Measuring the Effective Wavelength of CHARA Classic

This paper presents the first empirical measurement of the K'-band effective wavelength and bandwidth of the CHARA Classic beam combiner on the CHARA Array. Prior to this work, the accepted effective wavelength value used for CHARA Classic data (2.1501 mu m) came from a model of the system; it was not derived from measurements done on the system directly. We employ two data collection methods for our observations: using the Optical Path Length Equalizer (OPLE) cart to scan through the interference fringes and using the dither mirror to scan through the fringes. The two observational methods yield similar effective wavelength measurements (2.141 +/- 0.003 mu m with the OPLE cart and 2.136 +/- 0.002 mu m with the dither mirror). Both of these results are lower than the previously adopted effective wavelength value, but by less than 0.7%. The bandwidth values measured by the two methods differ from each other by almost 5% (0.334 +/- 0.002 mu m with the OPLE cart and 0.351 +/- 0.003 mu m with the dither mirror). Our results establish the first estimate of the uncertainty in the effective wavelength and bandwidth.