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Birefringent filter is physical optics instrument which base on principle of polarization interferometry。It consists of Iceland spar，quartz and KD*P birefringent crystal as well as polarizer，which is used to make solar monochromatic observation with very narrow transmission Band for research of chromosphere and magnetic field etc solar active phenomena。
It is indispensably special instrument for research on modern solarphysics。Since it came out at third decade last century，no more than other country except developed country such as French，Germany，USA and Russia etc were able to manufacture。On sixth decade last century，Nanjing Astronomical Instruments Factory，CAS (former of NIAOT) started to technically explore for research and manufacture of birefringent filter。
Since seventy decade，ten filters successively have been provided to Chinese observatory and station for solar research。In 1990 Chinese started entering into international market，seventeen filters have been exported to Japan and Korea including a tunable universal filter at overall waveband。Meanwhile three filters have been repaired。Because of many factors developed countries have early or late stopped commercial production of the instrument。NIAOT is the only provider for the instrument in the world。NIAOT continuously improve technology of filter。
At present WYKO interferometer which act as international standard has been applied to test fabrication quality of all optical crystal and glass component，which can achieve and excess diffraction limited image standard。Stability of woking temperature for the filter is better than similar product of other country，which position adjustment of transmission band is remote—controlled by computer。When entering into 21th Century，NIAOT has provided four birefringent filter for “Solar Magnetic Active Research Telescope－SMART” to Kyoto University of Japan，by which Japanese solarphysics scientist has obtained excellent observational result，among which one with f50mm has biggest aperture so far in the world。At present，infrared birefringent filter (at wavelength 1.083，1.564 micro) for Big Bear Lake Observatory of USA undertaken by NIAOT is underway。
Filter for SMART
Solar chromosphere image obtained from filter in SMART produced by NIAOT
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