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Primer - Astronomy with Binoculars

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[glow=red,2,300]Choosing Binoculars[/glow]

Binoculars (or bino's as their affectionately known) are often ignored by beginners, yet they offer a low cost start to the hobby. Portability and a wide field of view can often be a great place to start, Bino's offer this. One of the disadvantages is that, being hand held, they can give unsteady images. Obviously this is due to hand shake however can be overcome by using a tripod. Some recent binoculars have an image steadying mechanism, but this comes at a price!.

The performance of binoculars is specified using a two number system, such as 6×30 or 8×50. The first number represents the magnification, and the second the diameter of the objective lens in millimetres.

The object lens is the one at the front of the binoculars. The larger the diameter (or aperture as its know), the better the light gathering power. This is the most important property for observing the night sky. Beginners usually assume that the higher the magnification the better. In general it is often better to choose a pair of binoculars with a wide field of view, i.e. a lower magnification. Higher magnifications can resolve double stars, but narrows the field of view, making it difficult to find your way around the sky. It is important to note that higher magnifications also magnify the atmospheric instability. This causes stars to twinkle, and, of course, it also magnifies hand shake, making stars appear to jump around in the field of view. For this reason, 10 times magnification is considered the maximum for hand-held bino's.

The bigger the aperture of the objective lens the brighter the image. Most astronomical objects are hard to see not because they are small, or distant, and need more magnification, but because they are faint and need a larger aperture to gather more light. A pair of 8×50s collects twice the amount of light as 8×35s, and hence everything appears to be brighter.

Suitable binoculars are 10x50; 15x70 and 20x100 giving magnifications of 10 times, 15 times and 20 times with lens diameters of 50mm; 70mm and 100mm ( 2 inches; 2.75 inches and 4 inches) respectively. For best results, the ratio of aperture to magnification should not exceed 5, that is lens size (in millimetres) divided by the magnification should be no more than 5.

Bino's are good start point to the hobby, but equally an essential part of your kit bag even if you have a telescope!.

Clear skies


Source: R.M. Clarke. The salopain web - edited by R Hughes. 2005

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Excellent topic Rob. 

Binocs are my thing having sold them for the last twenty years and written articles and reviews on them.  Any questions chaps ... bring it on  8)

Steve :)

PS: The article doesnt mention Patrick Moores favourite (the 7x50), types (porro/roof) or exit-pupils. 

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  • 2 weeks later...

spotting scopes work well too...

Very true Adrius,

Astronomers tend to sniff at the suggestion of using spotting scopes because of the light reducing effect of the built in image erecting prism but in reality, the loss of light is quite negligible.  The top brands - Swarovski, Leica & Zeiss - produce state-of-the-art compact refractors with APO performance that, via an adaptor, can also be used with astro eyepieces. 

Before buying a 'proper' astro scope I used a Nikon ED78 Fieldscope with a 30x wide for fantastic wide-field views; the moon and Pleides were stunning and a number of Jupiter's weather bands were routinely visible.  I had intended buying the photo-adaptor as, being a camera/lens manufacturer, Nikon's is of excellent quality but ended up selling it to buy a Meade ETX105. 

My fondest memory is of the time I used it to show my mother Jupiter and four of its moons. 

Steve :)

Mind you, the 45 degree viewing position can put a crick in your neck, but then ... you can't have it all :)

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I've shown several people the night sky through my 10x50's (Meade's) - and the reaction is the same every time!


I tend to use them when observing while waiting for the scope to cool down or my eyes to dark adapt. Really ought to get a tripod adaptor for them...


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Currently I've got those and a pair of 15X70.
Cant say as i have used anything other than 10x50 bino's although i do fancy something with a little more power. Do you have them mounted Gaz?

I think M31 is the best object to view with Bino's personally and sharing that one always get's a WOW from people.


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No James 15x70 is just enough to be OK for me, they're not that bad. I had a pair of 20x60 that were too much, it's the mag not the weight that gives the shakes IMO.

I also had a pair of 30x80 that were quickly sold on...... :nono:

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