optics

Visual angle

The visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc. It also is called the object s angular size.
The diagram on the below shows an observer s eye looking at a frontal extent (the vertical arrow) that has a linear size S, located in the distance D from point O.
For present purposes, point O can represent the eye s nodal points at about the center of the lens, and also represent the center of the eye s entrance pupil that is only a few millimeters in front of the lens.
The three lines from object endpoint A heading toward the eye indicate the bundle of light rays that pass through the cornea, pupil and lens to form an optical image of endpoint A on the retina at point a. The central line of the bundle represents the chief ray
Likewise for object point B and its retinal image at b.
The visual angle V is the angle between the chief rays for A and B.










Diagram showing visual angle V.


Beam Expanders
A laser beam expander is designed to increase the diameter of a collimated input beam to a larger collimated output beam. Beam expanders are used in applications such as laser scanning, interferometry, and remote sensing. Contemporary laser beam expander designs are afocal systems that developed from well-established optical telescope fundamentals. In such systems, the object rays, located at infinity, enter parallel to the optical axis of the internal optics and exit parallel to them as well. This means that there is no focal length to the entire system.
Theory: Telescopes
Optical telescopes, which have classically been used to view distant objects such as celestial bodies in outer space, are divided into two types: refracting and reflecting. Refracting telescopes utilize lenses to refract or bend light while reflecting telescopes utilize mirrors to reflect light.
Refracting telescopes fall into two categories: Keplerian and Galilean. A Keplerian telescope consists of positive focal length lenses that are separated by the sum of their focal lengths (Figure 1). The lens closest to the source image, or the object being viewed, is called the objective lens while the lens closest to the eye, or image created, is called the image lens.
A Galilean telescope consists of a positive lens and a negative lens that are also separated by the sum of their focal length (Figure 2). However, since one of the lenses is negative, the separation distance between the two lenses is much shorter than in the Keplerian design. Please note that using the Effective Focal Length of the two lenses will give a good approximation of the total length, while using the Back Focal Length will give the most accurate answer.

Figure 1: Keplerian Telescope [View Larger Image]

Figure 2: Galilean Telescope [View Larger Image]

The Magnifying Power or the inverse of the magnification of the telescope is based upon the focal lengths of the objective and eye lenses.
(1)
(2)
If the magnifying power is greater than 1, the telescope magnifies; if the magnifying power is less than 1, the telescope minifies.