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Jul 23 13:05 2008
For an astronomer, binoculars may be very useful for several reasons: they are relatively inexpensive, have a large field of view and show images right side up (which makes finding things in the sky easier), are easily portable and require little to no setup.
So, how do you choose a good pair of binoculars for amateur astronomy?
There are different ways of categorising binoculars. But usually, they are distinguished by their magnification and the size of the aperture (
for example, 8 x 42
). A combination of a small magnification and large lens produces a brighter view. A 7x50 pair, for example, gives a brighter image than a 10 x 50, but the image magnification is smaller. A standard 7 x 50 pair is considered one of the best all-round binoculars for practicality, performance and price.
For high-powered observation, a magnification of 25x with a 100mm objective lens is recommended.
Observation binoculars come with either straight or angled eyepieces. The benefit of the angled design, usually 45º, is the ease of use and comfort of viewing. The angled eyepiece model is much more user-friendly, allowing more flexibility for people of different heights to use the binoculars without the need to continually adjust the tripod. It is also of benefit when the binocular is trained on the night sky. The straight eyepiece design model will require a higher adjustment of the tripod for each user and this may make the tripod less stable.
If youre forced to go out to the country where you cannot fix your binoculars to something, then consider image stabilised binoculars. These are far more expensive but have other applications as well.
Stabilisation may be enabled or disabled by the user as required. Stabilisation will allow binoculars up to 20× to be hand-held. Major brands making 'image stabilised binoculars include are Canon, Nikon, and Bushnell.
Digital binoculars are becoming increasingly popular - these capture a digital image seen through the binoculars.
Zoom binoculars have the facility to quickly and very efficiently zoom in on the object of interest, but the image quality is compromised in cheaper models. The extra workings and glass inside reduce the amount of light available, making them unsuitable for astronomy.
Avoid binoculars that claim to be "focus-free". Also be beware of advertising lingo like "high-powered". Increasing the magnification will decrease the brightness and field of view, which makes objects faint and fuzzy.
A lower magnification will maximise the amount of light transferred.
The larger the aperture, the brighter the image will be; but the greater the size, the binoculars will weight and cost more. For general astronomy use, choose binoculars with an aperture of 50mm. An observation binocular with a 60mm objective lens will still be fairly portable whilst an observation binocular with a 80 to 100mm + objective lens is far more suited for static use.
The size of the objective lens and the power of magnification are the two major factors that determine the light transmission of the binocular. For example, 100mm (the diameter of the objective lens) divided by 25 (
the power of magnification
) gives a figure of 4, which is the diameter (mm) of the exit pupil and indicates the amount of light reaching the eye. In general the larger the exit pupil diameter the brighter the binocular will appear and the better the resolution will be, enhancing colour and contrast perception, especially in low light conditions. However, as the human eye pupil dilates on average from 2.5mm to 7mm depending on light conditions it follows that an exit pupil above 7 is not beneficial as the human eye cannot accommodate it.
It is worth remembering that as we age our eye pupil does not dilate so much, so a large exit pupil of 7mm is not so important for a 50 year old person compared with a 19 year old. So a 4mm exit pupil on a 25x100 observation binocular will be more than satisfactory for most users in most conditions, whereas a 40x100 will only give 2.5mm exit pupil, drastically reducing the amount of light reaching the eye.
10x50 or 8x40 will have a 5mm exit pupil.
A standard 7x50 pair will have a 7mm exit pupil, the average human eye pupil size at night.
Most binoculars are not suitable for use with glasses (spectacles). You have to put your eye close to the eyepiece, but the glasses prevent you from getting close enough. You can of course take your glasses off to use the binoculars, but this can be a nuisance. It is possible to buy special binoculars which can be used with glasses.
Standard binoculars have eye relief ranging from only a few millimetres to 15 millimetres.
Long eye relief (
15 to 25 millimetres or more
) is necessary for eyeglass wearers. A poorly designed optical system may force the observer to press his or her eye close to the eyepiece in order to see an unvignetted image, or alternatively may have an exit pupil larger than the observer's pupil at a comfortable viewing position, resulting in wastage of light and a dimmer image.
The eyepieces of binoculars are usually permanently mounted in the binoculars, causing them to have a pre-determined magnification and field of view. Normally binocular eyepieces are 3-4 element with marginal correction for colour and edge sharpness. The correction on Siebert 6 element eyepieces are comparable to a Japanese made Meade 26mm Super plossl. The eyepieces do not add colour correction or false colour.
Wide-angle binoculars have a field of view that is wider than average (60 or higher).
The Field-of-view is the size of an area that can be viewed using the binoculars.
