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Your Telescope

What to look for in a telescope

Magnification: This is a property that you can safely ignore. The empirical rule for the magnification limit for a telescope is either 50x the aperture in inches or 2x the aperture in millimetres. This gives us the maximum magnification for a 60mm telescope of only 120x. Any manufacturer's claim very much outside these rules are designed to get you to part with your money.

Aperture and Light-Gathering Power: Telescopes are rated by their aperture. A 4-inch instrument has a main lens or mirror 4 inches in diameter. The larger the lens or mirror, the more light it collects, providing brighter and sharper images. An 8-inch telescope has four times the surface area, and therefore light-gathering power, of a 4-inch, making its images four times brighter.

Resolution: In theory, an 8-inch telescope can resolve twice as much detail as can a 4-inch instrument. The resolving power(RP) of a telescope can be roughly calculated as follows: The resolving power (in arc seconds) = 4.56 divided by the aperture (diameter) of the primary light gathering element in inches, or 116 divided by the same measurement in millimetres. This semi-emperically derived figure is known as the Dawes Limit (derived by William Dawes in the 19th century). So any claims by manufacturers of a specific RP is for that aperture of telescope, not only their specific model.

Focal Length: The distance between the primary light gathering optic and where the light paths cross over (the focal point). The eyepiece is set at this point. With Maksutov- and Schmidt-Cassegrains, the optical path is folded back on itself, making the tube shorter than the focal length.

Focal Ratio: The focal ratio (or "f" number) is the focal length divided by the aperture. For example, a 40" telescope with a focal length of 320" has a focal ratio of f/8. When photographing, the faster f/4 to f/6 lenses yield shorter exposure times, so they are known as fast. But when viewing is done by eyesight, image brightness depends solely upon the aperture. The "f" number has nothing to do with it, the eye cannot accumulate light, but sees only what it receives at any given instant.

Diffraction-Limited: A promise of diffraction-limited optics means aberrations in the optics are small enough that image quality is affected primarily by the wave nature of light and not by errors in the optics. This is more or less claiming that the instrument under discussion meets up with the the Rayleigh criterion, which states that the imaging process is diffraction-limited when "the first diffraction minimum of the image of one source point coincides with the maximum of another". This is the equivalent of an error of 1/4 of the wavelength of light - usually referred to as "the wavefront error". It is supposed to be the minimum standard for the affordable sort of telescope that an amateur would buy. Higher cost elescopes can do better with wavefront errors down to 1/6 or 1/8 wave. This improvement of performance comes at a high cost.

Central Obstruction: In a reflecting telescope, light is lost due to the secondary mirror sitting in the path of the incoming light. Image quailty is also lost due to diffraction from the obstruction. This obstruction should be stated in percentage terms. The central obstruction should be stated as a percentage of the diameter of the aperture. Therefore an 8" telescope with a 2 1/2" diameter secondary has a central obstruction of 31 percent. Some manufacturers, to make numbers appear smaller, give the obstruction as a percentage of area - in our example, 2.5%! It can be taken that a central obstruction (calculated by the first method) of 20 percent or lower produces a negligible effect.

Types of Primary Optics

Achromatic Refractor: Uses a doublet lens with elements made of crown and flint glass, with an f number usually in the range f/10 to f/15. Chromatic aberration is negligible.

Apochromatic Refractor: To eliminate false colour, some apochromatic refractors use triplet lenses with elements of super-extra-low dispersion glass. Others use flourite doublets or small corrector lenses near the focuser.

Newtonian Reflector: Invented by Isaac Newton in 1668, this classic design uses a concave primary mirror (preferably with a parabolic curve) with a flat secondary mirror.

Schmidt-Cassegrain: An aspherical corrector plate compensates for aberrations in the spherical mirror.

Maksutov-Cassegrain: Based on a design invented by Dmitri Maksutov in 1941, the Maksutov-Cassegrain uses a steeply curved corrector lens. All its surfaces are spherical and are therefore easy to mass-produce.

Schmidt-Newtonian: This is a hybrid design. It is usually f/4 or f/5, and combines a Schmidt corrector with Newtonian optics to reduce the off-axis coma inherent in fast Newtonians.

Maksutov-Newtonian: Usually made with an f number of f/6. This design claims viewing free of aberrations across a wide field of view at low power and refractorlike images at high power.

Dobsonian: In theory Dobsonians could be of any reflector design since the name refers to the type of mount not the optics. The base rotates and the 'scope itself tilts and the materials are chosen for their fricative qualities. Smooth enough to be aligned by hand power but with sufficient friction so that it stays in place while in use. Very popular with builders of home-made telescopes because of this simple, and economical design.

