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Humans’ fascination with the stars is as old as our ability to think and ask questions. For millennia we, as a species, were limited to observing the heavens with just our eyes. Of course, back then we were able to see more because light pollution didn’t exist, but making detailed observations was impossible. The invention of the microscope led to the development of the telescope, which allowed people to finally start exploring the larger universe. As technological advances were made and telescopes got bigger and better, their reach and the details they could resolve became broader and more intricate. These advances caused us to question doctrines that many believed were immutable, and doubt institutions we were taught infallible. Astronomy led the way to the acceptance of science, the rise of the Enlightenment, and incalculable advances not only to our understanding of our world and universe, but also the philosophical questions about our place in that world.
In the late 1500s, two Dutch eyeglass makers, father and son Zaccharias and Hans Janssen, developed and began experimenting on a crude microscopic device. Their work was disseminated, as inventions often are, and it wasn’t long before someone built on their microscopy work and reconfigured their lenses to bring distant objects closer. The first patent application for a telescope was by another Dutch eyeglass maker named Hans Lippershey, in 1608. The Janssens and Lippershey lived in the same town and evidence suggests that they not only knew each other, but influenced each other’s work. Compounding the confusion, yet another Nederlander, Jacob Metius, applied for a patent for a telescope a few weeks after Lippershey. The government of the Netherlands eventually rejected both applications because of the counterclaims, and, officials said, the device was easy to reproduce, making it difficult to patent. In the end, Lippershey is credited with inventing the telescope and the Janssens the microscope.
In 1609, famed Italian mathematician and scientist Galileo Galilei learned of the work being done with lenses in the Netherlands and began refining the Janssen system, eventually adding a focusing mechanism. He apparently developed the telescope on his own, and is the first known person to point a telescope skyward. He was able to make out mountains and craters on the moon, as well as a ribbon of diffuse light arching across the sky—the Milky Way galaxy. The sun had sunspots, and Jupiter had its own set of moons.
These first telescopes would be instantly familiar to modern people. They were Refractor-style scopes with a larger lens in the front and an eyepiece at the back—what we usually picture in our minds when we think of what a telescope looks like. These refractors had lenses that were upwards of 60-70mm in diameter and, given the lack of light pollution, allowed astronomers to view quite a lot. A limitation of these early refractors was color fringing, known as chromatic aberration, that is caused when light passing through a lens is broken up into various colors as the glass bends various wavelengths differently. This chromatic aberration affected the ability to see colors correctly and to clearly resolve certain celestial objects.
In 1668, Sir Isaac Newton, among his innumerable achievements, sought to solve the chromatic aberration problem. His solution was as simple as it was groundbreaking: Remove the lens from the equation completely. Newton replaced the primary lens with a polished, rounded, metal mirror. The light rays no longer passed through glass, so bright images were no longer surrounded by a colorful halo. Unfortunately, Newton couldn’t eliminate another common problem: spherical aberration (distortion)—especially at the edges of the field of view—caused by the shape of the primary mirror. He was also able to make larger mirrors more easily than comparable lenses, which allowed more light to be gathered, offering him better views than earlier, smaller refractor telescopes could produce.
Mathematicians attempted to solve Newton’s problem and, while calculations determined that a new type of mirror—called a Parabolic—was possible, it wasn’t until 1721 that John Hadley built a reflector telescope with a parabolic mirror that displayed very little spherical aberration.
Over the decades, many permutations and variations were developed. Some were successful, many others not so much. Through the 1800s, refractors and reflectors were refined and improved as the Industrial Revolution moved forward. They grew larger and the glass and grinding became more precise, but moving into the 20th century, the standard designs were reaching their maximum sizes.
Developments outside the realm of astronomy were being made that would help change the way telescopes were designed and built: as early as 1876, catadioptric lens systems were in use around the world in such diverse areas as lighthouse reflectors and microscopes. As it applies to optics, this system combines both lenses and mirrors to produce images without chromatic or spherical aberrations.
The first full–diameter corrector plate was used in Bernhard Schmidt's 1931 Schmidt camera. This was a wide-field photographic camera, with the corrector plate at the center of curvature of the primary mirror, producing an image at a focus point inside the tube assembly where a curved film plate or detector is mounted. The relatively thin and lightweight corrector allowed Schmidt cameras to be constructed in diameters of more than 50". Over the years, building on the catadioptric principals in general, and Schmidt’s design in particular, the camera was modified into an observation optic and a new category of optical tube assembly was created. Other innovators developed variations such as Schmidt–Cassegrain, Maksutov, Maksutov-Cassegrain, Argunov-Cassegrain, and Klevtsov-Cassegrain. All of these variants are grouped together in the catadioptric category and all use a combination of lenses and mirrors to correct for chromatic and spherical aberration in different ways by applying similar principals.
