GETTING STARTED IN ASTRONOMY
So, you’ve caught the “Astronomy bug”! Your imagination has been sparked, and you want a front row seat on the Universe. You want to join the ranks of telescope-equiped amateur astronomers!
Wait! Before you go out and spend your savings on optics that might hinder more than encourage your interest, consider the following: some preparation and information will assure that your telescope purchase really will reward you with easy access to the wonders of space.
A telescope is not like a television! Plug a TV in, turn it on, and everyone looking at it sees exactly the same thing. Aim a telescope at a globular star cluster and some people will be transported to the edges of our galaxy in an enthralling moment! Others will look through the same scope and see nothing, or maybe just a smudge. The difference is in the readiness of the observer’s eyes and mind to see and understand what is there!
Stay away from Department stores and other places where people with no telescope experience sell scopes in colourful boxes claiming huge magnification numbers (675 X for $149.95 etc!). Go instead to meetings and public observing events (Star Parties) put on by your local astronomy club (you will find listings on this site or at http://www.skypub.com). There you will find many friendly people with a lot of knowledge and skills to pass on. You’ll get to look through a variety of telescopes as well.
Go to your public library and your bookstore and look for astronomy books with an observing slant. I recommend particularly books by Terence Dickinson and David Levy. Go to your local newsstand and buy copies of Sky and Telescope, Sky News, and Astronomy magazines. You will find excellent books and book catalogs available through the publishers of these magazines.
Get a planisphere (rotating star-finder) and learn how to use it to identify stars and constellations up in the Sky at any time you may be looking. Redden your flashlight with red plastic or red nail polish (or even put it in a brown paper bag) so it no longer dazzles your eyes and spoils your night vision as soon as you look at your star-finder!
The books and magazines mentioned above also have monthly or seasonal star charts and lots of ideas for things to look for in the Sky. Look for something in the Sky every clear night! Cultivate an observer’s attitude. The Sky holds frequent surprises for those who look up!
DOUBLE YOUR PLEASURE!
“But”, you say, “What about the Scope!”. Be careful not to hurry past the the shortcut to stargazing satisfaction! The Sky unlocks its secrets to those who develop a friendship with it. This is not a realm to rush through on the Interstate Highway! If you’ve followed me to here its time to glue your eyes to a pair of binoculars. Take them out as far away from the glare of white lights and the glow of the city as you can (or use them in your backyard) and you’ll see endless vistas of starry patterns, nebulae, star clusters, moon craters, planets, and even galaxies. The information sources mentioned above will give you dozens of specific targets, but take time to just “drift” from starfield to starfield.
Next get a “Star Atlas” such as Sky Atlas 2000, which divides the sky up into a number of charts each showing the faint star patterns visible in binoculars but too faint to see without them. Use the charts to find star clusters, nebulae, and even galaxies! In this way you gain the skills that will soon allow you to find almost anything you want in your telescope.
Good general pupose binoculars that are also good for Astronomy include 7 x 35’s, 7 x 50’s, and 10 x 50’s. The first number is the magnification and the second is the diameter of the front (“objective”) lenses in millimeters. Fifty millimeters is about 2 inches. The first and the last binocular sizes above give 5 power for each inch of aperture (diameter). This is considered to be “low power”. Low power is very useful because it allows you to see a large patch of sky all at once. One of the problems with too much power in a telescope is that you will be getting a tiny “keyhole” glimpse of the sky, and it will be hard to get the object you are looking for into that minuscule window.
To picture what magnification means, note that the moon appears about the same size as an aspirin tablet held at arm’s length. Now imagine holding a golf ball at arm’s length. That’s about how big the moon will appear in 7 power binoculars (The diameter of a golf ball is about the length of 7 aspirins).
Next imagine a beach ball balanced on your outstretched hand. That would be about the size of the moon seen at 80 power, enough magnification to see hundreds of craters and jagged mountain peaks as well!
With binoculars you will even be able to identify the planets Venus, Jupiter, and Saturn. Venus will sometimes look like a dazzling dot and sometimes like miniature crescent moon. Jupiter looks like a tiny bright pinhead, and Saturn like a “-“.
When I use my telescope I always have my binoculars on the ready. It’s the best way to pinpoint a target to find with the telescope. By being a frequent observer and by exploring the sky with binoculars you are assuring that your first views through your own telescope will be great ones!
APERTURE, NOT MAGNIFICATION
Avoid scopes that are advertised on the basis of a high magnification number. These are designed only to generate quick profits for their distributor, not to help eager eyes see more! Real astronomical telescopes are described by the type and size of their light gathering optics, not by magnification.
The diameter of the lens or mirror which collects starlight and forms it into images is called the “aperture” of the telescope. Larger apertures make fainter objects visible and smaller details distinguishable. Forinstance a reflecting telescope with a main (primary) mirror 6 inches in diameter is said to have an aperture of 6-inches or 150 mm.
