From Jones and Bartlett, a book on Stem Cells from Dr. Ann A. Kiessling and Scott C. Anderson:


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February 16, 2004

Understanding Telescopes

Introduction

The primary goals of this article are to explain how telescopes work, what the major types and categories are, and how you can best choose a telescope for yourself or a budding young astronomer in your midst. We'll look at some baseline principles, the major types of optical systems, mountings, manufacturers, and of course, what you can actually see and do with any given telescope.

I think it is important to point out some things at the outset: while astronomy can be a casual hobby, it tends not to be. It rapidly engenders passion, and when astro-geeks get together, the passion reinforces itself. The planets, stars, clusters, nebulae, and space itself are profound things, an experience waiting to happen. When it happens to you, be prepared for your life and daily perspective to be altered by the general nature of the cosmos. When you fully understand the physical scale of the stars and galaxies, and the role that light (AKA "electromagnetic radiation") plays in our understanding, you will be changed.

When you have the experience of knowing that an individual photon traveled from the sun for several hours (at the speed of light), struck an ice crystal in the rings of Saturn, and then reflected back for several more hours, passing through your telescope's optical system, through the eyepiece, and onto your retina, you will truly be awed. You have just experienced "primary source" perception, not a photograph on the Web or TV, but the real deal.

Once this bug bites you, you may need counseling to prevent you from selling everything you own in order to get a bigger telescope. You have been warned.

Rules of Engagement

Before we look at the equipment and principles in detail, there are a few widespread myths that need clarification and correction.

  • Don't buy a "department store" telescope: While the price may seem right, and the pictures on the box look compelling, small telescopes found in retail stores are of consistently poor quality. The optical components are often plastic, the mounts are wobbly and impossible to point, and there is no "upgrade path" or ability to add accessories.

  • It's not about magnification: Magnification is the most over-hyped aspect used to lure uninformed buyers. It is actually one of the least important aspects, and is something you control based on your choice of eyepieces. Your most-used magnification will be a low-power eyepiece with a wide field of view. Magnification not only magnifies the object, but also the telescope's vibrations, its optical flaws, and the rotation of the earth (making tracking difficult). Far more important than magnification is light-gathering power. This is a measure of how many photons your scope collects, and how many make it to your retina. The larger the diameter of the primary optical element (lens or mirror) of the telescope, the more light-gathering power it has, and the fainter objects you will be able to see. More on that later. Lastly, the resolution of your telescope is also more important than magnification. Resolution is a measure of the ability of your optical system to discern and separate features that are close together, such as splitting double-stars, or seeing detail in the belts of Jupiter. Although theoretical resolution is determined by the diameter of your primary optical element (lens or mirror), it turns out that the atmosphere, and even your own eye, can be far more important. More on that later, too.

  • Computer pointing is not necessary: In the past several years, advanced mounts with GPS and computer pointing and tracking systems have come of age. These systems increase the cost of the telescope significantly, and don't add much value for beginners. In fact, they can be detrimental. Part of the reward of this hobby is to develop an intimate relationship with the sky - learning the constellations, individual stars and their names, the movement of the planets, and the locations of numerous interesting deep-sky objects. For technology junkies with laptops sporting observation-planning software, the computer pointing mounts can be fun. But don't consider it a critical buying decision for a first telescope.

  • If you're just curious: Don't rush out and buy a telescope. There are many ways to become more familiar with the hobby, including local observatory public observing sessions, local star parties put on by astronomy clubs, and friends-of-friends who may already be immersed in the hobby. Check out these resources, and the Web, before deciding if you should spend hundreds of dollars obtaining a telescope.

Optical Systems

Telescopes work by focusing light from distant objects to form an image. An eyepiece then magnifies that image for your eye. There are two primary ways to form an image:refracting light through a lens, or reflecting light off of a mirror. Some optical systems employ a combination of these approaches.

Refractors (above) use a lens to focus light into an image, and typically are the long, thin tubes most people think of when they imagine a telescope.

Reflectors use a concave mirror to focus light.


Catadioptrics use a combination of lenses and mirrors to form an image.

Concepts

Before we look at various types of refractors and reflectors, there are some useful concepts that help in overall understanding:

  • Focal Length: the distance from the primary lens or mirror to the focal plane.

  • Aperture: a fancy word for the diameter of the primary.

  • Focal Ratio: the ratio of the focal length divided by the aperture of the primary. If you are familiar with camera lenses, you know about F/2. 8, F/4, F/11, etc. These are focal ratios, which, in camera lenses, are changed by adjusting the "F-stop". The F-stop is an adjustable iris within the lens that modifies the aperture (while the focal length is constant). Low F-ratios are called "fast", while large F-ratios are "slow". This is a measure of the amount of light hitting the film (or your eye) compared with the focal length.

  • Effective Focal Length: for compound optical systems (employing an active secondary element), the effective focal length of the optical system is typically much larger than the focal length of the primary. This is because the curvature of the secondary has a multiplying effect on the primary, a kind of optical "lever arm," allowing you to fit a long focal length optical system into a much shorter tube. This is an important benefit of compound optical systems like the popular Schmidt-Cassegrain.

  • Magnification: magnification is determined by dividing the focal length of the primary (or the effective focal length) by the focal length of the eyepiece.

