The Chromospheric Telescope

Abstract

Aims. We introduce the Chromospheric Telescope (ChroTel) at the Observatorio del Teide in Izaña on Tenerife as a new multiwavelength imaging telescope for full-disk synoptic observations of the solar chromosphere. We describe the design of the instrument and summarize its performance during the first one and a half years of operation. We present a method to derive line-of-sight velocity maps of the full solar disk from filtergrams taken in and near the He i infrared line at 10 830 Å. Methods. ChroTel observations are conducted using Lyot-type filters for the chromospheric lines of Ca ii K, Hα, and He i 10 830 Å. The instrument operates autonomically and gathers imaging data in all three channels with a cadence of down to one minute. The use of a tunable filter for the He i line allows us to determine line-shifts by calibrating the line-of-sight velocity maps derived from the filtergram intensities with spectrographic data from the Tenerife Infrared Polarimeter at high spatial and spectral resolution. Results. The robotic operation and automated data reduction have proven to operate reliably in the first one and and half years. The achieved spatial resolution of the data is close to the theoretical limit of 2 arcsec in Hα and Ca ii K and 3 arcsec in He i. Line-of-sight velocities in He i can be determined with a precision of better than 3-4 km s −1 when co-temporal spectrographic maps are available for calibration. Conclusions. ChroTel offers a unique combination of imaging in the most important chromospheric lines, along with the possibility to determine line-of-sight velocities in one of the lines. This is of interest for scientific investigations of large-scale structures in the solar chromosphere, as well as for context imaging of high-resolution solar observations.

Sunspot splitting triggering an eruptive flare

“…The derivation of the LOS velocity has been explained in Bethge et al (2011), wherein the weight factor for each filtergram has to be determined through calibration with spectroscopic data. Since there were no spectroscopic observations of the He i triplet on July 2, we compute the LOS velocity from the mean weight factors determined by Bethge et al (2011). For this reason, and since we are dealing with a wide range of velocities from several km s −1 in the region of the flare to less than 2 km s −1 in the quiet Sun, we refrain from making quantitative estimates of the velocities.…”

Diagnostic potential of the Ca II 8542 Å line for solar filaments

“…To put the observations in context, we have made use of data provided by some of the band-pass filters (AIA; Lemen et al 2012) on board NASA's Solar Dynamics Observatory (SDO; Pesnell et al 2012), by the Global Oscillation Network Group (GONG; Hill et al 1994) and from the Chromospheric Telescope (Chrotel; Bethge et al 2011). We display in Fig.…”

“…Chrotel uses a tunable filter for observing the He i line, which allows us to estimate the line-of-sight velocity. Due to the presence of several line components and the weak absorption of the multiplet, the calibration of the LOS velocity in Chrotel is difficult (Bethge et al 2011). For this reason, we only show a Dopplergram computed from the subtraction of two filtergrams at ±0.7 Å in arbitrary units (see Fig.…”

Where does the Thermospheric Ionospheric GEospheric Research (TIGER) Program go?

“…This requires a telescope that observes the entire solar disk as often as weather conditions and observatory operation permit. These requirements are met by ChroTel, a full-disk telescope (Bethge et al, 2011) operated by KIS in the Observatorio del Teide on Tenerife. The telescope provides narrow-band images of three chromospheric lines, namely Hα, Ca K, and the He I 1083.0 nm line.…”