JeffPo's Setting Circles & Celestial Coordinates Page
Original article: 06/10/02
Updated: 11/03/05
Last update: 01/23/17 (added photo of telescope with setting circles pointed out)
Star charts can sometimes be a work of art. The constellations are always in their place and seem to dance from one season to the next. Familiar stars, with names that now roll off your tongue, are firmly anchored. There's just something magical about having a good star chart in your hands and using it to locate some distant galaxy. One particular method of finding objects is called star hopping. You know that your quarry is between this star and that star, so you point your telescope in that general direction, jumping from known star to known star until your desired object slides into view. Some people are quite handy at this method of hunting. But with the advent of computerized telescopes with so called GOTO technology, star hopping is starting to be viewed as a method of the past. Today's electronic telescopes simply require the user enter the object they want to view into a hand controller and the telescope will slew effortlessly to the exact region of sky where the object is located.
Ever wonder how the electronic telescope knows where the object is? Ever think about how the star chart maker knows to put a certain object in a certain place? The simple fact of the matter is that every object in the sky, indeed every point in the sky, is defined by a set of numbers. These numbers are its celestial coordinates. An object's coordinates precisely describe where that object is located, be it on a star chart or in the database of some computer program. With the right telescope mount, you can use this system of celestial coordinates to find any object in the sky.
Most people are familiar with terrestrial maps of earth. Locations are defined by latitude and longitude coordinates. Latitude is how far north or south something is, measured in degrees. From the equator, you measure from zero degrees until you reach the north or south pole, at 90 degrees. Longitude is how far east or west something is, again measured in degrees. From a starting point in Greenwich, England, you measure from zero degrees east or west until you reach the other side of the earth at 180 degrees.
Celestial coordinates work in a similar fashion. Object locations are defined by declination (DEC) and right ascension (RA). Think of the terrestrial system projected onto the sky. The celestial equator is the earth's equator projected onto the sky. I always like to think of the constellation Orion when I'm trying to visualize this because Orion is a very prominent constellation and the celestial equator runs very close to the belt stars. The north celestial pole (which is very near the star Polaris, commonly called the North Star) is the earth's north pole projected onto the sky. So, declination is similar to latitude. In fact, whatever latitude you are located at is how many degrees above the horizon the celestial pole is. So if you live in a city that is 35 degrees latitude in the northern hemisphere, the North Star will be 35 degrees above the northern horizon. This spot is stationary in the sky and all other objects seem to rotate around it. Okay, now back to what declination is. Declination is how far north or south an object is from the celestial equator, measured in degrees. From the celestial equator, you measure from zero degrees until you reach the north or south celestial pole at 90 degrees. Sometimes north declination coordinates are represented by positive numbers and south declination coordinates are represented by negative numbers. Right Ascension is similar to longitude, with the difference that right ascension is measured in hour angles (zero to 24...as in 24 hours in a day) instead of degrees. And it's all in one direction, which is east. Basically it's one complete loop of the sky, divided up into 24 hours. A good way to think of the starting point, or zero hour, is to think of a line drawn from the north celestial pole to the south celestial pole, that intersects the great square in the constellation of Pegasus.
So, declination (DEC) is expressed in degrees, minutes, and seconds of arc. And right ascension (RA) is expressed in hours, minutes, and seconds. This coordinate grid is "fixed" upon the sky. As the sky seems to move around the earth (from the earth's rotation), this coordinate grid rotates with it. The map, which is the sky, is always moving. And since the grid moves with it, that's how objects can have a specific, defined set of coordinates. Using an equatorial mount equipped with setting circles, you can "dial in" any object that you want to observe.
To help you better understand the celestial coordinate system and setting circles, let's walk through an example with pictures and illustrations. We'll start with an example of an alt-azimuth mount and transform it into an equatorial mount and then move on to actual pictures of manual setting circles and what they would look like when pointed at certain objects.
An equatorial mount is not as complicated as it seems or looks. Think of the most basic type of mount, an alt-azimuth mount. This is shown in the image labeled Fig. 1. This particular design is a Schmidt-Cassegrain telescope (SCT) on a fork mount. We are seeing it from the side, as it points to the left. The telescope can be moved up and down, and left and right to point at objects. Imagine that the telescope is parallel to the ground, basically pointing at something on the far horizon. Mentally spin the telescope, left or right, 360 degrees. Now think of an axis line going from the middle of the mount, straight up into the sky. The left and right motion of the scope pivots, or rotates around this vertical axis. Got it visualized?
