November 10, 2019 0

Astronomy & Telescopes : How to Use a Telescope With Setting Circles


So today we’re going to talk about setting
circles on telescopes. So first off, to back up a little bit, every object in the sky has
a coordinate associated with it. That coordinate is the right ascension and the declination,
which is basically like a latitude and longitude on the sky. So, the steady circles on the
telescopes you’ll notice are these dials. There’s one on this access of motion, and
then there is one on this access of motion. The steady circles are basically used to tell
you exactly where the telescope is pointed in the sky. So this circle here is giving
us our latitude. Which in astronomy we refer to as our declination. Our declination, the
nice thing about it is it doesn’t have to be set every time you use a telescope. Our
base, you’ll notice that if I move our telescope, it rotates around a shaft that’s tilted at
a specific angle. That angle is set to our latitude and that’s the key to making this
declination work. So just like if you happen to spin a globe and you hold your finger over
a specific location on the Earth, as that globe is spinning, you’re always standing
at the same latitude, but your longitude is changing constantly. So our longitude for
our telescope which we call right ascension, is this other dial here. This one does have
to be set though every time you use your telescope. So the deal is, you can either put your telescope
on an object that you know the coordinate of, let’s say Jupiter. You look up its coordinates
for that particular day and you move your telescope to where it’s pointed at Jupiter.
You declination should match up perfectly with the coordinate of Jupiter, but you’ll
have to turn your other setting circle so that the right ascension coordinate is now
in line with Jupiter. So there’s one other way to align our right ascension setting circle.
And that’s making use of the concept of local sidereal time. So, as the night progresses,
everything in the sky is moving around us including our coordinate system. So as the
sky rotates, our lines of right ascension, which are lines of longitude, are constantly
moving to the point directly overhead which we call our zenith. The line of right ascension
that happens to be passing directly through the zenith is called your local sidereal time.
So as time progresses, that coordinate, or the time progresses as well. So, to make use
of that, what we can do is make a mark on our telescope mount. It’s on the opposite
side of the rotation access as that line of right ascension. So on our telescope that
little mark has to be about right here. So we’re using an arrow to point out that mark
which is now also pointing at our local sidereal time that’s passing directly overhead. So
now to align our telescope, all we have to do is swing our right ascension setting circle
around until that particular time on the dial is in line with the mark that we just made.
And now, our telescope is aligned in right ascension. Okay, so now that all of our setting
circles are aligned with the celestial coordinate system on the sky, all we have to do to find
our object now is move the telescope until the markers on the telescope axis actually
point to the coordinates of the object in question. So, to do that, let’s say my object
has a declination of minus 10. So, when I move the telescope, I move this dial until
it’s marker is now at minus 10. Lock the telescope, and then likewise, I would do the same thing
for the right ascension. And so it’s as easy as that. So this telescope is basically the
little brother to this telescope we were just using. This is more of the type you would
see amateur astronomers using. But it uses setting circles in basically the same way.
So this dial here, you’ll notice that I’m moving, this is the setting circle for right
ascension. And then, on the arm here, the other axis of the telescope moves in. You’ll
see that there’s another setting circle and this happens to bee your declination. So it’s
basically the exact same set up, even though the mounts are slightly different.

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