  • Field-of-View: there are two ways to consider field of view (FOV). The actual FOV is the angular measurement of the patch of sky you can see in the eyepiece. The apparent FOV is the angular measurement of the field that your eye sees in the eyepiece. An actual field of view might be of a degree at low power, while the apparent field might be 50 degrees. Another way to calculate magnification is to divide the apparent FOV by the actual FOV. This results in exactly the same number as the focal length method described above. While apparent FOVs are readily obtained from the specs of a given eyepiece, the actual FOV is harder to come by. Most folks calculate the magnification based on focal length, and then calculate the actual FOV by taking the apparent FOV and dividing it by the magnification. For an apparent FOV of 50 degrees at 100X, the actual field is degree (about the size of the moon).

  • Collimation: collimation refers to the alignment of the overall optical system, making sure thing are properly aligned, and the light is forming an ideal focus. Good collimation is critical to getting good images in the eyepiece. Different telescope designs have various strengths and weaknesses with respect to collimation.

Types of Refractors

You may wonder, "Why are there different types of refractors?" The reason is because of an optical phenomena known as "chromatic aberration". Chromatic means color, and the aberration is due to the fact that light, when passing through certain mediums like glass, undergoes "dispersion." Dispersion is a measure of how different wavelengths of light are refracted by different amounts. The classic effect of dispersion is the action of a prism or crystal creating rainbows on the wall. As the different wavelengths of light are refracted by different amounts, the (white) light spreads out, forming the rainbow.

Unfortunately, this phenomena also affects lenses in telescopes. The earliest telescopes, used by Galileo, Cassini, and the like, were simple, single-element lens systems that suffered from chromatic aberration. The problem is that blue light comes to a focus at one location (distance from the primary), while red light comes to a focus at a different location. The result is that if you focus an object at the blue focus, it will have a red "halo" around it. The only way known at the time to reduce this problem is to make the focal length of the telescope very long, perhaps F/30 or F/60. The telescope used by Cassini when he discovered Cassini's Division in Saturn's rings was over 60 feet long!

In the 1700's, Chester Moor Hall exploited the fact that different types of glass have differing amounts of dispersion, measured by their index of refraction. He combined two lens elements, one of flint glass and another of crown, to create the first "achromatic" lens. Achromatic means "without color". By using two types of glass with different indices of refraction, and having four surface curvatures to manipulate, he produced a vast improvement in the optical performance of refractors. They no longer had to be massively long instruments, and subsequent developments over the centuries further refined the technique and performance.

While the achromat greatly reduced false color in the image, it did not eliminate it completely. The design can bring the red and blue focal planes together, but the other colors of the spectrum are still slightly out of focus. Now the problem is purple/yellow halos. Again, making the f-ratio long (like F/15 or so), helps dramatically. But that is still a long "slow" instrument. Even a 3" F/15 achromat has a tube around 50" long.

In recent decades, scientists have created exotic new types of glass that have extra-low dispersion. These glasses, known collectively as "ED", greatly reducing false color. Fluorite (which is actually a crystal) has virtually no dispersion and is used extensively in small to medium sized instruments, though at very great cost. Finally, advanced optics employing three or more elements are now available. These systems give the optical designer more freedom, having 6 surfaces to manipulate, as well as possibly three indices of refraction. The result is that more wavelengths of light can be brought to the same focus, almost completely eliminating false color. These groups of lens systems are known as "apochromats", which means, "without color, and we really mean it this time". The short hand for apochromatic lenses is "APO". Refracting telescope designs using APOs are now able to achieve low focal ratios (F/5 to F/8) with excellent optical performance and no false color; however, be prepared to spend 5 to 10 times the amount of money that would buy the same diameter achromat.

A final note about refractors is that they use a "closed-tube" design, helping to minimize convection currents (which can degrade images), and offering a system that rarely needs alignment. Unpack it, set it up, and you're ready to go.

Types of Reflectors

The main advantage of the reflecting telescope design is that it does not suffer from false color - a mirror is intrinsically achromatic. However, if you look at the above diagram for the reflector, you will note that the focal plane is directly in front of the primary mirror. If you place an eyepiece there (along with your head), it will interfere with the incoming light.

The first useful design for a reflector, and still most popular, was invented by Sir Isaac Newton, now called the "Newtonian" reflector. Newton placed a small, flat mirror at a 45 degree angle to deflect the light cone to the side of the optical tube, allowing the eyepiece and the observer to remain outside the optical path. The secondary diagonal mirror still interferes with the incoming light, but only minimally.

Herchel constructed several large reflectors that used the technique of "off-axis" focal planes, that is, diverting the light cone from the primary to one side where the eyepiece and observer could operate without interfering with the incoming light. This technique works, but only for long f-ratios, as we will see in a minute.

While the mirror conquered the color problem, it has some interesting problems of its own. Focusing parallel rays of light onto a focal plane requires a parabolic shape on the primary mirror. It turns out that parabolas are rather difficult to generate, compared to the ease of generating a sphere. Pure spherical optics suffer from the phenomena of "spherical aberration", basically, a blurring of the images in the focal plane because they are not parabolas. However, if the f-ratio of the system is sufficiently long (more than about F/11), the difference between the shape of the sphere and parabola is smaller than a fraction of the wavelength of light. Herchel built long focal length instruments that could take advantage of the ease of generating spheres, and use the off-axis design for observing. Unfortunately, this meant his telescopes were rather huge, and he spent many hours observing on a 40-foot ladder.

