Types of Telescopes

Fundamental Parts and Use of Telescopes[1]

Background

There are several characteristics that astronomers use to build a good telescope. One of these is field of view: the area that the instrument can see (Figure 1). Another attribute is angular resolution: the level and sharpness of detail an instrument can resolve. Professional telescopes see only a tiny area (small field of view) but in detail (good angular resolution) while other optical instruments see a larger field of view with worse angular resolution. More important than both attributes is light-collecting power. The amount of light an instrument can collect is directly related to the size of its lens or mirror. The human eye collects light through only a tiny area while some professional telescopes collect light through areas of over one hundred square meters.

Figure 1. The field of view is defined as how much of something you can see.

When using an amateur telescope, it’s often interesting to use different eyepieces. The focal length of the eyepiece working with the focal length of the objective lens or primary (objective) mirror magnifies the object. An eyepiece is typically described by its focal length (for example, 40mm). Though different eyepieces allow you to magnify what you’re seeing, they do not improve the image or give you more information. Professional telescopes seldom use eyepieces and instead use high quality digital cameras to capture images.

Question: Why would astronomers using professional telescopes choose digital images versus looking through eyepieces?

Required Materials

• Project Star telescope kit or a common retail refracting telescope
• A reflecting telescope: Newtonian or Cassegrain or other

 Learning Goals

• indentify parts of refracting and reflecting telescopes and binoculars.
• state the difference between refracting and reflecting telescopes.
• define field of view, angular resolution, and light-gathering power.
• characterize telescopes based on what objects look like when viewed.
• calculate the magnification of the telescopes.

Part I: Identifying parts of a telescope

We start by getting acquainted with the parts of a telescope.  We’ll be looking through two different types of telescopes: a refractor (refracting telescope), which uses lenses to gather and focus light, and a reflector, which uses a mirror or mirrors to focus light and pass the image to the eyepiece lens.

Refracting telescopes

In the simplest terms, a refractor has an objective lens that captures the light from objects and sends the image to its focal point. That focal point must match the focal point of the eyepiece lens if the image is to be in focus when we view through the eyepiece. Figure 2 shows the simplified light paths.

 Figure 2. The paths of light rays within a refracting telescope.

Figure 3 shows a “Project Star” refracting telescope put together from a kit, probably the simplest refractor possible. Note the parts of this telescope and how one would work with sliding the inner tube to match focal points.

 Figure 3. A simple refracting telescope from Project Star using two cardboard tubes and lenses.

Figure 4, on the other hand, shows a refracting telescope you could order. There are, of course, many additional parts that you would need to learn about to use it effectively. Start with labeling the following parts for both the main telescope and its finder (spotting) scope, the one sitting on top. The focal length of the objective lens is usually written on the telescope.

• Eye piece
• Focal length of the eyepiece ______ mm
• Objective lens
• Focal length of the objective lens ______ mm
• Focus knob
• Aperture size

Figure 4. A common beginning refracting telescope mounted on a tripod.

Refracting telescopes have lots of limitations. There is a limit as to how large they are because the objective lens becomes heavier and heavier. The telescope becomes extremely long to match the focal length of the objective lens. Lenses bend the light basing through them, with blue wavelengths bent more than red. This causes a “rainbow” of colors around images–chromatic aberration (Figure 5).

Figure 5. The cause of chromatic aberration through a convex lens.

Reflecting telescopes

Reflecting telescopes don’t have the shortcoming of refractors. Mirrors can be supported from behind and multiple mirrors operated in sync lead to mirrors of enormous size. The largest visible-light telescope currently in operation is at Gran Canarias Observatory, Canary Islands, and features a 10.4-meter (34-foot) primary mirror. Figure 6 details the main parts of two kinds of reflecting telescopes: A Newtonian and a cassegrain. Examine the sketches of this figure and then label the parts of the sample Newtonian and cassegrain reflecting telescopes (Figure 7 and Figure 8). Look for the focal length given for each telescope, usually on a label on the telescope.

Figure 6. The path light takes when using a Newtonian or a cassegrain telescope. The secondary mirror for the cassegrain telescope is mounted in the center of a plate of glass at the light entrance end.

Newtonian reflecting telescope

Label each of the parts listed for this telescope.

• Eye piece
• Focal length of the eyepiece _____ mm
• Finder scope
• Focusing knob
• Primary mirror
• Focal length of the primary mirror _____ mm
• Secondary mirror
• Aperture size

 Figure 7. A typical Newtonian telescope

Cassegrain reflecting telescope

Label each of the parts listed for this telescope.

• Eye piece
• Focal length of the eyepiece _____mm
• Finder scope
• Focusing knob
• Primary mirror
• Focal length of the primary mirror _____mm

• Secondary mirror
• Aperture size

Figure 8. A cassegrain telescope needs weights to balance the “tube” as it points to celestial objects.

Part 2: Viewing with different instruments

Let’s compare the characteristics of these different optical instruments. Pick a distant, single object to look at. Start with using just your eyes, sketch that object in the space provided. Then, using what equipment has been provided, sketch it as you see it through the Project Star, the sample refractor, the Newtonian, and the cassegrain telescopes. Pick a different eyepiece for the refractor or one of the reflecting telescopes and view the object again, sketching what you see. (You will find that you will need to refocus when changing eyepieces.)

When done using a telescope, kindly mess up the focus and point the telescope in a different direction so the next person can get practice in focusing and aiming each telescope.

Part 3: Summary

Use your observations to answer the questions below.

1. Which instruments invert the image?

2. Rank the instruments in order by their field of views (smallest field of view to largest).

3. How did you test or determine the field of view for each instrument?

4. Rank the instruments in order by angular resolution (worst angular resolution to best).

5. How did you test or determine the angular resolution of each instrument?

6. Rank the instruments in order by light gathering power (least to most). Did your ranking relate to sizes of the objective lens or primary mirrors?

7. How did you test or determine the light-collecting power of each instrument?

8. How is the focal length of the eyepiece related to field of view?

The magnification, M, you get depends on the focal length of the objective lens, f_obj or primary mirror and the focal length of the eyepiece f_eye: M = f_obj / f_eye. For example, if the focal length of the objective is 1000 mm and the eyepiece focal length is 40 mm,

M = f_obj / f_eye = 1000 mm / 40 mm =25

9. Calculate the magnification for your observations by filling in this table.

[1] Useful website for this activity: https://www.rocketmime.com/astronomy/Telescope/telescope_eqn.html