How to use an equatorial mount???

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wAi_kiT
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How to use an equatorial mount???

Post by wAi_kiT »

I've got a 4.5" Newtonian on equatorial mount. As I've noticed, there are 3 axis on the mount which, I don't really understand what it means. 2 of the axis have degrees fixed, but the RA axis, has its "hour" circle movable... Can anyone explain how do I make use of these 3 axis?
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Axis 1
Axis 1
06-09-06_0211.jpg (60.65 KiB) Viewed 8031 times
Axis 2
Axis 2
06-09-06_0213.jpg (62.02 KiB) Viewed 8031 times
Last edited by wAi_kiT on Thu Sep 07, 2006 10:38 am, edited 1 time in total.
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weixing
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Post by weixing »

Hi,
The one in the first picture is the DEC. The 2nd picture one should be the latitude or altitude adjustment screw.

I'll mention the second picture one first. For you to track object (either by motor drive or manually), you need to set the latitude adjustment on the mount to match the latitude of your observation site. In Singapore, it is around 1 degree... your second picture show that it's already in the correct setting for Singapore use.

The RA (the one with the "h" value) and DEC (your first picture) together form what we call Setting Circles or Manual Setting Circles. It's use to help you to locate object using the object coordinate.

Below is a simple description on how to use it:
To use the setting circles, you need to polar align your EQ mount... that is point the "N" on the mount towards North. Remember, our latitude is around +1 degree, so we are in the Northern Herisphere. To use the Setting Circles to help you find your target, first you need to know the coordinate of your target and a known object near it.

For example: You want to find M57 (Ring Nebula) and you can identify the bright star near M57, Vega. From the star chart, You know M57 and Vega coordinates:
Vega -> RA:18h 37m, DEC: +38 degree 47min
M57-> RA: 18h 54m, DEC: +33 degree 02min

So you point your scope toward Vega and after you move Vega to the center of your scope's field of view, you set the RA Setting Circle to Vega's RA coordinate... that's why RA circle is movable. Please note that on some EQ mount, both RA and DEC are movable and so need to set both.

After setting the RA Setting Circle, you move your scope and look at the RA and DEC Setting Circles and guide the scope until it point to the RA and DEC of M57. Now you had point the scope to where M57 should be.

Please note that the time took after setting the Setting Circle to guiding the scope to the target should be as short as possible... the star and object on the sky will "move" due to earth rotation.

Also, please note that the Setting Circle on most EQ mount is usually not very accurate because it is usually very small, so it'll only help you point your scope to around the target area... you'll need to use a low power eyepiece to search for the target.

Ok... Need to go for work already.

Happy observing and have a nice day.
Yang Weixing
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wAi_kiT
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Post by wAi_kiT »

Hmmm... So that means you need to locate an object first, to get the RA axis right, then you're able to use that axis onwards to locate other objects more easier? BTW, on the DEC axis, the 0 value means its due east right. So a value of DEC +90 will turn the telescope somewhere in the middle between easst and west?
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alvinsclee
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Post by alvinsclee »

Hi Wai Kit,
Reading celestial co-ordinates is very different from the way we read bearings on the map. On a flat map, bearings are read from magnetic North clockwise. It is a 2-D directional reference system.

With regard to celestial co-ordinates, we are talking about a sphere in 3-D, taking reference from Earth's axis of rotation (zero degrees DEC being the Equator) and the 'historical' "First point of Aries" during Spring Equinox in Northern Hemisphere (that is zero hours RA). I said 'historical' because the precession of Earth's axis complicate things further, causing this point to drift towards the direction of Pisces and Aquarius (note that we are now no longer having Spring Equinox in Northern Hemisphere at Aries, but at Pisces!).

