Principal focal length

Figure 2. The focal length is the distance between the principal plane and the focus, where the principal plane is defined as the surface where tlic input and out-pul light rays would intercept.

As the beam leaves the prism predisperser, it is focused on the entrance slit of the grating monochromator. The slit is curved, has variable width, and opens symmetrically about the chief ray (optical center line of system). The monochromator itself is of the off-axis Littrow variety (James and Sternberg, 1969 Stewart, 1970 Jennings, 1974) and uses a double-pass system described by McCubbin (1961). The double-pass aspect of the system doubles the optical retardation of the incident wave front and theoretically doubles the resolution of the instrument. The principal collimating mirror is a 5-m-focal-length, 102-cm-diam parabola. 

Figure 4.31 Data for the characterization of an electrostatic lens, (a) Positions of the focal and principal planes (left-hand and right-hand sides are indicated by the subscripts Y and r respectively) and their distances (optical sign conventions are disregarded, i.e., the distances are described only by their lengths). (b) Geometrical construction applied to image the arrow ye by means of characteristic asymptotic trajectories, (c) Geometrical construction for an asymptotic ray with a pencil angle a,e. The shaded areas are needed for the derivation of the linear and angular magnification factors of the lens. For details see main text.

Figure 9 illustrates an example in which the object has a uniform diffusion surface of area S, reflectance R, the focal length of the lens /, the distance from the surface of the object to the principal point on the front of the lens is a, and the distance from the principal point on the reverse of the lens to the sensor plane is b—giving the well-known formula ... 

A spherical concave mirror of radius R — CP = CQ reflects an object at O (mirror-object distance OP = u) into image at / (mirror-image distance IP= v). If object moves to infinity (u = oo), then all rays from that object will converge at the principal focus point F f—FP = (1/2) CP—RI2] that is, the principal focal length f is half of the radius of curvature. 

Based on the theory of simultaneous adjustment, such an adjustment was developed for calibration in the 2-D field, combined with a conventional photogrammetric method and the constraint condition for a principal point, focal length or scale factor, etc. 

Cardinal elements The focusing properties of optical components such as lenses are characterized by a set of quantities known as cardinal elements the most important are the positions of the foci and of the principal planes and the focal lengths. 

FIGURE 1 A thin lens, showing the conjugate points O and O, the primary and secondary focal points F and F, the primary and secondary focal lengths f and f, the object and image distances / and I, the object and image heights h and h, and the principal point P. 

If the telescope is to have a sizable field of view, as measured by the angle a, then the eyepiece could become prohibitively large. To reduce the diameter of the eyepiece, sl field lens is installed near the secondary focal plane of the objective. The field lens, which may be part of the eyepiece, directs the principal ray toward the axis and allows the diameter of the eyepiece to be reduced. Typically, the focal length of the field lens may be chosen so that the principal ray intersects the eyepiece at its periphery. This shifts the location of the exit pupil somewhat toward the eyepiece and slightly reduces the eye relief ht-tween the eye and the eyepiece. When the field lens is built into the eyepiece, the second lens in the eyepiece is called the eye lens. 

Focal length The distance along the optic axis of a lens that covers the distance from the lens to the principal focus. 

The most common sample compartment in high-resolution FT-IR spectrometers is the White cell, where the beam passes a path of 1-1000 m. In a long-path gas cell, however, a small f/number (focal length of the mirror/diameter of the pupil) and a large size of the circular image may lead to a loss of effectivity. In the Oulu interferometer this problem was solved by an alternative multiple-path gas cell, where the images at the focal planes occur on a circle around the principal axis of the cell. The aberrations remain small in spite of a small f/number, because the successive images are relatively near to the principal axis of the optics. 

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