arXiv subjects

Carsten Denker

Publications and source records attributed to Carsten Denker.

At least 19 recordsLinked to original sources

arXiv2025

The ORCA-TWIN qCMOS Experiment I. Science case and commissioning at Calar Alto Observatory

We describe a pilot study to explore a new generation of fast and low noise CMOS image sensors for time domain astronomy, using two remote telescopes with a baseline of 1635 km. The experiment involves direct imaging with novel qCMOS image sensor technology that combines fast readout with sub-electron readout noise. Moreover, synchronized observations from two remote telescope sites will be used to explore new approaches for measuring Solar System bodies, precision stellar photometry, and speckle imaging. A fast-track installation of an ORCA-Quest2 camera at the Calar Alto Observatory 1.23m telescope has demonstrated the potential of the qCMOS technology for time domain astronomy. Numerical simulations suggest that owing to sub-electron readout noise, qCMOS sensors outperform classical CCDs for high-cadence imaging on 1m-class telescopes. The small penalty for post-readout binning, that is almost insignificant in comparison to higher readout noise detectors, opens interesting applications for scene-dependent data processing in direct imaging, and potentially even for spectroscopy.

astro-ph.IM↗

arXiv2025

The calm before the storm: High spatial resolution mosaic of active region NOAA 14274 at the onset of an X1.2 flare

Active region NOAA 14274 produced some of the strongest flares of Solar Cycle 25, including the X1.2 and X5.1 flares on 10 and 11 November 2025, respectively. We present the first large mosaic of speckle-restored images obtained with the improved High-resolution Fast Imager (HiFI+) at the 1.5-meter GREGOR solar telescope at the Observatorio del Teide in Iza\~na, Tenerife, Spain. The observations were obtained approximately 30 minutes before the onset of the X1.2 flare. The active region exhibited strongly curved penumbral filaments, sunspot rotation, and shear motions along the polarity inversion line (PIL), which led to a highly stressed magnetic field configuration that stored sufficient energy to release multiple M- and X-class flares. The first flare signatures appeared as small-scale brightenings, each with a width of a few tenths of an arcsecond, that trace penumbral filaments in the trailing sunspot.

astro-ph.SR↗

arXiv2025

High-spectral Resolution, Multi-wavelength Center-to-limb Observations of the Sun

The center-to-limb variations (CLVs) of photospheric and chromospheric spectral lines were obtained in 2025 July and August using drift scans from the echelle spectrograph of the 0.7 m Vacuum Tower Telescope at the Observatorio del Teide (ODT) in Tenerife, Spain. This instrument can observe four spectral regions simultaneously, enabling multi-line spectroscopy with high spectral resolution of various activity features and the quiet Sun in the lower solar atmosphere. The initial results of Halpha observations demonstrate the diagnostic potential of drift scans obtained with a ground-based, high-resolution telescope. Data products include spectroheliograms and maps of physical parameters such as line-of-sight velocity, line width, and line-core intensity. The combination of the CLV from photospheric and chromospheric lines, as well as the wide range of formation heights of the selected lines, renders this dataset ideal for characterizing stellar and exoplanet atmospheres.

astro-ph.SR↗

arXiv2025

Differential rotation of solar {\alpha} sunspots and implications for stellar light curves

Differential rotation is a key driver of magnetic activity and dynamo processes in the Sun and other stars, especially as the rate differs across the solar layers, but also in active regions. We aim to accurately quantify the velocity at which round {\alpha}-spots traverse the solar disk as a function of their latitude, and compare these rates to those of the quiet-Sun and other sunspot types. We then extend this work to other stars and investigate how differential rotation affects the modulation of stellar light curves by introducing a generalized stellar differential rotation law. We manually identify and track 105 {\alpha}-sunspots in the 6173 {\AA} continuum using the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO). We measure the angular velocities of each spot through center-of-mass and geometric ellipse-fitting methods to derive a differential rotation law for round {\alpha}-sunspots. Results. Using over a decade of HMI data we derive a differential rotation law for {\alpha}-sunspots. When compared to previous measurements we find that {\alpha}-sunspots rotate 1.56% faster than the surrounding quiet-Sun, but 1.35% slower than the average sunspot population. This supports the hypothesis that the depth at which flux tubes are anchored influences sunspot motions across the solar disk. We extend this analysis to other stars by introducing a scaling law based on the rotation rates of these stars. This scaling law is implemented into the Stellar Activity Grid for Exoplanets (SAGE) code to illustrate how differential rotation alters the photometric modulation of active stars. Our findings emphasize the necessity of considering differential rotation effects when modeling stellar activity and exoplanet transit signatures

astro-ph.SR↗

arXiv2024

A Universal Method for Solar Filament Detection from H-alpha Observations using Semi-supervised Deep Learning

