Infra-red imaging systems and other optical systems

An optical system comprising:

• at least two variable focal length reflective optical components whose positions are fixed relative to one another;
• and
• control means for varying the focal length of said at least two variable focal length reflective optical components to control an image passing through said system.

An optical system for varying the width of an optical beam incident on the system, comprising:

• at least two variable focal length optical components whose positions are fixed relative to one another;
• and
• control means for varying said at least two variable focal length optical components such that the width of said optical beam is varied whilst substantially maintaining any collimation of said optical beam.

A method of dithering an image between first and second positions on an imaging detector, the method comprising the steps of:

• controlling a variable focal length optical device upon which radiation is incident so as to direct an image to one position on an imaging detector,
• distorting the device so as to direct the image to a different position on said imaging detector, and repeating the controlling and distorting steps so as to repeatedly dither the image on the imaging detector.

A method of providing a periodic reference signal in an electronic imaging detector, said method comprising the steps of:

• operating the detector in a mean scene radiation mode from time to time by varying a focal length of a variable focal length optical device to change its focal length so that radiation incident upon the imaging detector is uniform and has substantially no spatial information,
• using a signal produced by the detector during this mean scene radiation mode of operation as a mean scene reference signal, and
• controlling the focal length of the device to return to an imaging mode of operation after the reference has been obtained.

A method of minimising damage to detectors in an optical system and retro-reflection from an optical detector system, the method comprising the steps of:

• operating variable focus optical components of the system in a de-focus mode by varying the focal length of said optical component such that the captured radiation incident upon a detecting element is significantly de-focused.