Now hold onto your hats for this next one. Imagine tilting the entire mount until the vertical rotational axis for left and right directions, is pointing at the North Star, Polaris (we would really point at the north celestial pole, but Polaris is close enough for this article). This axis is now referred to as the polar axis of the mount. The telescope still moves up and down, and left and right relative to its mount, but now the entire thing is leaning toward the North Star. This magically transforms the mount into an equatorial mount. Now moving the telescope "left and right" is moving it in right ascension (RA). If you move it "up and down" it is moving in declination (DEC). The mount's polar axis is aligned with the earth's polar axis. Does it make sense?
If the telescope was pointed at the horizon when you "tilted" it over, the scope now points to some point along the celestial equator and at zero degrees declination. The only other thing to remember is that as you move toward the east, the right ascension numbers increase (until you hit 24, in which case you're back to your starting point of zero hour again, i.e you've done a "360", a loop). So, if an object was defined as having a Right Ascension of 2h 30m and a Declination of 15d 10m N, you would start at the zero hour of right ascension on the celestial equator and move east until you reach the 2 hour, 30 minute mark. Then, since the declination is North (if it had been marked S or with a negative, it would be south), you move up (north) from the celestial equator until you reach the 15 degree, 10 minute mark. There you will find the object.
This is an image of a Meade LX200 telescope on an equatorial wedge. The RED arrow points to the Declination setting circle and the GREEN arrow points to the Right Ascension setting circle.
Now let's go through the exercise of actually locating an object using mechanical setting circles. Let's assume we have already setup our telescope and have it polar aligned (i.e. we have aligned the right ascension rotational axis, which is the polar axis, with the north celestial pole...you know, the tilt). We first need to calibrate the setting circles. Actually, we only need to calibrate the right ascension circle because the declination dial doesn't move. To visualize this, imagine you are standing, facing due south, and you point your finger at the sky at about the 15 degrees north declination mark. And you stand that way all night long. Well, you will always be pointing at 15 degrees north declination. But as the earth turns, and the sky slides by, the right ascension you are pointing at will be constantly changing. If you happened to be initially pointing at 4hr 30m right ascension, two hours later you will be pointing at 6hr 30m right ascension. Unless of course you move your finger to keep it pointing at the original spot, which is exactly what the clock-drive does on your mount. Since declination does not move, the clock-drive only needs to rotate the telescope about the polar axis, in right ascension toward the west, at the speed of a clock to keep an object centered in the telescope. So, you really only need to calibrate the right ascension setting circle. The most common way of doing this is to point to a known object and rotate the right ascension setting circle dial until it matches the coordinates of this object. I keep a list of bright stars handy, along with their coordinates, for this purpose.
The bright star Sirius's coordinates are approximately 6h 45m right ascension, and -16d 43m declination. I always round the seconds off to the nearest minute. To calibrate the setting circles, we would point the telescope at Sirius then rotate the right ascension dial until it reads 6h 45m. Let's assume the telescope is now pointed at the star Sirius.
This is an image of my right ascension setting circle. It has been rotated so that the pointer is pointing to the right ascension coordinates of Sirius, 6h 45m RA. On my system, this is read on the outside ring of the dial. The inside or interior numbers are used if the observer is in the southern hemisphere. If I ever get confused, I just push my scope to the east and note which set of numbers has increasing values of RA.
This is an image of my declination setting circle. By default, since the actual dial doesn't move (see note below), it will be pointing at the declination coordinates of Sirius, -16d 43m DEC. Notice the negative sign in Sirius' declination? That means it is below the celestial equator which is a southern declination. Visualize the constellation of Orion. Remember that the celestial equator runs through the belt. Where is Sirius? It's down and to the left of Orion, which means it has to have a southerly, or negative declination since it below the celestial equator.
NOTE: Normally the declination circle is calibrated and set at the factory. But sometimes it is off. If the declination circle indicates a value other than the declination of the object it is pointed at, it will have to be calibrated. This as simple as loosening the locking knob, rotating the declination circle to read the correct value, and then tightening the knob. Once set, you should never have to do this again.
Now that the setting circles (really only the right ascension circle) are calibrated, the mount can be used to find an object. Let's imagine that we want to find the galaxy M65, located in the constellation of Leo. Using software, books, or whatever, we find that M65's coordinates are 11h 19m RA and 13d 5m DEC. It's now a simple matter of just moving the telescope until the pointers are pointing at these readings on the setting circles.
This image shows the right ascension dial once the telescope has been moved so that the pointer is aligned with the right ascension coordinates of M65, 11h 19m RA. Since the dial is only in 5 minute increments, I basically interpolate where I think 19m point is.
NOTE: Someone informed me that my "pointer" is actually a vernier scale, and can be used for greater accuracy. However, I have found that few people like how Meade divided its scale, and don't find the precision all that useful in the field. Since even with my interpolation I'm still able to put an object in the field of view of a 25mm eyepiece, I'm inclined not to worry with the vernier scale and just use it as a simple pointer.
