After all, not all telescopes have built-in computers and since even a small telescope will reveal objects that are too faint to be found with the naked eye, even from a dark sky site, there are many advantages to knowing how to find objects manually using setting circles.
You might think that that now Go-To computerised mounts are so affordable you don't need this knowledge, but its valuable knowledge because it means that you will be able to locate objects even when there isn't a convenient power source or if the batteries are flat.
Setting circles are found on equatorial mounts of all types, from the most basic to the most advanced. They are discs with lines and numbers engraved on them that encircle the mounts right ascension and declination axis. These lines and numbers correspond to the lines of right ascension (RA) and declination (dec) that the night sky is divided up into, like latitude and longitude on a map.
You'll find coordinates in star maps, star atlas and in astronomy software.
Getting Started with Setting Circles
The first thing you need to do so you can use your setting circles properly is to accurately get a good polar alignment of your telescopes mounts. This is actually possible with an equatorially aligned mount - if you have an altazimuth mount you won't be able to use it. A good low power eyepiece is helpful as it will give you a wide field of view - very useful in the hunt for elusive objects as it shows you stars in the context of the surrounding area.
It is easiest to set up the setting circle for declination first. Once it is set you can leave it alone for the rest of the observing session. Assuming your mount is polar aligned, do this by your telescope at Polaris and then adjusting the declination setting circle to read 89° 18', which is Polaris's declination. You've now set the setting circle so that future reading will be correct.
To use the right ascension circle you'll first need to find the right ascension and declination of a bright star that as near as possible to your intended target, so grab a star chart for the area your aiming for.
Let's say you're going to track down the galaxy M66 in Leo. In this instance, you can use the mag +2.2 star Denebola in the tail of Leo to set your RA setting circle by. Denebola has the right ascension of 11h 49.5m and declination of 14° 31'. So loosen the clutches on the mounts axis and swing the telescope over to the tail of Leo to centre the star in your eyepiece. Now make sure that the declination setting circle is reading 14° 31'. If it isn't, reset it. Next, set the RA setting circle to read 11h 49.5m.
Now you can move onto the trail of your target, M66, which has the coordinates RA 11h 20.7m Dec. 12° 56.2'. So loosen the clutches on the mount once again and gently swing the telescope towards the approximate area of the sky where M66 is going to be, carefully watching the setting circles until the pointers are reading correct coordinates. Then lock the clutches and you should be looking at M66 in the eyepiece.
Step by Step Guide to Settings Circles
Ensure your mount is level and polar align it as accurately as possible. Check alignment by watching a star near the celestial equator for several minutes. If the star wanders north or south in the field of view, adjust the polar axis west or east as appropriate.
Centre your telescope on Polaris and set the declination setting circle to its declination, which is 89° 18'. You will be able to double check that you have set this circle correctly when you find your first target star in steps 4 and 5.
Decide on an object to view, for example, M66 in Leo, and find it on your star chart. Make sure that it is not too faint to be seen with your telescope. Locate a bright star near your chosen object: e.g. For M66 chose mag +2.2 Denebola in the tail of Leo.
Loosen the clutches on your mount, swing your telescope around to Leo and centre it on Denebola. A good setup finderscope with crosshairs will help you locate and centre stars and other objects more easily.
With Denebola centred on your eyepiece, check that the declination setting circle is reading the correct declination of 14° 31'. If not, reset it now. Next, set the right ascension setting circle to read 11h 49.5m. Now you are ready to head to your target.
Carefully watch you setting circles and gently move your telescope so that they read RA 11h 20.7m, Dec. 12° 56.2'. M66 should be in the eyepiece. If it isn't trying slow sweeps in right ascension using your slow-motion control.