There are lots of households that have an old pair of binoculars lying about somewhere. Using binoculars for stargazing is a great way to start out, and many people have had their love of astronomy take off simply by taking a casual look through them.
With their ease of use, and a wide field of view, they are a great way to connect with the night sky.
In this article we’ll take a look at the different binocular styles, what you need to know about binoculars, and examine what makes the best binoculars for stargazing.
- The Two Different Binocular Styles
- Stargazing Binoculars – What You Need To Know
- What Do The Numbers Mean?
- Explaining Aperture
- Magnification ..What’s Best For You?
- What Is The Exit Pupil?
- Why Binoculars Have Optical Coatings
- Image Stabilized Binoculars – The Next Level
- Advantages And Disadvantages
- Choosing Binoculars – Striking A Balance
The Two Different Binocular Styles
Binoculars are basically two very small telescopes connected together, and are similar to a refracting telescope in that they have a lens at the front and an eyepiece at the back.
The big difference with bins is that they use prisms to fold the light making them much more compact and easier to use.
Binoculars come in two body styles based on the type of prisms used, the “Porro prism” design and the “Roof prism” design.
Porro Prism Binoculars
Porro prism binoculars are named after their creator Ignazio Porro who patented the system in 1854. It’s the classic design which most people would recognise as a typical pair of binoculars.
This type of binocular contain two prisms which offset the light path from the objective lens and out through the eyepiece. These prisms are set wider than the eyepiece tube which results the classic zigzag appearance of the binoculars. It also makes for a comfy fit in the hands.
Roof Prism Binoculars
To look at a pair of roof prism binoculars it would appear that light goes straight through the objective lens to the eyepiece. In fact, they still fold the light through prisms, but the design enables this to happen without offsetting the light path.
Roof prism binoculars have their name because the two prisms have a section in the design which resembles the roof of a building.
This type of binocular contains more complicated prisms and requires greater optical precision in the manufacture which can be reflected in its price.
On the plus side they have a more modern look, are lightweight and more portable and have become increasingly popular as optical coatings have improved.
Stargazing Binoculars – What You Need To Know
What Do The Numbers Mean?
When you buy a pair of binoculars you will notice that there are two numbers with an x in between them. This can be many combinations such as 8 x 30, 10 x 25, 12 x 42.
Take the first example 8 x 30 (as shown in the photo).
The first number represents the magnification, so ( 8 x ) magnifies everything 8 times. The second number (30) represents the aperture (or diameter) of the objective lens, in millimetres.
So in this case the binocular has an 8 x magnification, and an objective lens aperture of 30 millimetres.
Just to confuse the issue slightly there are also zoom binoculars. These are not recommended for stargazing because they require a highly precise manufacture to be any good, and too often this requirement is not met.
A zoom binocular will have three numbers for example 8-20 x 40 where 8-20 x represents the magnification range and 40 is the aperture.
Explaining Aperture
Aperture is the diameter of the two objective lenses at the front of the binoculars. The bigger the aperture, the better it’s light gathering properties.
When you’re buying a pair of binoculars for use during the day, lets say for bird watching, one thing you don’t have to consider so much is the aperture.
It’s not as important during the day, as there is plenty of light, so you can get by with smaller front lenses.
The best binoculars for stargazing however, have a requirement for larger front lenses. As the observing is done in the dark they need to have good light gathering properties, i.e., larger aperture.
When choosing the best binoculars for stargazing it’s a good idea to strike a balance between aperture and magnification
Magnification..What’s Best For You?
Magnification (or power) is an important factor when determining the best binoculars for stargazing.
Basically, the given magnification for any particular set of binoculars means that is how much closer an object looks through the binoculars, compared to the naked eye.
For example a pair of binoculars, with an 8x magnification, will show objects that appear 8 times closer than when viewed through the naked eye.
There are lots of different magnifications available in binoculars and as previously mentioned it is a good idea to strike a balance between magnification and aperture.
Any binocular with a 7 to 10x magnification is often regarded as a good mid range target.
Lower magnification binoculars are ideal for a lot of users as they have a brighter and steadier image and a wide field of view, making finding objects easier.
On the other hand there are users who are happy to have a narrower field of view, and a slightly dimmer image with more shaking, for the extra detail that higher magnification gives.
What Is The”Exit Pupil”?
If you hold a pair of binoculars out in front of you on a bright day, or at a bright wall, you’ll notice two round discs of light floating around behind the eyepieces.
These columns of light coming through the binocular to your eye are called the exit pupil.
Binoculars Showing ‘Exit Pupil’
The exit pupil is another factor to consider when determining the best binoculars for stargazing.
You can calculate the size of the exit pupil by taking the diameter of the objective lens and dividing that by the magnification of the binoculars.
For example, binoculars that are 7 x 50 (7x magnification and 50mm aperture) have an exit pupil size of 7.1mm. (50 / 7 = 7.1) So in this example the exit pupil size is 7.1mm.
This is important because the light from the bright disc (exit pupil) should ideally fit inside the pupil of your eye.
This, surprisingly, can be affected by your age, which makes this calculation potentially useful as we’ll see.
When adapted to the darkness, the pupil of the human eye has a diameter of around 7mm at the very most, and this relates to young people with healthy eyes.