The magnification of a telescope is not a fixed quantity like the engine size of a motorcycle. Magnification can be changed in a moment to suit conditions and your observing aims simply by slipping a different eyepiece into the focuser. As mentioned above a telescope can be used at low power to see a wide patch of sky, making objects easy to find and fit into the eyepiece view. Then a higher power eyepiece can be used to “zoom in” on fine details.
But there are limits. We are looking through an atmosphere that is nearly always in turbulent motion, and the view often shakes and shimmers through no fault of the telescope. When these motions smudge the images seen in the scope at higher power, more magnification will probably show no additional detail. Using excessive magnification will then produce a dim blurry image.
In addition, “blowing up” an image with high magnification spreads the light over a larger area, making the object lose its sparkle. Even on the best of nights the highest useful magnification is likely to lie between 30 power per inch of aperture and 50 power per inch. Thus the top power for a 6-inch telescope would probably be between 180 X and 300 X. (I have used the word “probably” because different eyes may have different needs!).
In contrast, a Christmas catalog I saw recently has a 60 mm ( 2 3/8″) objective and promises 575 power! This is a totally bogus 244 power per inch of aperture!
FOCAL LENGTH AND FOCAL RATIO
Stand near a window and hold up a pair of reading glasses. Each lens will form an image of the outdoor scene on a white card held in your other hand at the right distance from the glasses. The distance from one of the lenses to the card when you are forming the image of a distant object is the focal length of lens. Telescopes have a focal length too, and this will usually be similar to the length of the tube. (exceptions will be mentioned below.)
Divide the focal length of the lens by its diameter (aperture), and you get the focal ratio, or “f-number”. For example if you do the above experiment with “2 diopter” reading glasses the focal length of the lenses will be about 20 inches. If the lenses are 2 inches in diameter their focal ratio would be f/10 (20″ div.by 2″ = 10). You could actually use one of the lenses from the glasses to make a telescope, and we would say it was an “f/10 refracting telescope”.
You will also see “f” numbers if you look at the front of a camera lens. On a 35 mm camera you will see numbers like “f/2.8”, “f/1.8” or “f/1.4”. Smaller numbers mean shorter exposure times, which is important for action shots in low light conditions. Hence camera lenses marked “f/2.8” or “f/1.4” are often described as “fast”. By analogy with cameras, telescopes having focal ratios of f/6, f/5, or f/4.5 are called “fast”. But this labelling has no practical importance other than indicating a relatively “stubby” tube unless the telescope is being used as a sky camera or “astrographic” instrument.
Telescopes with focal ratios of f/8, f/10 or greater are often described as “long focus” scopes.
OPTICAL SYSTEMS FOR TELESCOPES
The light-gathering image-forming “objective” of a telescope can be a mirror, a lens, or a combination of the two. Reflecting telescopes have gently-curved concave objective mirror at the bottom of the tube. Refractors have an objective lens at the front end of the tube like a spy-glass.
DIAGRAM 2 – MIRROR-LENS SCOPES (Schmidt-Cassegrain or Maksutov Telescopes)
Telescopes such as Schmidt-Cassegrain and Maksutov Scopes actually have a lens (called a corrector) at the front of the tube and a concave mirror at the back. The advantage of this more- expensive optical system is compactness. In apertures up to 8 inches they can be carried in suitcase- sized boxes, not including the tripod. Many Maksutov telescopes show surprisingly detailed planetary images for such compact instruments, but for crisp fine details Schmidt Cassegrain Telescopes (SCT’s) are often outperformed by simpler longer-tubed telescopes. This is because most SCT’s have relatively large “secondary” mirrors interfering with incoming light.
DIAGRAM 3 – NEWTONIAN TELESCOPE
The simplest form of reflecting telescope was invented by Isaac Newton, and is called the “Newtonian Reflecting Telescope”. In addition to the concave primary mirror at the bottom of the tube it has a small flat tilted mirror (called the “secondary”) near the top of the tube to direct the focussed light sideways to the eyepiece. This simple optical system offers the greatest performance for a given price. Long focus reflectors (example f/10, focal length 10 times the mirror diameter) costing as little as $229.00 US often outperform refractors and “mirror-lens” scopes costing many times as much. Budget wise, bigger Newtonians are available for a given price, leading to brighter images of faint objects as well.
Short-focus (f/6 to f/4.5) Newtonian Reflecting Telescopes are conveniently compact, but to give sharp detailed images their mirrors need to have a parabolic curve polished into their front surface. They also need to be adjusted (collimated) to much tighter standards. Thus high performance short focus telescopes tend to be more expensive. Even f/8 reflecting telescopes need parabolic mirrors to deliver the sharpest possible images. Before choosing a 4 1/4-inch f/8 to 6-inch f/8 reflector, make sure the scope will have a parabolic mirror! (Longer focus f/10 reflectors have the advantage here, not requiring parabolization for sharp detailed images.)
