Thursday, November 7, 2013

Telescopes

There are three main telescope designs, refractor (lens-based), reflector (mirror-based), and compound (both lenses and mirrors). Each scope has benefits and drawbacks depending on what you will be observing and where you will be observing from.


1. Refractors. A refracting telescope is the oldest telescope design. The first refracting telescopes were spyglasses intended for terrestrial purposes that were turned skyward. Galileo’s pioneering discoveries were all made with a very basic refracting telescope. A refracting telescope uses a lens to gather the light. The lens is shaped so that the light is bent (refracted) down to a focus point. After the focus point, the image is flipped and continues a short distance before hitting a diagonal mirror which then flips the image back to right side up and directs it into the eyepiece at the same time. Because of this, refracting telescopes can also be used for terrestrial pursuits, such as bird watching. This is the basic anatomy of a refractor. Although two refractor designs will be dealt with, the difference in the two types is in the glass used, not the design.


Achromatic refractor. Achromatic refractors are refracting telescopes with a lens made of two individual pieces of glass. Commercially made achromatic refractors usually range from 2 to 6 inches in aperture. The focal ratio usually ranges from f/5 to f/12. While these are the most common standards, achromatic refractors can go over 6 inches in aperture or over f/12 in focal length. It is virtually unheard of for a refractor to be less than an f/5 length. Achromatic refractors provide unrivaled image sharpness because of a clear aperture and color contrast, which is created by using a lens. An achromatic refractor will easily out perform larger scopes of other designs in the area of contrast and sharpness. While usually marketed for planet and double star observing, achromats of about 3.5 inches or larger are really excellent all around telescopes, except in the faint galaxy department where more aperture is needed. Because of the high color contrast, refractors are by far the best type of telescope to use in light polluted settings because the object being observed stands out well in contrast to the dark background.

There are two major drawbacks with an achromatic refractor. First, the eyepiece is at the base of the tube, which means that the eyepiece can be quite low when observing objects high in the sky. However, the height problem can be fixed with mount extensions to raise the scope.

The second problem is chromatic aberration. Because the objective is made of standard glass, it is difficult to bring light to a single focus point at focal ratios under about f/12. The colors of the spectrum have different wavelengths, red having the longest and violet the shortest wavelength. The goal of the lens is to bring these colors together at a single point to create an image. However, the glass used in achromatic refractors can not accomplish this task completely unless the lens is a long focal length. Because of their shorter wavelength, colors on the violet end of the spectrum are not brought to focus with the rest of the colors. The result is a purple halo around bright objects, especially noticeable at higher powers. Views of chromatic aberration vary. While some people easily ignore it, other observers are driven crazy by the false color. Long achromats, about f/15 or longer, can virtually eliminate chromatic aberration on all but the brightest objects, but the length can be a problem, especially for larger aperture scopes.


Apochromatic/ED refractors. Apochromatic/ED (Extra Low Dispersion) are interchangeable names for refractors that retain all the high contrast and crystal clear sharpness of an achromat while eliminating the chromatic aberration. Without a doubt, apo refractors provide the best images per inch of aperture of any telescope design. The key to the apo refractor is in the glass. Apo refractors use anywhere from a two or sometimes even a five element lens design. With extra layers of high quality ED glass and strategic spacing in between, an apo refractor can bring all the colors of the spectrum to a single focus, eliminating any false color. Apo refractors are usually at least 2.5 inches in aperture and some companies even offer apos over a foot in diameter. The focal length is, like the achromat, at least f/5. Large apo refractors are an astrophotographer’s dream scope. For photography through a telescope, nothing can beat the combination of pinpoint sharpness, high contrast, and lack of false color that only an apo refractor can provide. Like other refractors, an apo can be raised with a mount extension to bring the scope up to a more reasonable height. With all of these features going for them, apo refractors are probably the closest thing to the perfect telescope. However, with apo refractors, there is a downfall. For all of these perks, any would-be apo owner will have to pay a premium price. A 3 inch apo optical tube with no accessories selling for $500 is a bargain. A fully outfitted 4.5 inch achromat with a tripod and accessories can be bought for about the same price. But if money is not an object, an apo refractor is probably the best way to go.


