Telescope Basics

Telescope Basics

A telescope is an instrument that collects and focuses light. The nature of the optical design determines how the light is focused. Some telescopes (known as refractors) use lenses and other telescopes, known as reflectors (Newtonians), use mirrors. Then, the Schmidt-Cassegrain and Maksutov telescopes use both mirrors and lenses. Each optical design is briefly discussed below: 

The Refractor

Developed in the early 1600s, the refractor is the oldest telescope design. It derives its name from the method it uses to focus incoming light rays. The refractor uses a lens to bend or refract incoming light rays, hence the name. Early designs used single element lenses. However, the single lens acts like a prism and breaks light down into the colors of the rainbow, a phenomenon known as chromatic aberration. To get around this problem, a two-element lens, known as an achromat, was introduced. Each element has a different index of refraction allowing two different wavelengths of light to be focused at the same point. Most two-element lenses, usually made of crown and flint glasses, are corrected for red and green light. Blue light may still be focused at a slightly different point. Higher priced units use ED (low dispersion) or apochromatic (APO) optical designs to virtually eliminate chromatic aberration.

The Newtonian

A Newtonian reflector uses a single concave mirror as its primary mirror. Light enters the tube traveling to the mirror at the back end. There light is bent forward in the tube to a single point, its focal point. Since putting your head in front of the telescope to look at the image with an eyepiece would keep the reflector from working, a flat mirror called a diagonal intercepts the light and points it out the side of the tube at right angles to the tube. The eyepiece is placed there for easy viewing. Newtonian Reflector telescopes replace heavy lenses with mirrors to collect and focus the light, providing much more  light-gathering power for the dollar. Because the light path is intercepted and reflected out to the side, you can have focal lengths up to 1000 mm and still enjoy a telescope that is relatively compact and portable. A Newtonian Reflector telescope offers such impressive light-gathering characteristics you can take a serious interest in deep space astronomy even on a modest budget. Newtonian Reflector telescopes do require more care and maintenance because the primary mirror is exposed to air and dust. However, this small drawback does not hamper this type of telescope’s popularity with those who want an economical telescope that can still resolve faint, distant objects.

The Schmidt-Cassegrain and Maksutov

The Schmidt-Cassegrain optical system (Schmidt-Cass or SCT for short) uses a combination of mirrors and lenses and is referred to as a compound or catadioptric telescope. This unique design offers large-diameter optics while maintaining very short tube lengths, making them extremely portable. The Schmidt-Cassegrain system consists of a zero power corrector plate, a spherical primary mirror, and a secondary mirror. Once light rays enter the optical system, they travel the length of the optical tube three times. Inside the optical tube, a black tube extends out from the center hole in the primary mirror. This is the primary baffle tube and it prevents stray light from passing through to the eyepiece or camera. The Maksutov optical system is similar to the SchmidtCassegrain but can have a secondary mirror or an aluminized spot in place of the secondary mirror. There are many variations of the Maksutov design.

Image Orientation

The image orientation changes depending on how the eyepiece is inserted into the telescope. When using the star diagonal with refractors and Schmidt-Cassegrains or Maksutovs, the image is right-side-up, but reversed from left-to-right (i.e., mirror image). If inserting the eyepiece directly into the focuser of a refractor or the visual back of the Schmidt-Cassegrain or Maksutov (i.e., without the star diagonal), the image is upsidedown and reversed from left-to-right (i.e., inverted). Newtonian reflectors produce a right-side-up image but the image will appear rotated based on the location of the eyepiece holder in relation to the ground. Newtonian reflectors are best for astronomical use where right-side-up does not matter.

