Question

Compare and contrast the Newtonian and Galilean telescope designs. Explain the operation of each using your knowledge of optics. Illustrate the operation of each device using ray diagrams. Describe the advantages and disadvantages of each type.

Make sure to include:

- A description of Newtonian and Galilean telescope designs in detail.

- Compares and contrasts the design, functions, advantages, and disadvantages of each telescope.

- Applies knowledge of lenses and or mirrors to explain how the telescope works. Explanation includes a ray diagram with at least two rays drawn from the object to the image.

1

Answer 1

Description:

Galilean Telescope: This is a Refracting type telescope made of lenses. A Galilean telescope has one convex lens and one concave lens. The concave lens acts as the eyepiece and the convex lens acts as the objective. The lens are situated on either side of a tube such that the focal point of the eyepiece and objective are same.

Newtonian Telescope: This is a Reflecting type telescope made of mirrors. It has a Primary mirror, which is a concave (parabolic) mirror, and a secondary mirror, which is a a flat mirror. The eyepiece is a convex lens as usual.

The ray diagrams for Galilean and Newtonian telescopes are shown in Figure 1 and Figure 2 below, respectively.

Figure 1: Galilean telescope schematic construction elements and ray diagram of working.

Figure 2: Newtonian telescope schematic construction elements and ray diagram of working.

Operation:

Galilean Telescope: The eyepiece is situated in front of the focal point of the objective. The distance between the lenses is equal to F. Since converging lenses are of positive optical power and diverging lense have negative power, the distance between the objective and the eyepiece is equal to sum of their focal lengths. The eyepiece intercepts the converging rays coming from the objective, making them parallel and and forms a magnified, erect and virtual image at infinity (QP). The magnification of the system is determined by the ratio of focal length of the objective to focal length of eyepiece.

Newtonian Telescope: As is clear from Figure 2, parallel incident light from a distant object, after getting reflected from the primary mirror, falls on the 45 deg flat secondary mirror. From here, the rays are reflected towards the eyepiece for viewing of object. The primary mirror is a parabolic mirror with a concave reflecting surface. This acts as a focussing mirror. The focussed light rays falling on the secondary mirror forms the image of the distant object through the eyepiece lens.

Advantages/Disadvantages:

Galilean Telescope: The Galilean telescope, although gives erect images, has the severe drawback of an extremely narrow field of view. This makes it, in practice, usable only for magnifications up to around thirty. They have chromatic aberrations and are expensive.

Newtonian Telescope:

Advantages

• They are free of chromatic aberration.
• Less expensive for any given objective diameter.
• Simple fabrication.
• A short focal ratio can be more easily obtained, leading to wider field of view.
• The eyepiece is located at the top end of the telescope tube. Along with short focal ratios, this allows for a much more compact mounting system which reduces cost and increases portability.

Disadvantages

• They suffer from coma. This is an off-axis aberration which causes inward flare of the image towards the optical axis. This flare is zero on-axis, and increases linearly with field angle, showing an inversely proportionality to the square of mirror focal ratio.
• Collimation is difficult for portable Newtonian tekescopes. This is because transport and handling may cause disalignment of the primary and secondary mirrors. This implies the telescope needs to be re-aligned (collimated) every time it is set up.