  Broad-band imaging and even imaging with a moderate bandpass (about 1 nm)
provides a "photon-rich" environment, where frame selection ("lucky imaging")
becomes a helpful tool in image restoration allowing us to perform a
cost-benefit analysis on how to design observing sequences for high-spatial
resolution imaging in combination with real-time correction provided by an
adaptive optics (AO) system. This study presents 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, where the speckle
masking technique is used to restore images with nearly diffraction-limited
resolution. 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, 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. However, MFGS still remains a good choice for determining image
quality without a priori knowledge, which is an important characteristic when
classifying the huge number of high-resolution images contained in data
archives. In addition, this investigation demonstrates that a fast cadence and
millisecond exposure times are still insufficient to reach the coherence time
of daytime seeing. Nonetheless, the analysis shows that data acquisition rates
exceeding 50 Hz are required to capture a substantial fraction of the best
seeing moments, significantly boosting the performance of post-facto image
restoration.
