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Prestonian

Hi All, Novice requires Dec R/A advice

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Hi All

This is my first post as a budding wannabe astromoner. I bought a Bresser 90/900 refractor telescope with a EQ mount. I was playing around with it last night but just need some advice on how I set it up correctly so that I can find objects by using co-ordinates. This is what ive done so far:-

1: - I have balanced the scope

2:- I have locked the base down so that I cannot swivel the mount around.

3: Ensured the tripod is on level ground

4:- I then turn the tripod so that it is facing North, then raise the latitude to 54 degrees to find the North Star, then lock it

I now have Polaris in my sights.

This is where the confusion begins. Im of the understanding that I now have to find a star nearby and try to locate it by only moving Dec and R/A, so im going to track one of the stars in Ursa Major as its the nearest star to Polaris (Dubhe)

What im confused about is what settings to place Dec and R/A once ive moved the telescope on these 2 axis to find Dubhe. Dubhe is shown as 50 Dec and 11hr R/A.

On Dec there is a dial going from 1-9 then back down to 1 (i think). I take it this is repeated 4 times around the dial to give 360 degrees?? So if I had the Dec set at 9 when pointing to the Polaris star, do I just move the dial either clockwise or anticlockwise until it lands on one of the 5's?? Im moving the tube clockwise to get from Polaris to Dubhe.

Reference the R/A setting, my dial goes from 0-23 but ive noticed above the number are another set of number in reverse So how do I set this to the 11th hour? Do I just spin it clockwise until I find No. 11, and which set of numbers do I use? The top row or bottom row?

Thanks in advance

Andy

ps... Great site!!

Once this is achieved, am I all set up for finding objects via co-ordinates??

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Hi Welcome to SGL :D

This is a personal view and others will have different opinions but, in 25 years of using telescopes I've never bothered to use the setting circles (the RA and Dec dials) to find objects to look at. My technique with equatorial mounts like you have is to set the latitude scale to 51.5 degrees (I live near Bristol), put the scope down with polar axis pointing roughly north (using a compass), and then start observing using a book with good finder charts or charts printed from the WWW to find objects.

When you have found something to look at you should be able to track it using the RA slow motion control plus a few dabs of the RA one if it starts to drift. At low power you will hardly need to track much at all.

I do worry that newcomers to the hobby get worried about doing everything "properly" rather than just getting out there, plonking the scope down and seeing what they can find using simple star charts.

I've given up using an EQ mount alltogether now and just use a manually operated alt-azimuth mount - up a bit, along a bit, etc, etc.

Thats my advice - keep it simple - use some good starcharts and maybe invest in a red-dot type finder :rolleyes: :rolleyes:

John

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Hi Andy, and welcome to SGL.

I agree entirely with what John as said to you, you are not going to be using your telescope for photography, and therefore a reasonable alignment on the celestial pole will be fine for your observing needs.

In my personal opinion, manufacturers who put setting circles on small telescopes, only do it for effect. The index marks on them are so obscure,and difficult to see in the dark, as to be next to useless. The field of view in a telescope can be very small, and unless using the circles can get you very close to the area where your target is, you will be groping about all night trying to get it in your eyepiece. They will only add to the frustration for you.

Get a good starmap, learn the constellations, find the object on a map you want to view, then use the prominent stars in a constellation near the object, to find you way to it. A good finder scope can help, and although the lens is not usually very large, they have a wide field of view, and sometimes a nebulous object can be detected in them. So if your finder scope is accurately aligned with your main scope, getting your target in the centre of the finder, will ensure you can see it in the main telescope.

I hope this will help you a bit anyway.

Ron. :D

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Hi Andy and welcome to SGL. Like the others have said I wouldn't bother with the setting circles. It's probably better in the long term to learn the constellations and star hop around.

cheers

Sam

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Hi guys

Thanks for the responses.

I did wonder last night when i was racking my brain about the setting circles whether I was overcomplicating things! :D

I will stick to learning the constellations and using the finder for now then. Ive got a good map of objects within Orion off the internet so that should keep me busy for a while.

I also got a couple of books and a planesphere with the scope, just a pity its always raining in Preston!

I managed to get a short look a Mars the other night, but thats about it since I built it on boxing day!