Filaments are omnipresent features in the solar atmosphere. Their location, properties and time evolution can provide information about changes in solar activity and assist the operational space weather forecast. Therefore, filaments have to be identified in full disk images and their properties extracted from these images. Manual extraction is tedious and takes much time; extraction with morphological image processing tools produces a large number of false-positive detections. Automatic object detection, segmentation, and extraction in a reliable manner allows to process more data in a shorter time. The Chromospheric Telescope (ChroTel), Tenerife, Spain, the Global Oscillation Network Group (GONG), and the Kanzelh\"ohe Observatory (KSO), Austria, provide regular full-disk observations of the Sun in the core of the chromospheric H-alpha absorption line. We present a deep learning method that provides reliable extractions of filaments from H-alpha filtergrams. First, we train the object detection algorithm YOLOv5 with labeled filament data of ChroTel. We use the trained model to obtain bounding-boxes from the full GONG archive. In a second step, we apply a semi-supervised training approach, where we use the bounding boxes of filaments, to learn a pixel-wise classification of filaments with u-net. Here, we make use of the increased data set size to avoid overfitting of spurious artifacts from the generated training masks. Filaments are predicted with an accuracy of 92%. With the resulting filament segmentations, physical parameters such as the area or tilt angle can be easily determined and studied. This we demonstrate in one example, where we determine the rush-to-the pole for Solar Cycle 24 from the segmented GONG images. In a last step, we apply the filament detection to H-alpha observations from KSO which demonstrates the general applicability of our method to H-alpha filtergrams.

astro-ph.SR↗

arXiv2023

Spectral Background-Subtracted Activity Maps

High-resolution solar spectroscopy provides a wealth of information from photospheric and chromospheric spectral lines. However, the volume of data easily exceeds hundreds of millions of spectra on a single observation day. Therefore, methods are needed to identify spectral signatures of interest in multidimensional datasets. Background-subtracted activity maps (BaSAMs) have previously been used to locate features of solar activity in time series of images and filtergrams. This research note shows how this method can be extended and adapted to spectral data.

astro-ph.SR↗

arXiv2022

Characterization of chromospheric activity based on Sun-as-a-star spectral and disk-resolved activity indices

The strong chromospheric absorption lines Ca H & K are tightly connected to stellar surface magnetic fields. Only for the Sun, spectral activity indices can be related to evolving magnetic features on the solar disk. The Solar Disk-Integrated (SDI) telescope feeds the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) of the Large Binocular Telescope (LBT) at Mt. Graham International Observatory (MGIO), Arizona, U.S.A. We present high-resolution, high-fidelity spectra that were recorded on 184 & 82 days in 2018 & 2019 and derive the Ca H & K emission ratio, i.e., the S-index. In addition, we compile excess brightness and area indices based on full-disk Ca K line-core filtergrams of the Chromospheric Telescope (ChroTel) at Observatorio del Teide, Tenerife, Spain and full-disk ultraviolet (UV) 1600~Å images of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). Thus, Sun-as-a-star spectral indices are related to their counterparts derived from resolved images of the solar chromosphere. All indices display signatures of rotational modulation, even during the very low magnetic activity in the minimum of Solar Cycle 24. Bringing together different types of activity indices has the potential to join disparate chromospheric datasets, yielding a comprehensive description of chromospheric activity across many solar cycles.

astro-ph.SR↗

arXiv2020

Classification of High-resolution Solar Hα Spectra using t-distributed Stochastic Neighbor Embedding

