did you see a Southern Hemisphere version???

Merlin

Local astronomy shop in Brisbane had them a while ago

Not sure u can get from Bintel

Check my club website other astronomy news as well

Club is co-hosing Star Stuff again this year down at Byron, first week-end in July, details club website

Also NASA is launching a new probe to our sun called Parker Solar Probe later this year

NASA have invited astronomy communities world wide to submit your name, to be included on microchip on the probe

Stay in touch

John

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