Thanks anyway, and i will let you know how i get on

Andy

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Lo Andy

I would go along with what everyone has said so far. Don't know if You are a member of an astronomy club but if not it is worth finding a local one and joining as you are probably going to have loads of questions and I'm sure someone at the club will be able to give you a hands on demonstration which will answer most of them, plus when you are shown how to do something it sinks in a lot quicker, well it does with me :D

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For completeness.....

What you are supposed to be able to do is this: Locate a star whose coordinates are known and centre it in the eyepiece. Then set the RA/DEC circles to those coordinates and lock the circles. Now, when you want to move to another object, you simply rotate the mount until the pointers on RA/DEC point to the new object coordinates. Simple!!

In practice, however, the circles are not accurate enough for this to work, which is a shame! Still, learning the skies is no bad thing :D

Oh, and the RA has the reversed settings for the Southern Hemisphere - you would use the top set of numbers....

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4:- I then turn the tripod so that it is facing North, then raise the latitude to 54 degrees to find the North Star, then lock it

I think step 4 is : I then turn the scope in declination until my tube is parallel to the RA axis. At that point, turning the scope in RA axis should keep the same target in view. Lock the declination at that point and point (by means of latitude screw and azimuth rotation of the whole tripod or just the mount) that axis to the Celestial North Pole (which is 3/4 or so of a degree away from Polaris). Once there, the axes must not be disturbed. The declination circle should read 90 degrees. Then you find a star on the equator, or near it and point the scope to it. Read off the RA position of the star from a star catalogue or planetarium program. Set the RA circle to that reading when you are pointing at the known star. That's it. Then test it!

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Hi again,

Just been outside to see if I can use the Dec/RA mainly out of curiosity more than anything. Cant see much from my garden as there is pretty bad light pollution around so I lined up Polaris which went pretty well (need to slightly align the finder next time out but other than that, it was all good :D ), I then proceeded to unlock Dec/RA and swing it round clockwise to Dubhe so that I could then adjust the Dec + R/A settings to their corresponding co-ordinates. I locked Dec and R/A once more but to my amazement, I could hardly move the R/A numbers around. Dec seemed to move but still pretty stiff.

I tried it last night in the kitchen (warmer temp) and it span around quite well but outside, it would hardly budge.

Any ideas how I can loosen it up a bit?? Is this a common problem??

Cheers,

Andy

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Sorry I cant comment on the stiffness of the RA&DEC as I'm a dob user but just a brief point.

I know its damned cold outside but you're better out there than in the kitchen :D

1 - Your dark adaption will be better.

2 - Your scope will cool down better avoiding the temperature differential currents. (Which cause unstable images)

3 - You'll not be looking through a double glazed pane of glass (which adds to distortion of the image you see)

Cheers

Paul

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haha :rolleyes::clouds2:

when i talked about the kitchen, I meant I was playing around with the dials and noticed they were fairly loose but obviously that must be down to the warmer temps indoors. I left the telescope outside for 15 minutes before I started using it tonight so by the time I needed to move the dials once id found the star, everything had tightened up, and its a problem I will face again next time I try it.

Ive just tried again in the kitchen and they are again much looser than they were outside in the colder climate. :D:rolleyes:

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I hope it's OK to tack this onto the end of the Dec / RA thread, but I do have a concern about how to use the EQ mount that came with my kit. I noticed John's attitude towards the EQ mount seemed to be that it was more bother than its worth.

What I've been doing is using the lattitude setting to move the telescope up and down and the declination for side to side movement. I suspect that What I'm doing is stupid, or at least silly :D Without touching the lattitude setting I can't really move the telescope up and down as the RA just seems to make it do a part circle around the mount. I have the telescope sitting directly above the mount.

Can anyone advise me on using it properly please? I'm not worried about the setting circles. That just seems like a chore!

Peace and clear skies,

Becky.

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And great it was too. I had a lot of fun using setting circles. Mind you, my old Vixen Super Polaris had a brilliant Vernier scale which made it extremely accurate. I found the trick was to work a bit ahead of time and place the scope a bit ahead of the target and wait for it to come into view, otherwise by the time you've faffed around you've missed it!!

Having said all that, other than goto nothing beats a telrad!

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I hope it's OK to tack this onto the end of the Dec / RA thread, but I do have a concern about how to use the EQ mount that came with my kit. I noticed John's attitude towards the EQ mount seemed to be that it was more bother than its worth.

