Point spread functions for the Solar optical telescope onboard Hinode

Abstract

Aims: We investigate the combined point spread function (PSF) of the Broadband Filter Imager (BFI) and the Solar Optical Telescope (SOT) onboard the Hinode spacecraft.
Methods: Observations of the Mercury transit from November 2006 and the solar eclipse(s) from 2007 are used to determine the PSFs of SOT for the blue, green, and red continuum channels of the BFI. For each channel, we calculate large grids of theoretical point spread functions by convolution of the ideal diffraction-limited PSF and Voigt profiles. These PSFs are applied to artificial images of an eclipse and a Mercury transit. The comparison of the resulting artificial intensity profiles across the terminator and the corresponding observed profiles yields a quality measure for each case. The optimum PSF for each observed image is indicated by the best fit.
Results: The observed images of the Mercury transit and the eclipses exhibit a clear proportional relation between the residual intensity and the overall light level in the telescope. In addition, there is an anisotropic stray-light contribution. These two factors make it very difficult to pin down a single unique PSF that can account for all observational conditions. Nevertheless, the range of possible PSF models can be limited by using additional constraints like the pre-flight measurements of the Strehl ratio.
Conclusions: The BFI/SOT operate close to the diffraction limit and have only a rather small stray-light contribution. The FWHM of the PSF is broadened by only ~1% with respect to the diffraction-limited case, while the overall Strehl ratio is ~0.8. In view of the large variations - best seen in the residual intensities of eclipse images - and the dependence on the overall light level and position in the FOV, a range of PSFs should be considered instead of a single PSF per wavelength. The individual PSFs of that range allow then the determination of error margins for the quantity under investigation. Nevertheless, the stray-light contributions are found to be best matched with Voigt functions with the parameters σ = 0.008 arcsec and γ = 0.004 arcsec, 0.005 arcsec, and 0.006 arcsec for the blue, green, and red continuum channels, respectively.