ChroTel - Chromospheric Telescope

ChroTel is a telescope to observe the solar chromosphere across the full disk. We acquire narrow-band filtergrams in the lines of Calcium II K (393.3 nm), H alpha (656.3 nm) and Helium I (1083.0 nm) with a cadence of less than a minute. As a special feature we can also derive He I Doppler maps (velocity maps) from these data. 

Latest ChroTel image: The chromosphere of the Sun

ChroTel is continously observing the Sun, weather permitting. The following display is showing the latest image. Examples of ChroTel images and a more detailed description of the instrument are provided below.

Calcium II K
H alpha
Helium 1083

Brief Introduction into ChroTel

spectrum with ChroTel-passbands

ChroTel is a 10 cm aperture telescope to observe the chromosphere of the Sun. It was developed at the Leibniz-Institut for Solar Physics together with the High Altitude Observatory (HAO) in Boulder, USA. With a spatial resolution of about 2 arcsec it observes the solar chromosphere in its most prominent lines un the near UV, the red and the infrared (Ca II K, Hα and He I) using a 2048x2048 pixel CCD. ChroTel uses a two mirror system ("turret") next to the VTT building that relays a stabilized image of the solar disc into a laboratory within the VTT building (click on the video on the right to see it in operation). The heart of the optical lab is a filter system consisting of three narrow-band Lyot filters, of which one is tunable. The He I filter is tunable and observations are taken at seven wavelength positions fully covering the whole spectral line, giving full-disk chromospheric Doppler maps. Images in all channels (and the seven He I bands) can be taken with a cadence of below one minute. The following intensity maps show examples in all three channels.

The operation of ChroTel is automatic, as far as possible. The data with the full cadence is temporarily stored on Tenerife and they are transferred to Freiburg. The ChroTel data also serve as live images for VTT and Gregor and are available over the internet. The scientific data of ChroTel are freely available for scientific and educational use.

Observing campaigns together with the Tenerife Infrared Polarimeter (TIP) at the VTT enabled us to calibrate the Doppler shift maps derived from the seven filtergrams of ChroTel in the Helium I channel.


The Doppler velocity data show interesting flow structures e.g. in low-lying cool loops and in active regions and underline the potential of ChroTel for the regular science operations. The performance of ChroTel is illustrated by the Doppler map in He I showing an active region and surrounding network. 

The main goals of the science operations of ChroTel are to study the dynamic response of the chromosphere to photospheric driving especially in the chromospheric network, large-scale structures and their disturbance such as flares and CMEs and the chromospheric source of the fast solar wind.