Date: May 23, 2011
Title: Binocular Astronomy
Organization: RapidEye Observatory – a private observatory in rural Lee County, NC
Links: http://www.rapideye.us/astro/RapidEye-ClearSky.html
Description: I’m often asked by my coworkers, friends, and relatives: “I’d like to buy my first telescope, whats a good one to get?” When I give them my standard answer “Binoculars” it is usually followed by an uncomfortable pause and a puzzled look on the face of the person asking the question.
Bio: I’ve been captivated by astronomy ever since I was a kid, living in NW Colorado where the Milky Way was bright enough to read by.
I can be found most clear nights in my pasture with either my 4.5″ SkyQuest, 10″ Deep Sky Hunter, 18″ Obsession, or my binoculars.
Sponsor: This episode of 365 Days of Astronomy is sponsored by — no one. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.
Transcript:
Podcast for 365 Days of Astronomy – May 23, 2011
Binocular Astronomy
Being an active visual backyard astronomer I’m often asked by my coworkers, friends, and relatives: “I’d like to buy my first telescope, whats a good one to get?” When I give them my standard answer “Binoculars” it is usually followed by an uncomfortable pause and a puzzled look on the face of the person asking the question.
I then go on to explain that a good pair of binoculars can be bought for under $100 (US) whereas a decent telescope starts at $200 (US) and realistically, a good telescope that will really allow you to see a lot, on a good mount that won’t shake in the lightest breeze, and a couple of extra eyepieces will usually cost closer to $400 (US) or more.
I also go on to explain that if they live in the city surrounded by light pollution, they will probably have to drive out to the country away from the city lights to see much beyond planets and the moon. Would you rather have to pack and unload 100 lbs of gear and accessories, or a pair of binoculars that will easily fit in a Cooper Mini!
Finally, I point out that most of the $200-$400 (US) telescopes are really dedicated astronomy instruments and if they want to do things like look at birds, boats, or other distant objects during the day, they’ll have to buy a more expensive telescope with additional accessories. Or, they could just buy a pair of quality binoculars!
OK, so what to look for when selecting a pair of binoculars? What features, specifications, and options work best without breaking the bank?
Lets start with the basic measurements. All binoculars will be labeled with 2 numbers, separated by an “X” Examples are 7×35, 8×40, or 10×50. The first number is the magnification the binoculars deliver. 7x power binoculars will make things look 7 times bigger than you’ll see with your naked eye. The second number is the size of the objective lens (the one that points away from you) measured in millimeters. So 7×35 binoculars have a 35mm objective lens and 10×50 binoculars have a 50mm objective lens. Pretty straight forward so far, right?
Comparing different magnifications are easy for most folks to understand, as well as the trade offs: a pair of 10 power binoculars will show you more detail than a pair of 7 power binoculars; however, any shaking or instability in holding the binoculars will make using the higher power more difficult. Also, all things being the same, the higher power binoculars will also have a narrower field of view.
Comparing different objective lens sizes is a little less intuitive. In general, the larger the objective lens is, the more light it will collect and the brighter the image will be when compared to another binocular of the same magnification with a smaller objective lens. This is true, up to a point. Your eyeballs have an iris built into them that opens and closes to allow more or less light to pass through depending on how bright the light hitting your eye is. This opening is measured in millimeters and typical values for an average person range from 6-8mm, fully dilated in very dark conditions to 1mm or less under very bright conditions. If the size of the image leaving the binoculars, also known as the exit pupil, is larger than the opening of your eye, light will be wasted hitting the closed down iris.
Fortunately the exit pupil for binoculars is easy to calculate. Divide the size of the objective lens by the power of the binoculars. So 10×50 binoculars have a 5mm exit pupil and 10×30 binoculars have a 3mm exit pupil. When comparing those two binoculars, the only time you will see a difference between them is when your eyes are starting to adapt to darkness and the iris is opened more than 3mm, at which point, the larger binoculars will give a brighter image and you’ll see more detail in objects. When working in dark conditions, like when star gazing away from light pollution, you should target binoculars with a 5-7mm exit pupil. If working from the suburbs where there are lights on the neighbor’s house, street lights, and sky glow, you should be targeting binoculars with a 3-5mm exit pupil.