The 20th Century saw the rise of giant research telescopes, from the 60" Mount Wilson Observatory to the 238" BTA-6 in Russia. A persistent problem from the beginning of reflector telescopes was that the mirrors had to be removed and re-silvered to maintain their high reflectivity. When dealing with smaller apertures, this was an inconvenience. With these massive mirrors, this became a real problem. In 1932, a physicist at the California Institute of Technology successfully devised a method to aluminize a mirror through a process known as thermal vacuum evaporation. This not only revolutionized the research telescope industry, it helped set the stage for the rise of the amateur astronomer. The BTA-6 is notable for another milestone: it was the first computer-controlled telescope that helped move the massive optical tube assembly and mount. As we moved through the middle of the 20th Century, technological advances began coming faster and faster, and each advance set the stage for dawn of the Digital Age.
Just as the aluminization process represented a leap forward in mirror technology, a small telescope manufacturer in California, known as Celestron, developed a method to mass-produce Schmidt corrector plates using a vacuum to pull the glass into a curved mold. This enabled the company to lower the cost of Schmidt-Cassegrain telescopes drastically, and opened the amateur market to a wider audience. While Celestron was making reflectors and Cassegrains, a rival company was focusing on refractors: Meade Instruments. Realizing that Celestron was cornering the market on Cassegrains, Meade entered the market and a period of innovation, spurred by competition, helped the amateur astronomy field flourish.
The greater portion of astronomical history relied on manual manipulation of the mount to track objects across the night sky. The problem with handling the mount is that it causes vibrations that interfere with the observation process. A logical step was retrofitting motors onto manual mounts to reduce the vibrations and allow for greater concentration during observation sessions. As the new century approached, and technology shrunk, mount makers began integrating small servo and stepper motors into their offerings. It was only a matter of time before the computer revolution would hit astronomy.
Mounts had been computer controlled since the 1970s, but they needed to be tethered to a computer. And remember: back then there were no MacBook Airs and, even into the ’90s, laptops were still heavy and prohibitively expensive and astronomy software was very rudimentary. In the late 1990s, Meade released a revolution: the AutoStar hand controller. This computer controller, first introduced on the company’s LX90 ETX, was fairly easy to use with a menu-driven user interface. While you still had to align the scope properly and learn basic astronomy, the ETX changed amateur astronomy. It was small, lightweight, with an integrated motorized mount and, most importantly, the AutoStar plugged directly into the mount and was powered by the same AA batteries driving the motors. At the dawn of the 21st century, hundreds of years of progress finally came together that allowed the wide-scale development of amateur astronomy: easy-to-produce optical systems with practically no aberrations, virtually vibration-free motors, self-contained computer controllers.
Now that we’re firmly entrenched in The Future, we’re seeing the continued evolution of the computer-controlled telescope. From mobile device interfaces, to using GPS and high-resolution digital cameras to automate the alignment process, technology continues to drive the consumer and research telescope markets. In the last few years, Celestron announced and then rolled out the first line of consumer scopes that have integrated local Wi-Fi to connect the mount directly to a smartphone or tablet running an astronomy app. Both Meade and Celestron have proprietary versions of digital alignment aids (StarLock and StarSense, respectively) which are outfitted with digital cameras that take pictures of the night sky and automatically set up the scope for viewing, constantly checking and correcting its tracking during your observation session.
With Internet connectivity and superior processing power, unheard of when the AutoStar was released, you can now use your smartphone or tablet to display a virtual planetarium with all the objects you can see, and with a tap on the screen your mount will go to that object. Want to know the history of that constellation you’re looking at? There’s an app for that. Want a guided tour of the night sky on any given night? No problem. Want that guided tour with audio and/or video commentary? You can have that, too.
The Optical Tube Assembly, or OTA, is the main part of the telescope. It gathers light and it’s where the eyepiece and all optical accessories go.
The Mount is what the OTA is attached to and is responsible for the how the user aligns, moves, and tracks celestial objects. A more detailed explanation on the different mounts is below, but for now you just need to know that there are three principal types: Alt-Azimuth (AZ or Alt-Az), German Equatorial (EQ), and Motorized. Motorized mounts can be of either Alt-Az or EQ, but are usually set aside to differentiate them from manual mounts.
Go-To is a term that gets used a lot and is relatively new to the amateur astronomer. It’s applied to a motorized mount that is partially or wholly computer controlled.
Aperture is the diameter, usually measured in millimeters, of the objective (primary) lens or mirror of the telescope. Essentially, the larger the aperture, the brighter images will appear, and the deeper into space you will be able to see.
Focal length is the measurement, again in millimeters, from the objective to the eyepiece. This length directly affects the magnification potential of the telescope when paired with an eyepiece.
Focal Ratio is a term that will be familiar to photographers, but it is important to certain astronomers, as well. This term is defined as the ratio between the focal length of the scope and the aperture. A 100mm aperture 1500mm focal length telescope will have a focal ratio of f/15. The obvious question is why knowing this is important. There are several answers.
The f-number can give you an idea of the overall size and portability of the scope if you’ve never seen it before—smaller f/ratios equal shorter focal lengths, and therefore shorter OTAs. Let’s say you’re considering buying a 12" f/5 or a 12" f/15 Dobsonian. Just by looking at the f/ratios, you can tell that the f/5 one will have much shorter tube length and can probably be handled by one person, whereas the f/15 will be fairly massive. Specifically, the f/5 will have an optical tube a little over 5' long, while the f/15 OTA would be more than 15 feet long.