Several inventors created additional "compound" reflectors, employing a secondary to pass the light back through a hole in the primary mirror. Some of these types are the Gregorian, the Cassegrain, the Dall-Kirkham, and the Ritchey-Cretchien. All of these are folded optical systems, where the secondary plays an important role in creating long effective focal lengths, and differ mainly in the types of curvature employed on the primary and secondary. Some of these designs are still favored for professional observatory instruments, but very few are available commercially for the amateur astronomer today.

 


The presence of a secondary mirror is an important aspect of Newtonians, and indeed almost all reflector and catadioptric designs. First, the secondary itself obstructs a small part of the available aperture. Second, something must hold the secondary in place. In pure reflecting designs, this is usually accomplished with the use of thin vanes of metal in a cross, called a "spider". These are made as thin as possible to minimize obstruction. In catadioptric designs, the secondary is mounted on the corrector plate, and there is therefore no spider involved. The small loss of light-gathering power in these designs is of almost no consequence since inch-for-inch, reflectors are less expensive than refractors, and you can afford to purchase a slightly larger instrument. However, an effect called "diffraction" is more important than the light-gathering power concern. Diffraction is caused when light passes near edges of things on its way to the primary, causing them to bend and change direction slightly. Additionally, secondaries and spiders cause scattered light - light coming in from off-axis (i. e. , not part of the patch of sky you are viewing), and bounce off the structures and into and around the optical system. The result of diffraction and scattering is a small loss of contrast - the background sky is not as "black" as it would be in the same size refractor (of equal optical quality). Not to worry - it takes a highly seasoned observer to even notice the difference, and then it is only noticeable under ideal circumstances.

Types of Catadioptrics

One of the problems with pure reflecting optical designs is spherical aberration, as noted above. The design goal of catadioptrics is to take advantage of the ease of generating spherical optics, but fix the problem of spherical aberration with a corrector plate - a lens, subtly curved (and therefore generating minimal chromatic aberration), to correct the problem. There are two popular designs that achieve this goal:the Schmidt-Cassegrain, and the Maksutov. Schmidt-Cassegrains (or "SCs") are perhaps the most popular type of compound telescope today. However, Russian manufacturers have, in the past few years, made significant inroads with various "Mak" designs, including folded optical systems and a Newtonian variant - the "Mak-Newt". The beauty of the folded Mak design is that all the surfaces are spherical, and the secondary is formed by merely aluminizing a spot on the back of the corrector. It has a long effective focal length in a very small package, and is a preferred design for planetary observing. The Mak-Newt can achieve fairly fast focal ratios (F/5 or F/6) using spherical optics, without the need for the (by-hand) optical figuring needed for parabolas. Finally, both Mak designs result in closed-tubes, minimizing convection currents and dust gathering on the primaries.

Types of Eyepieces

There are more eyepiece designs than there are telescope designs. The most important thing to keep in mind is that the eyepiece is half of your optical system. Some eyepieces cost as much as a small telescope, and generally, they're worth it. The past two decades have witnessed the emergence of a variety of advanced eyepiece designs, and there are many considerations to make in choosing an appropriate design for your telescope, your uses, and your budget.

There are three major format standards for telescope eyepieces:0. 956", 1. 25", and 2". These refer to the eyepiece barrel diameters, and the type of focuser they fit into. The smallest 0. 965" format is most commonly found on Asian-imported beginner telescopes found in retail chains. These are generally of low quality, and when it comes time to upgrade your system, you're out of luck. Don't buy a department-store telescope!. The other two formats are the preferred system in use today by the majority of amateur astronomers worldwide. Most intermediate or advanced telescopes come with a 2" focuser and a simple adapter that also accepts 1. 25" eyepieces. If you anticipate getting a modest size telescope and taking it to dark skies to observe nebulae and clusters, you're going to want some of the better 2" eyepieces, and you should make sure you get a 2" focuser.

Eyepieces are constructed of lenses, and thus we have the same issue of chromatic aberration that we had in the case of the refractor. Eyepiece design has evolved over the centuries in step with the overall advancements of optics and glass. Modern eyepiece designs use achromats ("doublets") and more advanced designs (involving "triplets" and more), along with ED glass to maximize their performance.

One of the original optical designs came from Christian Huygens in the 1700's that used two simple (non-achromatic) lenses. Later, the Kellner employed a doublet and a simple lens. This design is still popular in low cost, beginner telescopes. The Orthoscopic was a popular design throughout the 1900's, using dual doublets, and is still favored by hard-core planetary observers.

 


In the past two decades, exploiting advances in glass, optical design, and ray-tracing software, manufactures have introduced a wide variety of new designs, most all of which try to maximize the apparent field of view (which also increases the actual field of view at a given magnification). Eyepieces before this were limited to 45 or 50 degrees apparent FOV.

The first and foremost of these is the "Nagler" (designed by Al Nagler of TeleVue), which is also dubbed the "Space-Walk" eyepiece. It provides an apparent FOV of over 82 degrees, giving the feeling of immersion. The FOV is actually larger than what your eye can take in during any one glance. The result is that you must actually "look around" to see everything in the field. Numerous other manufactures have produced similar, very wide field eyepieces in only the last five years varying from 60 degrees to 75 degrees in apparent FOV. Many of these offer an excellent value, and produce a far better experience for casual observers than the low-end designs that come bundled with most beginner telescopes (where the feeling is like looking through a wrapping paper tube).