Focusing 

To focus your refractor or Newtonian telescope, simply turn the focus knob located directly below the eyepiece holder. The Schmidt-Cassegrain focusing mechanism controls the primary mirror which is mounted on a ring that slides back and forth on the primary baffle tube. The focusing knob, which moves the primary mirror, is located on the rear cell of the telescope to the right, or just below (on some models) the star diagonal and eyepiece. Turn the focusing knob until the image is sharp. If the knob will not turn, it has reached the end of its travel on the focusing mechanism. Turn the knob in the opposite direction until the image is sharp. Once an image is in focus, turn the knob clockwise to focus on a closer object and counterclockwise for a more distant object. A single turn of the focusing knob moves the primary mirror only slightly. Therefore, it will take many turns (about 30) to go from close (near) focus to infinity. For astronomical viewing, out of focus star images are very diffuse, making them difficult to see. If you turn the focus knob too quickly, you can go right through focus without seeing the image. To avoid this problem, your first astronomical target should be a bright object (like the Moon or a planet) so that the image is visible even when out of focus. Critical focusing is best accomplished when the focusing knob is turned in such a manner that the mirror moves against the pull of gravity. In doing so, any mirror shift is minimized. For astronomical observing, both visually and photographically, this is done by turning the focus knob counterclockwise. Note: If you wear corrective lenses (specifically glasses), you may want to remove them when observing with an eyepiece attached to the telescope. However, when using a camera you should always wear corrective lenses to ensure the sharpest possible focus. If you have astigmatism, corrective lenses must be worn at all times.

Calculating Magnification

You can change the power of your telescope just by changing the eyepiece (ocular). To determine the magnification of your telescope, simply divide the focal length of the telescope by the focal length of the eyepiece used. In equation format, the formula looks like this:

Let’s say, for example, you are using a 25mm eyepiece that may have been supplied with your telescope. To determine the magnification you simply divide the focal length of your telescope (for this example we will assume your telescope has a focal length of 1000 mm) by the focal length of the eyepiece, 25 mm. Dividing 1000 by 25 yields a magnification of 40 power. Although the power is variable, each instrument under average skies has a limit to the highest useful magnification. The general rule is that 60 power can be used for every inch of aperture. For example, the telescope above is 4 inches in diameter. Multiplying 4 by 60 gives a maximum useful magnification of 240 power. Although this is the maximum useful magnification, most observing is done in the range of 20 to 35 power for every inch of aperture which is 80 to 140 times for the telescope used in the example. You can determine the magnification for your telescope the same way. 

Note on Using High Powers – High powers are used mainly for lunar and planetary observing where you can greatly enlarge the image, but remember that the contrast and brightness will be low due to the high magnification. High powers can be used occasionally when conditions allow – you will achieve the power but the image will be dark with low contrast because you have magnified the object to considerably. For the brightest images with the highest contrast levels, use lower powers.

Determining Field of View

Determining the field of view is important if you want to get an idea of the angular size of the object you are observing. To calculate the actual field of view, divide the apparent field of the eyepiece (supplied by the manufacturer) by the magnification. In equation format, the formula looks like this:

As you can see, before determining the field of view, you must calculate the magnification. Using the example above, we can determine the field of view using the same 25 mm eyepiece. The 25 mm eyepiece has an apparent field of view of 50°. Divide the 50° by the magnification, which is 40 power. This yields an actual field of 1.25°. To convert degrees to feet at 1,000 yards (which is more useful for terrestrial observing) simply multiply by 52.5. Continuing with our example, multiply the angular field of 1.25° by 52.5 and this produces a linear field width of 65.6 feet at a distance of one thousand yards.

General Observing Hints 

When using any optical instrument, there are a few things to remember to ensure you get the best possible image. 

• Never look through window glass. Glass found in household windows is optically imperfect, and as a result, may vary in thickness from one part of a window to the next. This inconsistency can and will affect the ability to focus your telescope. In most cases you will not be able to achieve a truly sharp image, while in some cases, you may actually see a double image. 

• Never look across or over objects that are producing heat waves. This includes asphalt parking lots on hot summer days or building rooftops. 

• Hazy skies, fog, and mist can also make it difficult to focus when viewing terrestrially. The amount of detail seen under these conditions is greatly reduced. 

• If you wear corrective lenses (specifically glasses), you may want to remove them when observing with an eyepiece attached to the telescope. When using a camera, however, you should always wear corrective lenses to ensure the sharpest possible focus. If you have astigmatism, corrective lenses must be worn at all times.