The older a person is, the less the pupil dilates in dark conditions.
The rule of thumb is that from the age of around 30 generally, you lose 1mm off the maximum dilation of the pupil every 10 to 15 years.
So if, like me you’re in your 50’s (sigh!) or older, then you probably have a maximum pupil dilation in dark conditions of around 5mm.
This means the extra light collected by a big pair of binoculars, as in the example above with exit pupil 7.1mm, isn’t all going to fit in the pupils, and just goes to waste.
It’s worth noting that if you increase either the size of the objective lens (the aperture), or the magnification it has the opposite effect on exit pupil size. The larger the objective lens the larger the exit pupil, whereas the greater the magnification the smaller the exit pupil. The larger the objective lens the larger the exit pupil, whereas the greater the magnification the smaller the exit pupil.
Why Binoculars Have Optical Coatings
When light travels through a lens some light is reflected off the surface reducing contrast and therefore the image quality.
Standard optical glass can reflect up to 4% of the light that hits it’s surface. If you consider that a pair of binoculars may have as many as 16 air to glass surfaces, that potentially adds up to a lot of light wastage!
For this reason manufacturers deposit thin chemical coatings on lens surfaces which improves light transmission and reduces reflective loss.
These coatings are applied with incredible precision a few millionths of an inch thick which results in greater light transmission for brighter, sharper contrast images.
There are four main types of optical coating for binoculars.
- Coated…An anti-reflective coating, typically of magnesium fluoride is applied to one or more of the lens surfaces.
- Fully Coated…Binoculars have a thin anti-reflective light transmission coating on all air-to-glass surfaces to increase light transmission.
- Multi-Coated…Multiple layers of coatings on one or more of the lens surfaces.
- Fully-Multi Coated…Binoculars have multiple coatings on every lens surfaces which provides the highest light transmission and contrast.
Another type of coating which is exclusively for roof prism binoculars is called Phase coating, and this is applied to the prisms rather than the lenses.
As we’ve seen, Roof prism binoculars have a design where the eyepiece is in-line with the objective lens whereas on Porro prism models the eyepiece is offset from the objective lens.
With the Roof prism design, the light is folded back on itself through a prism for a short distance. When this happens the waves of light, which were lined up perfectly when they entered the binocular, exit the prism slightly out of phase.
Phase coating is composed of a thin layer of dielectric material and this in effect delays the rays of light enough for them to come back into phase. This produces better brightness, contrast and clarity.
As you might expect, the better the coatings, the more expensive the binoculars.
Image Stabilized Binoculars – The Next Level
The problem with magnification is that the more you have, the more pronounced the shaking of the image tends to be when you hold a pair of binoculars.
Image stabilized binoculars have been developed to prevent this. It’s the kind of technology that’s been around for years in cameras and camcorders and has now been passed on to binoculars.
Image stabilizing binoculars have built-in electronic sensors which activate when the image becomes shaky. This can then adjust the angle of the prisms that are built into the binoculars, adjust the lenses of the binocular or even a combination of both.
As you may imagine, with all this wizardry, image stabilizing binoculars don’t come cheap.
Also, you couldn’t say that they are a vital bit of kit (especially if you’re just starting out), but if your budget will allow and for stargazing in particular, these little marvels take observing to the next level.
Advantages And Disadvantages
Advantages
- Depending on the pair you pick and the observing conditions, it’s possible to see up to 50 times more stars with binoculars than with the naked eye.
- Binoculars are the simplest way to observe the night sky. They are highly portable, so can be used for a quick 5-minute look outside, or taken with you if you want to travel to a rural area with a darker sky.
- The great advantage of binoculars over a telescope is their wide field of view, making it easier to scan the night sky and recognise patterns, such as constellations.
- Observing with both eyes feels more natural, and your brain can actually form a better image when looking through both eyes with binoculars.
- Stargazing binoculars can be used effectively for other types of observations, such as bird watching.
Disadvantages
- Most binoculars are around 7x to 10x magnification and while this gives a wide field of view it’s not enough to show detail on planets.
- Having a pair of binoculars with a relatively high magnification can result in a comparative loss of light, and contrast, to the image viewed.
- High power binoculars often have larger lenses to compensate for loss of light, and this can make them more difficult to hold steady.
Choosing Binoculars – Striking A Balance
Binoculars are a great starting point for stargazing because they are so easy to use. You can grab a pair and be outside looking at the night sky in no time.
There is a lot of variation in sizes for binoculars. They can range from the very small, to some monsters that are so big they are impossible to hole steady and require a tripod.
When choosing the best binoculars for stargazing a good pair of Porro prism binoculars are, for many, the best choice.
Even though they are generally heavier, they are comfortable to hold and due to their less complicated design are more often than not comparatively cheaper.
Roof prism binoculars do have advantages. They are lightweight compared to the Porro prism binoculars and although they may not have quite as high a quality of image, this is quite often made up for by having better optical coatings.
If you’re really looking to treat yourself, or someone else, you could go for one of the amazing image stabilized binoculars, but only if you have deep pockets!
The chances are that most people will be interested in a good quality all-rounder that ticks as many boxes as possible. If that’s the case, to strike a balance, a good quality 10 x 50 pair of binoculars would be a fine choice.
