Technical Aspects of Binoculars

The amount of sky you can view through a telescope is called the real (true) field of view and is normally measured in degrees of arc (angular field). The larger the field of view, the larger the area of the sky you can see. However, many people want to k

Everthing you need to know about binoculars and telescpoping gear: 


The objective lenses of binoculars are the front lenses. The diameter of one of these lenses, given in millimeters, will be the second number describing a particular binocular. Hence, a 7x42 binocular has an objective lens of 42mm. The diameter of the lens determines the light gathering ability of the instrument, with the greater light gathering ability of a larger lens translating into greater detail and image clarity. This is especially useful in low light conditions and at night.

Doubling the size of the objective lenses quadruples the light gathering ability of the binocular. For instance, a 7x50 binocular has almost twice the light gathering ability of a 7x35 binocular and four times the light gathering ability of a 7x25 binocular. This might lead you to assume that bigger is better when it comes to the diameter size of the objective lenses, but in reality the size of the lens must be considered along with exit pupil and intended usage to determine the best binocular for you.

Magnification (Power)
Magnification is the degree to which the object being viewed is enlarged. For example, with a 7x42 binocular, the number 7 represents the ""binocular power"". A binocular of the power 7 magnifies an image to seven times the size it would be when viewed by the normal, unaided human eye. The level of power affects the brightness of an image, so the lower the power of a binocular, the brighter the image it delivers will be. In general, increasing power will reduce both field of view and eye relief.



The amount of sky you can view through a telescope is called the real (true) field of view and is normally measured in degrees of arc (angular field). The larger the field of view, the larger the area of the sky you can see. However, many people want to know what the linear field of view is. Linear field of view is calculated by multiplying the angular field of view by 52.5. For example, if the eyepiece being used with your telescope gives you an angular field of view of 0.5 degrees, then the linear field of view is 26 feet at 1000 yards (0.5x52.5). In reality, 1 degree = 52.365 feet but in the optical industry it is rounded to 52.5.


Exit Pupil

The diameter, in millimeters, of the beam of light that leaves the eyepiece of a pair of binoculars is the ""exit pupil"". The larger the exit pupil, the brighter the image obtained will be. Having a large exit pupil is advantageous under low light conditions and at night. For astronomical applications, the exit pupil of the binocular should correspond with the amount of dilation of your eye's pupil after it has adapted to the dark. This number will be between 5mm and 9mm. 9mm of dilation is the maximum amount for the human eye, and this number tends to decrease with age.

To calculate the exit pupil, divide the size of the objective lens by the magnification of the binocular. For example, the exit pupil of 7x42 binoculars is 42 ÷ 7 = 6mm.

Eye Relief - This refers to the distance, in millimeters, that an optical instrument can be held from the eye and the full field of view can still be comfortably observed. Eyeglass wearers in particular benefit from longer eye relief.

Near Focus (Binoculars) - This is the nearest distance you can focus your optics visually or photographically for close terrestrial observing.

Prisms - A binocular's prisms serve to invert the image and come in one of two basic designs: Roof or Porro prisms. By design, roof prisms are more lightweight and compact, for portability. Porro prisms are designated either BK-7 or BAK-4. Both are economical and highly effective designs. The finer glass in the BAK-4 design is of high density and virtually eliminates internal light scattering, producing sharp, well defined images.

StarBright® XLT  - An Optical System Breakthrough!

One of the most important factors in the evaluation of a Schmidt-Cassegrain telescope's optical performance is its transmission -  the percentage of incoming light that reaches the focal plane.  The design of the XLT System accomplishes two crucial objectives - 1. To develop a coating system that is optimized for visual use and, 2. To optimize the coating system and optics for CCD/Photographic imaging.

The StarBright® XLT High Performance Optical System design consists of:

1. Unique enhanced multi-layer mirror coatings  Our mirror coatings are made from percise layers of Aluminum (Al), SiO2 (Quartz), TiO2 (Titanium Dioxide), and Si02.  Reflectivity is fairly flat across the spectrum, optimizing it for both CCD imaging and visual use.

2. Multi-layer anti-reflective coatings  Made from precise layers of MgF2 (Magnesium Flouride), and HfO2 (Hafnium Dioxide), which costs nearly $2000 per kilogram.

3. High Transmission Water White glass  Our Schmidt-Cassegrain optical system with StarBright® XLT coatings use Water White glass instead of Soda Lime glass for the corrector lens.  Water White glass transmits about 90.5% without anti-reflective coatings.  That is 3.5% better transmission than uncoated Soda Lime glass.  When Water White glass is used in conjunction with StarBright® XLT's anti-reflective coatings, the average transmission reaches 97.4% - an 8% improvement!

These three components of our StarBright® XLT coatings result in one of the finest coatings available.  The peak transmission for the system is 89% at 520 nm.  The overall system transmission is 83.5% averaged over the spectrum from 400 to 750 nm.

Refractor Telescopes
A refractor telescope uses a lens as the primary. The lens at the front of the telescope bends the light passing through it until it comes to a single point called the “focal plane”. The long, thin tubes of refractor telescopes look much the same as those Galileo used centuries ago. High quality optical glass and multi-coatings provide today’s sky watchers views Galileo never dreamed of. The refractor type of telescope is very popular with individuals who want mechanical simplicity, rugged reliability and ease of use. Because the focal length is limited by the length of the tube, refractor telescopes become quite bulky and expensive beyond a four inch aperture. This limits the light gathering properties of refractor telescopes, but they are an excellent choice for beginners and those who prefer simple operation and versatility. Refractor telescopes are also a popular choice because of their unobstructed view, high contrast and good definition.