As far as astrophotography is concerned; the f/ratio plays an important role. The smaller the ratio, the “faster” the scope is, making the exposure times required for capturing images shorter. Having shorter exposure times means that any tracking errors will be less noticeable while providing you more time to take more images that you then can stack in post production.
Magnification is the number of times in size an object appears, compared to viewing it with the naked eye. A magnification of 32x means what you are looking at will look thirty-two times larger than when viewed unmagnified. The magnification is calculated by dividing the eyepiece focal length into the telescope focal length. So, a telescope that has a 1500mm focal length, using a 25mm eyepiece will produce a magnification of 60x, and a 10mm eyepiece produces 150x. As you can see, the longer the telescope focal length and the shorter the eyepiece focal length, the higher the magnification achieved.
Coatings are microns-thin and applied in multiple layers to optical surfaces to increase the performance of the scope. When applied to lenses, these coatings help to prevent incoming light from being reflected from the surface (and thereby lost), and will be optimized for nighttime viewing of celestial objects—generally focusing on accentuating certain wavelengths for improved viewing. When applied to mirrors (whether primary, secondary, or those in diagonals), they increase reflection with the intention of achieving 100% reflection. The best coatings are dielectric, which are able to reach upwards of 99+%.
Glass is what the lenses are made of. With most decent (and some not-so-decent) models, the lenses will be made of optical glass—already superior to conventional glass—to help reduce spherical and chromatic aberrations and to produce clear and crisp images. Better scopes will employ extra low-dispersion (ED) or fluoride glass for superior aberration correction.
I’ve been mentioning chromatic and spherical aberrations a lot in the preceding paragraphs, so let’s look a bit deeper into what those terms actually are.
Chromatic Aberration Different colors of light have different wavelengths and pass through the glass at different, but predictable, speeds: Shorter wavelengths travel faster than longer ones, so when they come out the other side of the lens, the various colors of light from a single object gets to your eye at different times.
The shape of a mirror or lens may cause this aberration, as well. In regard to lenses, the shape of the lens causes it to be thicker or thinner in certain points; as a result, the light passing through the thicker part will take longer than that passing through the thinner areas. For mirrors, the light in the center reflects straight down the OTA, but the light at the edges needs to travel farther, again causing the light to strike your eye at different times. In extreme cases, the distortion is so bad that you’ll see a halo around objects that can interfere with your observation. As we’ve seen, chromatic aberration has been a problem since the very first telescope was developed, and many different telescope designs, optical coatings, and glass have been employed specifically to correct this.
Spherical Aberration This has also been around from the beginning of astronomy. It is caused by the curvature of the mirrors or lenses required to focus the light to a single point. In order to see an image, the light entering the optical system from a large mirror or lens needs to be focused to a single small point—the focal point—so you can see the object with your eye. If the grind, polish, or placement of the lenses or mirrors within the optical path isn’t perfect, the light might not be focused correctly and overshoot or fall short of the focal length. This will result in distortion and/or the inability to achieve sharp focus.
The development of the reflector helped to correct for chromatic aberration, but the mirror, by its very nature, had inherent spherical aberration. To correct this, the catadioptric class of telescopes was developed through the use of corrector plates. With refractors, multiple lenses stacked at the front of the telescope helps this correction. Every refractor on the market today is doublet—meaning there are two lenses—that help correct both aberrations. Higher-end refractors will be in a triplet configuration that adds a third lens for further correction. Triplets are optimal, and essential for astrophotography.
This section deals with the gap between what you expect to see through a telescope and what you will actually be able to see. Most people barely remember a time, or haven’t lived in a world, without the Hubble telescope, nor can they remember a time when they couldn’t immediately check out pictures on the Internet. There is an entire generation that has literally grown up on high-resolution images of space and the universe in general; and the Moon, planets, galaxies, nebulae, and any number of celestial objects in particular.
As a result, we’ve come to expect Saturn and its rings to look like what we see when we Google it on our HD screens. It won’t. It’s going to be small—really small. It’s going to look like Saturn, but just a smaller version of it, compared to what we see on our iPad, laptop, or 4K television. But here’s the thing: with a telescope, you get to see it for yourself. It’s an incredibly personal experience. New astronomers, of any age, need to be aware of this before investing in a telescope. As a telescope user and astronomy advocate, I suggest that anyone with an interest should take a spin around the Internet and make sure their interest survives. No matter how much money you pour into buying a telescope, there’s very little chance that anything you look through will compare to images that are taken in space and professionally manipulated.
And by “professionally manipulated” I’m not saying “Photoshopped.” The images you see on the Internet are, virtually without exception, a composite of dozens, or hundreds, or thousands of images taken over the course of a night, or a month of nights, using physical filters at the time the exposure was taken, or digital filters applied during the stacking process. Incalculable amounts of manipulation can be applied with editing software to make that one “perfect” picture. But remember: no amount of Internet surfing can replace the absolute thrill of seeing a planet or galaxy or nebula for the first time with your own eye.
