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The nominal focal length of the camera is 700 mm. The nominal field of view is 38.518.8 mm; the effect of vignetting at larger field diameters is illustrated in Fig. 5. It is important to note that the unvignetted field of view is smaller than many of the detectors used with the instrument; thus part of the detector area will be vignetted.
Two LED's are mounted within the camera, facing the detector. These were originally intended as a source of undispersed light, for use in flatfielding the detector. A red LED is provided for use with CCD detectors, and a green LED for use with the IPCS-II. To date these LEDs are not routinely used, except for occasional engineering purposes.
Between the camera and detector are two shutters. One of the shutters is a safety shutter, which opens slowly but can close very quickly. This is used to protect the IPCS detector; if the IPCS is overilluminated, or the panic button is pressed, then the shutter will close. The other shutter mechanism was orignally intended to carry a 'pinhole mask'; this is a mask consisting of a pattern of holes on a regular grid. The idea was that using this mask in conjunction with the flatfielding LED would make it possible to project a set of reseau marks onto the IPCS detector, in order to map the geometric distortion. The mask has not been useful in practice, and the mechanism has been removed. (?)