Summary. Refractors, of all the telescope designs, are the most expensive per inch of aperture. But if you have the cash, a refractor is well worth the extra investment. Refractors offer unrivaled image clarity, making them the obvious choice for anyone who likes to observe planets and/or double stars. The internal baffling of the tube can greatly reduce internal glare, boosting the color contrast of your targets. The high color contrast of a refractor lends itself nicely to light polluted areas. Another perk of a refractor is the greater light transmission. Refractors average at least 90% light transmission, with some premium refractors being tested at as much as 98% light transmission. On the other hand, the best reflectors only transmit about 75% of the light collected to your eye. Because of the high contrast and greater light transmission, a refractor can outperform larger scopes of other designs for even deep sky objects, especially from cities and suburbs. Another perk of a refractors are their size, which makes them easily portable. Achromatic refractors, especially those over 4 inches, are great all around performers and the economical refractor choice for a beginner. Another big advantage: a closed-tube design, which means no internal dust, and the fact that it is virtually impossible to knock a main lens out of alignment without trying.




Reflectors. Physics pioneer Sir Isaac Newton, bothered by the lack of high quality refractors on the market at the time, built the first reflecting telescope around 1668 (exact dates can vary). Reflecting telescopes use a set of mirrors to gather light. The primary mirror for a reflector is housed in the rear of the tube. The light enters the tube, bounces off of the main mirror toward a small secondary mirror, which then directs the image up through the eyepiece. Unlike refractors, the image in reflectors stays upside down.


The modern reflector is essentially unchanged in design from the first one built by Newton and has the mirror arrangement as described above. Commercially built Newtonian reflectors can range anywhere from about 4 ½ inches to three feet, yes three feet, in diameter. The focal ratio of a reflector is typically an f/4 to f/8 range. Because of their smaller aperture, small aperture reflectors are typically longer in focal length. Because of their large diameter, large reflectors are usually at the short end of the length spectrum. An example of this fact is the following comparison. A six inch, f/8 reflector is a common design at about 48 inches long. A ten inch, f/4.5 is another common design, about 45 inches long. A long focal ratio, large aperture reflector would just be too difficult to handle. Not many people would want to mess with a ten inch, f/8 reflector about 80 inches long. The focal ratio determines what the telescope is best suited for. Long 6 inch, f/8 reflectors are good all around performers, but are somewhat limited to a narrow field of view because of the long focal length. An f/4.5, ten inch reflector is best suited to deep sky observing because of its large aperture and short focal length, which allows for a generous field of view. Probably a good, middle of the road choice is an eight inch, f/6. 

Advantages of the reflector include the mirror light collection system itself. With a mirror, chromatic aberration is not an issue. Also, a mirror located at the rear of the tube is less prone to collecting dew than a lens at the front of a refractor or compound design. The reason many beginners go for reflectors is the cost. A Newtonian reflector is the cheapest design per inch of aperture of all telescopes. However, there are drawbacks to a reflector.

One drawback is due to the mirror system. The pair of mirrors must be kept in line. Collimating, is the process of aligning mirrors and this will undoubtedly have to be done sooner or later. When it comes to handling, Newts are the most fragile of telescopes. Another drawback of the mirror is that the reflective coating will need to be replaced with enough time. Bulkiness is an issue for some people. Many reflectors are typically about four feet long with apertures of up to a foot. This may prove too large for some people to want to carry outside very often. For many astronomers, by far the biggest complaint about Newtonian reflectors is the secondary mirror. The small secondary mirror is supported by a four pronged “spider” at the front of the tube. The “legs” lead to diffraction spikes appearing on stars. While to some people, the spiking is aesthetically appealing, it is an annoyance to others. Even for people who think the spiking just adds to the beauty of the stars, the secondary mirror obstructing the tube definitely degrades the image sharpness/contrast. Because of the obstruction caused by the secondary mirror, no reflector will ever match a refractor in the clarity and resolving department. However, the clarity and resolving ability of a reflector is not bad at all, it just is not quite as good as a refractor. Also, while refractors are greatly limited in size because a lens is only supported on its periphery, a mirror can be supported underneath, allowing for giant telescopes. Some companies offer ready built reflectors of up to three feet in aperture.


Summary. The simple Newtonian reflector is by far the cheapest telescope design around. Because of the cost, a six to eight inch reflector is often considered an ideal choice for a beginning astronomer. Reflectors offer some decided advantages over other designs. First is the low cost. Reflectors are also the least likely design to be effected by dew formation. Another benefit of the mirror is the lack of false color. The older Newtonian design offers these benefits but has some disadvantages. Mirrors will occasionally have to be realigned and refinished. Diffraction spikes provide varying reactions. But the undeniable drawback is the clarity. While Newtonians can split double stars and resolve detail in the cloud bands of Jupiter, a refractor will always be better for clarity and contrast. The new clear aperture reflector design offers great potential. With the lack of false color and clarity, apo refractors may soon be challenged as the best telescope design around.