Finally, there’s the effect that the Quality of the telescope has on what can be seen. As we’ve discussed, certain things can improve image quality, such as ED or fluoride glass, specialized optical coatings, and the overall precision of the grind and engineering. Be aware that skimping on the quality will have a significant effect on what and how well you can see. At the end of the day, you may want to forego a large aperture on a low-quality scope for a smaller aperture, higher-quality scope that costs the same.
In our discussion of the evolution of telescopes, we outlined the three basic kinds: refractor, reflector, and catadioptric. Now, we come to the part where we discuss which one to acquire. Sadly, there is no answer that will satisfy everyone (or anyone, for that matter). All types have their strengths and weaknesses, so the choice you have to make is going to be based on what you want to see and what you want to spend. What we’re discussing here is strictly based on optical performance; later we’ll bring mounts and tripods and other support systems into the mix to give you the full picture.
Refractors The simplicity and reliability of the design makes it easy to use and requires little maintenance. These are excellent for observing objects within our solar system—planets and the Moon and, with the right accessories, they can be used for terrestrial viewing. Since the optical system is basically a straight line, there are no obstructions from secondary mirrors as there are in Newtonians or catadioptrics. With optical options like triplet configurations and specialized glass, aberrations can be virtually eliminated.
There are a few downsides, however. They tend to be more expensive per inch of aperture than the other two designs. The lens systems tend to make them heavier than similarly sized Newtonians and catadioptrics. And because of their limited available apertures, they tend to have difficulty seeing dim deep-space objects.
Finally, the overall superior optical performance of a refractor makes it an ideal platform for astrophotography or astro-imaging.
Reflectors Utilizing a large primary mirror, the Newtonian gives you greater value per inch of aperture, since making a mirror is less labor-intensive than making lenses. However, in order to get the light focused and into an eyepiece, it is bounced from a secondary mirror placed near the front of the OTA facing the primary mirror. The secondary mirror is set at a 45-degree angle to send the image into the side-mounted eyepiece. This secondary mirror causes a slight obstruction to the light entering the OTA, which results in light loss. Additionally, in traditional reflectors, the OTA is open to the elements, so they tend to require a certain amount of maintenance to keep the mirror free from dust, dirt, and pollen. A variation of a Newtonian is the Schmidt-Newtonian, which places a corrector plate at the front, thereby helping to reduce spherical aberration and sealing the system for easier maintenance.
Because you can get large-aperture telescopes out of the mirrors, reflectors are ideal for seeing the deep-sky objects that refractors often miss, such as galaxies and nebulae.
On a side note, large reflectors that can reach apertures of several feet with optical tubes up to 10' long or longer are identified as Dobsonians. These over-sized OTAs are capable of seeing incredibly dim objects, but are very bulky and heavy—usually requiring disassembly for transport. Many Dobsonians are trailer-mounted and simply towed behind a car or truck to the observation site, or permanently installed in backyard observatories.
Catadioptrics These are defined by their large magnification potential and their short optical tubes. Utilizing a folded light path, light enters through a thin, aspheric correcting plate, reflects from a spherical primary mirror back up the tube, where it is reflected again from a small, secondary mirror toward the back to the optical tube and out an opening in the rear of the instrument to form an image at the eyepiece. Manipulating the light in this manner creates a mathematical focal length much longer than size of the optical tube.
This optical configuration creates a compact and portable OTA that is virtually maintenance free and easy to use. It offers larger aperture per inch than refractors, but tends to be more expensive than similarly sized reflectors. Catadioptrics are excellent for all types of near and deep-sky viewing, except for extremely dim objects. This configuration shares the secondary mirror obstruction that we discussed above with reflectors.
The mount you choose is just as important as the optical tube assembly. As we touched upon earlier, mounts can be loosely grouped into two categories: Alt-Azimuth and Equatorial. Each of these allows you to move the telescope to track objects in the sky. Basic earth science teaches us that the Earth rotates, so as you observe an object, it will move across your field of view, creating the need to move the telescope accordingly. To visualize this movement, think of when the Sun rises in the East, moves across the sky, then sets in the West—every observable object in the sky tracks a similar path, with the exception of the Pole Star, which is set directly above the North Pole and creatively called the Pole Star. All calculations and coordinates for celestial navigation in the Northern hemisphere are taken from their position relative to the Pole Star. The speed at which an object moves is relative to the distance it is from Earth: The Moon moves very fast, requiring nearly constant tracking; while a deep-sky object like a galaxy moves fairly slowly. Similarly, the higher the magnification, the faster it moves versus a lower magnification.
Alt-Azimuth This is the most common and basic mount. It has two perpendicular axes on which the scope moves: up/down (Alt or Altitude) and left/right (Az or Azimuth). Lower-end models will require you to grab the OTA to move it by hand, while others offer knobs or flexible cables to make adjustments. The downside to these mounts is that to track objects as they move across the sky, you need to manipulate each axis constantly and simultaneously—imagine drawing an arc on an Etch-a-Sketch.
As an aside, Dobsonians tend to use a modified Alt-Az mount that rests on the floor or ground, owing to their large apertures and long tubes. While they are considered Alt-Az mounts, they look and operate slightly different and very few of them are motorized; but since Dobsonians are almost exclusively used for extreme deep-sky objects, the amount of correction required is greatly reduced.
