Hints, Tips, Links, and Information
1) PUBLICATIONS AND OTHER RESOURCES FOR BEGINNING ASTRONOMERS
2) TELESCOPE BUYING TIPS
If you don't know the terms in bold print, you can look them up in the definitions that follow.
DEFINITIONS:
aperture: the diameter of the main mirror of reflectors or the front lens on a refractor or catadiooptric, given in inches, millimeters, or centimeters (note: 1 inch = 2.5 centimeters; 1 centimeter = 10 millimeters).
catadioptric: uses lenses and mirrors. More expensive than similar sized reflectors.
eyepiece or ocular: a system of lenses at the eye position of a telescope. Astronomical telescopes have the provision of changing eyepieces to alter magnification. When purchasing eyepieces, be sure to buy the diameter that matches the size of your drawtube. Drawtubes of at least 1 ¼ inches diameter are preferred over smaller 0.965-inch drawtubes.
finderscope: a small wide-field crosshair telescope attached to the main telescope. It's an aiming device.
focal length: the distance from a lens or mirror in which light from distant objects comes to a focus. For example, the focal length of a magnifying glass can be determined by concentrating sunlight through the lens onto a sheet of paper. When the sun's light is concentrated into the smallest point possible, the distance between the paper and the lens is the lens' focal length. Eyepieces also have an associated focal length.
f-ratio: this is a proportion of the telescope's focal length divided by the telescope's aperture. An f/8 telescope's focal length is 8 times its aperture. When calculating f-ratio, be sure to use the same units of measurement for aperture and focal length.
GO-TO telescopes: these use computer control to automatically slew to celestial objects. They are expensive for the size, and a larger non-computerized telescope could be had for the same money. However, if you feel that you want a GO-TO telescope, don't cheat yourself by not learning the night sky. This is half the experience!
light gathering power: ability of a telescope to show faint objects. This is more important than magnification. It is primarily determined by the telescope's aperture. Doubling the aperture increases light gathering power fourfold.
magnification (or magnifying power): the degree of enlargement provided by a telescope or pair of binoculars. The magnification of astronomical telescopes can be varied up or down by changing the eyepiece. Binoculars are of fixed magnification. A magnification of 30x provides an enlargement of 30 times the diameter of the object when viewed with the naked eye.
mount: a mechanical device supporting a telescope. Altazimuth are simple mounts that are easy for the beginner to use. Equatorial mounts are more sophisticated and can track objects with only one motion, rather than two (as required by the altazimuth. However, equatorials require more set-up time and have a greater learning curve. Equatorials are often equipped with electric drives that keep objects in the field of view without manual tracking.
reflector: a telescope using a curved mirror as the primary light collector (a good dollar value).
refractor: a telescope using a lens as the primary light collector. This is the instrument most people think of as a telescope. It is more expensive than similar sized telescopes of other designs, but well made refractors are great performers.
resolution: the ability of a telescope to show fine detail on the moon or planets, or to resolve close double stars or tight star clusters. Doubling the aperture doubles the resolving power.
tripods and piers: these stands support the telescope's mount. A tripod is a wooden or metal stand with three legs. A pier is usually a metal pipe with legs. Most beginner scopes have tripods, which are more easily portable, but should be of sturdy construction.
- Advertisements that emphasize the magnifying power (magnification) of their telescope as the main selling point usually represent inferior products. Telescopes seldom give good views at powers exceeding 50x per inch of aperture. This means that a telescope of 3 inches (75 millimeters) aperture has a maximum useful magnification of 150x. For most views with any telescope, except for views of the brighter planets and the moon, magnifications from 30 to 50x give the most pleasing view.
- Look for a product with a good mount and a sturdy pier or tripod. If these components are undersized, the telescope will be too shaky to give satisfactory views. Remember that a telescope working at even a modest magnification of 30x not only magnifies the celestial object 30 times, but magnifies vibrations in the mount equally.
