Paper detail

Photometric properties of new solar H alpha commercial Fabry-Perot etalons. Application to the analysis of the chromospheric fringe

We consider the use of the commercially available Fabry-Perot etalons (FP) for the imaging of the solar chromosphere in the H alpha line of HI. Three etalons of 40, 60 and 90 mm diameter were accurately analysed. At normal incidence the maximum transmission wavelength as well as the finesse were evaluated. Polynomial curves precisely fitting the transmitted central wavelength variations when using a quasi-parallel beam from a point-like source are presented. Further calibrations have been done with photometric accuracy using a laboratory set-up comprising i/ a 16 bits CCD camera; ii/ a Littrow spectrograph of a spectral power 110000 giving a linear dispersion of 0.0058 nm/pixel and iii/ an artificial Sun used as a light source and iv/ precisely adjustable in position optical components, including the F-P etalons. In addition, a precise laboratory wavelength calibration was performed using a low pressure deuterium 2D spectral lamp simultaneously illuminating the adjustable entrance slit using a splitter before. The variations of the FWHM of the spectral transmission variations as a function of the incidence angle of a parallel beam are also given for each etalon. Consequences resulting from the use of a low but significant aperture/ratio are tentatively discussed for the first time. An application to a precise photometric work of solar physics interest when using limb filtergrams is illustrated and discussed, with emphasis on the photometric accuracy resulting from the use of such etalons put before the entrance aperture of an imaging telescope. Monochromatic images of the solar chromosphere shell in the vicinity of the polar and equatorial limbs were made using a small telescope, in order to deduce the variation of the typical average thicknesses at poles and equator interpreted as a prolateness effect of the chromospheric shell observed during the last minimum of solar activity (2009).