German Equatorial Much more precise and more intricate than an Alt-Az, the German Equatorial (EQ) mount has two axes: one that controls declination and another that describes an arc that matches the curvature of the Earth—the right ascension. The mount needs to be aligned with the Pole Star (Polaris in the Northern Hemisphere) and once that’s done, if you know the coordinates of a celestial object you can find it and—most importantly—track it simply by turning the right ascension adjustment. There’s a pretty steep learning curve to figuring out how to use an EQ mount properly, so you need to be prepared for a lot of research and reading in the weeks and days leading up to your first observation session with one.
Speaking from experience, for math or science nerds, there are very few things as cool as going through all that prep work, setting up the mount under the stars, properly aligning it, moving the scope to the selected coordinates, then looking in that eyepiece and seeing Saturn for the first time. The thought of that first experience for me still brings goose bumps.
While most commercially available mounts fall into these two categories, there are variations that need to be mentioned, mainly because they are growing in popularity as they fall in price.
The first is motorized mounts. As the name implies, these have motors for tracking. Both EQ and Alt-Az can be motorized, but the question arises as to why you would want to? Primarily, the advantage of adding motors is to increase precision and decrease vibration. The motors used on mounts are generally stepper servo-motors that are capable of moving at various speeds with very little vibration. This allows you to spend more of your concentration observing than turning knobs. Also, anytime you touch the scope in any way, you cause vibrations—tiny, slight, minute, vibrations—but when you’re looking through an eyepiece with high magnification, those little vibrations translate into scene-obliterating ocular earthquakes. Hyperbole? Not really. If you can use a simple hand controller to direct the scope rather than touch it, that’s the way to do it. Depending on the mount and manufacturer, you may be able to retrofit motors onto manual mounts or they may come integrated into the mount at the time of purchase. Often, you will see a “standard” manual model and an upgraded version with motors.
The second variation is computer controlled. Having the mount motorized is a prerequisite to having it computer controlled. In the past, most computerized mounts relied heavily on the user properly aligning the scope, and knowing what they wanted to see.
Almost as important as your OTA and mount is the platform on which you put your rig. The platform will give your telescope stability and will affect vibrations from handling the scope, the wind blowing, or even ground vibrations from nearby people, cars or equipment. When making your decision, you need to consider how and where you will be using your telescope: Will you keep it in a backyard observatory with a retractable roof? Will you keep it set up in your garage and wheel it out on a dolly? Will you break it down and set it up close to home, or will you be trekking out far away from civilization? Each of these options will help you decide on what kind of support system you will want to use.
Beyond this, another thing to be mindful of is how high it can go and where will the tripod/mount/OTA combination place the eyepiece. For example: The eyepiece on Newtonian OTA with an EQ mount will be in a radically different position than a refractor on the same mount. If the eyepiece is set very low, causing you to lean over or squat, the increase in blood pressure to your head can cause your eye to change shape and affect your ability to see properly. You need to make sure that the platform you use allows you to observe in a natural position throughout your entire range of motion, from the horizon up to the zenith.
Far and away, most people will choose a tripod. As you might imagine, all tripods are not created equal. They will vary greatly based on size, weight, material, and stability. Be mindful of the carrying capacity rating for the tripod to make sure it can even physically handle the OTA and mount, with room for any accessories and counterweights you might use down the road. Generally speaking, you will want to get the largest one you can use. If you are observing from your backyard, a large and heavy tripod might not be a huge inconvenience but if you are hiking miles into the back woods, hauling it might not be easy—or even possible. For dolly mounting, weight becomes less of an issue, so again, bigger is better. Also, since one of the principal tasks of the tripod is to reduce or eliminate vibrations, the heavier it is, the less it will be affected by wind and other forces.
Useful accessories to improve the experience with a tripod are anti-vibration pads. These extremely common pads are placed on the ground for each of the three tripod legs to stand on. They absorb tiny vibrations and steady the rig. Second is a mini-pier. This accessory is placed between the top of the tripod and mount to add height for more comfortable viewing, as described above. The mini-pier is purpose-built to add height without sacrificing stability.
A less popular, but much more stable platform is a pier. It is usually a large pedestal-type mount that would get bolted to a concrete pad or footing. This rock-solid platform is meant for telescopes that are permanently set up in observatories, which includes backyard sheds with retractable/removable roofs.
So far in our discussion of OTAs, we’ve described the way the light is gathered and how it is directed to a single point for observation. This point is where the eyepiece is located. Specifically, on refractor and reflectors, the eyepiece sits in a drawtube that is part of the focuser. Simply put, the focuser allows you to move the eyepiece forward or back to bring the image in the eyepiece into focus for observation. There are two types of focusers: rack-and-pinion and Crayford style.
