Optical boresignting is process to align optical axis of single system or  a series of optical or electro-optical systems with a certain reference optical axis or mechanical axis. As optical/electro-optical systems we understand thermal imagers, TV cameras, laser range finders, optical sigths, night vision devices,  telescopes, teodolithes,  etc.  used in many applications.  Basically the aim of boresighting is to achieve a situation when:

1. optical axis of any single optical/electro-optical system is aligned with a reference mechanical axis
2. optical axises of all sub-systems of a multi-sensor  electro-optical surveillance system are parallel,
3. point 1 and point  2 are achieved for all mode of work  of the systems and for all focusing setting,
4. optical axises of dual channel imaging systems (field glasses, dual channel night vision devices) are parallel.

Practically this means that the aim of boresighting process is to achieve a situation when:

1. an imaging system is looking at a point of exactly known coordinates relative to the reference mechanical axis,
2. all subsystems of a bigger electro-optical imaging systems are looking into the same point (assumption: the distance is hundreds times bigger than the focal length of the optics of the subsystems)
3.  point 1 and point  2 are achieved for all modes of field of view of the systems and for all focusing setting of the optics of the systems,
4. two human eyes shall see the exactly the same image when looking via dual channel systems.

     When the boresighting process is not carried out properly we can get situation when:

1. the thermal imager shall generate an image of a slightly different area that the area imagined by the TV camera (the two images are slightly displaced); laser range finder measured a distance but not to the point marked on an image generated by the thermal imager; laser pointer showed a certain  point but it was not the point required,
2. all imaging systems generated image of exactly the same area; the laser range finder measured a distance to the point marked on an image generated by the thermal imager but such a situation existed for case of long distance  targets say  10 km (focusing of the optics  to optical infinity, optics working in narrow field of view mode). For a case of short distance targets say 200 m (focusing of the optics set to short distance, optics working in wide field of view mode) all imaging systems generate images of slightly different area; the laser range finder does not perform properly.
3. two channels of dual channel night vision devices generate slightly displaced images. Human eyes accommodate to the situation and the brain create a sharp image of the scenery when the boresingting error between two channels is not too big (not more than about 5 deg). However, when the boresighting errors is significant (over about 1 deg) then after some time human observer shall have a headache, be tired and  quality of the perceived image shall sharply deteriorate.

Fig.1. Example of bad boresighting of multisensor surveillance system (magnified image generated by a thermal imager with superimposed artificial spot corresponding to real spot irradiated by  laser range finder)

To summerize, the consequences of  using badly aligned optical system  or electro-optical systems can be very serious.