Posted by: Kash Farooq | December 18, 2010

Binoculars for Astronomy

I’ve been studying astronomy with the Open University for a while now. I became hooked after doing an introductory astronomy “short science course”. After the course finished I knew I wanted to continue studying it and started investigating telescopes.

I read loads of blog posts and reviews. One piece of advice was repeated again and again: start with a pair of binoculars. Binoculars are far easier to use, much cheaper (mine cost me about £20, though I can’t find any that cheap now – try this search) and it is far easier to “learn the sky” with binoculars. There are cheap books available specifically about observing the night sky with binoculars. Also, if it turns out that you are not that interested in stargazing…well, you’ve still got a decent pair of binoculars that you can take on holiday with you and use for nature watching.

So, I bought a pair of binoculars, realised that I was still interested and wanted to see more. Then, I bought a 5″ telescope. I still use my binoculars; even when I have the telescope in front of me. It is far easier to locate on object with binoculars and then try to find it with the telescope. Finding stuff with a telescope can be difficult. And, of course, binoculars are ideal if you just want to have a quick look at something for a couple of minutes. Zero set-up time.

It also turns out that some celestial objects actually look better through my binoculars than my telescope. Pleiades is one of them.

Pleiades (Seven Sisters, Messier object 45)

Pleiades (Messier object 45, also known as The Seven Sisters) is an open cluster of stars that are 440 light years away. It does not look like this through binoculars, but it still looks pretty spectacular. Image credit: NASA

It can be seen with the naked eye. It looks like a little fuzzy patch in the sky and you may be able to make out several of the stars, even from a light polluted city. Through binoculars it fills your view and looks beautiful. I showed Pleiades to a friend through my binoculars, someone who had grown up easily recognising Pleiades but just viewing it as the fuzzy cluster of stars. She was stunned.

Through my telescope, with a smaller, more magnified, field of view, I cannot see the whole of the star cluster in one shot. Instead I’m looking at a small subset of Pleiades at a time and it kind of spoils it. Binoculars win.

The rest of this post will summarise what I learnt when researching what to look for when buying binoculars for astronomy. A huge credit goes to “carlos_dfc” who replied to someone’s forum question.

Binocular Sizes: Magnification and Objective Aperture

Good “sizes” for astronomy are: 7×50, 8×42, 8×56, 9×63, 10×50, 10×70, 12×60, 15×70, 20×80, 22×100.

10×50 appear to be the “binoculars of choice” for amateur astronomers and that’s the size I’ve gone for.

Let me explain what these numbers mean.

The first bit is the magnification. So “10x” means the object will appear 10 times bigger. The second number is the aperture objective (e.g. 50 mm, 60 mm). This is basically a measure of how much light can get into your binoculars. You may think that you should just go for the biggest magnification combined with the biggest aperture that you can afford. This isn’t the case. A big aperture means more weight, which means it will be difficult to keep the binoculars steady. Ditto with the magnification. When you get above, say, 12x magnification, you need to mount the binoculars. You won’t be able to keep them steady to make any decent observations. Though, if you already have a camera tripod, you may want to go for bigger magnification and aperture – you would need a bracket (about £10) to fix the binoculars to a tripod.

The aperture objective size is probably more important than the magnification. Getting more light into your binoculars will allow you to see fainter objects. A 70 mm aperture will allow about twice as much light to enter as a 50 mm aperture. You can work this out by simply calculating the area of a circle using ∏r2 (it gives surface areas of about 7800 mm2 vs. 15400 mm2).

Zoom

Never buy a pair of binoculars with “zoom”. If you see a magnification given as a range (e.g. 6x – 12x) – avoid. The technology involved to perfectly align optics to give differing magnification is not something you’ll find in a cheap pair of binoculars.

Binocular Optics

The advice I saw was:

“Go for Bak-4 Porro Prisms. Avoid BK-7”

I haven’t really investigated any further than that!

I also saw some advice on coatings for optics:

“Coated optics are OK – Multi-coated is better – Fully coated is better still – and Fully Multicoated is best.”

“The colour of the coating is also a big clue, the best coatings usually have either a blue, purple, or green tinge. Red (Ruby) and Amber coatings are to be avoided.”

Exit Pupil

You calculate exit pupil by taking the size and dividing the second number by the first. So, for 10×50, you have 50 mm/10 = 5 mm exit pupil. You want to go for something between 5 mm and 7 mm.

So, what is it?

From Wikipedia:

“In optics, the exit pupil is a virtual aperture in an optical system. Only rays which pass through this virtual aperture can exit the system”

In binoculars, the exit pupil is a measure of the diameter of this virtual aperture. Basically it indicates the size of the disc of light that leaves the eyepiece and enters your eye. And you want the size of this disc to be as close to the diameter of your pupil.

So, there is no point going higher than 7 mm exit pupil. This would probably be a waste of money; most people’s pupils do not go wider than 7 mm and this value decreases with age!

In summary

You can pick up a pair of binoculars that satisfy these rules for reasonably low prices. You’ll be amazed at how much more you can see with binoculars than you can with just the naked eye. Even in a light polluted city – look up at the sky and you’ll see one or two stars; then look through your binoculars and see hundreds.

So, what have you got to lose?

A Google Search to find suitable binoculars for astronomy

Click me!


Responses

  1. Great blog entry! I’m glad you mentioned using binoculars along with a telescope, and that they aren’t just used as a pre-telescope option.

  2. You have a rare skill Kash; the ability to communicate complex ideas in a lucid and concise manner. Top post.

  3. Hey thanks for emailing me this link – I have been thinking about doing this for ages now and your post is now making me think I should definitely do it soon!! Thanks for all the tips😀

  4. Settled for these: http://www.celestron.com/astronomy/celestron-skymaster-15×70.html

    They’re quite heavy so can only really be used handheld for a short while, but fortunately I have a tripod for stabilisation. I’ve used binoculars along with a telescope before, but always had a rubbish bins. I also focused on the scope. This time I wanted a pair of bins that were actually pretty decent. The info on this page was very useful!

  5. […] discs. My binoculars are woefully small, and not really up to the task. Kash Farooq has written this post on how to get a decent pair of binocs at a reasonable price. Even with poor binoculars, you will be […]

  6. I want to see a close up of the stars from my attic window at night, will 10×50 binoculars really make much of a difference? As I have seen some going for £70

    • You’ll never be able to see “a close up of the stars” with binoculars, or even with a telescope. In fact, even the best telescopes in the world can’t show you the detail of a star.

      Here is an example what one of the biggest telescopes in the world can do http://apod.nasa.gov/apod/ap090805.html

      With binoculars you may be able to resolve some “double” stars (i.e. split a point of light in two, revealing two stars and not just the one you can see with the naked eye). And of course, you’ll see much more stars in general, and some star clusters.


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