Most drawtubes are offered in 1.25" diameter, so they will accept 1.25" eyepieces. In this discussion, the 1.25" refers to the diameter of the barrel, not the focal length of the eyepiece. These are the workhorses of eyepieces, the most popular, and offer the most variation across the board. Larger-diameter focusers and eyepieces push into 2" and even 3" behemoths. Why would you need a 2 or 3" eyepiece? Field of view and eye relief. A 2" eyepiece with the same focal length of a 1.25" will have a larger field of view and longer eye relief, which pulls your eye back from the eyepiece so you won’t have to touch it—which would cause vibration. Adapters are available for 2 and 3" focusers that will allow you to use smaller-diameter eyepieces in them for greater versatility. For astrophotography, a larger focuser allows more light coverage for bigger imaging sensors.
A rack-and-pinion style has a grooved rail, usually on the bottom of the drawtube, which sits on a matching grooved wheel. The axis of the wheel will have generally have knobs on each end that you turn to move the tube in and out for focusing. This kind of focusing is very popular and easy to produce and use. The downside is that the resolution power is dependent on the size of the grooves—smaller grooves allows for more precise focusing. Additionally, when you release the knob after focusing it, there can be a backlash where the rail and wheel settle, causing it to lose that fine focus.
The more precise Crayford-style fixes the backlash and resolving issue by using a tension system. A spring holds a steel rod tightly against a smooth drawtube. When the focusing knob is turned, the tension of the rod pushing against the tube causes it to move forward or back. This allows extremely precise focusing without backlash. Crayfords tend to be more expensive and will usually only be standard on higher-quality OTAs, but there are many after-market focusers that you can retrofit onto OTAs with rack-and-pinion focusers. The principal downside to a Crayford is that if you have heavier accessories, like a big astrophotography rig, it might not be strong enough to support it. Rack-and-pinions generally have greater weight capacity, so if you’re planning on having a substantial rig hanging off your focuser, you may want to consider the rack-and-pinion and sacrifice that ultra-fine focus for stability.
Catadioptric OTAs use an internal focusing system that does not fall into either of the above categories. Located at the back of the OTA next to the eyepiece holder, the internal focuser will move the primary mirror forward and back to achieve focus. This relies primarily on a screw and is often as precise as a Crayford, but there tends to be a lag between when you move the screw and see the correction, so be patient and move slowly when using this kind of focuser. To change from 1.25 to 2 to 3" eyepieces, you can use adapters as mentioned above, or simply get a different eyepiece holder all together.
A telescope doesn’t have an intrinsic magnification. In order to obtain any magnification you need an eyepiece. Just as an OTA has a focal length, so does the eyepiece. In order to figure out the magnification you’ll be observing with, there’s a simple calculation you need to perform: OTA focal length / Eyepiece focal length. For example, if your OTA has a 1000mm focal length and you use a 25mm eyepiece you will observe at 40x. Replace the 25mm eyepiece with a 10mm in the same OTA the magnification changes to 100x. Now, don’t just go out and buy the smallest focal length eyepiece and expect to stare into the eye of G-d. Magnification by itself is useless. Extremely high magnification will produce an image of a planet that is very large, but you will not be able to see details and it will quite probably be very shaky. A good rule of thumb is to max out your magnification at 20-30x per inch of aperture. So you shouldn’t go higher than 160-240x with an 8" OTA, or 100-120x with a 4". This will allow you see the images without affecting your ability to discern detail.
Just like the optics used in the OTA, the optics of eyepieces are important, as well. Quality varies greatly with ED glass, anti-reflection coatings, and lens elements that create wide and ultra-wide fields of view. You’re better off buying one excellent eyepiece with a medium magnification than multiple lower-quality ones.
Filters are used to emphasize or eliminate certain wavelengths of light to improve image quality. My colleague, Cory Rice, has written an article specifically on astronomy filters, so I won’t go into too much detail here, but I will touch on a few points. Like everything else we’ve discussed, quality matters, so look for glass filters that have their tints added when the glass is being produced, as opposed to applied as a coating after. Filters can be used individually or stacked for greater effect and can truly enrich what you see.
First, if you’re going to observe the Moon, you must have a Moon filter. The moon effectively acts as a giant reflector for the Sun. As a result, when it’s at more than half phase, it gets extremely bright, and during a full moon, you can cause permanent damage to your eye if you view it without a filter.
Second, multiple filters can—and should—be used for observing the same object. For example, you can use a #15 Deep Yellow filter to bring out Martian surface features, and a #25A Red to increase definition of the Martian polar ice caps and maria. If you get a set of filters, and you should, consider getting a filter wheel. This will attach to the focuser and would be loaded with different filters. When you’re observing, simply rotate the wheel to a different filter so you don’t have to remove the eyepiece and change them individually.
Third, specialized filters allow you to see the unseeable. Certain nebula filters will emphasize certain wavelengths by excluding all other wavelengths, thereby allowing a very specific narrow band through, and will show parts of a nebula that the human eye couldn’t detect otherwise. Similarly, light-pollution filters help filter out ambient light to make celestial images brighter, with greater detail.
Several accessories can be used to increase the value of your eyepieces or correct for imperfections in the optical system. All of these accessories enhance image quality and will make a huge difference for both observation and imaging. As always, quality matters.
Barlow lenses of 2x or 3x are popular because they allow you to double or triple the magnification of each of your eyepieces without greatly affecting your ability to focus and resolve the images that might occur if you used a shorter focal length eyepiece to achieve a similar magnification. As mentioned above, if you have a single high-quality eyepiece, picking up a quality Barlow lens will be less expensive, while giving you two magnifications from one eyepiece.