- The aperture of a telescope is its most important feature. For satisfactory views, a refractor should have a minimum aperture of 70 mm (2.75 inches); a reflector should have an aperture of at least 100 millimeters (4 inches); and a catadioptric should have an aperture of at least 90 millimeters (3.5 inches). Larger telescopes than these will show even more, but don't buy a telescope that's too bulky for your physical ability or living situation. Smaller telescopes will lack sufficient light gathering power (often called light grasp) and resolving power (or resolution) to give good views of a wide variety of celestial objects.
- Long f-ratios, f/8 or more, are better for the planets and moon; short f-ratios, f/6 or less, are best for wide field views of star clusters, nebulas, and galaxies (deep sky objects); moderate f-ratios, f/6 to f/8, are good general purpose telescopes.
- Look for quality accessories with the telescope. Avoid cheap eyepieces (sometimes called oculars) such as Hygenian and Ramsden. Eyepieces such as Kellners, achromatic Ramsdens, Edmund RKEs, orthoscopics, and Plossls are good eyepiece designs often supplied as package deals with new telescopes. The Plossl design is perhaps the best type for the money.
- The telescope should be provided with a finderscope with a magnification of 6 to 8x and a minimum aperture of 30 millimeters (i.e., a 6x30 or 8x50 finderscope).
- Buyers of computerized "GO-TO" telescopes can get by with a 5x finderscope; however, GO-TO technology should not substitute for learning ones way around the night sky. You're cheating yourself of a rewarding experience if you don't at least learn the major stars and constellations.
- Consider your physical ability and living situation before purchasing. How much weight can you carry? Will you have to lug the instrument up stairs? Will you be transporting the telescope to a dark location in the country for better views? Large telescopes are great, but only as long as they're not collecting dust in a closet because they're too large to use often.
- Consider stargazing with a pair of 7 to 10x binoculars before graduating to a telescope. Although any binocular will show you more than your naked eye, choose a pair with front lenses of at least 35 millimeters aperture. Little can be seen with binoculars over 10x magnification unless they are mounted on a tripod. Examples of good "stargazing" binoculars are 7x35, 8x40 and 7x50.
- Whether you use a telescope or binoculars, you need good reference materials to help you navigate. The lists of references on this website are a good place to start.
- Do your homework before you buy. Members of your local astronomy club can be a great help.
DEFINITIONS:
aperture: the diameter of the main mirror of reflectors or the front lens on a refractor or catadiooptric, given in inches, millimeters, or centimeters (note: 1 inch = 2.5 centimeters; 1 centimeter = 10 millimeters).
catadioptric: uses lenses and mirrors. More expensive than similar sized reflectors.
eyepiece or ocular: a system of lenses at the eye position of a telescope. Astronomical telescopes have the provision of changing eyepieces to alter magnification. When purchasing eyepieces, be sure to buy the diameter that matches the size of your drawtube. Drawtubes of at least 1 ¼ inches diameter are preferred over smaller 0.965-inch drawtubes.
finderscope: a small wide-field crosshair telescope attached to the main telescope. It's an aiming device.
focal length: the distance from a lens or mirror in which light from distant objects comes to a focus. For example, the focal length of a magnifying glass can be determined by concentrating sunlight through the lens onto a sheet of paper. When the sun's light is concentrated into the smallest point possible, the distance between the paper and the lens is the lens' focal length. Eyepieces also have an associated focal length.
f-ratio: this is a proportion of the telescope's focal length divided by the telescope's aperture. An f/8 telescope's focal length is 8 times its aperture. When calculating f-ratio, be sure to use the same units of measurement for aperture and focal length.
