@article{DenkerDinevaBalthasaretal.2018,
  author    = {Denker, Carsten and Dineva, Ekaterina and Balthasar, Horst and Verma, Meetu and Kuckein, Christoph and Diercke, Andrea and Manrique Gonzalez, Sergio Javier Gonzalez},
  title     = {Image Quality in High-resolution and High-cadence Solar Imaging},
  series = {Solar physics : a journal for solar and solar-stellar research and the study of solar terrestrial physics},
  volume    = {293},
  journal   = {Solar physics : a journal for solar and solar-stellar research and the study of solar terrestrial physics},
  number    = {3},
  publisher = {Springer},
  address   = {Dordrecht},
  issn      = {0038-0938},
  doi       = {10.1007/s11207-018-1261-1},
  pages     = {24},
  year      = {2018},
  abstract  = {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 imaging with high spatial resolution 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. The 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 contrastrich features and their neighborhood, which were part of Active Region NOAA 12661. Modifications of theMFGS algorithm uncover the field-and structure-dependency of this imagequality 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.},
  language  = {en}
}
@article{DierckeDenker2019,
  author    = {Diercke, Andrea and Denker, Carsten},
  title     = {Chromospheric Synoptic Maps of Polar Crown Filaments},
  series = {Solar physics : a journal for solar and solar-stellar research and the study of solar terrestrial physics},
  volume    = {294},
  journal   = {Solar physics : a journal for solar and solar-stellar research and the study of solar terrestrial physics},
  number    = {11},
  publisher = {Springer},
  address   = {Dordrecht},
  issn      = {0038-0938},
  doi       = {10.1007/s11207-019-1538-z},
  pages     = {17},
  year      = {2019},
  abstract  = {Polar crown filaments form above the polarity inversion line between the old magnetic flux of the previous cycle and the new magnetic flux of the current cycle. Studying their appearance and their properties can lead to a better understanding of the solar cycle. We use full-disk data of the Chromospheric Telescope (ChroTel) at the Observatorio del Teide, Tenerife, Spain, which were taken in three different chromospheric absorption lines (H alpha lambda 6563 angstrom, Ca II K lambda 3933 angstrom, and He I lambda 10830 angstrom), and we create synoptic maps. In addition, the spectroscopic He I data allow us to compute Doppler velocities and to create synoptic Doppler maps. ChroTel data cover the rising and decaying phase of Solar Cycle 24 on about 1000 days between 2012 and 2018. Based on these data, we automatically extract polar crown filaments with image-processing tools and study their properties. We compare contrast maps of polar crown filaments with those of quiet-Sun filaments. Furthermore, we present a super-synoptic map summarizing the entire ChroTel database. In summary, we provide statistical properties, i.e. number and location of filaments, area, and tilt angle for both the maximum and the declining phase of Solar Cycle 24. This demonstrates that ChroTel provides a promising data set to study the solar cycle.},
  language  = {en}
}