The Hα spectral line is a well-studied absorption line revealing properties of the highly structured and dynamic solar chromosphere. Typical features with distinct spectral signatures in Hα include filaments and prominences, bright active-region plages, superpenumbrae around sunspots, surges, flares, Ellerman bombs, filigree, and mottles and rosettes, among others. This study is based on high-spectral resolution Hα spectra obtained with the echelle spectrograph of the Vacuum Tower Telescope (VTT) located at Observatorio del Teide (ODT), Tenerife, Spain. The t-distributed Stochastic Neighbor Embedding (t-SNE) is a machine learning algorithm, which is used for nonlinear dimensionality reduction. In this application, it projects Hα spectra onto a two-dimensional map, where it becomes possible to classify the spectra according to results of Cloud Model (CM) inversions. The CM parameters optical depth, Doppler width, line-of-sight velocity, and source function describe properties of the cloud material. Initial results of t-SNE indicate its strong discriminatory power to separate quiet-Sun and plage profiles from those that are suitable for CM inversions. In addition, a detailed study of various t-SNE parameters is conducted, the impact of seeing conditions on the classification is assessed, results for various types of input data are compared, and the identified clusters are linked to chromospheric features. Although t-SNE proves to be efficient in clustering high-dimensional data, human inference is required at each step to interpret the results. This exploratory study provides a framework and ideas on how to tailor a classification scheme towards specific spectral data and science questions.

astro-ph.SR↗

arXiv2020

Wavelength Dependence of Image Quality Metrics and Seeing Parameters and their Relation to Adaptive Optics Performance

Ground-based solar observations are severely affected by Earth's turbulent atmosphere. As a consequence, observed image quality and prevailing seeing conditions are closely related. Partial correction of image degradation is nowadays provided in real-time by adaptive optics (AO) systems. In this study, different metrics of image quality are compared with parameters characterizing the prevailing seeing conditions, i.e., Median Filter Gradient Similarity (MFGS), Median Filter Laplacian Similarity (MFLS), Helmli-Scherer mean, granular rms-contrast, differential image motion, and Fried-parameter r0. The quiet-Sun observations at disk center were carried out at the Vacuum Tower Telescope (VTT), Observatorio del Teide (OT), Izana, Tenerife, Spain. In July and August 2016, time-series of short-exposure images were recorded with the High-resolution Fast Imager (HiFI) at various wavelengths in the visible and near-infrared parts of the spectrum. Correlation analysis yields the wavelength dependence of the image quality metrics and seeing parameters, and Uniform Manifold Approximation and Projection (UMAP) is employed to characterize the seeing on a particular observing day. In addition, the image quality metrics and seeing parameters are used to determine the field-dependence of the correction provided by the AO system. Management of high-resolution imaging data from large-aperture, ground-based telescopes demands reliable image quality metrics and meaningful characterization of prevailing seeing conditions and AO performance. The present study offers guidance how to retrieve such information ex post facto.

astro-ph.IM↗

arXiv2020

Measuring the Etalon Quality of the GREGOR Fabry-Pérot Interferometer

Imaging spectropolarimetry is an important observational tool in solar physics because of fast-cadence spectral scans with high-spectral resolution, large field-of-view, and its inherent suitability for post-facto image restoration. Fabry-Pérot etalons are the key optical elements of these instruments. Their optical quality critically defines the instrument's performance. The two etalons of the GREGOR Fabry-Pérot Interferometer (GFPI) were used for more than 10~years, raising questions about the potential deterioration of etalons coatings. We present an assessment of the etalons optical quality, describe the inspection method based on Zernike polynomials, discuss the field dependence of the finesse and its consequences for instrument design, and investigate the impact of the measurement technique to achieve plate parallelism. We find that extended exposure to sunlight affects the etalon coatings, i.e., lowering the peak transmission and leaving an imprint of the pupil of the GREGOR solar telescope on the etalon that is directly exposed to sunlight. The finesse of both etalons, however, remains high so that the impact on imaging spectropolarimetry is negligible.

astro-ph.IM↗

arXiv2019

Cloud model inversions of strong chromospheric absorption lines using principal component analysis

High-resolution spectroscopy of strong chromospheric absorption lines delivers nowadays several millions of spectra per observing day, when using fast scanning devices to cover large regions on the solar surface. Therefore, fast and robust inversion schemes are needed to explore the large data volume. Cloud Model (CM) inversions of the chromospheric H$α$ line are commonly employed to investigate various solar features including filaments, prominences, surges, jets, mottles, and (macro-)spicules. The choice of the CM was governed by its intuitive description of complex chromospheric structures as clouds suspended above the solar surface by magnetic fields. This study is based on observations of active region NOAA 11126 in H$α$, which were obtained 2010 November 18-23 with the echelle spectrograph of the Vacuum Tower Telescope (VTT) at the Observatorio del Teide, Spain. Principal Component Analysis (PCA) reduces the dimensionality of spectra and conditions noise-stripped spectra for CM inversions. Modeled H$α$ intensity and contrast profiles as well as CM parameters are collected in a database, which facilitates efficient processing of the observed spectra. Physical maps are computed representing the line-core and continuum intensity, absolute contrast, equivalent width, and Doppler velocities, among others. Noise-free spectra expedite the analysis of bisectors. The data processing is evaluated in the context of "big data", in particular with respect to automatic classification of spectra.