What I've been doing is using the lattitude setting to move the telescope up and down and the declination for side to side movement. I suspect that What I'm doing is stupid, or at least silly :D Without touching the lattitude setting I can't really move the telescope up and down as the RA just seems to make it do a part circle around the mount. I have the telescope sitting directly above the mount.

Can anyone advise me on using it properly please? I'm not worried about the setting circles. That just seems like a chore!

Peace and clear skies,

Becky.

Hi Becky

The trick with an EQ mount is to get it set up and aligned - then it makes sense.

1. Set the latitude scale to equal your latitude on the ground - you can get this off the internet very easily

2. Set up the mount so that the counter weight shaft points north - this is usually over one of the tripod legs, but not always

Now, as all the stars rotate around the Pole Star their movement in the sky describes an arc. Your telescope will now, for a given declination setting, do the same.

I suspect that your tripod was not aligned north - hence it didn't make sense when moving it.

Does this help?

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Hi Becky

The trick with an EQ mount is to get it set up and aligned - then it makes sense.

1. Set the latitude scale to equal your latitude on the ground - you can get this off the internet very easily

2. Set up the mount so that the counter weight shaft points north - this is usually over one of the tripod legs, but not always

Now, as all the stars rotate around the Pole Star their movement in the sky describes an arc. Your telescope will now, for a given declination setting, do the same.

I suspect that your tripod was not aligned north - hence it didn't make sense when moving it.

Does this help?

It does help, thank you. I set my lattitude for Birmingham and pointed it where I thought north was. It was broken cloud, so there was little chance of fixing on the pole star. The Moon was lovely, but no point trying to see anything else. I found then that the other controls make a lot more sense. I think I understand declination now (as degrees between the equator and the celestial pole?). RA still has me baffled. Is it simply a measure of 'how far round'? I looked it up in a book and wished I hadn't! Something about aries at the vernal equinox or something! Thanks Again.

Peace and Clear Skies,

Becky.

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Yes they both can be confusing at first!! Lets get you used to using the telescope first THEN we'll confuse and baffle you :D

Glad to have been of assistance!

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Without touching the lattitude setting I can't really move the telescope up and down as the RA just seems to make it do a part circle around the mount. I have the telescope sitting directly above the mount.

I bought my wife a Skylux from Lidl and we had the same confusion initially by the circular movement of the RA. However, once you align the scope with Pole, if you then move the scope 90deg down thru Dec you will find that the scope then moves in circular fashion although off axis along the celestial equator. The Dec control then moves it up and down with respct to the celestial equator. However, because the sky appears to spin on the polar axis, as you move up in Dec closer to 90deg, the RA circle becomes smaller and smaller until finally at 90deg when lined up with the celestial pole, it becomes a matter of spin.

We hovever have had the additional frustration in that, when moving the scope using the Dec and RA knobs and cables, apart form this taking ages to do, when we attempted to swing the scope around from pole to the Great Orion Nebula and Mars, we found that the RA and Dec cables ran into each other or the scope tube. I had to remove the RA cble to prevent this from happening. Is this something we are doing wrong, or this this a limitation of the EQ mount?

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Its a limitation of the mount - although you may find that you can attach the RA to the other side of the worm to prevent this....

Welcome to SGL by the way :D

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RA still has me baffled. Is it simply a measure of 'how far round'? I looked it up in a book and wished I hadn't! Something about aries at the vernal equinox or something! Thanks Again.

Peace and Clear Skies,

Becky.

As the stars rotate across the sky due to the Earth's rotation, the "how far round" is worked out in hours. If you wait an hour, everything has moved one hour in RA. The zero point is irrelevant, unless you were at the argument as to where to put it donkey's years ago, like the Greenwich meridian. Its just a point thats been agreed is all.

Due to the orbit of the Earth, the stuff in the opposite direction to the sun that we can see when it gets dark changes with the seasons so sky time (sidereal time) and normal time are out of sync, otherwise you could find a star by waiting until its RA matched the time and the star would be due south. You need to add a fiddle factor to allow for the amount that the RA time is out compared to real time so that doesn't work well. The way to go is find a star that you know, point the 'scope at it and set the RA scale so that its set to the RA for the star and you're set. You have just calibrated for the difference in time and all the RA numbers can be read off the scale.

