RA DEC observing for dummies?

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RA DEC observing for dummies?

Postby mikaselm » Fri Jul 10, 2015 12:12 am

Hello, Everyone,

I'm hoping someone can give me a little more information about RA Dec coordinates and why they may or may not work for observing with the BRT. I'm a total noob when it comes to using coordinates to find things, I've only ever observed named or cataloged objects. I recently stumbled across some images of the cartwheel galaxy, or the funky red spiral star near it, however, and wanted to see if I could observe this for myself. Now one of the things that I just LOVE is the new request creator that shows me when optimal viewing is for a particular object (this request is gonna sit for six months, ya know...). Makes it nice and easy, and Wikipedia is good about giving me Ra Dec coordinates for non named objects. I've noticed that sometimes when I put in the coordinates, I get the little green months telling me that it's a good time to observe, however, when I put my coordinates for the cartwheel galaxy in, (Right ascension 00h 37m 41.1s Declination −33° 42′ 59″) the whole year is red, bad observing.

I'm sure there's a reason for this, but I'm not familiar enough with coordinates to know what it is. Is it that this object never rises high enough, or that it's too dim for this scope, or something like that, perhaps?

Thanks for any help you can offer!


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Re: RA DEC observing for dummies?

Postby Ed » Fri Jul 10, 2015 1:12 am

So it sounds like first you could do with a basic intro to how the radec coordinate system works.

You will be aware that we can specify the position on the earth with latitude and longitude. Latitude is how far north or south you are. Longitude is how far east/west you are from some arbitrary position (greenwich). If you were to project that grid onto the night sky you would basically have right ascension and declination. (Although, it is normal to measure right ascension in hours (0->24) rather than degrees like the equivalent longitude (-180->180)).

So just as the north pole is latitude 90 degrees North, the pole star (which is right above our north pole) is declination +90 degrees. A star right above the equator would have declination zero. A star above the south pole declination -90. And then right ascension is how far around the sky from some arbitrary line the thing is.

If our telescope were on the North pole, we would only be able to see stars in the top half of the sky. This would be the case all year round. A star with declination zero would be just on the horizon. Anything with negative declination would be completely invisible.

If our telescope were perfectly on the equator, it would be quite different. The north star would be on the northern horizon. The southern pole star would be on the opposite horizon. The entire sky would go over your head.

We are situated 30 degrees north of the equator. That means we can see all of the northern sky plus 60 degrees down into the southern sky (if we look right to the horizon). But we don't observe at the horizon. We only observe 30 degrees above the horizon. So that chops off 30 degrees of the southern sky, meaning we can see 30 degrees into the southern hemisphere.

So if you see an object with a declination less than -30, it's not going to be observable.

With regards right ascension, as we move around the sun, the part of the sky the sun appears to be in changes. That is to say, the sun's right ascension changes over the course of the year. (It's declination also bobs up and down a bit as a result of the tilt of the earth). So things with the same right ascension will be visible at the same time of year.

You can sort of see all this by changing the values of the coordinates in the request constructor and seeing the visibility graph change.

In case you haven't figured out the units yet: RA is in hours. Each hour is made up of 60 'minutes'. Each minute is divided into 60 'seconds'. So 1 and a half hours is 1 hour 30 minutes, written 01h 30m 00s. Declination is between -90 and 90, and each degree is divided into 60 minutes and again each minutes has 60 seconds. Only now rather than m for minutes and s for seconds ' and " are used instead...

It's helpful to bear in mind that our pointing accuracy is only good within a few minutes, so specifying the seconds wont make a right lot of difference. If imaging with cluster, even the minutes aren't really necessary as the field of view is 4 degrees across.

In case you haven't worked it out yet, there are various ways to copy and paste radecs into the request constructor boxes in one go. Experiment with that. It's easier than typing them out.

I hope that helps.

Posts: 31
Joined: Tue Nov 25, 2014 6:56 pm

Re: RA DEC observing for dummies?

Postby mikaselm » Fri Jul 10, 2015 2:58 am

Thank you so much!! That's a fantastic description, and I think I actually understand now. I really appreciate you taking the time to explain that for me :)

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