## Info

* deformed hyperboloids of conic constants K — —4.6, Kp.p. — —4.7.

t parented to a flattened ellipsoid, K = Kp.p. — +0.1.

The axial separation from the cornea first surface to retina is 24.2 mm.

* deformed hyperboloids of conic constants K — —4.6, Kp.p. — —4.7.

t parented to a flattened ellipsoid, K = Kp.p. — +0.1.

The axial separation from the cornea first surface to retina is 24.2 mm.

Fig. 1.33 Horizontal section of the human eye

For objects at infinity, the efl is 22.4 mm in the image space, thus equivalent to an efl of 16.7 mm in the air. In the central field of about 5°, the resolution in the object space is 1 arcmin corresponding to 5 ¡im on the retina. The low resolution imaged field reaches 100° from the axis in the temporal direction and 60-80° in the other directions (Fig. 1.33). The aperture of the iris, which is the input pupil, varies from 2 to 8 mm and takes the value of 4 mm for normal daytime conditions.

### 1.12.2 Eyepiece

If the field is magnified and observed with the eye, the processor is an eyepiece. Eyepieces have a very different task from that of the objectives because of the location of the exit pupil: this pupil must be with some clearance after the last lens of the eyepiece - called the "eye lens," the opposite is the "field lens" - in order for the observer to position his eye's pupil, the iris, on it. This axial clearance varies from 5 to 20 mm and is called eye relief. Also, the diameter of the exit pupil shall be small enough for all of the beam of light to enter the iris. Another complication in eyepiece design is the increased field of view which is in inverse proportion to the diameter of the pupil. For example, imaging a (1/3)° field with a 1 m aperture telescope, the apparent field with a 5 mm pupil must be 200-times greater, thus 67°, requiring a complicated design with several elements.

The raytrace design of eyepieces leads to systems of several glass types having up to eight elements with spherical surfaces, or with less elements but using aspherical surfaces. Available eyepieces have an apparent field of view going up to 80° which requires the eye to pivot to accurately observe subfields. Typical Zeiss eyepieces are the classical Plossl with four elements, and the Erfle with five elements (Fig. 1.34). Descriptions and comparisons of some eyepieces are given by Taylor [153] and Smith [146].

Fig. 1.34 Left: Plossl eyepiece (type 2-2). Right: Erfle eyepiece (type 2-1-2)

1.12.3 Interferometer

If the image of a same object is issued from two or several separated apertures, or if a wavefront is separated in two wavefronts by a beam splitter or a phase mask, which causes the recombined waves of light to add and subtract, the processor is an interferometer.

Examples of interferometers are infrared Fourier transform spectrometers (cf. Sect. 2.5.1), stellar interferometers, pioneered by E. Stephan and A. Michelson, telescope arrays pioneered by A. Labeyrie (cf. Sect. 2.5.2), wavefront analyzers and some coronographs. For some interferometers requiring a field compensation, variable curvature mirrors have been installed at the retro-reflected focu of the mobile carriages (Chap. 2).

### 1.12.4 Coronograph

If a faint object is detected at the immediate proximity of a bright object, the processor is a coronograph. The first coronographs used a spatial filter at the focal plane, a Lyot stop, to block the bright image and transmit the surrounding light to a next imaging system.

An appropriate modification of the spatial transmission function of the pupil, known as apodization, allows the suppression of the secondary maxima. In astronomy, this facilitates the detection of a faint object when close to a bright punctual object. New type coronographs use interferometric techniques such as with a phase mask or are based on the incident flux division by pupil reversal and phase shift of one beam. The beam recombination provides destructive interferences at the center of the field. For instance, four-quadrant phase masks were proposed and elaborated by D. Rouan et al. (2000). A new concept providing an achromatic destructive interference over one octave at the center of the field was recently introduced by D. Rouan et al. (see details in Lena, Rouan et al. [1.96]). It consists of cellular phase shifter mirrors - called chessboard mirrors - introduced in each arm of a nulling interferometer. The number of shifting cells is determined from integer roots of polynomial relations (diophantine optics). Exoplanet detection will benefit from this concept.

### 1.12.5 Polarimeter

If the processor is a polarizer, such as the N-shaped Wollaston prism or a semi-wave plate, that causes the light waves to oscillate in a single plane, this instrument is a polarimeter. A usual convention is to describe linear polarization as the orientation of the electric field vector.

### 1.12.6 Slit Spectrograph

If the processor is a diffraction grating or a prism, the light entering through a slit is angularly dispersed with respect to the wavelengths from a spectrum: this instrument is a spectrograph. There exists reflective and transmission gratings which are deposited on plane, spherical, or aspherical substrates. The line distributions generated by holographic processes provide constant- or variable-spacing diffraction gratings.

Although a concave, toroid, reflective grating with constant-spaced lines, or a concave spherical concave grating with variable-spaced lines, are efficient single surface slit spectrographs for ultraviolet studies (cf. Sects. 3.5.7 and 7.8.2), spectrographs are generally designed with a collimator optics before a plane or plane-aspheric grating and a camera optics after it. If a reflective grating reflects the dispersed light back to the collimator so that the collimator also serves as a camera, this arrangement is called a Littrow's mountings spectrograph. Some spectrographs require an internal re-imaging of the pupil - as for example for cross dispersion spectrographs using both echelle and classical gratings - which is provided by additional mirrors or lenses. Such a design is called a white pupil mounting or Baranne's mounting as originated by A. Baranne [11, 12]. One usually distinguishes between slit, long-slit, and multi-slit spectrographs; the latter case is called a multi-object spectrograph (Mos).

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