Ritchey-Chretien: A specialized Cassegrain telescope designed to eliminate coma. Both its mirrors are hyperbolic. This design is free of third-order coma and spherical aberration, although it does suffer from fifth-order coma, severe large-angle astigmatism, and field curvature problems. When focused midway between the sagittal and tangential focusing planes, stars are imaged as circles, making the RCT well suited for wide field and photographic observations. It has a very compact design for a given focal length and offers good off-axis optical performance. One final note... cheap they are not!

Adaptive and Multi Mirror optics: These are technological means to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion. It is commonly used on astronomical telescopes to remove the effects of atmospheric distortion, or astronomical seeing. Adaptive optics works by measuring the distortion and rapidly compensating for it either using deformable secondary mirrors or material with variable refractive properties and/or multiple moveable mirrors. Typically, the computers driving the system samples distortion 40,000 times a second and makes adjustments to compensate. It is not commonly appreciated that some of the ground based massive MMTs can take better photographs than Hubble. Don't even think about this technology! Even those in financial charge of governments and Scientific Institutions have conniptions at the cost.

Types of Eyepieces

Eyepiece A combination of lenses, also known as an ocular, used to magnify the image formed by the objective of a telescope. In practice, eyepieces contain at least two lenses: the field lens, which faces the objective and collects the light from it, and the eyelens, which faces the observer and magnifies the image. A field stop (a circular aperture) inside the eyepiece limits the field of view, helping to give it a sharp edge. There are various types of eyepiece design, from very simple achromatic lenses to complex eyepieces with many optical elements. Usually, the more complex an eyepiece, the more optical corrections, and the better eye relief and wider field of view. On the other hand, adding many glass surfaces dims the image and may also increase internal reflections, or "ghosting."

Huygenian eyepiece One of the earliest compounds lenses, introduced by Christiaan Huygens in 1664. It uses two plano-convex lenses, both mounted with the convex surface toward the objective. Huygenians offer good eye relief but suffer badly from spherical aberration and various other defects. They are often included with the least expensive and least effective amateur telescopes but are only useful at focal ratios faster than about f/12.

Ramsden eyepiece Another old design, invented by the English instrument-maker Jesse Ramsden (1735-1800) in 1782. In this case, the two plano-convex lens both have their flat sides facing outward. Ramsdens are less prone to spherical aberration than are Huygenians and are free of coma, but fare badly with regard to chromatic aberration and have poor eye relief and prominent ghosts. Their apparent field of view is quite small, as little as 30°, and they don't perform well at focal ratios shorter than about f/9. However, good ones are surprisingly effective at longer focal ratios for lunar, planetary, and double-star work.

Kellner eyepiece A three-element, fairly aberration-free improvement on the Ramsden thanks to the addition of an achromat for the eyelens. Sometimes called achromatic Ramsdens, Kellners have a fairly wide field of view, up to 50°, and good eye relief, but need excellent internal coatings to minimize ghosting. They work best in long focal length telescopes and offer a good balance between performance and economy.

Orthoscopic eyepiece A four-element eyepiece (typically with a simple lens nearest the eye and a cemented triplet further away), more expensive than a Kellner, considered among the best eyepieces for lunar, planetary, and double-star work, and a good choice for eyeglass wearers because of their long eye relief. Although an excellent all-round performer offering crisp images, the orthoscopic has a somewhat restricted field of view, in the 35° to 50° range. It was invented by the German physicist Ernst Abbe (1840-1905) in 1880. See also Brandon eyepiece.

Plössl eyepiece A four- or five-element design (consisting of two doublets, sometimes with an intermediate lens) offering excellent performance, especially in the 15- to 30-mm size range. Plossls provide good color correction, flat field, adequate eye relief (except for 10 mm and shorter lenses), and a moderate field of view (40° to 50°); some astigmatism is present, especially at the edge of the field.

Wide Field Eyepieces Eyepieces that offer a field of view of 60° to 80°, providing spectacular panoramas of the night sky, but at some cost, both financially and observationally. They often won't work in certain telescope drawtubes and with certain telescope designs; instruments lacking good optical performance around the edge of field, will have their worst traits emphasized; and at high magnifications with some telescopes, some ultra-wide eyepieces are unusable.

Erfle eyepiece The earliest type of wide-field eyepiece to be developed, in 1917 by Heinrich Erfle (1884-1923). Erfles uses three lenses, at least two of which are doublets to give a field of view of up to 68°. Although they suffer from ghost images and astigmatism at the edges of the field, which makes them unsuitable for planetary viewing, they are relatively inexpensive.

König eyepiece A short-focal-length version of the Erfle, giving high magnification and a field of view of up to 70°. Königs may contain anywhere from four to seven simple lenses, grouped into various combinations of cemented doublets and singlets. They are named after the German optician Albert König (1871-1946).

Nagler eyepiece A seven-element design, introduced in 1982, with a remarkable 82° field of view. Naglers come in 2-inch barrel size only, and are heavy – up to 1 kg. Although expensive, they are an excellent choice for observers with very fast focal ratio telescopes (up to f/4).