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ChroTel: a new robotic solar telescope

A recently built telescope on Tenerife will observe eruptive events in the solar chromosphere and help to gain physical insights into the underlying triggering mechanisms.
9 September 2008, SPIE Newsroom. DOI: 10.1117/2.1200808.1260

The chromosphere is a thin dynamic layer in the solar atmosphere, located between the surface and the corona. It is not visible to the naked eye, except for a few seconds during a total eclipse. The spectacular and energetic processes occurring here can be visualized using narrow-band optical filters. For better insights into the as yet poorly understood physical mechanisms causing these chromospheric events, continuous high-quality observations are required. Some of these phenomena are very short lived and highly unpredictable.

ChroTel, a new solar telescope nearing completion at the Teide Observatory on Tenerife, Spain (see Figure 1), allows near-simultaneous observations of three different solar chromospheric layers. This is achieved by recording narrow-band images at different wavelengths, employing a short cadence of about a minute. The potential combination of this imaging capability with coordinated spectroscopic observations using the existing nearby high-resolution solar telescope will lead to the establishment of a unique astronomical facility. The images will reveal information about the highly energetic physical processes and their signatures in the chromosphere. The onset, evolution, and dissolution of eruptive events such as prominences, flares, and coronal mass ejections can be examined throughout almost the entire chromospheric altitude range, thus allowing us to uniquely determine the associated plasma motions.

Figure 1. ChroTel telescope with its turret pointed at the sun (right). The box on the left-hand side houses the prime focus and some of the reimaging optics.

ChroTel's main scientific objectives include measuring the dynamic response of the solar chromosphere to photospheric driving of the ejections. Models1 predict that magneto-convection at the solar surface can lead to convective collapse, which causes chromospheric plasma to rapidly accelerate toward lower altitudes. Chromospheric Doppler-shift observations of the full solar disk will provide the spatial coverage required to further investigate the physical conditions associated with these processes.

Second, ChroTel will probe large-scale structures in the solar chromosphere, in particular plasma-flow behavior during and after eruptions. He i filtergrams will provide unique opportunities to investigate the relevant Doppler shifts. The telescope's full-disk coverage will ensure that all events are recorded.

Third, the telescope will enable studies of the fast solar wind. Close examination of the Doppler shifts associated with the He i absorption line may show the signatures of solar-wind outflows deep inside the chromosphere. If so, this will help to clarify whether the solar wind is injected deep in the chromosphere, forming a continuous flow away from the sun, or at higher levels in the transition region to the corona.

The new facility2 (see Figure 2) is a refractor with an effective aperture of 100mm and a focal length of 2250mm. Its mechanical design is a robotically operated domeless fully encapsulated turret system. The domeless construction was one major reason for the use of a turret. All drives, optics, and light paths are located inside the protective housing but accessible for maintenance. A motor-driven shutter that will open only during observations protects the entrance window.

Figure 2. Mechanical design of the telescope with the position-sensitive detector (PSD), the two-mirror turret system (M1 and M2, where M1 is hidden behind the tube in this figure), the elevation and azimuth drives, the M3 mirror feeding the light horizontally into the telescope, and the telescope lens (L1).

ChroTel uses a three-stage guiding and pointing system. First, the sun's position is computed from the local time. This calibrates the telescope drives to the correct speed for proper guiding. Second, a slow-guider system detects deviations from the nominal solar position and sends the requisite correction signals to the motor drives. Third, a servo system using a limb sensor and a piezo-driven tip-tilt mirror3 measures and corrects for fast image motion. This leads to an overall pointing precision of better than 0.5 arcseconds.

The focal-plane instrumentation4 consists of three narrow-band Lyot-type filters for wavelengths around 393nm (a resonance line of ionized calcium), 656nm (the neutral hydrogen-α line), and 1083nm (a neutral helium line). The first two filters have fixed throughputs and bandwidths of 0.03 and 0.05nm, respectively. The helium filter5 was rebuilt—based on an existing fixed-wavelength Lyot filter—as a tunable filter with a step width of 0.014nm. This allows measuring the solar-plasma motion via Doppler shifts of the helium line.

The sun is imaged onto a CCD with 2048×2048 pixels, with a pixel scale of about 1 arcsecond. The telescope's field of view is 0.6°, i.e., the full solar disk and about 100 arcseconds of the outer atmosphere are recorded simultaneously (see Figure 3 for an example). Operation of ChroTel will start in 2009. Synoptic images at low cadence will be made available on the Web immediately.6 The full observational sequences will be stored for follow-up off-line analysis.

Figure 3. Image of the sun taken at 393nm with a bandwidth of 0.03nm, observed on 25 July 2007. North is at the top. The chromospheric network is clearly visible, as well as some magnetic activity (bright) on the eastern limb (left).
Ch. Bethge, D. F. Elmore, R. Friedlein, C. Halbgewachs, M. Sigwarth, and K. Streander made important contributions to the ChroTel project.

Wolfgang Schmidt, Thomas Kentischer, Hardi Peter
Kiepenheuer-Institut für Sonnenphysik
Freiburg, Germany

Wolfgang Schmidt is a senior scientist at the Kiepenheuer Institute for Solar Physics (KIS) and an associate professor at the University of Freiburg, where he also obtained his PhD. He is currently a coinvestigator of the Solar Lite technology project and of the Sunrise balloon telescope.

Thomas Kentischer is a staff scientist at KIS. He also obtained his PhD at the University of Freiburg and specializes in optical instrumentation. He is the project scientist for both ChroTel and the Visible Tunable Filter for the Advanced Technology Solar Telescope.

Hardi Peter is a staff scientist at KIS and an associate professor at the University of Freiburg. He obtained his PhD at the University of Göttingen, Germany. His scientific interests focus on the solar corona. He is the ChroTel principal investigator.

Michael Knölker
High Altitude Observatory
Boulder, CO

Michael Knölker received his PhD from the University of Freiburg, Germany. He has been the director of the High Altitude Observatory since 1997, following an appointment as staff scientist at KIS.