Astronomical Binoculars and Telescopes
One need not invest thousands of dollars to enjoy examining the night sky. Giant binoculars appropriately supported on stable tripods provide fantastic views of planets and deep space objects. Both small refracting telescopes as well as larger reflectors can provide sharp, clear, stable images of the night sky and can be the basis for a lifelong hobby.
There are many sizes, shapes, and qualities of telescopes and binoculars available. An understanding of the following optical principles can help a person select the best equipment for the job and for the money.
Telescopes and binoculars collect much more light than the eye. They then magnify the resulting image with an eyepiece. Light collecting ability depends on diameter (aperture) of the large lens or mirror (primary).
Curved surfaces of lenses or mirrors focus a large amount of light by bending or reflecting incoming light rays so that they come together at the focal point. Focal distance is measured from lens or mirror to focal point.
Excessive curvature decreases performance for two reasons.
- First, obtaining high degrees of consistent mirror curvature (no spherical aberration) requires extremely careful manufacturing and exceedingly high quality materials.
- Second, lenses bend different colors of light differently, causing colors to focus at slightly different points (chromatic aberration), i.e. greater curvature, greater rainbow effect.
High curvature, high quality lenses or mirrors are very difficult and costly to manufacture, so these components are of lower quality in inexpensive telescopes. In telescopes with larger lenses or mirrors, excessive bending or diversion of light is avoided by focusing light at larger distances from the primary. Large focal lengths reduce the need for drastic curvature.
Small focal ratios (focal length divided by aperture) provide wide fields of view but low magnification and are best for extended, deep space objects. Large focal ratios provide high magnification and small fields and are best for planetary viewing. Moderate focal ratios suit both applications.
Since magnifying power is equal to focal length of primary divided by focal length of eyepiece, high magnification is achieved with very long primary focal length (large focal ratio) or very short eyepiece focal length.
Very long primary focal lengths require either long instruments or instruments that increase light path length by multiple internal reflections. The former increases cost slightly and decreases convenience, and the latter increases costs more significantly.
Eyepiece focal lengths may be reduced if eyepiece diameter is also reduced in order to avoid excessive bending or diversion of light. However, this smaller diameter reduces size of the field of view.
The following tips are useful when selecting instruments:
1. Small instruments should have low magnifying powers because they collect insufficient light for large magnification.
In small instruments, high power eyepieces produce very dim images and/or the images may suffer from drastic spherical and chromatic aberration.
If higher quality high power (expensive) eyepieces are utilized, the aberration may be reduced, but the field of view may be so small that finding and keeping objects in the field of view is very difficult and time consuming.
Also such instruments are not very forgiving of turbulent, moist air or light pollution.
The rule of thumb is that instruments are limited to about 50 to 60 power per inch of aperture, or 2.0 to 2.2 power per millimeter.
Binoculars will not typically approach this limit, but telescopes often will. Some manufacturers promote 200X-500X small telescopes with eyepieces of such short focal length that they are literally unusable for astronomical (or other) viewing. A power of 120X is more appropriate.
2. Low quality instruments which often carry a low price tag but provide unsatisfactory service and/or end up in permanent storage in the back closet typically have:
- plastic lenses
- glass lenses that are not fully multicoated or lenses that are too small (less than 1.25 inches in diameter)
- delicate tubes and finder scopes
- coarse, erratic, and inconvenient adjustment mechanisms
- inappropriate, fragile, shaky instrument mounting systems & tripods
3. Good astronomical instruments include the following:
- small aperture, moderate to large focal ratio equatorial achromatic refractors for excellent planetary viewing and photography
- giant, well supported binoculars for general sky surveys
- medium aperture equatorial reflectors for good general purpose observation and photography
- large aperture high power dobsonian reflectors or schmidt/maksutov cassegrain catadioptrics for deep space objects
- small aperture, small to moderate focal ratio equatorial apochromatic refractors for superb planetary viewing and photography and precise deep space observation.
Far more prospective enthusiasts have been driven away from astronomy by inadequate instruments than by any other factor.
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Reindeer are 5.5 to 7.5 feet long, including the tail, and weigh 200 to 400 pounds.
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