A final consideration in eyepiece selection is "eye relief". Eye relief refers to the distance your eye must be from the lens of the eyepiece to be able to see the entire apparent FOV. One of the drawbacks of the designs such as the Kellner and Orthoscopic is limited eye relief, sometimes as small as 5mm. This doesn't usually bother folks with normal eyesight, or those who are simply near-sighted or far-sighted, because they can remove their glasses and use the telescope to focus ideally for their vision. But for some people with astigmatism, their glasses cannot be simply removed, and this introduces the need to accommodate the extra distance required by their glasses and still allow them to see the entire field. Typically, eye relief of more than 16mm is adequate for most eyeglass wearers. Many of the new, wide-field designs sport an eye relief of 20mm or more. Again, the eyepiece is half of your optical system. Make sure you match your eyepiece selection to the overall quality of your optics, and to your needs as an individual observer.

Popular Telescope Designs

Achromatic refractors are popular in the F/9 to F/15 range, with apertures from 2" to 5" at reasonable cost. There are several fast achromats (F/5) offered as "rich-field" telescopes because they give wide fields of view at low power, ideal for sweeping the Milky Way. These designs will show substantial false color on the moon and bright planets, but this will not be noticeable on deep-sky objects. To get both fast optics and no false color, you must go with an APO design at considerable cost. APOs are available from select manufactures (often with long waiting lists) in designs from F/5 to F/8, in apertures from 70mm to 5" or 6". The larger ones are very expensive (more than $10,000) and are the domain of the true fanatics in the hobby.

The popular Newtonian designs range from Richfield 4. 5" F/4's to the classic 6" F/8, probably the most popular entry-level telescope. Larger reflectors (8" F/6, 10" F/5, and so on) are gaining wide popularity because of the low cost and portability of the "Dobsonian" mount (more on that later) and increasing availability from numerous manufacturers, including kit offerings. Large Newtonians tend to have faster f-ratios to keep the tube length under control. Mak-Newts are mostly found in the F/6 range.

The Schmidt-Cassegrain is probably the most popular design with more advanced amateurs - the venerable 8" F/10 SC has been a classic for 3 decades. Most SCs are F/10, although some F/6. 3's are on the market. The trouble with fast SCs is that the secondary needs to be significantly larger, obstructing 30% or more. Overall, the F/10 design is ideal for a general mix of deep-sky observing as well as planetary and lunar.

The up-and-coming Maksutovs are generally in the F/10 to F/15 range, making them somewhat slow optical systems that tend not to be ideal for expansive Milky Way and deep sky viewing. However, they are ideal systems for planetary and lunar observing, rivaling far more expensive APOs of the same aperture.

Mounts

The telescope mount is definitely as important, if not more important, than the optical system. The best optics are worthless unless you can hold them steady, point them accurately, and make fine adjustment in the pointing without undo vibrations or backlash. There are a variety of mount designs, some optimized for portability, with others optimized for motorized/computerized tracking. There are two basic categories of mount designs:alti-azimuth, and equatorial.

Alti-Azimuth

Alti-azimuth mounts have two axes of motion:up-and-down (alti), and side-to-side (azimuth). A typical camera tripod head is a kind of alti-azimuth mount. Many small refractors on the market employ this design, and it has advantages of being convenient for terrestrial viewing as well as sky viewing. Perhaps the most important alti-azimuth mount is the "Dobsonian", almost exclusively used for medium to large Newtonian reflectors.

John Dobson is a legendary figure in the San Francisco Sidewalk Astronomer community. Twenty years ago, John was seeking a telescope design that was highly portable, and offered the ability to bring fairly large instruments (12" to 20" apertures) out to the public, literally on the sidewalks of San Francisco. His design and construction techniques created a revolution in amateur astronomy. "Big Dobs" are now one of the most popular telescope designs seen at star parties all over the world. Most telescope vendors today offer a line of Dobsonian designs. Before this, even an 10" reflector on an equatorial mount was considered an "observatory" instrument - you wouldn't generally move it around due to the heavy mount.

Generally, alti-azimuth designs are smaller and lighter than equatorial mounts offering the same level of stability. However, to track objects as the Earth rotates requires motion on two axes of the mount instead of just one as for equatorial designs. With the advent of computer control, many vendors now offer alti-azimuth mounts that can track the stars, with some caveats. A 2-axis mount suffers from "field rotation" over long periods of tracking, meaning that this design is not suitable for astrophotography.

Equatorial

Equatorial mounts also have two axes, but one of the axes (the "polar" axis) is aligned with the axis of rotation of the Earth. The other axis is called the "declination" axis, and is at right angles to the polar axis. The key benefit of this approach is that the mount can track objects in the sky by rotating only the polar axis, simplifying tracking, and avoiding the problem of field rotation. Equatorial mounts are fairly mandatory for astrophotography and imaging efforts. Equatorial mounts must also be "aligned" to the Earth's polar axis when they are set up, making their use somewhat less convenient than alti-azimuth designs.

There are several types of equatorial mounts:


German Equatorial: the most popular design for small to medium sized scopes, offering great stability, but requiring counterweights to balance the telescope around the polar axis.


