Focal length

Planes constructed at the focal points perpendicular to the optical axis are called focal planes. Parallel rays, even if not parallel to the axis, intersect in the focal plane. Corresponding points in the object and image space— points that satisfy an object-image relation—are called conjugate points. Planes within the points are conjugate planes. The distances between conjugate points and the surface are called conjugate distances [2]. 

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Fig. 2 shows the CFRP-sandwich specimen and the transducer mounted on the scanner. Fig. 23 presents a C-scan of the specimen as first interesting result. Only the defects visible from the outside are indicated. The distance between transducer and specimen was smaller than the focal length, so that the angle of incidence at the edge of the sound beam converts the longitudinal waves to Rayleigh-waves in the specimen. These waves provide a very sharp image of the surface. This method opens the possibility for a non-contact acoustic microscope. 

At its simplest, the mirror is a circular disc, of diameter 2a and mean thickness f, simply supported at its periphery (Fig. 7.2). When horizontal, it will deflect under its own weight M when vertical it will not deflect significantly. We want this distortion (which changes the focal length and introduces aberrations into the mirror) to be small... 

Natural crystals, synthetic crystals, and glasses often contain small bubbles that preserve samples of the fluid from which the crystals grew or of the atmosphere over the glass melt. Using a long focal length lens, the laser beam can be focused into inclusions at some depth below the crystal or glass surface. The Raman spectra then permit the identification of molecular species dissolved in the aqueous solutions or of components in the gas bubbles. ... 

Spectrometer Resolution (cm-1) Stray light Type Focal length (m) Gratings ... 

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.

For any two-mirror system we can use geometric optics to derive equations that describe some important parameters. (For derivations see Schroeder, 2000). The effective system focal length can be determined in terms of the individual focal lengths and mirror separation ... 

Given the focal length of the primary and the required system focal length one can determine the secondary focal length by ... 

Conversely the separation can be determined from the primary focal length, the magnification of the secondary and the back focal length. 

Conversely the two focal lengths for primary and secondary can be determined given a requirement on the mirror spacing, d and back focal length e by the following two equations. 

The Schwarzshield telescope which has a much larger secondary than primary, a large obscuration and is physically much larger than it s focal length (a big cost increase). 

Primary diameter, primary focal lengths (size)... 

We will use a specihc example to demonstrate the problems likely to be encountered when testing. Assume we have an extremely large telescope with around 1000 - 1 m hex segments whose prime focal length is 50 m, or vertex radius is 100 m. The inner ring of segments will have their centers at an aperture radius h of 3.5 m and the outermost ring at 14.5 m. 

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