Improved SOT (Hinode mission) high resolution solar imaging observations

Abstract

We consider the best today available observations of the Sun free of turbulent Earth atmospheric effects, taken with the Solar Optical Telescope (SOT) onboard the Hinode spacecraft. Both the instrumental smearing and the observed stray light are analyzed in order to improve the resolution. The Point Spread Function (PSF) corresponding to the blue continuum Broadband Filter Imager (BFI) near 450 nm is deduced by analyzing i/ the limb of the Sun and ii/ images taken during the transit of the planet Venus in 2012. A combination of Gaussian and Lorentzian functions is selected to construct a PSF in order to remove both smearing due to the instrumental diffraction effects (PSF core) and the large-angle stray light due to the spiders and central obscuration (wings of the PSF) that are responsible for the parasitic stray light. A Max-likelihood deconvolution procedure based on an optimum number of iterations is discussed. It is applied to several solar field images, including the granulation near the limb. The normal non-magnetic granulation is compared to the abnormal granulation which we call magnetic. A new feature appearing for the first time at the extreme-limb of the disk (the last 100 km) is discussed in the context of the definition of the solar edge and of the solar diameter. A single sunspot is considered in order to illustrate how effectively the restoration works on the sunspot core. A set of 125 consecutive deconvolved images is assembled in a 45 min long movie illustrating the complexity of the dynamical behavior inside and around the sunspot.

Proper Motions of Sunspots’ Umbral Dots at High Temporal and Spatial Resolution

“…Exposure time is 102 ms, the cadence is 6.4 seconds and the field of view is approximately 56 × 28 . All of the images are first corrected for dark current, flat field and bad pixels that are removed using fg prep routine and then co-aligned by means of two dimensional cross correlation algorithm involved in fg rigidalign routine (both of the codes are from Solar Soft library) and then deconvolved with the PSF deduced in the previous paper (Goodarzi et al 2015). 90 successive blue continuum images taken between 00:19:51 and 00:29:20 with 6.4s time interval have been used to consider proper motions of fine structures inside sunspot umbra in high resolution, when adding images (at least 3 images averaging) to improve the signal to noise ratio.…”

“…This is especially important when fine and low intensity structures are concerned such as umbral dots. We described in our previous paper (Goodarzi et al 2015) how important it is to take into account the far wings of the point spread function (PSF) and only with this accountancy it is possible to remove stray light coming from far distances, indeed from the whole bright solar disk. We performed it by adding the Lorentzian function that has extended wings to the Gaussian function that describes the core of the PSF.…”

“…After deducing the PSF precisely, a deconvolution procedure was performed using the iteration method (Goodarzi et al 2015(Goodarzi et al , 2016. Correction with this method leads to a higher contrast and a more deeper evaluation of the images that are now free of stray light.…”

“…1, but to consider them with more details and with a higher resolution, we made velocity map for a small part of the umbra-penumbral boarder (also called the periphery of the umbra) and using a much smaller FWHM of the windowing Gaussian function (0.2 arcsec). This is made achievable since that using accurately deconvolved images free of scattered light (Goodarzi et al 2015) and after improving the S/N ratio at an optimum level to warranty the reproducibility of the results, the most significant intensity distributions (see the next part) are used. The selected part is shown with red arrow in top part of Fig.…”

“…Because of the small size of the umbral dots, and their varying size and velocity within a few minutes, studying motions of UDs needs high spatial and temporal resolution. So we use the best available observations of the partial Sun free of turbulent Earth atmospheric effects from the Solar Optical Telescope (SOT) onboard the Hinode spacecraft, after significantly enhancing the visibility and contrast by an optimum Maxlikelihood deconvolution with the Point Spread Function (PSF) deduced in a preceding paper (Goodarzi et al (2015), further designated paper I) and improving S/N ratio by co-adding several images. The Fourier local correlation tracking (FLCT) technique have been used with different time intervals, field of view and FWHM of the windowing Gaussian function for constructing velocity map of the sunspot UDs with different resolution to see any faint velocity changes in time.…”