Fork mounts: popular design for Schmidt-Cassegrains, with the base of the fork being the polar axis, and the arms of the fork being declination. No counterweights are needed. Fork designs can work well, but are usually large compared with the telescope; small fork designs suffer from vibration and flexure. Fork designs have difficulty pointing near the north celestial pole.

Yoke mounts: similar to the fork design, but the forks continue past the telescope, and join together above the telescope in a second polar bearing, offering improved stability over the fork, but resulting in a fairly massive structure. Yoke designs were used in many of the world's great observatories in the 1800's and 1900's.

Horseshoe mounts: a variant of the Yoke mount, but employing a very large polar bearing with a U-shaped opening at the top end, allowing the telescope tube to point to the north celestial pole. This is the design used on the Hale 200" telescope at Mt. Palomar.

Key Considerations for Mounts

A stated, the telescope's mount is a critical part of the overall system. When choosing a telescope, mounting considerations play an important role in your ability and willingness to use it, and ultimately governs the types of activities you can undertake (such as astrophotography). Below are some of the key considerations you should make.

  • Portability: assuming you don't have a backyard observatory, you will be moving and transporting your telescope out to an observing site. If you have dark skies with minimal light-pollution where you live, this may only mean moving the telescope from the closet or garage into the back yard. If you have substantial light pollution, you will want to take your scope to a dark-sky site, preferably on a mountain top somewhere. This implies transporting the scope in your car. A large, heavy mount can make this a chore. Furthermore, if astrophotography is not a main consideration, the task of setting up and aligning an equatorial mount might not be worth the effort.

  • Stability: the mount's stability is measured by the amount of vibration that the telescope experiences when "nudged", when focusing, changing eyepieces, or when a slight breeze blows. The time it takes these vibrations to dampen out should be around 1 second or so. Dobsonian mounts generally have excellent stability. German equatorials and fork mounts, when properly sized to the telescope, also exhibit good stability, although they tend to weigh more than the telescope itself by a significant margin.

  • Pointing and Tracking: to really enjoy observing, the telescope must be easy to point and aim, and the mount should allow you to carefully track the object you are observing, either by nudging the telescope, by using manual slow-motion controls, or with a tracking motor (a "clock drive"). The higher the magnification you are using (such as for planetary observations or splitting double stars), the more critical the tracking behavior of the mount. Backlash is one good measure of the mount's tracking ability:when you nudge or move the instrument a slight bit, does it stay where you aimed it, or does it move back slightly? Backlash can be a frustrating behavior of a mount, and usually means the mount is either poorly manufactured, or is too small for the telescope you have mounted.

It is difficult to get a feel for mount behavior from a catalog or web site. If you can, go to a telescope store (there aren't very many) or a high-end camera dealership that carries major-brand telescopes for a touch-and-feel evaluation. Additionally, there are many resources, message boards, and reviews of equipment available on the Web and in astronomy magazines. Perhaps the best form of research is to attend a local star party held by your neighborhood astronomy club where you can see a variety of telescopes, talk to their owners, and have the opportunity to observe through them. Help in locating these resources is provided in a later section.

Finder Scopes

Finder scopes are small telescopes or pointing devices affixed to the main tube of your telescope to aid in locating objects that are too faint to see with the naked eye (i. e. , almost all of them). The field of view of your telescope is generally quite small, about one or two diameters of the moon, depending on your eyepiece and magnification. Generally, you use a low-power, wide-field eyepiece first to locate an object (even bright ones), then change eyepieces to higher magnifications as appropriate for the given object.

Historically, finder scopes were always small refracting telescopes, similar to a binocular, offering a wide field of view (5 degrees or so) at low power (5X or 8X). In the past decade, a new approach to pointing arose using LEDs to create "red-dot finders" or illuminated reticle projection systems that project a dot or grid onto the sky at no magnification. This approach is very popular because it overcomes several use-difficulties of traditional finder scopes.

Traditional finder scopes are difficult to use for two main reasons:the image in the finder scope is inverted, making it hard to correlate the naked-eye view of the star pattern with what is seen in the finder, and also making it hard to make adjustments left/right/up/down. Additionally, getting your eye to the eyepiece of the finder can be challenging at times since it is fairly close to the main telescope tube, and in many orientations, may have you straining your neck in awkward positions. While it is true that with practice, the orientation problem can be mitigated, and it is also possible to purchase correct-image finder scopes (at increased cost), the jury of the astronomical community has clearly spoken - projection finders are easier to use and much less expensive.

Filters

The last part of the optical system to understand is the use of filters. There are a wide variety of filter types used for various observing needs. Filters are small disks mounted in aluminum cells that thread into the standard eyepiece formats (another reason for getting 1. 25" and 2" eyepiece, and not a department store telescope!). Filters fall into these main categories:

  • Color Filters: red, yellow, blue, and green filters are useful for bringing out detail and features on planets such as Mars, Jupiter, and Saturn.

  • Neutral-Density Filters: most useful for lunar observing. The moon is really bright, especially when your eyes are dark-adapted. A typical neutral-density filter cuts out 70% of the moon's light, allowing you to see details of craters and mountain ranges with less eye discomfort.

