Choosing A Refractor Telescope

I truly hope that your interest in coming to this hub-page is because you are interested in Astronomy as a hobby. I was 11 years of age when my father bought us our first Refractor telescope. It was only a 3" telescope, but it sure opened a new world for me and my brother Steven. There were six children in our family, and as I remember, even the four smallest kids were picked-up and allowed to view the moon, planets, sun and stars. And, that said, everyone of us were struck by the beauty we saw.

Oh sure, I grew up during the 50's and 60's, when space was made out by John Kennedy to be the new frontier and challenge for America. Surely that helped excite us and make us aware of the new frontier, but it was not the reason dad bought us that scope. He bought it because he truly wanted to share with his children. He always had a penchant for involving us in his hobbies, and this was a new one for him, and he chose to share it with us. My father was learning at the same time we were, and he knew that was special to me and my older brother.

The small refractor telescope we had was not very powerful, especially in the city of Los Angeles with all of the ambient light interfering with our view. The sun was wonderful to view with filters, the moon was nice, but not in full detail. Saturn was well defined, but we could not count the rings. Mercury proved a problem to catch with the city putting up a false horizon. We saw Venus in all it's glory over and over. You might think this was boring, but we did not. It was not long before we took our first family trip to Randsburg, out in Death Valley, California. That trip amplified the hobby of Astronomy for all of us. Suddenly the sky was ablaze with so many objects and opportunities to view, that our hobby took on a whole new life.

We actually located ourselves in a small meteor crater not far from an old gold mine. There was a fire ring, hot chocolate, good food and all of us sharing in a new adventure. The desert night was so clear it seemed unreal compared to the night sky of the city. With that clarity came viewing opportunities that are hard to compare with or explain fully here. When you take a telescope out to the desert or mountains, suddenly, star clusters can be seen with the naked eye. Sure, they may appear dim to the eye, but train the scope on them and you will see a lot more definition than you can imagine. This is truly the opportunity you have with Astronomy. You and your family with friends can share something very educational and exciting together. I promise, it is something you will take part in the rest of your lives.

I will introduce you to the refractor telescope and how it functions. I'll provide you with enough information to make an informed choice about whether this scope is right for you. The suggestions I will make are based on my personal experience, and my intention that you choose something that will give you good viewing pleasure at the lowest cost. As a beginner Astronomer, you should not be investing in a large expensive telescope until you have learned the astronomical tables and how to apply them, as well how to photograph the heavenly objects of choice. Learning the ropes is key here, and a great investment in money is not needed. So I will suggest quality scopes in the range of $200.00 to $750.00, and present accessory and learning options for you to consider.

ClickHere: Quality Telescopes And Mounts

I think the refractor telescope is possibly the best design overall. It can be an expensive choice when choosing one with a large enough objective aperture lens. The refractor scope uses a totally different approach to producing an image than a reflector telescope. I am going to explain the Newtonian Refractor telescope and how it works so you can determine weather it is the choice for you.

The refractor telescope was the earliest and first telescope design. It appears in the Netherlands in about 1608, and was credited to three individuals, Lippershey, Janssen and Middleberg. Galileo Galilei happened into knowledge of the design while in Venice 1909, and constructed a version of his own. Many people were discussing the design ideas during that period of time, but Galileo was the first to build an operational version and lay claim to it.

All refractor telescopes use the same design features. The combination of an objective (primary) lens, and some type of eyepiece (secondary lens) to gather more light for the human eye to focus on. This concept presented the viewer with a bright clear and magnified virtual image. The principles of these early scopes was in-line. Modern refractor telescopes commonly use a prism encased in a special housing to reflect the objective image 90° upward through the secondary lens and into the viewers eye.

See images one and two at right. There are two types, the Star Diagonal, which simply reflects 90° upward and corrects for what is called chromatic aberration, and the Amici roof prism that corrects for both image reversal, chromatic aberration and image reversal. These prisms are housed in easily inserted assemblies that accommodate insertion of various lenses of different power ratings, and focal adjustment schemes allowing you to adjust focal lengths.

IMAGE CREATION AND REFRACTION

As can be seen in the Galilean Focal plane below, the original concept used two lenses that were convergent-divergent in relation to each other. The main objective (primary) lens was plano-convex and the secondary divergent viewing lens was plano-concave. This approach produced images that were properly upright at the viewing end. Because of flaws in the design such as the shape of the lenses and narrow field of view, the images were blurry and somewhat distorted. As you can see in the image below, parallel rays of light from a distant object (y) would be brought to focus along the focal plane of the front objective lens. (L1/y') However, the diverging eyepiece (L2) lens intercepts these rays and renders them parallel again, but traveling at a larger angle to the optical axis. This leads to an increase in the optical size, which is apparent magnification. The final image (y") is a virtual image located at infinity and retains the proper orientation rather than being inverted 90 degrees. At it's base, refraction and the resulting telescope name, derives from the characteristics of these lenses and the process of light ray bending imposed upon the rays being trapped and manipulated by them along the focal plane.

The design was improved by Johannes Kepler in 1611, when the eyepiece (secondary lens) was replaced with a convex lens. As can be seen in the first focal plane image above right, the improved arrangement allowed that the light rays emitted from the eyepiece were convergent in nature. This allowed for a much wider field of view and greater eye relief, but the resulting image was 90° inverted (upside-down).