So why not just go with super large objective lens binoculars like 10×70’s or 12×80’s? Two reasons really. Firstly, cost – the larger the lenses the more expensive the binoculars. Secondly, is weight. The larger the lenses are the more they weigh, and the harder they are to hold steady.
Typical limitations for hand holding binoculars are 10X power and 50mm objective lenses. There are people that are very steady handed and/or strong and can effectively use stronger and larger binoculars, but for most folks, 10×50 binoculars are the upper threshold for hand viewing and many people find that 8×40 binoculars are even better.
There are several other considerations when looking at binoculars such as prism glass, coatings, roof prism vs porro prism, and each has their trade offs. For brevetys sake, suffice it to say that for most folks looking to get a good pair of budget binoculars should look for, porro prisms, BAK4 glass, and fully multicoated optics. Resources I have listed below can go into more detail what those specifications mean.
One thing to consider when observing with binoculars is comfort. Standing for extended periods of time while holding a pair of binoculars up to your eyes will cause your arms and shoulders to get tired and you’ll have a hard time holding the image steady and appreciating the view. There are many solutions to this problem like specialized binocular mounts and image stabilizing binoculars, but I find that the cheapest and easiest solution is a reclining lawn chair. Lay back, relax, get comfortable, and prop your elbows up on the armrests of the chair. You’ll find that you can not only hold the binoculars very steady, but that you can hold them for a very long time before you’re arms become fatigued.
OK, so now that you’ve got a respectable pair of binoculars and your lawn chair out of the garage, what can you see with them? Lets start with the easiest target in the night sky – the moon. Even at 7x power, hundreds of craters, all the lunar maria (large dark areas), and dozens of bright rays (crater ejecta) are easily visible. Sky & Telescope sells a large laminated lunar map that is very inexpensive and easy to use and can quickly serve as a guide to features you are seeing. Sit back in your lawn chair, lay the map in your lap, and take your time – you’ll be amazed how once you start to find and identify big craters like Tyco and Plato, how many others will jump out at you.
Also, this time of year there are some very nice open clusters that will look outstanding in binoculars, even from suburbs of many cities. Messier 44, the Beehive Cluster and Mel 111 in Coma Berenices will jump right out of the sky in binoculars and are easy to locate using the free downloadable monthly sky maps available at skymaps.com. And over the next few months, the core of the Milky Way will start to make an appearance and you’ll be in prime binocular observing time! The star clouds of Sagittarius and Scutum can really only be appreciated in the wide field of view offered by binoculars. And from a truly dark location, like on your summer camping trip to the country, you may even be able to see various “Dark” nebula. Again, turn to skymaps.com for free printable maps for each month to know where to look.
If after a few evenings with the binoculars you are finding that astronomy isn’t your thing, well, you still have a nice pair of binoculars that you can use for hiking, boating, birding, and sporting events instead of a telescope and tripod sitting in the hall closet collecting dust.
But hopefully, otherwise you wouldn’t be listening to this podcast, you’ll find that after spending a couple of evenings under the stars soaking up the views with a pair of binoculars that you’ll want to see some different objects, and aren’t sure where to turn too – there are lots of great resources for helping binocular astronomers. Some of my favorites are:
“Observing the Night Sky with Binoculars: A Simple Guide to the Heavens” by Stephen James O’Meara. This is relatively recent publication and is easy to find at most book stores and internet retailers. Mr. O’Meara is easily one of the most accomplished visual astronomers of our time and his books are a real joy to use in the field.
“Touring the Universe Through Binoculars” by Philip S. Harrington. This book is out of print, but still easy to find. It may be 20 years old, but when it comes to low power visual astronomy, not much has changed in 20 years. Mr. Harrington also writes a monthly binocular observing column over at CloudyNights.com – just search for “Binocular Universe” at CN and you’ll be able to download all of his articles and charts for free.
Finally, Gary Seronik has a monthly column in “Sky & Telescope” with objects to find as well as a book called “Binocular Highlights: 99 Celestial Sights for Binocular Users” which is a collection of his back articles and charts condensed into a very easy to use field guide.
So if you are considering buying your first telescope or know someone that is, keep in mind what noted astronomy author Phil Harrington always says, “Two eyes are better than one”!
End of podcast:
365 Days of Astronomy
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