  • Light-Pollution Filters: light pollution is a pervasive problem, but there are ways to mitigate its effect on your observing enjoyment. Some communities mandate Mercury-Sodium vapor streetlights (especially near professional observatories) because these types of lights emit light at only one or two discreet wavelengths of light. Thus, it is easy to manufacture a filter that eliminates only those wavelengths, and allows the rest of the light to pass through to your retina. More generally, both wide-band and narrow-band light-pollution filters are available from major vendors that help substantially in the general case of a light-polluted metro area.

  • Nebula Filters: if your focus is on deep-sky objects and nebula, other types of filters are available that enhance the specific emission lines of these objects. Most famous is the OIII (Oxygen-3) filter available from Lumicon. This filter eliminates almost all the light at wavelengths other than the Oxygen emission lines generated by many interstellar nebulae. The Great Nebula in Orion (M42) and the Veil Nebula in Cygnus take on an entirely new aspect when viewed through an OIII filter. Other filters in this category include the H-beta filter (ideal for the Horsehead nebula), and various other more general-purpose "Deep Sky" filters that enhance contrast and bring out faint detail in many objects, including globular clusters, planetary nebula, and galaxies.

Observing

How to Observe: The most important aspect of a quality observing session is dark skies. Once you have experienced truly dark-sky observing, seeing the Milky Way appear as storm clouds (until you look closely) you will never again complain about loading up the vehicle and driving perhaps one or two hours to get to a good site. The planets and moon can generally be observed successfully from almost anywhere, but the majority of sky gems require excellent observing conditions.

Even if you are only concentrating on the moon and planets, your telescope must be set up in a dark location to minimize stray, reflected light getting into your telescope. Avoid streetlights, neighbor's halogens, and shut off all the outdoor/indoor lights you can.

Importantly, consider the dark-adaption of your own eyes. Visual purple, a chemical responsible for increasing the acuity of your eyes in low-light conditions, takes 15-30 minutes to develop, but can be eliminated immediately by one good dose of bright light. That means another 15-30 minutes of adaption time. Besides avoiding bright lights, astronomers use flashlights with deep red filters to help navigate their surroundings, view start charts, check their mount, change eyepieces, and so on. Red light does not destroy visual purple like white light does. Many vendors sell red-light flashlights for observing, but a simple piece of red cellophane over a small flashlight works just fine.

In the absence of a computer-pointed telescope (and even if you have one), obtain a quality star chart and learn the constellations. This will make it abundantly clear which objects are planets, and which are merely bright stars. It will also increase your ability to locate interesting objects using the "star hopping" method. For example, the supernova remnant known as the Crab Nebula is just a smidgen away to the north from the left horn of Taurus the Bull. Knowing the constellations is the key to unlocking the vast array of wonders available to you and your telescope.

Finally, become familiar with the technique of using "averted vision". The human retina is composed of differing sensors called cones and rods. The center of your vision, the fovea, is mainly composed of cones that are most sensitive to bright, colored light. The periphery of your vision is dominated by rods, which are more sensitive to low light levels, with less color discrimination. Averted vision concentrates the light from the eyepiece onto the more sensitive part of your retina, and results in an ability to discern fainter objects and greater detail.

What to Observe: A thorough treatment of the types and locations of the objects in the sky is far beyond the scope of this article. However, a brief introduction will be helpful in navigating the various resources that will help you find these spectacular objects.

The moon and planets are fairly obvious objects, once you know the constellations and begin to understand the movement of the planets in the "ecliptic" (the plane of our Solar system), and the progression of the sky as the seasons pass by. More difficult are the thousands of deep-sky objects - clusters, nebula, galaxies, and so on.

In the 1700's and 1800', a comet hunter named Charles Messier spent night after night searching the skies for new comets. He kept running into faint smudges that did not move from night to night, and so were not comets. For convenience, and to avoid confusion, he constructed a catalog of these faint smudges. While he did discover a handful of comets during his life, he is now famous and best remembered for his catalog of over 100 deep sky objects. These objects now bear their most-used designation stemming from the Messier catalog. "M1" is the Crab Nebula, "M42" is the great Orion nebula, "M31" is the Andromeda galaxy, etc. Finder cards and books on the Messier objects are available from many publishers, and are highly recommended if you have a modest telescope and dark sky availability. Additionally, a new "Caldwell" catalog gathers another 100 or so objects that are of similar brightness to the M-objects, but were overlooked by Messier. These are ideal starting places for the beginning deep-sky observer.

In the early half of the 20th century, professional astronomers constructed the New Galactic Catalog, or "NGC". There are approximately 10,000 objects in this catalog, the vast majority of which are accessible by modest amateur telescopes in dark skies. There are several observing guides emphasizing the most spectacular of these, and a high-quality star chart will show thousands of NGC objects.

When you understand the vast array of objects up there, from the galaxy clusters in Coma Berenices and Leo, to the emission nebula in Sagittarius, to the range of globular clusters (like the amazing M13 in Hercules) and planetary nebula (like M57, "the Ring Nebula" in Lyra), you'll begin to realize that every patch of sky contains marvelous sights, if you know how to find them.

Imaging

Like the observing section, a treatment of imaging, astrophotography, and video-astronomy is far beyond the scope of this article. However, it is important to understand some of the basics in this area to help you make an informed decision about which type of telescope and mounting system is right for you.

The simplest form of astrophotography is to capture "star trails". Set a camera with a typical lens on a tripod, point it at a star field, and expose the film for 10 to 100 minutes. As the earth rotates, the stars leave "trails" on the film depicting the rotation of the sky. These can be very beautiful in color, and especially if pointed toward Polaris (the "north star") showing how the entire sky rotates around it.

