R.A/Dec co-ord questions.

[pipnina]

While I've heard R.A/Dec co-ordinates aren't overly useful for lower end EQ mounts sue to their settings circles not having enough resolution, I figured that, since my scope has a field of view of at least 1.5 degrees, using settings circles for a lot of objects (especially larger ones) wouldn't be too difficult.

What makes R.A/Dec co-ords more appealing to me is also how, when planning tonight's viewing session using dso-browser.com, I can just list the name of the object, its co-ordiantes and the best time to observe it.

However, my question is: Aren't the co-ordinates always changing?

If I open stellarium and click on any object, its co-ordiantes change as time progresses. Does this mean that, if I see co-ordinates listed, they're only right once per day?

Thanks!

    ~pipnina

[kens]

The RA and Dec of an object never change (well except for planets, comets and such like). But a given RA will change its position in the sky through the night, rotating around the celestial pole. You can see this clearly in Stellarium, Select an object and  you'll see that its RA is constant even as the object moves through the sky.

What this means is that if you set your RA setting circle when you find an object then, say, 15 minutes later you want to find something else, your setting circle will be 15 minutes out.

Using Stellarium there is a better way. Get it to show Hour Angle (HA). When you identify an object set the hour angle on the RA setting circle. 15 minutes later, when you look for another object the setting circle will still be correct. What happens is that the HA of an object is always changing as you can see in Stellarium

[Cornelius Varley]

The RA/Dec coordinates represent fixed points on a revolving sphere - the sky as we see it.  These points are stars/galaxies etc. We are at the centre of this sphere which rotates around the Earth once every 24hrs. In reality it is we who are rotating. but from our perspective the sky rotates around us. The RA component of the coordinate is in hours/minutes/secs , ranging from 0h 00m 00s to 23h 59m 59m and the Dec part is in degrees - how far above or below the object is from the plane of the ecliptic. Another way of locating an object in the sky is by it's alt / azimuth coordinates. These coordinates are constantly changing, which is probably what you observed in Stellarium.

[pipnina]

The RA and Dec of an object never change (well except for planets, comets and such like). But a given RA will change its position in the sky through the night, rotating around the celestial pole. You can see this clearly in Stellarium, Select an object and  you'll see that its RA is constant even as the object moves through the sky.

What this means is that if you set your RA setting circle when you find an object then, say, 15 minutes later you want to find something else, your setting circle will be 15 minutes out.

Using Stellarium there is a better way. Get it to show Hour Angle (HA). When you identify an object set the hour angle on the RA setting circle. 15 minutes later, when you look for another object the setting circle will still be correct. What happens is that the HA of an object is always changing as you can see in Stellarium

Does this make using RA/Dec co-ords to find objects a little tedious? By the time I've got my setting circle aligned to betelgeus to observe other objects I made note of the co-ords for and moved to them... The setting circle will be incorrect?

And If I use the other method you mentioned I'd have to look for objects at a specific time so i couldn't just line up my setting circle and go?

This sort of leads into another thread I made regarding the way I plan my observing sessions: stargazerslounge.com/topic/239428-does-this-seem-like-a-reasonable-observing-plan

Unfortunately it didn't end up going well last night with this plan due to the mist and these issues using setting circles on an EQ mount.

    ~pipnina

[kens]

Dec coords are ok but using RA can be tedious. You need to keep moving the RA setting circle to keep it up to date. If you know the time when you set it, then next time you can advance by time elapsed since then e.g. if you look at the RA setting circle 10 minutes after you set it you should first advance  it 10 minutes.

That's why I prefer using hour angles but you need a computer to show Stellarium while you are observing

[acey]

I use a dob so never bother with setting circles, but I use RA/Dec co-ords all the time, because catalogues such as the NGC use them to give the positions of objects. Once you know the co-ords, you know where to find the object on a detailed star atlas, and then you know where to find it in the sky. For finding which hour of RA is on the meridian at a given time, the easiest thing is to use a planisphere. Since the NGC is ordered by RA, you then know the sort of number range of objects that will be best placed at that time. Dec is important for knowing how high the object will be in your sky, and whether it will be worth attempting. I didn't bother much with RA/Dec when I was starting out in deep-sky, looking for Messiers etc, but when seeking out large numbers of very faint fuzzies they become pretty much essential, just like latitude and longitude on a terrestrial map.