Field flatteners help to eliminate distortion across the entire field of view so everything appears on the same plane from the edge to the center. This allows for a more immersive visual experience and better imaging.
Coma correctors will further correct for chromatic aberration and allow you to better split double stars and achieve tack-sharp focus of bright objects.
Astigmatism correctors improve image quality by correcting lens astigmatism that can prevent the ability to achieve perfect focus.
Diagonals Diagonals are used to make viewing more comfortable or to correct the image orientation. When viewing celestial objects with any telescope, the way light is manipulated through the optical path to your eye, a normal image is seen upside down and backward. So to track the Moon as it moves to the left in your eyepiece, you have to move the OTA to the right. Now, if you’re looking at Saturn upside down and backward, it probably won’t cause any disorientation because you have no frame of reference to what is “right-side up.” But if you decide to use your telescope for viewing a boat out on the water, you will definitely have some disorientation if the ocean is on top and the boat is up-side-down. This is where the diagonals start to come into play.
Chances are, if you have a Newtonian or Dobsonian, you won’t need them, as these have the eyepieces placed in very comfortable positions and they’re not really ideal for terrestrial viewing. You will primarily use diagonals on refractors and catadioptrics. With these, the eyepieces are set at the backs of the OTAs, so when it is pointed at or near the zenith, the eyepiece has a tendency to be pointed at the ground. Using a diagonal—either 45-degree or 90-degree—brings the eyepiece into a more comfortable viewing position. There are three different kinds of diagonals: star, erector, and flip. Keep in mind that adding a diagonal will further bend or reflect the light, so there’s the potential for light loss. Quality diagonals will use optical glass with dielectric reflective coatings to minimize light loss.
Star diagonals simply reflect the light, and the image will be corrected vertically but not horizontally, so the image will be right-side up and backward. Whether you’re the scope for astronomical or terrestrial viewing, your image will be right-side up, but you’ll just need to move the scope right to track it left. It’s not a big deal, but it does take a little getting accustomed to.
If you know you’ll be using the telescope for both astronomical and terrestrial viewing, consider getting an erector prism. Unlike a Star diagonal, which is a simple mirror, erectors use a prism to correct the image orientation both horizontally and vertically for a more natural view and easier tracking. If you’re going to invest in an erector prism, aim for a high-quality one to reduce image degradation.
Flip mirrors will have two eyepiece holders: one in line with the focuser drawtube and one at a 90-degree angle. A lever is used to control a mirror to send the light straight to one eyepiece or up to the 90-degree-orientated eyepiece. This is used for several purposes: for one, it allows the use of two different focal length eyepieces and/or filters without needing to change the eyepieces completely. In addition, you can have an imaging system on one side and an eyepiece on the other. In between exposures, you simply flip the mirror from imager to eyepiece to confirm your subject is still centered and focused, allowing you to make corrections quickly and easily.
Whether your mount is manual, motorized, or computer controlled, at some point during your set-up and alignment process you’ll need to start your set-up procedure by finding at least one star. Even with a low-power eyepiece, scanning the sky looking for the one star out of thousands—even the brightest ones—will be virtually impossible. For this reason, you’ll want a finderscope. These come in various configurations, from small 6x25 mini-telescopes to unmagnified pointers. Regardless of the kind, your finderscope will need to be aligned with the OTA so that they are both pointed at the same point. This is easily accomplished using large objects like the moon or a distant street light, and shouldn’t take more than a few minutes.
Standard finderscopes are just small, low-power telescopes. Larger OTAs may push the size up to 10x50, since you’ll probably be observing deep-sky objects and will need to reach farther for finding those objects.
Dot pointers are very popular and easy to use. They are generally unmagnified and are simply a small window, a couple of inches across at most, with a red dot projected into the center. With both eyes open look through the pointer and align the dot on your subject. These are ideal for computerized scopes or EQ mounts where you’ll need to find one, two, or three bright stars to start, then use the computer controller to do the rest.
Illuminated finderscopes are a variation of the standard ones from above, but they will have an illuminated reticle denoting the center. Usually dimmable, they allow for more precise alignment versus a non-illuminated finder.
Worthy of an article longer than this one, astrophotography and astro-imaging are part of an emerging field with innumerable permutations. You can spend upwards of $10,000 on an imaging rig, on top of whatever you’ve spent on your OTA, mount, and platform. Or you can spend $50 on a smartphone adapter that attaches to your eyepiece. For the average user, there are some basic setups that won’t break the bank.
Using your DSLR will work, but DSLRs usually come equipped with a pre-installed IR filter in front of the imaging sensor. This filter is fine for taking pictures on the ground, but will affect your pictures of the stars. There are a few ways to mount your camera on your telescope, but the most popular method is to use a T-ring adapter that goes into your focuser and a T-ring for your camera’s specific bayonet mount. The adapter and ring screw together. You’ll also need a remote trigger and you’ll have to deactivate the autofocus and other automatic functions to give you complete control of the camera and sensor. Doing it this way with digital cameras is orders-of-magnitude easier, since you can simply delete bad photos as you refine your settings. In the pre-digital age, you could literally spend thousands of dollars on film rolls just tweaking your rig without getting one usable image.