Binoculars are advertised with their field of view specified in one of two ways: angular field of view, and linear field of view. Angular field of view is typically specified in degrees, while linear field of view is a ratio of lengths. For example, a pair of binoculars with a 5.8 degree (angular) field of view might be advertised as having a (linear) field of view of 305 feet per 1000 yards or 102mm per meter. As long as the field of view (FOV) is less than about 10 degrees or so, the following approximation formulas allow one to convert between linear and angular field of view. Let A be the angular field of view in degrees. Let L be the linear field of view in feet per 1000 yards. Let M be the linear field of view in millimetres per meter. Then:
A = 0.0191 \times L
A = 0.0573 \times M
L = 52.4 \times A
M = 17.5 \times A
Generally, higher powered binoculars give you a smaller field-of-view and the opposite is true for lower powered binoculars. Field-of-view may also be expressed in two ways; as the width in feet at 1,000 yards, or in degrees of field. When expressed in feet the field is called linear, and when expressed in degrees it is called angular. To convert angular field to the more practical linear field, multiply the angular field by 52.5.
For astronomy, a wide field of view is desirable because if offers a more pleasant viewing experience, and you can see more of the sky at a better edge performance compared to a narrower field.
Binoculars use prisms to provide correctly oriented images, and to shorten the optical path.
The typical binocular design can be porro-prism or roof prism.
The size and design of the prisms will affect sharpness, with quality optical glass delivering clarity from edge to edge of the field of view (sometimes called a "flat field").
Porro-prisms have objective lenses that are spaced farther apart than the eyepieces. Porro-prisms are bulky but usually perform better and cost less then roof-prisms. Porros yield a better three-dimensional image. Roof-prisms dominate the consumer market. The objective lenses line up directly with the eyepieces, resulting in a streamlined, compact and lightweight binocular. But roof-prisms usually cost more and lose more light to reflection, which is a disadvantage for astronomers but not for daytime terrestrial viewing.
Prisms are located inside binoculars that are like mirrors. It is a reflective coating on glass that bends and refracts light to bring the subjects you are looking at to your eyes. The BAK-4 prism is made of a high quality glass and produces sharp images and good edge to edge sharpness. Generally, higher quality binoculars will use BAK-4 prisms in the construction process. Phase coated prisms have a coating process that enhances the resolution and contrast of images coming through the binocular and are generally applied only on more expensive binoculars.
If the prisms are made of BaK-4 Barium crown glass the exit pupils will be round and evenly illuminated. If the prisms are of BK-7 Borosilicate crown glass you will notice squarish, grey edges in the exit pupils.
Roof Prism System
In roof prism binoculars the prisms overlap closely, allowing the objective lenses to line up directly with the eyepiece. The result is a slim, streamlined shape in which the lenses and prisms that magnify and correct the image are in a straight line. An Abbe-Koenig prism is a type of reflecting prism used to invert an image (rotate it by 180°). The prism is named after Ernst Abbe and Albert Koenig. The prism is made from two glass prisms which are optically cemented together to form a symmetric, shallow V-shaped assembly.
A Schmidt-Pechan prism is a type of optical prism used to rotate an image by 180°. They are commonly used in binoculars as an image erecting system.
The prism consists of two glass prisms separated by an air-gap. Multiple total internal reflections of the light cause a vertical flipping of the image; a "roof" section of the second prism also flips the image laterally, together causing a 180° rotation of the image. The image's handedness is not changed.
Compared to the double-Porro prism or Abbe-Koenig designs, the Schmidt-Pechan is much more compact. However, the large number of reflections and glass/air transitions of the light make the prism more lossy than the other designs. Some of the surfaces must be optically coated for efficient internal reflection, since the light is incident at an angle less than the critical angle.
The multiple internal reflections also cause a polarisation-dependent phase-lag of the transmitted light, in a manner similar to a Fresnel rhomb. This must be suppressed by special phase-correction coatings to avoid unwanted interference effects on the image.
Phase-corrected prism coating and dielectric prism coating are recent (in 2005) effective techniques for reducing reflections. Light reflected from one roof surface is 1/2 of a wavelength shifted from the light hitting the other roof surface, sometimes referred to as "out of phase" or "phase shift". Although the light waves are subsequently forced back together when they reach the viewer's eye, this phenomenon results in reduced contrast and image resolution. This effect does not occur in Porro prism designs.
Porro Prism System
In porro prism binoculars the objective or front lens is offset from the eyepiece. Porro prism binoculars provide greater depth perception and generally offer a wider field of view. A Porro prism binocular will inherently produce an intrinsically brighter image than a roof prism binocular of the same magnification, objective size, and optical quality, as less light is absorbed along the optical path. However, as of 2005, the optical quality of the best roof-prism binoculars with up-to-date coating processes as used in Schmidt-Pechan models is comparable with the best Porro glasses.
Of course, you will want to consider the following factors:
* Price: pay for the features you get and dont pay for the ones you dont want
* Coatings: reduce glare, and protect against water and other potential damage
* Quality of construction: the grade of glass, the quality of the prisms and the material used in the barrel are just a few of the factors to be mindful of. BK-7 borosilicate flint glass are of lower quality; for optimal optics, make sure you have BaK-4 barium crown glass prisms
* Long eye relief is a worthwhile feature for eyeglass wearers.