Kaptain Klevtsov

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Thanks for that information Kaptain. I think I'm *starting* to get it. Let's see if I'm getting it right now.

RA is a measure of time which due to seasons is sometimes ahead of earth time and sometimes before, probably being right at the Equinoxes. RA is also known as sidereal / star time.

If I set up by finding a known star, does that mean that the RA 'clock' is ticking and my calibration is only good for a very short time?

I imagine declination must be seasonal too? Therefore if noting else, I understand that RA and Dec are not fixed astronomical coordinates.

I've so much to learn. This is fun :D

Peace and Clear Skies,

Becky.

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RA is a measure of time

No! The RA coordinate of a star stays the same during one night. It changes very slowly over years though if the star is in relative motion with respect to the Sun. That's why you will see RA coordinates given with the qualification of something called "epoch" (something like J2000.0)

Go outside and turn towards the North. Tilt your head back to look up to zenith (dead above you), that's the meridian line you've just scanned across the sky. Then turn towards the South and do the same, that's the souther meridian line. This meridian line is unique to each observer. At 10pm exactly, you can note which star is on that line. Observers at the same geographical longitude as you and not too far in latitude will also see the same star on their meridian. Suppose someone sees the same star on their meridian but at 11pm. They are "one hour" away in terms of geographical longitude. Suppose that at 10pm they see a different star on the meridian. The difference between these two stars is "one hour" of RA.

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That explanation helps me understand a bit about RA. I thought it varied with time, but you say that it's pretty much the same for one night. Does that mean it's the declination which changes and is used to track an object across the sky? I imagine one must vary or things wouldn't move.

I'm starting to get pretty annoyed that I can't get my head around this. I'm sure I'm building a whole mental block about RA/DEC! Hopefully one day it will just click into place. I hope when we get rid of this cloud and actually get to mess about with the scope it will become clear. I wish I had gone out last night, it was clear about 2am here- but I really need to discipline myself to get up and go out at that time!

A lot of folks have said just learn the sky and point the telescope. Maybe I'm getting myself all worked up about being able to do it properly.

Peace and Clear Skies,

Becky.

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You should imagine the sky like a globe of the Earth, with longitude and latitude lines on it. The difference is, however, that you are inside the imaginary huge globe with lines of declination going east west and the right ascension lines going from the north celestial pole (very close to Polaris) to the south celestial pole which is below the horizon from the UK. As the stars track across the sky, the lines go with them. The star positions are described by these imaginary lines and they stay more or less exactly in the same spot. Some stars move, but its painfully slow needing photographs years apart to even notice the movement. To all intents and purposes, the stars are fixed in their positions and so are so many degrees north or south of the celestial equator and at a certain position east - west in right ascension.

The celestial equator is an imaginary line across the sky at 90 degrees to the celestial poles. If you took a set square and sighted along one edge at Polaris, the view along the other edge at right angles to the first edge would point at the celestial equator. By maintaining the view down one edge and rotating the set square, you would sweep out the curve of the celestial equator.

The meridian which Themos mentioned (North, through straight up to South) stays still relative to you but sweeps across the sky due to the rotation of the Earth. The view straight up will change by one hour of right ascension every hour of time (or close enough for the pedants :D ) but stay the same in declination.

Kaptain Klevtsov

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Does that mean it's the declination which changes and is used to track an object across the sky? I imagine one must vary or things wouldn't move.

No, the declination does not change either. Declination is a measure of how close a star is to Polaris (more accurately, the point in the sky that represents the Celestial North Pole but that's less than a degree away from Polaris). Stars stay the same distance from Polaris through the night. Again, over years you might see some stars move with respect to others.

So, neither RA or Dec vary during the night (and day). Imagine a huge bubble around the earth, much bigger than the earth. Point to anywhere in the sky and you are pointing at a point in the bubble as well. The bubble is far from the earth so it doesn't have to rotate like the earth does. Imagine that it doesn't. RA and Dec are coordinates on that bubble. Stars are fixed on the bubble. The earth is rotating inside this bubble so it looks, from the earth, like it's the bubble that rotates. The coordinates that change for an earthbound observer are the altitude (how high from the horizon) and the azimuth (the far along the horizon one needs to turn before looking up). If you were on a tumbling spaceship in orbit, you might find some other (but similar) coordinates useful. So, remember: the varying coordinates are Alt/Az, the unchanging ones are Dec/RA.

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