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• Print publication year: 2017
• Online publication date: January 2018

# 9 - Simple Optical Instruments I: The Eye and the Magnifier, Eyepieces, and Telescopes

## Summary

Objectives

1. Develop basic skills to make and manipulate simple optical instruments: Magnifier, telescope, and eyepiece.

2. Determine the locations of aperture stops, field stops, entrance and exit pupils, and entrance and exit windows of the systems.

3. Formulate general practical rules about how the performance of these optical instruments influences their image-making properties.

Background

Real optical systems are always limited in size, so it is important to know how the size of an optical system affects its performance. Consider the simplest optical system composed of one thin lens, as shown in Fig. 9.1. As we increase the angle between the incident ray emitted from the on-axis point O and the optical axis, the ray will eventually hit the edge of the lens. For angles beyond this slope angle, the light rays will not be imaged by the lens. An axial ray traced near the outer edge of the system (e.g. the lens rim) is termed a marginal ray. The physical opening in an optical system that limits the amount of light that can be collected is called the aperture stop of the system (the lens rim serves as the aperture stop in Fig. 9.1). A chief ray is a ray propagating through the center of the aperture stop.

An aperture stop is a very important concept in optical system design because it determines the ability of the optical system to collect light. In other words, the aperture stop limits the brightness of the image. The image of the aperture stop, as seen from the object space (the space to the left of the first element of the optical system), is called the entrance pupil of the system. The image of the aperture stop as seen from the image space is called the exit pupil of the system. In our simple example of an aperture stop, entrance and exit pupils coincide (Fig. 9.1).

There is a simple algorithm (described below) to find the entrance pupil, the aperture stop, and the exit pupil.(1) Make images of all the optical elements of the optical system as seen from the object space.