Refractor Advantages:
• Easy to set up and use
• Simple and reliable design
• Little or no maintenance
• Excellent for lunar, planetary and binary star observing especially in larger apertures
• Good for terrestrial viewing
• High contrast images with no secondary mirror or diagonal obstruction
• Color correction is good in achromatic designs and excellent in apochromatic and fluorite designs
• Sealed optical tube reduces image-degrading air currents and protects optics
• Objective lens is permanently mounted and aligned

Refractor Disadvantages
• More expensive per inch of aperture than Newtonians or Catadioptrics
• Heavier, longer and bulkier than equivalent aperture Newtonians and Catadioptrics
• The cost and size factors limit the practical maximum size primary to smaller apertures
• Some color aberration in achromatic designs (doublet)

Newtonian Reflector Telescopes
A Newtonian reflector uses a single concave mirror as its primary. Light enters the tube traveling to the mirror at the back end. Light is then “bent” forward in the tube to a single point, its focal plane. A flat mirror called a “diagonal” intercepts the light and points it out the side of the tube at right angles to the tube through the eyepiece. The eyepiece is placed there for easy viewing. Newtonian Reflector telescopes replace heavy lenses with mirrors to collect and focus the light, providing much more light gathering power for the money. You can have focal lengths up to 1000mm and still enjoy a telescope that is relatively compact and portable. Newtonian Reflector telescopes do require more care and maintenance because the primary mirror is exposed to air and dust. However, this small drawback does not hamper this type of telescope’s popularity with those who want an economical telescope that can still resolve faint, distant objects. Newtonian reflectors produce a “right-side-up image” but the image will appear rotated based on the location of the eyepiece holder in relation to the ground. Newtonian reflectors are best for astronomical use where right-side-up does not matter.

Newtonian Advantages
• Lowest cost per inch of aperture compared to Refractors and Catadioptrics since mirrors can be produced at less cost than lenses in medium to large apertures
• Reasonably compact and portable up to focal lengths of 1000mm
• Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters due to the generally fast focal ratios (f/4 to f/8)
• Adequate for lunar and planetary work
• Good for deep sky astrophotography (but not as convenient and more difficult to use than Catadioptrics)
• Free of color aberration due to the use of a primary mirror

Newtonian Disadvantages
• Generally not suited for terrestrial applications
• Slight light loss due to secondary (diagonal) obstruction when compared with Refractors

Schmidt-Cassegrain Telescopes
Schmidt-Cassegrain telescopes are in a category of optics called Catadioptrics. Catadioptrics use a combination of mirrors and lenses to “fold” (reflect) the light path and form an image. There are two popular designs: the Schmidt-Cassegrain and the Maksutov-Cassegrain. In a Schmidt-Cassegrain, the light enters through a thin aspheric Schmidt correcting lens. It then strikes the spherical primary mirror. It is reflected back up the tube and intercepted by a small secondary mirror which reflects the light out an opening in the rear of the instrument where the image is formed at the eyepiece. Catadioptrics are the most popular and most modern type of telescope optical design and are marketed throughout the world in 3.5” and larger apertures. Catadioptric telescopes combine the practical advantages of lenses and mirrors while eliminating their disadvantages. They offer the clarity and contrast of refractors with the low aberration of reflectors. Catadioptrics have an average focal ratio of f/10 which is wide enough for all types of photography. They are also easier to maintain because all optical elements are solidly mounted and rigidly collimated. Catadioptric telescopes provide the best possible combination of light gathering power, long focal length, portability and affordability.

Schmidt-Cassegrain Advantages
• Best all-purpose telescope design
• Combines the optical advantages of both lenses and mirrors while eliminating their disadvantages
• Excellent optics and razor sharp images over a wide field
• Excellent for deep sky observing and astrophotography
• Very good for lunar, planetary and binary star observing
• Excellent for terrestrial viewing and photography
• Focal ratio generally around f/10
• Closed tube design reduces image-degrading air currents
• Extremely compact and portable
• Easy to use
• Durable and virtually maintenance free
• Large apertures at reasonable cost and less expensive than equivalent aperture refractors
• Most versatile type of telescope
• More accessories available than with other types of telescopes
• Best near focus capability of any type of telescope

Schmidt-Cassegrain Disadvantages
• More expensive than Newtonians of equal aperture
•Slight light loss due to secondary mirror obstruction compared to refractors

Maksutov-Cassegrain Telescopes
The Maksutov-Cassegrain telescopes are in a category of optics called Catadioptrics. Catadioptrics use a combination of mirrors and lenses to “fold” (reflect) the light path and form an image. There are two popular designs: the Schmidt-Cassegrain and the Maksutov-Cassegrain. The Maksutov-Cassegrain is similar to the Schmidt-Cassegrain with essentially the same advantages and disadvantages. It uses a thick meniscus correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt’s which gives it slightly better resolution for planetary observing.

Advantages of Maksutov-Cassegrain Compared to Schmidt-Cassegrain
• Smaller secondary obstruction results in a slight increase in planetary detail and contrast
• Less expensive to manufacture
• Longer focal lengths resulting in higher magnifications for planetary viewing

Disadvantages of Maksutov-Cassegrain Compared to Schmidt-Cassegrain
•Slightly heavier because of the thick meniscus correcting lens
•Increased time to reach thermal stability in larger apertures over 90mm
•Longer focal lengths resulting in smaller field of views