The Edmund Scientifics Astroscan: an Ideal Small Telescope for Beginners
Patrick Draper pdrap@pdrap.org
September 8th, 2004
Reproduction is allowed with permission
Illustration 1:The Edmund Scientifics Astroscan
The Edmund Scientifics
Astroscan is a small telescope that delivers a lot of performance in a
compact package. It was introduced almost 30 years ago, won the
Industrial Design award in 1976, and is still a terrific telescope
despite the age of the design. Today, a beginning star gazer shopping
for their first telescope will find a rich variety of excellent
telescopes to choose from, and they owe it to themselves to consider the
Astroscan.
Illustration 2:The Newtonian telescope
Hundreds of different telescope designs are marketed to amateur astronomers, but they fall into two basic categories, refractor and reflector. The refractor telescope is the type with a large lens on one end of a long tube, and an eyepiece on the other end. The lens at the front is called the objective lens, and it forms an image by focusing light into the eyepiece in the same way a magnifying glass focuses an image. When most people think of a telescope, they are thinking of a refractor telescope. The reflector telescope uses a curved mirror at the back of the telescope instead of a lens at the front to focus light. Light enters the front of the telescope, bounces off the curved mirror at the back of the telescope, and moves back towards the front of the telescope to an angled flat mirror where it bounces out the side of the telescope into the eyepiece. Both types of telescope create excellent images. The reflector has the advantage of price over the refractor because making a the objective lens of a refractor is a more expensive than fabricating the mirror in a reflector. A 6 inch refractor telescope will cost many thousands of dollars, but a 6 inch reflector telescope will cost only a few hundred. Very advanced amateur astronomers sometimes prefer refractor telescopes because they are sometimes better at viewing specific types of astronomical objects, such as planets or globular clusters, at very high levels of magnification. For the purpose of general viewing of a wide variety of targets in the sky, a reflector telescope will serve just as well. The Astroscan is an example of a reflector telescope.
The most important component of any telescope, no matter how small or large, is the telescope mount. The mount's job is to hold the telescope steadily, allow the user to aim the telescope, and (if equipped) to track the stars as the Earth rotates. The importance of a steady telescope mount cannot be overemphasized. A very small telescope standing on a sturdy mount will be more useful than a large telescope standing on a wobbly mount. Because a telescope is an instrument that provides magnification, any slight movement of the telescope will be amplified when looking through the eyepiece. The quality of a telescope's optics don't matter if the object being viewed is jumping randomly around the field of view. A way to test a mount is to look through the telescope and bump the tube. The vibrations will be visible in the eyepiece, and should dampen out within a half-second. If the vibrations take longer to dampen, then the mount is wobbly, and will be difficult to use. The telescope bearings should also fit tightly together, yet move freely. As the tube is pushed to view a target, the target should remain centered in the field of view as the tube stops. If there is any undesirable slop in the construction of the mount bearings, the scope will settle a bit when it stops moving, and the position of the image will shift. Centering an object in a cheaply mounted telescope can be an exercise in frustration.
There are mounted several types of telescope mounts, but telescopes marketed to beginners tend to use two types, the altitude-azimuth (alt-az) mount, and various types of motorized "star seeking" robotic mounts. The altitude-azimuth (alt-az) mount allows a telescope to move up and down, and rotate from side to side. The motorized robotic mounts are relatively new on amateur class telescopes, and will accept commands on a hand held computer controller module to automatically point the telescope at any astronomical target in the computer's database. The motorized robotic mounts will also move the telescope to track a target as the Earth rotates. The target finding features are especially beneficial to a person who wants to see a variety of objects but isn't interested in memorizing their locations. If the amateur astronomer's goal is to learn where in the sky astronomical targets are located, it is better to stay away from the motorized mounts.
Illustration 3: The Astroscan Base
The Astroscan's mount is a very simple and stable type which works like a ball and socket. The telescope body is rounded like a sphere at the bottom end, and the whole telescope rests on a curved base, padded with felt. Viewing any object in the sky is as simple as holding the telescope and pushing it in the right direction. The curved metal base is only a couple inches high, and can sit on any sturdy platform such as a picnic table. Edmund Scientifics also sells an optional tripod which will hold the curved base and the telescope at an adjustable eye level if a table is not convenient.
Illustration 4: The Astroscan Resting on its Base
The Astroscan has a primary mirror diameter of 105mm (4.25 inches). The diameter of the mirror determines how much light the telescope gathers, and is the main way that a telescope optical system is described. Typical amateur telescopes typically range from about 90mm to about 300 millimeters (12 inches). There are those who own telescopes much larger than that, but they are generally very experienced amateurs with large budgets. In comparison to other amateur telescopes, the Astroscan is quite small, meaning that the number of objects visible with it is less than the number of objects visible in a larger scope. This is not a terrible problem though, because most amateur astronomers spend most of their time viewing the brightest objects in the skies. There is a group of 110 such bright objects known as the Messier objects that includes many types such as galaxies, globular clusters, nebulae, open clusters, and supernova remnants. All of these are visible using the Astroscan.
Illustration 5: View of the Astroscan's Primary Mirror
The other number important to the description of a telescope optical system is the f-number. This is a ratio of the diameter of the primary mirror to the focal length of the primary mirror. For example, the Astroscan has a primary mirror diameter of 105mm, and a focal length of 440mm, giving an f-number value of f/4.2. The f-number is a general indicator of some important characteristics of a telescope, such as field of view, magnification, and image brightness. Telescopes with higher f-number values will tend to have narrower fields of view, higher magnification, and lower image brightness. A telescope with the same sized primary mirror and a lower f-number value will have a wider field of view, lower magnification, and higher image brightness. The actual performance of a telescope is also dependent on the eyepiece focal length. Because there is a large variety of high quality eyepieces available, the preference has shifted over the years to smaller f-numbers, because it allows the telescope to be smaller, shorter, and more portable. The Astroscan is sometimes referred to as a rich field telescope, meaning that it has very low f-number value. This makes the Astroscan particularly suited to scanning over the large, bright, glowing clouds of gas and dust that are especially conspicuous along the summertime Milky Way. Astronomers using the Astroscan really do see a view that is rich with stars and brightly glowing nebulae.