GO-TO telescopes: these use computer control to automatically slew to celestial objects. They are expensive for the size, and a larger non-computerized telescope could be had for the same money. However, if you feel that you want a GO-TO telescope, don't cheat yourself by not learning the night sky. This is half the experience!
light gathering power: ability of a telescope to show faint objects. This is more important than magnification. It is primarily determined by the telescope's aperture. Doubling the aperture increases light gathering power fourfold.
magnification (or magnifying power): the degree of enlargement provided by a telescope or pair of binoculars. The magnification of astronomical telescopes can be varied up or down by changing the eyepiece. Binoculars are of fixed magnification. A magnification of 30x provides an enlargement of 30 times the diameter of the object when viewed with the naked eye.
mount: a mechanical device supporting a telescope. Altazimuth are simple mounts that are easy for the beginner to use. Equatorial mounts are more sophisticated and can track objects with only one motion, rather than two (as required by the altazimuth. However, equatorials require more set-up time and have a greater learning curve. Equatorials are often equipped with electric drives that keep objects in the field of view without manual tracking.
reflector: a telescope using a curved mirror as the primary light collector (a good dollar value).
refractor: a telescope using a lens as the primary light collector. This is the instrument most people think of as a telescope. It is more expensive than similar sized telescopes of other designs, but well made refractors are great performers.
resolution: the ability of a telescope to show fine detail on the moon or planets, or to resolve close double stars or tight star clusters. Doubling the aperture doubles the resolving power.
tripods and piers: these stands support the telescope's mount. A tripod is a wooden or metal stand with three legs. A pier is usually a metal pipe with legs. Most beginner scopes have tripods, which are more easily portable, but should be of sturdy construction.
3) CLEANING OF OPTICAL SURFACES
DO NOT!
DO:
EQUIPMENT LIST
CLEANING PROCEDURE - SMALL REFRACTOR OR CATADIOPTRIC
CLEANING PROCEDURE -- SMALL REFLECTOR
FORMULA FOR OPTICAL CLEANING FLUID
4) USEFUL ASTRONOMY LINKSwww.scopereviews.com Hands-on reviews of many different telescopes. www.darksky.org Home page of the International Dark-Sky Association (IDA), the leading organization fighting for more efficient and less polluting outdoor lighting. http://www.polcouncil.org Home page of the Pennsylvania Outdoor Lighting Council, a state based organization sharing similar goals with the IDA. http://www.bbastrodesigns.com/collimateF3/CollimatingF3Telescope.html Help with telescope collimation (adjustment). www.skyandtelescope.com The home page of Sky and Telescope magazine. This page contains a wealth of information for the budding astronomer. www2.astronomy.com The home page of Astronomy Magazine, an excellent non-technical publication, the best bet for beginners. The web page is one of the best sources of information. aa.usno.navy.mil/ An excellent source of free astronomical data, such as the rising and setting of celestial objects. www.oras.org The home page of ORAS. https://3ap.org The home page of the Amateur Astronomers Association of Pittsburgh, a large organization with 500+ membership. http://www.cpoclub.org The home page of the Central Pennsylvania Observers based in State College, PA. http://www.martzobservatory.org The home page of the Marshal Martz Memorial Astronomical Society, based in Jamestown, NY. http://www.telescope.com Home page of Orion Telescope and Binocular Center. An excellent mail order dealer, offering excellent support and advice. http://www.highpointscientific.com Home page of High Point Scientific, a first-class on-line astronomy equipment business favored by several ORAS members. http://www.homeadvisor.com/r/?p=25176&preview=true This page has some excellent information on space and astronomy for beginners. It was recommended by one of our readers! Click here for additional links! 5) USEFUL TELESCOPE FORMULAS
MAGNITUDE LIMIT:
8.8 + 5(log A) = ML where A = telescope aperture in inches ML = limiting magnitude RESOLVING POWER: R = 4.56/A where A = telescope aperture in inches R = telescope resolving power in seconds of arc MAGNIFICATION: MAGNIFICATION = TELESCOPE FOCAL LENGTH/ EYEPIECE FOCAL LENGTH Units can be in inches or millimeters, but they must be consistent. MAXIMUM USEFUL MAGNIFICATION: Mu = 50(A) where Mu = maximum useful magnification A= telescope aperture in inches MINIMUM USEFUL MAGNIFICATION: Mm = A/0.28 where Mm = minimum useful magnification A = telescope aperture in inches TELESCOPE FIELD OF VIEW: FT = FA/M where FT = field of view of telescope in degrees FA = apparent field of view of eyepiece in degrees M = magnification of telescope and eyepiece combination |
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