How to… master setting circles
Puzzled by the dials on your equatorial mount? Here's how to make them work for you
They can be very confusing at first, but those dials on your equatorial mount are there for a purpose: to help you locate objects quickly and easily in the night sky. There are two dials, one on the right ascension (RA) axis and one on the declination (Dec) axis. They correspond to the lines of latitude and longitude (RA and Dec) which are used as coordinates in the night sky.
To use them to find objects with some degree of accuracy requires that your equatorial mount is properly polar aligned. The polar axis of the mount must point to the north celestial pole, if you are in the northern hemisphere, or the south celestial pole if you live south of the equator. Once your setting circles are set up properly, you can use them to 'dial' the coordinates of the object you want to look at. As you can imagine, this saves a lot of time hunting for those elusive faint objects which you have always wanted to see. If you are careful and methodical, even the most basic of setting circles can get you in the right area of sky, and probably have the object within the field of view of a low-power eyepiece.
The easiest circle or dial to set up is the declination circle. You only need to do this once, or at most once an observing session. The RA circle is a little more 'user intensive', as you need to reset it after every object which you visit, but, with a little practice, you will soon get the hang of using it. You will need a good star chart with the RA and Dec coordinates marked, and which shows many of the deep sky objects you might like to see. You will also need slow motion controls on your mount – that is the knobs, often on flexible shafts, that you turn by hand – or you will need electric drives with a hand control. These will enable you to gently zero in on your target when you are getting near to it, without disturbing the set up of the setting circles themselves.
Start with easy to find, bright objects, so you can understand how it all works, and with a little practice, you will soon be finding objects that you never thought you would spot, and enjoying the full capabilities of your telescope.
"This saves time hunting for those elusive faint objects"