astro-ph.SR↗

arXiv2019

Chromospheric Synoptic Maps of Polar Crown Filaments

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 6563A, CaII-K 3933A, and HeI 10830A), and we create synoptic maps. In addition, the spectroscopic HeI 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 declining phase of Solar Cycle 24. This demonstrates that ChroTel provides a promising data set to study the solar cycle.

astro-ph.SR↗

arXiv2019

Dynamics and connectivity of an extended arch filament system

In this study, we analyzed a filament system, which expanded between moving magnetic features (MMFs) of a decaying sunspot and opposite flux outside of the active region from the nearby quiet-Sun network. This configuration deviated from a classical arch filament system (AFS), which typically connects two pores in an emerging flux region. Thus, we called this system an extended AFS. We contrasted classical and extended AFSs with an emphasis on the complex magnetic structure of the latter. Furthermore, we examined the physical properties of the extended AFS and described its dynamics and connectivity. At the southern footpoint, we measured that the flux decreases over time. We find strong downflow velocities at the footpoints of the extended AFS, which increase in a time period of 30 minutes. The velocities are asymmetric at both footpoints with higher velocities at the southern footpoint. The extended AFS was observed with two instruments at the Dunn Solar Telescope (DST). The Rapid Oscillations in the Solar Atmosphere (ROSA) imager provided images in three different wavelength regions. The Interferometric Bidimensional Spectropolarimeter (IBIS) provided spectroscopic H$α$ data and spectropolarimetric data that was obtained in the near-infrared Ca II 8542 Å line. We used He II 304 Å extreme ultraviolet images of the Atmospheric Imaging Assembly (AIA) and LOS magnetograms of the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) as context data.

astro-ph.SR↗

arXiv2019

Background-subtracted Solar Activity Maps

We introduce the concept of a Background-subtracted Solar Activity Map (BaSAM) as a new quantitative tool to assess and visualize the temporal variation of the photospheric magnetic field and the UV 160 nm intensity. The method utilizes data of the Solar Dynamics Observatory (SDO) and is applicable to both full-disk observations and regions-of-interest. We illustrate and discuss the potential of BaSAM resorting to datasets representing solar minimum and maximum conditions: (1) Contributions of quiet-Sun magnetic fields, i.e. the network and (decaying) plage, to solar activity can be better determined when their variation is measured with respect to the background given by "deep" magnetograms. (2) Flaring and intermittent brightenings are easily appraised in BaSAMs of the UV intensity. (3) Both magnetic-field and intensity variations demonstrated that the flux system of sunspots is well connected to the surrounding supergranular cells. In addition, producing daily full-disk BaSAMs for the entire mission time of SDO provides a unique tool to analyze solar cycle variations, showing how vigorous or frail are the variations of magnetic-field and intensity features.

astro-ph.SR↗

arXiv2018

Calibration of full-disk He I 10830 Å filtergrams of the Chromospheric Telescope

The Chromospheric Telescope (ChroTel) is a small 10-cm robotic telescope at Observatorio del Teide on Tenerife (Spain), which observes the entire Sun in H$α$, CaII K, and HeI 10830Å. We present a new calibration method that includes limb-darkening correction, removal of non-uniform filter transmission, and determination of HeI Doppler velocities. Chromospheric full-disk filtergrams are often obtained with Lyot filters, which may display non-uniform transmission causing large-scale intensity variations across the solar disk. Removal of a 2D symmetric limb-darkening function from full-disk images results in a flat background. However, transmission artifacts remain and are even more distinct in these contrast-enhanced images. Zernike polynomials are uniquely appropriate to fit these large-scale intensity variations of the background. The Zernike coefficients show a distinct temporal evolution for ChroTel data, which is likely related to the telescope's alt-azimuth mount that introduces image rotation. In addition, applying this calibration to sets of seven filtergrams that cover the HeI triplet facilitates determining chromospheric Doppler velocities. To validate the method, we use three data sets with varying levels of solar activity. The Doppler velocities are benchmarked with respect to co-temporal high-resolution spectroscopic data of the GREGOR Infrared Spectrograph (GRIS). Furthermore, this technique can be applied to ChroTel H$α$ and CaII K data. The calibration method for ChroTel filtergrams can be easily adapted to other full-disk data exhibiting unwanted large-scale variations. The spectral region of the HeI triplet is a primary choice for high-resolution near-infrared spectropolarimetry. Here, the improved calibration of ChroTel data will provide valuable context data.