Chromatic aberration is caused because light of different colours does not bend the same amount when passing from glass to air. Blue light, for example, will not focus to the same plane as red light. The effect can create a "ring" of colour around point sources of light, and results in a general blurriness to the image. Chromatic aberration is minimised by using an achromatic doublet (or achromat) in which two materials (
usually crown and flint glass
) with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction.
The problem can be reduced in several ways. One method is to apply a thin film to the eyepiece element that corrects. The more traditional approach, however, is to eliminate the aberration by using multiple elements of different types of glass and curvature.
An apochromat is a lens or lens system which has even better correction of chromatic aberration, combined with improved correction of spherical aberration. Apochromatic lenses are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. Apochromats are much more expensive than achromats.
Antireflection coatings lens coatings reduce the amount of light reflecting off of the lens and allow more light to pass through. Without coatings, up to 50% of the light entering the binoculars is lost to reflections from the many glass surfaces within. The more expensive brands will have multiple coatings on ALL the lenses which will help to give the brightest and clearest images.
The most used and least expensive coating is a single-layer of magnesium fluoride (MgF
), but there are also modern broadband multicoatings. Magnesium fluoride, which is also hard-wearing, reduces reflections from 5% to 1%. Modern lens coatings , such as zinc sulphide or titanium dioxide, consist of complex multi-layers and reflect only 0.25% or less to yield an image with maximum brightness and natural colours.
To save money, some optics manufacturers coat only some of the air-to-glass surfaces.
Common Antireflection coatings often look somewhat bluish (since they reflect slightly more blue light than other visible wavelengths), though green and pink tinged coatings are also used.
Binoculars that have so-called "ruby" coatings intended to reduce glare in bright light and improve the contrast between brown and green objects. Avoid any binocular that uses these coatings, it will perform poorly for astronomical use.
But generally, as far as binoculars go, better coatings mean higher cost!
Coated (C) - One or more surfaces are coated.
Fully-Coated (FC) - All air-to-glass surfaces are coated but plastic lenses may not be.
Multi-Coated (MC) - One or more surfaces are coated.
Fully Multi-Coated (FMC) - All air-to-glass surfaces are coated.
Binoculars come with two types of focusing mechanisms. Most people opt for the centre-focus model, which uses a centrally mounted wheel to adjust both eyepieces at once. There is also a separate adjustment for the right eyepiece, which helps to correct for any difference in near or farsightedness between your eyes.
The second focusing system uses individually focused eyepieces and has no centrally located focusing mechanism. Even though focusing is slower compared to the previous model, binoculars that use individual focus tend to be more rugged and less prone to moisture infiltrations.
Hermetically sealed binoculars filled with dry gas (usually nitrogen) will not be susceptible to clouding due to condensation at low temperatures; this will also help to prevent mildew, although air may leak in over a period of years if the binoculars are not overhauled.
Modern binoculars have a hinged construction that enables the distance between eyepieces to be adjusted to accommodate viewers with different eye separation. This adjustment feature is lacking on many older binoculars.
Because binoculars are basically two small telescopes mounted side by side, an error in collimation (
optical and mechanical alignment
) can lead to numerous problems including eyestrain and double-images. For most binoculars collimation problems are not immediately obvious when you first pick the instrument up and view through it. If after using the binoculars for several minutes your eyes feel uncomfortable as they compensate for the barrel misalignment, most probably the binoculars are out of collimation, which means that the two barrels don't point in the same direction. This is a serious problem, and you shouldn't buy those binoculars.
Keep your binoculars in their protective carrying case to prevent dust and grit getting into the mechanism. This can clog up the oils and make the controls grind which could eventually seize up. Also avoid knocking them-prisms are often mounted lightly and a bump can misalign one, causing double vision. For the best view keep the front and rear lens surfaces clean with optical cleaning fluid and a soft lint free cloth.
The following gives a guide as to what to expect to pay:
8x30 binoculars from £30
7x50 binoculars from £49
10x50 binoculars from £49
20x80 binoculars from £450
With the 20x80 binoculars you will need a tripod stand, which may cost around £50.
May 5 16:31 2008
Carl Zeiss Jena 7x50 Binoctar.
These binoculars were produced by the Eastern Germany Carl Zeiss company (DDR).
The exit pupil size is perfect for astronomy and low-light nature watching.
The rubber-armoured Porro-prism design will help protect the glasses.
The multi-coated BaK-4 lenses produce clean luminous, high contrast images with impressive colour rendering, and are ideal for astronomy.
The individual eyepiece focusing is ideal for astronomy work; where the protection from dust and moisture is of particular importance.
FOV 128m at 1000m (7.3 degrees)
Exit Pupil [mm]: 7.14
Lens coating: T3M Multi-coated
(251kb, 1280 x 960)
(490b, 1280 x 958)
(193kb, 1280 x 652)
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