This image shows the declination dial once the telescope has been moved so that the pointer is aligned with the declination coordinates of M65, 13d 5m DEC. Since the dial is only in 1 degree increments, I basically interpolate where the 5m point is.
NOTE: Someone informed me that my "pointer" is actually a vernier scale, and can be used for greater accuracy. However, I have found that few people like how Meade divided its scale, and don't find the precision all that useful in the field. Since even with my interpolation I'm still able to put an object in the field of view of a 25mm eyepiece, I'm inclined not to worry with the vernier scale and just use it as a simple pointer.
If we did everything correctly, we should be able to look through the eyepiece of the telescope and see the galaxy M65. And if the telescope has a clock-drive, the galaxy should stay in the field of view as long as we want to observe it. When we want to observe another object, we just move the telescope until the pointers are pointing at the listed coordinates for the new object.
Some considerations and pitfalls
What if the object is not in the eyepiece when you take a look? Well, we'll assume that given the type of telescope and the sky conditions, it should be visible. There are a number of reasons why it might not be in the eyepiece. First of all, you could have made a mistake. You might have transposed a couple of numbers, or moved north when you should have moved south. Did you calibrate on the right star? Are you sure you have the right coordinates? Are you using the correct, northern hemisphere right ascension dial? Assuming you have the right numbers, just take your time and dial in one coordinate at a time. I usually will read then dial in the right ascension. Then I go back and read the declination, and dial it in.
What if you have all the right numbers, and did everything by the book, but still don't see the object? Some mounts are more accurate than others. For example, my declination circle is only divided by whole degrees. That means I have to guess and estimate at where the minute markings are. This naturally introduces some error. Most of the time, if I'm careful the object is there when I take the first look. If not, it's usually within an eyepiece field of view away, so I just pan a little bit in declination, followed by right ascension. Use a low power eyepiece so that you have the widest possible field of view. And of course, if your polar alignment is way off, that will affect your accuracy as well. Also, although you shouldn't have to worry with calibrating the declination circle, sometimes it is a little off when it comes from the factory. One way to determine if your declination circle is aligned properly is to move the telescope until it reads 90 degrees north. In the case of my telescope, that means the tube will be parallel with the forks. If I have an object centered in the telescope's field of view, like a star, and spin the telescope about the polar axis (i.e. in RA movement), that object should stay in the center. If it doesn't, then the telescope is not really pointing exactly at 90 degrees. To correct this, you would move your telescope in declination until the object finally does stay in the middle of the field of view during a spin. Then you would adjust your declination circle until it reads 90 degrees. This should be a one time procedure. I haven't adjusted my declination circle since I initially done it a few years ago.
What if you found the fist object okay, but when you decided to go for a second one, you missed? Assuming the miss is not because of a mistake or inadequate setting circles, it could be related to the type of mount and how the clock-drive, if any, functions. If your mount does not have a clock-drive, that means that it will not track to keep the object within the telescope's field of view. You'll have to move the telescope every now and then in right ascension (toward the west) to keep the object within sight. For every minute that goes by after initially pointing at the object, your right ascension circle will be a minute in error. So if you observe an object for 20 minutes, your right ascension setting circle will be off by 20 minutes. Before going on to another object, you have to recalibrate the setting circle. Just move the right ascension dial back to read the coordinates of the object you are viewing before moving to the next object. Even if you have a clock-drive on your mount, you may still have to perform this procedure. My German equatorial mount has a clock-drive but the right ascension dial must be reset before going to another object. On the other hand, my SCT mount actually moves the right ascension dial as it moves the telescope so once I have made the initial calibration, I don't have to readjust it. Although I have noticed that sometimes during the night it will get a little off and I'll have to sync it up again. And if I just can't figure out why I can't find an object, I'll start over with pointing at a known star and making sure the right ascension setting circle is properly calibrated.
Conclusion
Some mounts have better setting circles than others. I have found the ones on my SCT to be quite accurate. But with most people buying electronic GOTO telescopes today, they don't have much use for mechanical setting circles. And for those that don't have the GOTO scopes, they seem to rely more on star hopping as a method for finding objects. But setting circles can have their place. They are generally quicker than star hopping when going to a target that isn't sufficiently surrounded by bright stars. They are also helpful in crowded areas like the galaxy clusters in Virgo and Coma Berenices, where star hopping is more confusing because of the sheer number of galaxies. Then there's the satisfaction of using them. It kind of feels like making something with hand tools instead of power tools. For me, finding objects with setting circles is the most efficient use of my time when observing. What ever your reason for considering the use of setting circles and celestial coordinates, I hope this article helped in some way.