astro-ph.SR↗

arXiv2018

The effects of stellar activity on optical high-resolution exoplanet transmission spectra

Chromospherically sensitive atomic lines display different spectra in stellar active regions, spots, and the photosphere, raising the possibility that exoplanet transmission spectra are contaminated by the contrast between various portions of the stellar disk. To explore this effect, we performed transit simulations of G and K-type stars for the spectral lines Ca II K at 3933 Å, Na I 5890 Å, H I 6563 Å (H$α$), and He I 10830 Å. We find that strong facular emission and large coverage fractions can contribute a non-negligible amount to transmission spectra, especially for H$α$, Ca II K, and Na I D, while spots and filaments are comparatively unimportant. The amount of contamination depends strongly on the location of the active regions and the intrinsic emission strength. In particular, active regions must be concentrated along the transit chord in order to produce a consistent in-transit signal. Mean absorption signatures in Na I and H$α$ for example, can reach $\approx 0.2\%$ and $0.3\%$, respectively, for transits of active latitudes with line emission similar in strength to moderate solar flares. Transmission spectra of planets transiting active stars, such as HD 189733, are likely contaminated by the contrast effect, although the tight constraints on active region geometry and emission strength make it unlikely that consistent in-transit signatures are due entirely to the contrast effect. He I 10830 Å is not strongly affected and absorption signatures are likely diluted, rather than enhanced, by stellar activity. He I 10830 Å should thus be considered a priority for probing extended atmospheres, even in the case of active stars.

astro-ph.EP↗

arXiv2014

Chromospheric Rapid Blueshifted Excursions Observed with IBIS and Their Association with Photospheric Magnetic Field Evolution

Chromospheric rapid blueshifted excursions (RBEs) are suggested to be the disk counterparts of type II spicules at the limb and believed to contribute to the coronal heating process. Previous identification of RBEs was mainly based on feature detection using Dopplergrams. In this paper, we study RBEs on 2011 October 21 in a very quiet region at the disk center, which were observed with the high-cadence imaging spectroscopy of the Ca II 8542 A line from the Interferometric Bidimensional Spectrometer (IBIS). By using an automatic spectral analysis algorithm, a total of 98 RBEs are identified during a 11 minute period. Most of these RBEs have either a round or elongated shape, with an average area of 1.2 arcsec^2. The detailed temporal evolution of spectra from IBIS makes possible a quantitative determination of the velocity (~16 km/s) and acceleration (~400 m/s^2) of Ca II 8542 RBEs, and reveal an additional deceleration (~-160 m/s^2) phase that usually follows the initial acceleration. In addition, we also investigate the association of RBEs with the concomitant photospheric magnetic field evolution, using coordinated high-resolution and high-sensitivity magnetograms made by Hinode. Clear examples are found where RBEs appear to be associated with the preceding magnetic flux emergence and/or the subsequent flux cancellation. However, a further analysis with the aid of the Southwest Automatic Magnetic Identification Suite does not yield a significant statistical association between these RBEs and magnetic field evolution. We discuss the implications of our results in the context of understanding the driving mechanism of RBEs.

astro-ph.SR↗

arXiv2014

Digitization of sunspot drawings by Spörer made in 1861-1894

Most of our knowledge about the Sun's activity cycle arises from sunspot observations over the last centuries since telescopes have been used for astronomy. The German astronomer Gustav Spörer observed almost daily the Sun from 1861 until the beginning of 1894 and assembled a 33-year collection of sunspot data covering a total of 445 solar rotation periods. These sunspot drawings were carefully placed on an equidistant grid of heliographic longitude and latitude for each rotation period, which were then copied to copper plates for a lithographic reproduction of the drawings in astronomical journals. In this article, we describe in detail the process of capturing these data as digital images, correcting for various effects of the aging print materials, and preparing the data for contemporary scientific analysis based on advanced image processing techniques. With the processed data we create a butterfly diagram aggregating sunspot areas, and we present methods to measure the size of sunspots (umbra and penumbra) and to determine tilt angles of active regions. A probability density function of the sunspot area is computed, which conforms to contemporary data after rescaling.

astro-ph.SR↗