How To Use The Setting Circles On Your Telescope To Find Objects

If you have an equatorial mount, being able to use its setting circles will help quickly find objects in the night sky without computers.

Go-To computerised mounts have dropped in price in recent years and are now so affordable that you don't need to know about setting circles. However, this valuable knowledge is important because it means you can locate objects even when there isn't a convenient power source or the batteries are flat.

Setting circles can be found on almost all equatorial mounts, from the most basic to the most advanced. They are rotating discs with engraved lines and numbers that encircle the right ascension and declination axis. These lines and numbers correspond to the lines of right ascension (RA) and declination (dec), which the night sky is divided into, like latitude and longitude on a map.

When reading about objects to observe on websites or star maps, they'll often give coordinates in terms of right ascension and declination. With calibrated setting circles, it's just a case of slewing the telescope so these lines.

Getting Started with Setting Circles

The first thing you need to do to calibrate and use your setting circles properly is an accurate polar alignment of your telescope mount. A good low-power eyepiece is helpful as it will give you a wide field of view. It is very useful in the hunt for elusive objects as it shows you stars in the context of the surrounding area.

The declination setting circle is the easiest, so we usually do that first. Assuming your mount is correctly polar aligned, point your telescope at Polaris and then adjust the declination setting circle to read 89° 18', which is Polaris's declination. This is usually done by loosening a grub screw and turning the setting circle until 89& 18' is on the marker and tightening the grub screw again.

To use the right ascension circle, you'll first need to find the right ascension and declination of a bright star that is as near as possible to your intended target, so grab a star chart for the area you're aiming for.

Let's say you want to view the Galaxy M66 in Leo for this example. We can use the magnitude +2.2 star Denebola in the tail of Leo to set the RA setting circle. Denebola has a right ascension of 11h 49.5m and declination of 14° 31'. So loosen the clutches on the mount's axis and, slew the telescope over to the tail of Leo, and centre the star in your eyepiece. Next, make sure that the declination setting circle is reading 14° 31'. If it isn't, reset it. Set the RA setting circle to read 11h 49.5m.

Now you can start locating M66, which has coordinates RA 11h 20.7m Dec. 12° 56.2'. Loosen the clutches on the mount again and gently swing the telescope towards the rough location of M66, carefully watching the setting circles until the markers show the correct coordinates. Lock the clutches, and you should be looking at M66 in the eyepiece.

Step-by-Step Guide to Using Settings Circles

Step 1 - Polar Alignment

Ensure that your telescope mount is level and polar align it as accurately as possible using the instructions below. Observe a star near the celestial equator for several minutes and check the alignment. If the star wanders to the north or south in the field of view, adjust the polar axis west or east as appropriate.

Step 2 - Calibrate Declination Setting Circle on Polaris

Aim your telescope at Polaris and set the declination setting circle to its declination, 89° 18'. You can double-check that you have set this circle correctly when you find your first target star in steps 4 and 5.

Step 3 - Decide What to View and a Bright Target Nearby

Decide on an object to view, for example, M66 in Leo, and find it on your star chart or planetarium software. Make sure it is not too faint to be seen with your telescope. Find a bright star near your chosen object; for example, for M66, you could choose Denebola in the tail of Leo.

Step 4 - Align onto Nearby Bright Star

Loosen the clutches on your mount, slew your telescope to Leo, and centre it on Denebola. A good finderscope with crosshairs will help you locate and centre stars and other objects more easily.

Step 5 - Verify Declination and Set Right Ascention

With Denebola centred on your eyepiece, check that the declination setting circle reads the correct declination of 14° 31'. If not, reset it now. Then, set the right ascension setting circle to read 11h 49.5m.

Step 6 - Slew to your Target and Start Viewing M66

Gently slew your telescope toward M66 whilst watching the setting circles. Slowly swing the telescope so the setting circles read RA 11h 20.7m and Dec. 12° 56.2'. M66 should be in the field of view of the eyepiece. Try slow sweeps in the right ascension using your slow-motion control if it isn't. It'll be close as long as the setting circles are fairly accurate.