How to… star-hop your way around the night sky
Make navigating the night sky easy with our guide
There are thousands of objects in the night sky within the grasp of binoculars and small telescopes which are just too faint to be seen with the naked eye. The question is though, how do you find them? Even if you have a clever, computerised GOTO mount, there is a lot of fun to be had, and much to discover, hunting down objects using the simple method of 'star hopping'.
There are a couple of things you need to know about your binoculars or telescope before you start: firstly their field of view. On binoculars, this is usually printed or engraved on them. Secondly, you need to understand the limiting magnitude of your binoculars or telescope – in other words how bright or faint the stars and other objects can be seen through them. There is no point attempting to look for an object that is just too faint too be seen, unless you have a large telescope! To work out the field of view that you get with any given eyepiece, you need to divide the 'apparent field of view' of the eyepiece, again often marked on the eyepiece itself, for example 50°, by the magnification you get with it.
To work out the magnification, divide the focal length of the eyepiece, say 25mm, into the focal length of your telescope. For example, a 1,000mm focal length telescope divided by a 25mm focal length eyepiece gives a magnification of 40x. Quite straightforward really! The actual field of view, with such a telescope and eyepiece, is 1.25 degrees.
Once you have all this information, you can select your target, and by choosing a nearby brightish star, you can hop to it, one field of view at a time. The limiting magnitude of standard 10x50 binoculars is around 11, so don't look for any objects fainter than this. For an 80mm aperture telescope, it is around magnitude 12 and a 150mm aperture scope, it is about 13.5. You need to make allowances downwards, for light pollution and poor conditions. Mark on your star chart using a compass the field of view which you are using, and how many fields of view it will take to get to you required object.
"There is a lot of fun to be had, and much to discover, hunting for objects"