There are now several types of approaches to imaging astronomical objects, thanks to the advent of CCDs, digital cameras and camcorders, and continuing advances in film techniques. In any of these cases, an equatorial mount is required for accurate tracking. In fact, the best astrophotos taken today employ an equatorial mount several times more massive and stable than would be required for simple visual observing. This approach relates to the need for stability, breeze-resistance, tracking accuracy, and minimized vibrations. Typically, good astro-imaging also requires some kind of guiding mechanism, often meaning the use of a second guide scope on the same mount. Even if your mount has a clock drive, it is not perfect. Continual corrections are required during a long exposure to make sure the object stays in the center of the field, to an accuracy that is near the resolution limit of the telescope being used. There are both manual guiding approaches and CCD "auto-guiders" that come into play in this scenario. For film approaches, "long exposure" can mean 10 minutes to more than an hour. Excellent guiding is needed during the entire exposure. This is not for the faint-hearted.

Piggy-back photography is substantially easier, and can give excellent results. The idea is to mount a normal camera with a medium or wide-field lens on the back of a telescope. You use the telescope (with a special illuminated reticle guiding eyepiece) to track a "guide star" in the field. Meanwhile, the camera takes a 5 to 15 minute exposure of a large patch of sky at a fast setting, F/4 or better. This approach is ideal for vista shots of the Milky Way or other star fields.

Below are a few images taken with a 35mm Olympus OM-1 (still a preferred camera among astrophotographers) with exposures ranging from 25 minutes to 80 minutes on fairly standard Fuji ASA 400 film.

More advanced imaging techniques include hyper-sensitizing film to increase its sensitivity to light, using sophisticated astro-CCD cameras and auto-guiders, and performing a wide variety of post-processing techniques (such as "stacking" and "mosaic alignment") on digital images.

Upper Left:M42, The Great Nebula in Orion; Upper Right, Sagittarius Star Field (piggy back); Lower Left:the Pleiades and reflection nebula; Lower Right, M8, the Lagoon Nebula in Sagittarius.


If you like imaging, are a technophile, and have patience, the field of astro-imaging may be for you. Many amateur imagers today produce results that rival the achievements of professional observatories only a few decades ago.

Manufacturers

With the recent rise of popularity of astronomy, there are now more telescope manufacturers and retailers than ever before. The best way to find out who they are is by going down to your local, high-quality magazine rack and picking up a copy of Sky and Telescope or Astronomy magazines. From there, the Web will help you get more detail on their offerings.

There are two major manufacturers that have dominated the marketplace for the past two decades: Meade Instruments and Celestron. Each has a several lines of telescope offerings in the refractor, Dobsonian, and Schmidt-Cassegrain design categories, along with other specialty designs. Each also has comprehensive eyepiece sets, electronics options, photo and CCD accessories, and much more. Both operate through dealer networks, and pricing is set by the manufacturer. Don't expect to bargain or get a special deal other than close-outs and seconds.

Close on the heels of the big two is Orion Telescopes and Binoculars. They import and re-brand several lines of telescopes, along with reselling selected other brands. Orion is a good source for a broad selection of quality, entry-level telescopes. It is also a great source of accessories, such as eyepieces, filters, cases, star atlases, mounting accessories, and more. Sign up for the catalog on their web site - it too is full of useful, general-purpose information.

Televue (www.televue.com) is a purveyor of very high quality refractors (APOs) and premium eyepieces (Naglers and Panoptics). Takahashi (www.takahashiamerica.com) produces world-renown fluorite APO refractors. In America, Astro-Physics (www.astro-physics.com) has produced perhaps the highest quality, most sought-after APO refractors; they typically have a 2 year waiting list, and their telescopes have actually appreciated in value on the used market over the past decade.

Obsession Telescopes was the first, and still most highly rated, producer of premium large Dobsonians. Sizes range from 15" to 30". Be prepared to get a trailer to move one of these telescopes to dark skies. See www.obsessiontelescopes.com.

Resources

The Web is full of astronomical resources, from manufacturer's web sites to publishers, classifieds, and message forums. Many individual astronomers maintain sites showing their astrophotography, observing reports, equipment tips and techniques, etc. A comprehensive listing would be many pages. The best bet is to start with Google, and search on a variety of terms, such as "telescope observing techniques", "telescope reviews", "amateur telescope making", etc. Also search on "astronomy clubs" to find one in your area.

Two sites are worth mentioning explicitly. The first is the Sky & Telescope web site (www.skyandtelescope.com) which is full of great information about observing generally, what's up in the sky right now, and past equipment reviews. The second is Astromart (www.astromart.com), a classifieds site dedicated to astronomy equipment. High quality telescopes don't really wear out or have many problems due to use, and they are usually meticulously cared for. You might want to consider obtaining a used instrument, especially if the seller is in your area and you can check it out in person. This approach also works well for obtaining accessories like eyepieces, filters, cases, etc. Astromart also has discussion forums where the latest chatter on equipment and techniques is abundant.

Books

There are thousands of books on astronomy. Below are some recommendations suitable for folks new to the field. Click on the links to read more about them at Amazon. By the way, Amazon is also an authorized dealer of both Meade and Celestron telescope lines.