Much greater magnification was made possible by this arrangement, but other image aberrations were manifest too. Chromatic aberration became evident and is defined by a halo of colored light surrounding viewed objects. Bright astronomical objects were invariably surrounded by a rainbow of colors which distorted the viewable image. Spherical aberrations were apparent in these telescopes and was manifest by a squashing of the image giving it a somewhat ovoid appearance. This was as if the telescope had astigmatism, a common defect in the human eye where the eyeball is ovoid shaped. It was later theorized that white light was composed of a spectrum of colors and that the prism effect of refracted light separated and made evident the color spectrum through magnification. It was specifically the characteristics of light refraction magnification that caused this phenomena to occur. Sir Issac Newton visited this concept of light in the late 1660's and introduced the Reflector telescope as a solution.

REFRACTOR IMAGE IMPROVEMENTS

Later, Achromatic and Apochromatic refractor telescope principles were developed, and corrected the chromatic and image inversion aberrations of these type telescopes. It was not until 1733 that an Achromatic Lens was invented by Chester Hall, an English Barrister. The patent came in 1758 by John Dolland. The new design did way with the need for very long focal lengths by using an objective (primary) lens made of two pieces of glass with different dispersions. The 'crown' and 'flint' glass approach effectively corrected for chromatic and spherical aberration. Both sides of each lens is ground and polished, then assembled together. (See second focal plane image above right) Achromatic lenses are corrected to bring two wavelengths (red and blue) into focus on the same plane.

The Apochromatic lens creation brought with it an approach that brought extra-low dispersion materials used to bring three wavelengths of light (red, blue and green) into focus on the same focal plane. The residual error spectrum of these lenses is much lower than the Achromatic approach, and used glass than contained elements of fluorite, which brought about the creation of ED glass used to make the lenses. This approach made it possible to build high-end refractor telescopes for the amateur telescope market. Telescopes with 22" objectives were made possible, but most commercially manufactured refractor lenses run in the 3" to 6" range and are of very high quality for amateur astronomy use. These telescopes are expensive to purchase also, and should be considered by experienced astronomers and astrophotographers who can find use to justify cost. Then comes the Newtonian approach using prism mirrors and bringing down cost for most of us amateurs as follows.

NEWTONIAN REFRACTOR TELESCOPES

A Newtonian Refractor is a more cost effective solution to the Achromatic and Apochromatic approach at correcting chromatic, spherical and image inversion aberrations in refractor telescope design. As Newton discovered in 1669, white light is composed of a spectrum of color in the RBG ranges. It is in the separation and magnification of light rays that diffusion of color impedes an images quality and he proposed using mirrors to collect and amplify light as a form of magnification. The Newtonian refractor telescope design uses a prism approach to correct the and reassemble the focal plane into a viewable image on the secondary end of the optical telescope tube. A cheaper Galilean plano-convex lens, which is ground on one side only, is used as the objective (primary) lens. The focal plane is the same as image one above left shows, however, the focal plane is directed into a Star Diagonal, or Amici Roof Prism instead of another plano-convex or plano-concave lens. This assures a cost effective approach because expensive lens needs are eliminated entirely and a quality image that is corrected can be attained.

The Star Diagonal prism is depicted in images three and five respectively above left. As can be seen in image five, the focal plane comes to rest on the lower exposed portion of the prism (E) and is bent, or reflected upwards 90°, striking that surface and finally exiting thorough an inserted secondary lens of varying power rating. (See lower refractor telescope image) This is accomplished by the fact the back surface planes of the prism as light strikes it (E) is coated with aluminum oxide creating a mirror surface. So also is the prism surface depicted by the reflection of light rays at a 45° angel above the plane (F), effectively bending the light 90° in total through the secondary lens. A Star Diagonal corrects for image inversion effectively flipping the image right-side up, and allows for comfortable viewing of images that are at zenith above the telescope. If the scope must point upwards to view a celestial image, it is difficult to engage your viewing eye on the secondary lens at the rear of the scope. The diagonal makes this viewing a more comfortable one. It does not correct for spherical or chromatic aberration.

An Amici Roof prism corrects for all aberrations discussed so far. It was named after it's inventor Giovanni Amici and is a type of reflecting prism used to deviate a beam of light by 90° while simultaneously inverting the image. It is shaped as a standard right angle prism with an additional 'roof' section consisting of two other faces meeting at 90° on the longer slope side of the triangle edge. Total internal reflection from the roof section flips the image laterally. The left or right handedness of the image is left unchanged. As seen in the image above, the prism orientation is swung backwards in comparison to the Star Diagonal prism. The non-dispersive Amici prism roof surfaces are usually coated to provide a mirror surface for inversion of the image. both the Amici and Star Diagonal prisms come in 1.25" - 2" diameters. The 1.25' is commonly used to view images with the eye and the 2.0" diameter is commonly used to fit a camera for astrophotography purposes. Secondary viewing lenses are produced in mutiple power ratings and in a 1.25" and 2.0" version too.

In summary, the Newtonian approach is both cost effective and a solution for the common aberrations attributable to light refraction. It is a mix between reflector telescope and the original Galilean refractor telescope designs. I will be recommending three basic refractor and three intermediate options below. All of these scopes will possess a long focal length to give a clear crisp image view, and to intruduce minimum chromatic-spherical aberration. If you are looking to buy a more expensive refractor telescope using the Achromatic and Apochromatic lens configurations, I will direct you to my website below. I hope this article has been informative and helpful in making a determination about astronomy as a hobby and which telescope option you may be interested in. Below the article I have provided likes to more hubs written by myself and others, that can be helpful in your future learning process.