Cyril Bazin, Serge Koutchmy
We consider the use of the commercially available Fabry-Perot etalons (FP) for the imaging of the solar chromosphere in the H line of HI. Three etalons of 40, 60 and 90 mm of diameter were evaluated and accurately analysed. At normal incidence the maximum transmission wavelength is 656.285 nm for the 60 and 40 mm etalons FP. The finesse has been evaluated at 13.3 for the FP 60mm, 8.7 for the FP 40 and 13.9 for the FP 90 mm. Shifts of the central wavelength as a function of the incidence angle were accurately measured. Polynomial curves precisely giving the transmitted central wavelength variations when using a quasi-parallel beam from a point-like source are presented. Further calibrations have been done with a photometric accuracy using i/ a 16 bits CCD camera; ii/ a Littrow spectrograph of a spectral power 110000, a linear dispersion giving a 0.0058 nm/pixel resolution and iii/ feed by an artificial Sun used as a light source and iv/ precisely adjustable in position optical components. The precise laboratory wavelength calibration was performed using a low pressure deuterium spectral lamp illuminating the entrance slit. The variations of the FWHM of the transmission curves as a function of the incidence angle are also given for a typical etalon. Consequences resulting from the use of a significant aperture/ratio are tentatively discussed for the first time. An application to a precise solar photometric work using filtergrams is illustrated, with emphasis on the photometric accuracy resulting from the use of such etalons put before the entrance aperture of the imaging telescope. Monochromatic images of the solar chromospheric shell near the limbs were made, in order to deduce the variation of its thickness at poles and equator in order to measure the prolateness effect during this current minimum of solar activity (2009).
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http://arxiv.org/abs/1209.1027
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