Since 1994, the Navy Precision Optical Interferometer (NPOI) has operated at visual wavelengths (450 to 850 nm). Its primary Classic backend is a pupil-plane combiner that disperses the light at a resolution R ≈ 50, uses avalanche photo-diodes as photon-counting detectors, and scans interference fringes by modulating the delay at 1 kHz. The newer NPOI image-plane combiner, VISION (Tennessee State University), which is similar to CHARA’s MIRC and is currently being upgraded, dispenses with delay modulation. We are now developing a third backend to expand into the near infrared. Its primary purpose will be to stabilize the NPOI for high-resolution observations by bootstrapping from the infrared to visual wavelengths.
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