Eyepiece imagers are growing in number and popularity. These are simply an image sensor and electronic shutter tethered to a computer and controlled by imaging software. Designed to drop into the focuser, they are capable of high resolution and able to capture thousands of images a second. Using the capture software, you can then stack, filter, and fiddle with these images in thousands of ways to create a single composite photo. They come in both color and monochromatic versions, and certain models will accept standard eyepiece filters for filtering at the time of image capture.
Adapters are available for smartphones and point-and-shoot cameras, but these aren’t going to produce high-quality images; think mostly along the lines of social-media-postable pictures of the moon, and maybe some constellations.
A subset of astrophotography is wide-field astrophotography which is using a standard DSLR and lens to capture swaths of the night sky.
To do this right, you’ll need long exposure times, which means you’ll need a mount—preferably EQ or motorized. The mount needs to keep the same star field centered during the entire exposure period, and if the camera doesn’t move as the stars move, you’ll get star trails, which are cool if that’s what you’re going for, but annoying if it’s not.
There are adapters and plates you can use for standard EQ or motorized alt-az mounts but, if you want to observe, you’ll need to swap your camera for an OTA or bring two rigs. Some mounting systems come with a ¼"-20 screw on the top of the mounting rings for the OTA, which allows you to mount your DSLR piggy-back style on your telescope and capture wide-field photos while you’re observing or imaging specific objects.
Recent years have seen purpose-built wide-field mounts for photo tripods that work similarly to EQ mounts, but are smaller and more compact for DSLR use, with alignment tools and right-ascension tracking, different speeds and the ability to use them in the Northern or Southern hemispheres.
Everything we’ve been discussing so far has dealt directly with observing the night sky. But there is an array of accessories to make the experience better or easier. This is by no means a complete list, just some of the more popular ones.
Power supplies and battery stations All motorized and computer-controlled mounts require power. Usually they run on batteries. Usually a lot of AA batteries—6, 8, 10, or 12, depending on the size of the mount. If you’re observing at home or near a car, you can usually get 110VAC or 12VDC adapters to power your mount. If you’re using your car, take care… you may enjoy a night of sky-gazing only to realize your car battery is dead and you’re stranded. An alternative to power adapters is a battery station. Heavy, but portable, these employ a bank of rechargeable deep-cycle batteries that will power your mount for tens of hours; more than enough for a night or weekend away off the grid. Several will have 12VDC, USB, and 5VDC ports to power your mount and other accessories, smartphones, or cameras. Some even have booster cables (just in case).
Flashlights An often overlooked tool, the simple flashlight is a must-have. You’ll need one that has a dedicated red light to allow you to see without affecting your night-adjusted vision. The light will let you maneuver around your observation sight without tripping over your tripod, choose eyepieces and accessories, or consult star charts. Many people use white lights with red filters for versatility and variable power for convenience. There are also headlights for hands-free illumination.
Star charts have been around since astronomy’s early days and are inexpensive yet invaluable tools to show you what is available for you to see at any given time or day. Sometimes the old way is still the best way.
Books, either digital or print, are still the best way to learn about astronomy. They provide great detail on what you can see, how to find it, what you’ll be looking at, and a wealth of other information you didn’t know you needed.
Software can do similar things that books and star charts can. Some basic software is just digital observatories that can show you detailed charts, including co-ordinates for all objects. Other software will control mounts similar to the hand-controllers and apps, but with the power of the Internet behind it. Still more software packages can be used to process and manipulate your images to make them better and more impactful.
Cleaners are pretty straightforward; you’ll need to wipe dust, dirt, pollen, or dew off your optics to see clearly. Cleaners for telescopes will be designed to leave neither residue nor scratches, to keep your lenses and mirrors pristine.
Miscellaneous items can be pop-up observatories, like tents for your telescope, seats with accessory trays or pockets, hoods to block out stray light, or eye patches that allow you to work with light while you dilate your observing eye. And don’t forget a folding table for all your accessories and peripheral gear.
Astronomy is a niche but emerging hobby that gets more popular as technology makes it easier to get out and get observing. You’ll need to go into this hobby knowing that a great rig will set you back many thousands of dollars, and even competent setups will still cost more than most people think. The best way to gauge your interest level, what you’re most eager to see, and what kind of OTA, mount, and platform are the best for you is to start with the Internet. Join forums and ask questions.
Next, join an astronomy group. You’ll be surprised how many are around and right in your neighborhood. Go to star parties and observation get-togethers. Introduce yourself and explain that you’re just getting ready to buy your first scope—we’re a community of sharers, and you are sure to be invited to view through everyone’s rig. Being exposed to the community and a wide variety rigs will help you narrow your choices.
Finally, go to a reputable dealer. That’s where you’re going to find the best brands, knowledgeable people, all the accessories, and you’ll know that your scope is new and undamaged.
The Universe is an incredible and awe-inspiring place, and you’re a part of it. Get out there and see it.