The magnification of a telescope is calculated by dividing the focal length of the primary mirror by the focal length of the eyepiece. The Astroscan is supplied with a 28mm eyepiece, resulting in a magnification of just under 16 times. Sixteen power might seem low when department store telescopes commonly advertise magnifications of more than 500X, but sometimes lower is better. High magnifications make a telescope difficult to use in several ways. The images are bigger, but because the light is spread out over a wider area, they are much dimmer. Doubling the magnification of an image causes it to become four times dimmer. Motion of the telescope is also amplified. At low powers a slight touch of the telescope tube might cause a slight amount of wiggling in the image, but at high powers the same touch will cause the image to gyrate wildly. The field of view is also much narrower at high powers, and that can make finding an object in the sky much harder. Atmospheric turbulence, which causes the stars to twinkle, can become overwhelming at high powers. If the atmosphere isn't perfectly still, it might be impossible to see anything but a blurry, moving shape. Eyepieces have a property called eye relief, which is usually greater in low magnification eyepieces. Eye relief is a measure of how close the eye has to be to the eyepiece to see an image, and if it's very short, people with eyeglasses might not be able to get close enough to the eyepiece. The reason telescope advertisers like to highlight the magnification has more to do with marketing than with the best way to use the instrument. Don't be fooled; a telescope used at relatively low power is far more enjoyable and useful for almost everything. A good rule of thumb is that the highest reasonable magnification is 25X for every inch of aperture. Therefore a 4-inch telescope such as the Astroscan can provide a maximum of 100X, under the best atmospheric conditions.
Illustration 6: 28mm RKE Eyepiece
A telescope's eyepiece is the lens (actually set of lenses) closest to the eye. There are many different types, and they range in price from very inexpensive, to many hundreds of dollars. The most expensive types use glass manufactured with rare-earth elements such as Lanthanum, and utilize as many as 6 or 7 separate pieces of glass. Though there are very good uses for those eyepieces, there are some excellent designs that are very inexpensive. The standard barrel sizes are 1.25 inches and 2 inches barrel diameter. The cheapest telescopes used to be equipped with eyepieces having a barrel diameter of 0.96 inches, which were best avoided. The Astroscan uses eyepieces with a diameter of 1.25 inches. In years past a single RKE type eyepiece having a focal length of 28mm was supplied with the telescope. Currently is it supplied with 28mm (16X) and 15mm (30X) Plossl type eyepieces. Detailed information on the RKE and Plossl eyepiece designs can be discovered on the internet. For the beginner, it is sufficient that both the RKE and Plossl type eyepieces are excellent low-cost designs, and will provide sharp, clear images with the Astroscan telescope.
Illustration 7: Angular Diameter of the Moon
The finder is used to point the telescope at a target in the sky. Many times, a finder is an actual telescope, smaller and less powerful than the main telescope. The field of view of a finder telescope is large, which allows objects to be found easily. The Astroscan with its low power eyepiece has a field of view that that has an angular diameter of 3°. The angular diameter is a measurement of size that is based on the same angles that a protractor measures. For comparison, the full moon has an angular diameter of 0.5°. The field of view of the Astroscan with the 28mm (16X eyepiece) is therefore six times the angular diameter of the full moon. With such a large field of view, the Astroscan has little need of a smaller telescope to act as a finder scope, so it relies on a metal sighting device that attaches to the telescope. The observer looks through two metal rings at the area of the target to aim the telescope.
Illustration 8: Rigel Quickfinder reflex finder
A more modern sighting device called a reflex finder can be used with the Astroscan. A reflex finder works like a head-up display in a fighter jet. A slanted piece of glass reflects an image of a bullseye or cross hairs back to the observer. The slanted glass also allows starlight to pass through, so the effect is as if a glowing red bullseye is floating in the sky over the stars. Pointing the telescope is as simple as looking through the reflex finder and placing the bullseye on the part of the sky to view. The original reflex finder was called the Telrad. The Telrad cannot be used with the Astroscan because of the bulbous shape of the telescope. The Telrad also has significant weight, unbalancing the telescope. Fortunately, there is another brand of reflex finder called the Rigel Quickfinder which is very light in weight, and shaped so that the view port is extended away from the telescope. The Rigel Quickfinder is not supplied with the Astroscan, but can be purchased separately for about $40.
Illustration 9: Astroscan with Rigel Quickfinder
Astronomy is an exciting hobby that can last for a lifetime, and a good telescope can only enhance the experience. A telescope that is well-built will provide enjoyment for years. Growth in the hobby will of course lead to the purchase of larger and more capable telescopes. In that eventuality, a quality instrument will retain value, and can provide another beginner many years of enjoyment. Too many poor quality department store telescopes are used once or twice, then banished to the closet forever. Though the Astroscan was designed almost 3 decades ago, its combination of quality, excellent optics and mount, and overall thoughtful design ensures that it will remain an excellent small telescope for the beginning amateur astronomer for a very long time.
Specifications
Primary mirror diameter: 4.25 inches (105 mm), parabolic, f/4.2
Magnification with 28mm eyepiece: 16 power
Price: $199, basic package
Edmund Scientifics
60 Pearce Ave.
Tonawanda, NY 14150-6711
800-728-6999
716-874-9091
http://www.scientificsonline.com
Catalog number: 3002001