[hgjevans]

There is another way to use the RA/Dec coordinates with an EQ mount - it suits me, but I admit it's not for everybody. And that is to work out in advance of the observing session the relative coordinates for an object, starting from some nearby easily identifiable star.

For example, I know that if I start from Dubhe in the Plough and go up about 7.5° in declination, and back by about 1hr 8' in RA, then I'll land pretty much exactly on Bode's Nebulae. And I also know that one full turn of the dec control on my EQ mount corresponds to 2.5° of declination, and similarly one turn of the RA control gives me 10' of right ascension. Which means that I can just count off about three turns of Dec and nearly seven turns of RA, and into the eyepiece they pop - every time.

It takes some planning, as I say, but it works for me. 

[chiltonstar]

I correct the RA figure by rotating the scale until it's right on an obvious nearby star, and then start a count-up timer. I then correct the RA value for the object I'm looking for by the elapsed time in minutes.

Another method - easier but not always possible - is to find an obvious star just N or S of the object your looking for, centre on it and then adjust the Dec scale accordingly.

Chris

[Gazabone]

What really helps is attaching a RA motor. I'm using a eq3-2 and with a 1000mm scope with a 24mm 68 deg five ep I get my target in the ep 1st time 95% of the time. Possibly a bit tedious but I've made a list of "sign point" stars (1 or 2 per constellation) to set my RA circle and if I move across a fair big of sky to look at a new target I can get there with the setting circles in not much more time than a goto scope.

Personally I find settings circles much much easier to find faint targets than star hopping; for some reason I seem to find star hopping in a telescope quite difficult.

A good one I find to practice on is M81 and M82 galaxies:-

Centre your scope on Dubhe (your dec scale should say 61deg 45 mins) then adjust your RA scale to read 11hours and just under 4 mins. Then move the scope so your RA reads 9hours 56 mins and your dec to 69 deg 4 mins and you should see M81. You may even have M82 in view too.

Part of the trick is using a wide field ep which will help you deal with any inaccuracies such as lack of resolution of the circles (as you say), setting of the circles and pointing of the scope.

What I will say is that there is something quite satisfying in looking for something that's not visible to the naked eye, using the circles then looking in the ep and there is your target. I really like the idea of there being no computers involved, with a lot of goto set ups, if the computerisation fails your observing is screwed up.

I'll emphasise that a motor (clockwork is ok but battery powered is better imho) is extremely helpful and gets over the sky moving whilst you're making the adjustments to find your target.

Either way, good luck with it, once you've cracked it, you'll love it. For me it's an enjoyable dimension to my astronomy rather than any sort of chore.

[ronin]

Pipnina, RA is a measure of time.

It relates to where an object is at a given time in relation to the Greenwich Meridian (I think it is still Greenwich), or what the time is relative to Greenwich when the object is on the local meridian.

So when you set your circles you need to take into account the actual time in terms of GMT and the "local" time after that you would point the scope to the RA specified and hopefully it is still above the horizon.

As an object is moving it is the sort of odd way to still give it a fixed location so it can be found.

The whole process is about as clear as mud, and in general it is a sticky, gloopy, smelly mud.

[acey]

RA is not a measure of time, it's a measure of position (analogous to longitude) on the celestial sphere. It's measured in hours, minutes etc, but the RA of a star doesn't change. What changes is the value ("hour") of RA that is on the meridian (line from southern horizon to Polaris) at a given time. It's equivalent to a measure of time if you have a sidereal clock, which few people do.

[Roy Challen]

As acey says, RA is a measure of position with the 0 point being the First Point Of Aries. This point does actually change position but over a very long period of time. If I remember correctly it is due to Precession. Our star atlases are adjusted every 50 years- we are currently using coordinates for Epoch 2000. I might be wrong on all this but I'm going by memory, so feel free to correct me!