The Optical Tube Assembly

• Refractor Telescope
• Newtonian Reflector Telescope
• Schmidt-Cassegrain Telescope

The ED Apochromatic Refractor Telescope

The Refracting Telescope was the telescope design used by Galileo to usher in the age of telescopic astronomy. With his small refractor, Galileo was the first person to witness the craters of our moon, discover four other moons around Jupiter and determine the phases of Venus. Galileo did NOT make the first telescope. No one knows for certain who or when the first telescope was designed. Some historians feel it was a spectacle maker from Holland, Jan Lippershey. But archaeologists have unearthed glass in Egypt dating about 3500 BC.

The Galilean refractor placed a convex (curved outward) objective lens in front of a concave (curved inward) ocular lens at a given distance. This produced an extremely narrow, upright image. Johannes Kepler improved on Galileo's design by swapping the concave eyepiece for a double convex lens system just behind the prime focus. The Keplerian refractor was far superior and many modern refracting telescopes continue to use the design.

Unfortunately, both the Galilean and the Keplerian designs have several optical problems. Chromatic aberration is the result of light focusing at different places resulting in an image that appears blurry and surrounded by a halo of color. The ED Apochromatic refractor, shown below, uses modern optical design to correct for chromatic aberration by using a precisely computed combination of lens surface radii and air spacing distance between the elements of the objective. The resulting telescope design brings all visible wavelengths of light to virtually the same focus resulting in a clear image.

How a Apochromatic Refractor works

Light enters from the left and passes through the hard (KF3) glass element of the objective lens (1), then through the ED (extra-low dispersion) glass element (2). This two lens objective system permits the telescope to bring all visible wavelengths (colors) of light to the same focal point (3). Everything is housed in the optical tube (4). The focuser (5) moves the eyepiece through the focal point (3).

More about the Apochromatic Refractor

Apochromatic refractor (or Apos for short) are not light buckets like a Newtonian or Schmidt-Cassegrain. The cost per aperture unit on Apochromatic refractor is the highest for any telescope. If you are looking for the finest textbook perfect, high contrast, high resolution images of the sky and money is no problem; than a modern Apochromatic refractor is the telescope for you.

The Newtonian Reflector Telescope

Named for its inventor, Sir Isaac Newton, no other telescope type gives more aperture for the money. Newtonians can produce moderate to high powered images of the moon and planets. Deep sky fans like the Newtonian's ability to produce large panoramic views of star fields. Commercially made instruments range in size between 3 inches to over 2 feet in diameter with focal ranges from f/3.5 to f/10. Both rich and normal fields of view are supported by this class of telescope. The following figure shows the basic makeup of the Newtonian reflector telescope.

How a Newtonian telescope works

Light enters from the left and passes the entire length of the optical tube(6) to the primary mirror(1) where it is reflected back up the tube to the secondary mirror(2) mounted in the center of the optical tube by the spider(3). The secondary mirror reflects the light at a 45° angle so that it passes into the eyepiece set at the focal point(5) located at the end of the focuser(4).

Newtonian Reflectors give a correct image which is rotated at an angle depending on the eyepiece angle with respect to the vertical.

More about Newtonians

Newtonians are great telescopes for the beginner. A small outlay of money will give the user a telescope with a good sized 8 to 10 inch aperture. Best of all, you do not have to throw out your initial investment when you decide to add the little extras to your scope. Many companies offer upgrade options like digital setting circles, clock drives, motorized focusers, dew removers, transport systems, DC converters and much more. All are easily retrofitted onto your existing instrument.

The only real downside to a Newtonian is its larger size. This makes the scope a little inconvenient when transporting it to remote viewing locations.

The Schmidt-Cassegrain Telescope

The Schmidt-Cassegrain class of telescope was named for the German astronomer Bernhard Schmidt and French sculptor Sieur Cassegrain. Known as a Schmidt-Cas or just SCT, this hybrid telescope design has outpaced both the reflectors and refractors to become the most popular telescope among amateur astronomers.

How a Schmidt-Cassegrain telescope works

Light enters from the right and passes through a correcting plate (1). This correcting plate is a thin, two sided aspheric correction lens. The corrected light passes the entire length of the optical tube to a spherical primary mirror(2) where it is reflected back up the tube to the convex secondary mirror(3). The convex secondary mirror increases the effective focal length of the primary mirror. The resulting light is returned back down the scope into the primary baffle tube (4) and focuser (5) to the eyepiece set at the focal point(6) located on the end of the focuser. The primary baffle tube passes through a hole in the primary mirror.

More about the SCT

Commercial SCTs range in apertures from 4 to 16 inches. The SCTs greatest asset is its compact design. No other class of telescope fits as large of aperture and long focal length into such a short tube assembly. This makes the SCT easy to store and transport. The optics tend to stay cleaner longer since they are enclosed within the telescope assembly.

SCTs do have a few drawbacks. SCTs produce fainter images with less contrast than other telescope designs with similar aperture sizes. This is due to the comparatively large secondary mirror required to reflect the light back the eyepiece. SCTs are notorious for dewing up very quickly on the correcting plate. You will here many hair dryers going at a star party drying off the correcting plate on SCTs. If you are looking at a SCT you should also look for a dew removing system of some type. SCTs also tend to have a higher price tag than other types of telescopes.

If small size is important and you have the extra money, a SCT is the scope for you.