Here is one link you can read more about celestial co-ordinates:-
http://www.astronomyforbeginners.com/as ... sphere.php
Cheers!
Last edited by alvinsclee on Thu Sep 07, 2006 11:43 am, edited 3 times in total.
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Post by wAi_kiT »

Thanks Alvin for showing me how to use the mount :D However, I still have a lot more questions!!! :oops: In the picture shown below, my Celestron Equatorial Mount have 2 markings on the RA axis. One is of coZ the painted arrow (ie. between the R and the A), and the other one is the metal needle (somewhere slightly below). So which one do I set the RA value to? The metal one or the painted one? :roll:

And in the picture, you see that there are 2 24hours markings on the RA axis... Which one to use? The top or the bottom?

Last question, when using the Starry Night software, the info they give for RA and DEC comes in 2 types, one is the J2000 and the other one is JNow. I noticed that the values are quite close to each other. So whats the difference between them and which set of values do I use?
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RA axis
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alvinsclee
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Post by alvinsclee »

OK, looking over the top of the celestial sphere (true North), RA is measured from "First point of Aries" anticlockwise... that is, it increases as we go eastwards.

Then, there is a 'more convenient' unit that is exactly the other way round (clockwise). It is called the "Hour Angle" (HA), increasing as we go westwards. It is more natural in the sense that you can easily calculate the difference in hours that a certain celestial object has passed by (or will pass by) with reference to the current meridian. Note that some computer-generated star maps are printed out in HA instead of RA. Have a good look at the RA setting circle and you will figure what I mean. To know more about hour angle:
http://www.astro.ufl.edu/~oliver/ast372 ... coords.htm

The "J" refers to Julian Epoch. "J2000" would refer to the beginning of Julian Year 2000. As you already know, the precession of Earth's axis moves the reference RA point. Hence, star maps published in the 19th century would definitely be different from those published in the 21st century... etc. To be more precise, the reference year must be mentioned. Therefore, if you are referring to a map published around year 2000, then "J2000" will match that map closely (within the limits of an epoch). See this link on epochs:
http://en.wikipedia.org/wiki/Epoch_%28astronomy%29

Hopefully, I did not confuse you with all these info. Here is a link to read more about time measurement:
http://skytonight.com/howto/basics/3304611.html
Last edited by alvinsclee on Wed Sep 06, 2006 8:29 pm, edited 10 times in total.
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wAi_kiT
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Post by wAi_kiT »

Thanks Alvin again!!! Think I understand le... Thanks a lot dude!
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weixing
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Post by weixing »

Hi,
Do you have the manual for this scope??
Hmmm... So that means you need to locate an object first, to get the RA axis right, then you're able to use that axis onwards to locate other objects more easier?
If I'm not wrong, the RA Setting Circle of your mount do not move together when your mount move, so you have to do all the above again for every object you want to observe.
In the picture shown below, my Celestron Equatorial Mount have 2 markings on the RA axis. One is of coZ the painted arrow (ie. between the R and the A), and the other one is the metal needle (somewhere slightly below). So which one do I set the RA value to? The metal one or the painted one?
Should be the metal one.
And in the picture, you see that there are 2 24hours markings on the RA axis... Which one to use? The top or the bottom?

If you know how RA works, you should know which one to use... :)

Hint:
1) Your EQ mount is design to be used on both Northern and Southern Hemisphere.
2) RA value will increase as it goes eastwards.

Have fun and have a nice day.
Yang Weixing
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Debash
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Post by Debash »

Check out AstronomyBoy

http://www.astronomyboy.com/eq/
Debash
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Post by wAi_kiT »

To Weixing:

Hi there, thanks for ya help! The RA circle on the mount moves together when i turn the RA axis... Hmm... So how does this thing work since it moves together when you turn the RA axis? Is the painted arrow of any use?

In books, they often quote that the 0hr is denoted as the first point of aries... So what does that means?

Thanks! :P

To Debash:

Thanks dude for showing me how to tilt the telescope on the equatorial mount. Hmmm... But I guess the problem is that I'm still blur in using the DEC and RA circles rather than orientating the telescope. Well, thanks for ya help! :D
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