Thursday 3 March 2016

Binoculars and a very wild duck

Isn't it about time you take your binoculars that've been collecting dust for years out of the closet? It's really a shame you haven't used them more often! In general, people believe that you need a real telescope, and a very big one too, in order to see something interesting in the night's sky. This couldn't be further from the truth! Ordinary field binoculars, the one you've used on the odd occasion for spotting birds or spying at your neighbour when he was building that new shack, are just the perfect tool to explore the heavens and especially so if you haven't got a lot of experience yet and you're still hesitating to spend a lot of money on a telescope. Not only do they have more light gathering power than an entry-level telescope (!), which is so incredibly important as I shall explain hereafter, but they also use low magnifications and therefore make it easy to scan the sky and find various objects. Take a look at the numbers that are probably written on its back. Chances are that it's a "7x50" or similar, which indicates that its magnification is 7 times and that its lenses have a diameter of 50mm. 

Now what does all of this mean? Contrary to popular belief, the most important job of a telescope is not to magnify but... to capture light. Much more than we humans can with our teeny-weeny eyes and then concentrate it so that it all fits into our pupil. So you have to see the lens or primary mirror of a telescope as its "eye" as this will be an indication of how much more a certain telescope will make you see. Suppose that the fully dilated human pupil has a diameter of 7mm. Then an entry-level refrator (or lens telescope) with a diameter of 60mm will capture 73 times more light because the surface of its lens is 73 times bigger than the surface of our pupil. Or in other words, objects appear 73 times as bright as they do with the naked eye. With an 8" (200mm) telescope, the light gathering power increases to 816 times the human eye and my 18" (457mm) telescope is 4.262 times as powerful! However, you shouldn't get overexcited because we're talking zero magnification here. Binoculars and telescopes magnify and the bigger the telescope, the bigger the minimum magnification will be. I'm not going to bother you with the technical explanation of all of this, but suffice to say that if a telescope magnifies 40 times, the light it captures is also spread over a surface 40 times as large. So if we take our 60mm entry-level telescope, at 40x it will show you the object much larger than with the naked eye, but not even twice as bright anymore. An 18" telescope at 80x on the other hand, will still show you the object 53 times as bright as with the naked eye. So in the end, the most important thing you're looking for in a telescope is aperture. We can discuss the technicalities of different telescope designs for hours or even weeks but in the end there's just no match for size. 

But let's go back to our ordinary 7x50 binoculars. Yes, their aperture's only 50mm, smaller than the entry-level telescope of my example. But... they've got two lenses so their light gathering power actually doubles, without even mentioning the effect of observing with two eyes instead of one (binocular summation factor). What's more, because of their very low magnification the image remains extraordinarily bright, which is an absolute bonus when you're trying to observe very large but faint objects such as the With Head nebula. Now do you understand why all serious astronomers will never let go of a good pair of binoculars, even when they've got a telescope the size of a house?

Take a look at my sketch. The object is M11, or for some reason that is totally beyond me nicknamed the Wild Duck cluster. It's an object you can't miss in the summer sky, in the small constellation of Scutum (the Shield), just below Aquila (the Eagle). With its 2.900 stars it's one the most compact and richest clusters in our galaxy and perhaps it does deserve to be magnified a little more through a telescope. But look how brightly and elegantly it appears in the large field of view of my binoculars! Every single star can still be resolved like the grains of a small heap of salt that someone accidentally spilled on the table. And that, dear readers, is poetry, isn't it? :-)


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