Field mirror

For example, in the spot pattern shown in Fig. 11.1b, the beam enters the cell at the gap marked 0 in the field mirror and, after multiple reflections, exits at the gap in the field mirror on the opposite side, marked 28. A total of 28 passes (or 2n + 2, where n is the... 

The number of reflections is also limited by the size of the image striking the entrance and the size of the mirror M2. As seen in Fig. 11.1b, the images that are refocused from Ml and M3 onto the field mirror M2 are stacked beside each other. The width of M2 therefore determines how many of these images can be accommodated (i.e., how many reflections are possible). 

Variations of the White cell are also in use. For example, Horn and Pimentel (1971) added a corner mirror assembly to redirect the beam that would normally exit the cell back into it. This doubles the number of passes, giving four rows of spots on the field mirror. The image pattern for such a design is shown in Fig. 11.1c (Hanst and Hanst, 1994). 

More complex multiple-reflection systems that give a much greater number of traversals have also been developed. For example, Tuazon et al. (1980) describe a system using four collecting mirrors that focus the light onto four field mirrors. The advantages and disadvantages of such multiple-mirror cells are discussed by Hanst (1971) and Hanst and Hanst (1994). 

Figure 11.2. Position of the images on the field mirror of a multipass gas cell. (Reproduced from [6], by permission of Academic Press copyright 1979.)...

A White ceU consists of a tube, typically made from glass or metal, with mirrors at each end. As seen in Figure 4.31, one end of the cell contains a spherical mirror caUed the field mirror, and the other end of the cell contains a pair of spherical mirrors caUed the objective mirrors. The focal lengths of these mirrors are identical and equal the length of the tube. For example, a 1-meter-long White ceU requires mirrors with a 1-meter focal length. Once the infrared beam enters the ceU it is brought to a focus in the focal plane of the field mirror. The beam expands to fill one of the... 

To view all or part of the focused FOV, it is common to insert a field mirror into the beam at 45 deg to divert the desired portion of the field out to an eyepiece or a TV monitoring camera. The undiverted light to be observed passes on to the instrument. The diverted light can be used for guiding purposes or to make different observations. 

A gold-coated parabolic field-mirror collimates and redirects the light to a gold-coated fold mirror. The fold mirror is positioned in a baffle tube just imder the slit masks. An image of the infrared secondary mirror is formed on a pupil mask just after the fold mirror. This mask is designed to blodc out extraneous radiation. 

Only one channel operates at a time. The channel selection is obtained with two field mirrors supported by a wheel. The field mirror in operation b in the focral plane of the telescope, covering the 3 arcmin field of view, and reflecrting the light into one or into the other channel. 

Fig. 1. Schematic lay-out of the IR camera, ISOCAM. The optical beam enters in the camera through the entrance wheel and can be directed to a short or a long wavelength channel by field mirrors fixed on the selection wheel. Each channel includes a filter wheel and a lens wheel.

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