Image Quality in High-Resolution and High-Cadence Solar Imaging

C. Denker,  E. Dineva,  H. Balthasar, M. Verma, C. Kuckein,  A.
Diercke, S.J. Gonzalez Manrique

Abstract. High-cadence imaging and frame selection ('lucky imaging') in
combination with real-time correction by an adaptive optics (AO) system are
essential for image restoration to obtain nearly diraction-limited solar
images. We present high-cadence (160 Hz) G-band and blue continuum image
sequences obtained with the High-Resolution Fast Imager (HiFI) at the
1.5-meter
GREGOR solar telescope. HiFI employs two synchronized large-format and
high-cadence sCMOS detectors. The 'Median Filter Gradient Similarity' (MFGS)
image quality metric is applied, among others, to AO-corrected image
sequences
of a pore and a small sunspot observed on 2017 June 4 and 5. A small
region-of-interest (ROI), which was selected for fast imaging performance,
covered these contrast-rich features and their neighborhood, which were
part of
active region NOAA 12661. Modifications of the MFGS algorithm uncover the
field-
and structure-dependency of this image quality metric. In addition, we
demonstrate that this fast cadence and millisecond exposure times are still
insucient to reach the coherence time of daytime seeing. However, frame
selection significantly aids post-facto image restoration, if the data
acquisition rate exceeds 50 Hz. This poses challenges for largeformat and
high-cadence detectors, which are proposed for the next generation of 4-meter
aperture solar telescopes.