Refracting telescopes cost MUCH more for a given aperture, but can deliver very sharp images because there is no secondary in the way of the incoming light. However objective lenses, except on very expensive refractors, suffer from the tendency of light to split into rainbow colours when passing through glass. This is why Newton invented his telescope! Very long focus refractors (f/15 usually) have this tendency under control well enough to display fine detailed images. But to get sharp “colour-free” images with a conveniently short tube a refractor needs to be of the very expensive “Apochromatic” type.
For example, an excellent 6-inch Newtonian Reflector, which shows a Universe of fascinating planet and starscapes, costs less than $400.00 US. A 6-inch f/15 refractor on a good mount would cost about 4 times that price, and a first rate 6-inch “Apo” would set you back between five and ten thousand dollars! Frequently the $400.00 Newtonian would show you just as much (and be much easier to set up to boot!)
Many telescope mounts allow you to point your scope anywhere in the sky by swinging it from side to side and up and down. This is the easiest and most direct way to get the scope aimed where you want, and it also provides the most effective way to keep the scope steadily aimed at your chosen sky scene. This type of mount is sometimes called “altazimuth”. The most effective variety of altazimuth mount for a reflecting telescope is named after its inventor, John Dobson (hence the term “Dobsonian” mount). Dobsonian mounts use teflon bearings mounted on a strong boxy structure called a “rocker”, which turns on a sort of “lazy susan” base. Since the rocker is a strong vertical structure it supports the scope very steadily, and small gentle pushes by hand keep the scope on effortlessly track.
Other telescope mounts tilt their main turning axis until it is aligned parallel to Earth’s axis. The main turning axis of the mount is then called a “Polar Axis”, and the mount is called “Equatorial”. When this alignment is done by the user a slow turn of the polar axis (perhaps by a “clock drive” motor) will allow the scope to automatically follow the stars by countering the rotation of the Earth.
This tracking of the stars is necessary for long-exposure photography or imaging of faint stellar objects. It is not the most effective way to enjoy viewing sky wonders by eye. Tilting one of the telescopes axes without opening the door to backlash, shakes, and vibrations requires costly engineering and the highest quality manufacturing. Good equatorial mounts are expensive, costing hundreds or thousands of dollars more than your are likely planning on. Inexpensive ones deliver frustratingly little observing ease for the effort required to set them up properly. The hallmark of a cheap equatorial mount is an image that “zig-zags” or even disappears when you put your hand on the focus adjustor!
ACTION PLAN FOR SCOPE BUYING:
Want a scope that won’t break the budget and will reward your investment many times over? Realize that there is no single telescope which will forever satisfy your every astronomical aim, now and in the future!
The scope for NOW is the one you can take out to your backyard (or put in the back seat of your car) easily and set up quickly, with only a few moments of adjustment allowing rewarding clear steady views of Moon mountains and craters, Saturn’s rings and other planetary details, star clusters, and dark sky views of nebulae and galaxies.
For all the reasons explained above the scope that best fits this description is a Dobsonian 6- inch f/8 reflector with a parabolic mirror (See the end of this article). If you are on a tighter budget consider a 4 1/4-inch f/10 Newtonian Reflector on a Dobsonian mount.
The wealth of personal discoveries you will make with THIS high-quality always-ready scope will allow you find out what kind of astronomical experience you enjoy most! You are now equipped to plan your eventual “ADVANCED” telescope purchase. If you are excited about astrophotography through a telescope you will now want to evaluate high-quality (expensive!) equatorial mounts. If you are interested in computers you will want to explore interfacing of telescopes to computers and electronic astronomical imaging. If tracking down the faintest of nebulae and galaxies is your ambition you will want to consider large aperture newtonians or schmidt-cass scopes. In short you are now ready to plan purchases of more specialized and expensive equipment knowing that you will be making rewarding investments!
BE A SCOPE WIZARD! … A SHORTCUT
The people who use telescopes with the most skill, success, and pride are those who have made their own scope. This used to mean months of planning and study and a year or more of work. NOT ANY MORE! Now anyone can enjoy the thrill of seeing the Universe with the scope they built just hours before! With Stargazer Steve’s Deluxe Reflector Kits you open the all-inclusive box after lunch, follow the step-by-step video and illustrated print manual learning as you go, and your scope is ready when night falls!
Sky and Telescope Magazine (Dec/99) has chosen Stargazer Steve’s 6-inch Deluxe Reflector Kit as one of the “25 TOP HOT PRODUCTS of 1999!
Sky News (Nov-Dec/99) awarded a 5-star rating (***** out of five !) in reviewing this telescope!
THIS ARTICLE COPYRIGHT 1999 STEVE DODSON – MAY BE REPRODUCED IN ENTIRETY FOR PERSONAL USE ONLY. TO OBTAIN PERMISSION TO USE IN OTHER WAYS CONTACT THE WRITER AT email@example.com
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