Binocular Astronomy

Tuesday 21st July, 2015


Our visiting speaker was Steve Tonkin who is originally from Zimbabwe. As a boy he saw Sputnik travel overhead and his interest in space and astronomy continued into adulthood as he undertook searches for distant supernovae. Unfortunately, a back injury meant he could not continue his search, as it entailed manoeuvring a large telescope, and so he decided to switch to using binoculars to study the night sky.

The title of his talk was "Binocular Astronomy" and he has published a book (now in its second edition) with this title in the Patrick Moore Practical Astronomy Series. He also writes a regular article each month on binocular objects for the Sky at Night magazine as well as being heavily involved in astronomy outreach and education. Each month he publishes a free newsletter "The Binocular Sky" which can be downloaded from his website

He began his talk by reinforcing the view that our own members always give to newcomers to astronomy — it is best to start your exploration of the night sky using a pair of binoculars. He explained that there are many advantages to beginning the hobby this way as binoculars are portable, easy to set up and you can use both your eyes. Being portable means that you can easily take them with you on your travels and away from light pollution. It is advantageous to use a tripod to ensure a steady view through the binoculars and he commented that it takes him less than ten minutes to attach even his large binoculars to their mount.

He emphasised that using both eyes to examine faint objects and the detail in objects made a big difference. This is due in part to the phenomenon known as binocular summation, which basically means that two eyes gather more light than one eye and the signals from each eye are added together but the random neural noise is cancelled. The other advantage of using both eyes is that each eye will be able to see that part of the image that is hidden by the other eye's blind spot. This is the location on the retina in each eye where the optic nerve enters.

He said that if asked what sort of binoculars to buy for astronomy he would recommend Porro-prism 10 x 50. A Porro-prism is a type of prism which alters the orientation of an image and the figures "10 x 50" relate to the magnification (10) and the aperture (50mm). Two things to definitely avoid are zoom binoculars or ruby coatings.

He suggested starting binocular observations by selecting easy objects such as the Moon and planets. As the Moon waxes and wanes the shadow (known as the terminator) sweeps across its disc and different surface details are more visible. To look at fainter objects such as galaxies or nebulae it is best to wait until the Moon is not present in the night sky. This point in the Moon's orbit is known as New Moon and occurs when the Moon lies between the Earth and the Sun. Other popular targets are Saturn with its rings and Jupiter with its four innermost moons. A little more tricky to locate are the ice giants Uranus and Neptune which need a detailed current star chart to track them down.


This article was written for the club news column of the Stratford Herald. The actual lecture explained the subject at a deeper level.