Telescope Power: Fantastic Activities & Easy Projects for Young Astronomers by Gregory L. Matloff. A great starter book for kids. The book introduces the amateur to the stars and telescopes with dozens of projects and activities. Learn how to read a start chart, set up and take care of your scope and pick out accessories. A great stater!

40 Nights to Knowing the Sky by Fred Schaaf. A night-by-night sky-watching primer. Schaaf provides a step-by-step program to keep amateurs involved for forty nights of entertaining skywatching. Each lesson builds on the last, and they take you from rank newbie to seasoned stargazer. Learn about the motions of the heavenly orbs in a concise, hands-on way.

Nightwatch by Terence Dickinson. A Practical Guide to Viewing the Universe. This spiral-bound field-guide steers you through all the problems and tricks of modern-day stargazers, from dealing with light pollution to working with telescope mounts. The beautiful amateur photos in the book are a complete inspiration to the budding astronomer. Who knows, perhaps one of your shots will make it into the next edition...

Turn Left at Orion by Guy Consolmagno. A Hundred Night Sky Objects to See in a Small Telescope--and How to Find Them. This guide lets even city dwellers find something worthwhile to look for through the air pollution and glare of street lights. A great guide to finding stars and galaxies.

Deep Sky Companions by Stephen James O'Meara. In the 1700s, Charles Messier cataloged all the fuzzy spots in the sky that might be confused with his favorite objects: comets. These were soon found to encompass some wonderful deep-sky objects like nebulae and galaxies, and they have become a favorite set of objects for generations of astronomers.

Observing the Caldwell Objects by David Ratledge. This is a guide to 110 famous non-stellar objects, including nebulae, clusters, galaxies, x-ray objects and more. Each object gets its own double page spread along with complete technical details, including position and NGC number. A large fold-out map locates all of the Caldwell objects in the sky.

The Observer's Sky Atlas by Erich Karkoschka. With 50 Star Charts Covering the Entire Sky. This is the book you'll take into the field with you each night. Each object of interest has a nicely described neighborhood picture helping you to locate even the dimmest objects.

More advanced observers will ultimately need the three volume set, Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System by Robert Burnham. These volumes are all-time classics in the field of deep sky observing, with complete coverage of thousands of celestial object well within the reach of any good three- to twelve-inch scope.

Telescopes

Here are some good telescopes to start with. Just click on the link for more info or to buy:

The Celestron Nexstar 114 GT is a good low-end computerized telescope, suitable for budding astrophotographers. It's ready to go with minimal assembly, and you just need to dial in the date, your location and the direction for north. After that, the computer takes over and can find anything you need in the night sky. It's a 114mm Newtonian reflector and comes with two eyepieces (10mm & 20mm). With a serial cable, you can even connect it to your computer and control it from the warm indoors.
The Celestron Firstscope 90 EQ is a good 90mm entry-level refractor that can double as a spotting scope for viewing wildlife. It includes two good (but not great) eyepieces for viewing both planets and stars. Its equitorial mount lets you manually follow stars, or you can add an optional motor drive.
The Konus Konusky 200 Newtonian is a nice mid-level 7.8" Newtonian telescope with a two-axis motor-driven equitorial mount. A big Newtonian like this has a lot of light gathering power to give you bright views of nebulas, galaxies and planets. The battery-powered motor tracks planets and the moon for hands-off viewing or photography. Because of its size, it can take a little longer to set up, so keep that in mind if you're not serious about stargazing. The scope includes two eyepieces, 10mm and 25mm, a heavy-duty tripod and a large finderscope.
The Meade 8" 203mm LX90GPS is a fine Schmidt-Cassegrain telescope with an 8 inch mirror and UHTC coatings. Not only will it follow stars and planets, it can track and even talk to GP satellites. When it does, it downloads the precise time and location and then automatically aligns the scope. It can even find specific stars to automate the alignment process. You can set up and start viewing with a minimum of fuss. A fine scope with good optics.
The Orion SkyQuest XT8 is an inexpensive Dobsonian with a terrific amount of light-gathering power for the buck. This is an ideal scope for the beginner, sturdy and simple to set up and operate. It will reveal nebula, galaxies and planets with ease. It doesn't have any fancy motors or computerized star finders, so there is nothing to break. Of course, that means you'll have to find the stars yourself, but you may actually learn more that way!
The Orion SkyQuest XT12 IntelliScope is an advanced Dobsonian telescope with terrific light-gathering power. This scope is not for beginners, and takes a bit of wrangling to move around. But twelve inches of mirror gathers a lot of light and can peer very deeply into space. A great scope for a dedicated stargazer.

A note on light pollution

If you are interested in the fight against light pollution, or want to know what you can do in your local community to improve the situation, visit the International Dark Sky Association (IDA) at www.darksky.org.

Next?

If you haven't already done so, get out there and do some observing with friends or a local astronomy club. Amateur astronomers are a gregarious bunch, and given the chance, will generally tell you more about any given topic than you can possibly absorb in one sitting. Next, inform yourself with magazine sources, web searches and sites, and a visit to the book store. If you find you really have the bug, then decide your parameters and constraints to narrow down your telescope choices in terms of size, design, and budget. If that is all too much work, and you just want to get a telescope yesterday, then spring for the classic 8" SkyQuest from Orion.

Happy Star Trails!


Copyright (c) 2008 by Chuck Fuller