Compound. The compound design is a relatively recent idea that uses a system of mirrors and lenses to produce an image. The light enters the telescope through either a front plate or corrector lens. From there, the light travels to the rear of the tube to the primary mirror. After reflecting off of the primary mirror, the light travels forward to a small secondary mirror attached either to the corrector lens or front plate, depending on the design. From the secondary mirror, the light is directed back toward and through a hole cut in the primary mirror, finally reaching the eyepiece. Because of the back and forth reflecting, a compound design telescope tube is much shorter than either a reflector or refractor, despite being of often longer focal lengths. The two most common designs here are very similar visually to a beginner, the differences in the fine details.


Maksutov (Maks) use a spherical, meniscus shaped corrector lens in the front of the telescope. This corrector lens is the distinguishing feature of a Maksutov design. Maks are of often a longer focal ration than the Schmidt design (mentioned later). The longer focal length results in a less steeply curved primary mirror, which means less need for image correction. For the secondary mirror, Maks use an aluminized section on the back of the corrector lens. This is easier than mounting an actual mirror but by using the back side of the corrector as a secondary virtually fixes the focal ratio of a Mak, usually at f/15. Although the field of view of a Mak is quite small, the small secondary ups the contrast compared to a Schmidt. Also, because of the use of a glass corrector lens in the front, Maks are usually limited to about six inches. Any greater aperture would be too front heavy because of the large lens.


Schmidt. The Schmidt design uses an aspherical corrector plate as the front objective with a secondary mirror mounted on the back of the plate. The corrector plate of a Schmidt is more complex that the corrector lens of a Mak. The Schmidt corrector plate appears flat, but is actually thicker in the middle and around the edge. Unlike the Mak, the Schmidt design uses a real mirror mounted on the back of the corrector plate. Unfortunately, the secondary obstruction of the Schmidt is the largest percentage of any telescope design, which degrades image sharpness. The good news about the Schmidt is that it is shorter, typically f/10, allowing for a wider field of view than the Mak. Because of this, Schmidts are better for deep sky viewing. Because of the lighter corrector plate, Schmidts can be built bigger, up to two feet for commercially built models.

As with everything else, though, there are disadvantages. Like Newts, compound designs are obstructed by a secondary mirror, which will degrade image clarity/sharpness somewhat. Like refractors, with front-mounted optics, these scopes are also more prone to dewing up, especially considering that, unlike refractors, they are often sold without dew shields! AS for the biggest problem of the compound design it all has to do with the focusing. Unlike refractors and Newts, which are focused by racking the focuser in and out, this changing the length of the scope, the compounds are focused by moving not the eyepiece, but the main mirror, which creates two problems. First, and most common, mirror flop. Swinging a compound scope around the sky can actually defocus the scope because the main mirror will slide around, albeit slightly. Second, because the tube is not sealed, moisture can actually get inside the scope, resulting in inner dew and, if the scope in not allowed to dry, fungus.


Summary. When it comes to portability, nothing beats a compound design. Models under six inches are an ideal scope for any astronomer who likes to travel. The great advantages of the compound design revolves around the short tube design. With the short tube, looking into the eyepiece will never feel like a stretching exercise. Large scopes are still compact for their aperture and are a common choice for anyone considering building an observatory with a permanently mounted telescope. Like the reflector, chromatic aberration is never an issue. Maksutovs have small fields of view but provide clearer, higher contrast images because of the smaller secondary mirror. Schmidts are shorter and can be built bigger, but at the price of having a substantially larger secondary mirror than the Mak. Either way, size wise, nothing beats a compound design.





What is best?
The question of what is the best telescope has no right or wrong answer. What constitutes an ideal scope is determined by where the scope will be used and the preferences of the observer. In only a few situations can any actual recommendation be given with confidence. If you live in a light polluted city where only planets and bright stars are visible, a small refractor is the way to go, unless you know of a dark site you can go to in the country. Even a 60mm refractor at high power can be used to successfully reveal the cloud bands of Jupiter, the rings of Saturn, and split many double stars. From a bright city setting, trying to find deep sky objects will be a futile search, making a large aperture, deep sky scope useless for its intended purpose. For observers living under dark skies where the Milky Way is easily visible, aperture should be the goal. Because deep sky objects will be easily seen, the largest telescope that can be easily taken outside would be the ideal scope. Just be careful not to go too big. A scope too big to transport easily will probably hardly ever get used. A small compound, under 6 inches, is ideal for anyone who likes to take their hobby out on the road. Some small scopes can be made to fit a lightweight camera tripod, which are perfect for traveling astronomers. For anywhere in between, and recommendations are hard to give. A great piece of advice for someone buying a telescope is to go to a star party and look through as many types of telescopes as possible. The best way to discover your personal preferences is to look, reading descriptions can only go so far. According to many astronomers, the best telescope is one that will be used and not left to sit and collect dust in a closet.


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