Instrumentation

GRASSP (GRAnada Sprite Spectrograph and Polarimeter) is an instrument designed, developed and partially built and tested at the IAA with the goal to detect and characterize the optical emissions from different types of Transient Luminous Events (TLEs).

 

TLEs are enormous electrical discharges taking place in the stratosphere and mesosphere of the Earth that were first detected in 1989.


Images of different TLEs recorded with GRASSP from Calar Alto during the 2013 observational campaign. The top right image corresponds to an ELVE. The other images are examples of different types of Sprites (M. Passas et al., 7th Triennial Special Issue of the IEEE Transactions on Plasma Science (Images in Plasma Science), August 2014).

 

The analysis of TLE optical features will allow us to improve our knowledge on how the Earth upper atmospheric electrical activity can affect the chemical and electrical properties of the atmosphere.

GRASSP is a ground observation facility built to complement lightning-related TLE and Terrestrial Gamma ray Flashes (TGF) observations from space that will be carried out by two european space missions: ASIM (of ESA) and TARANIS (of CNES) will be launched in May 2016 and Nov 2016, respectively. GRASSP is a two-arm instrument on a ALT-AZ mount equipped with a medium spectral resolution (R = λ / Δλ = 1600; Δλ = 0.45 nm) spectrograph and a six channel imaging polarimeter that is able to derive the four Stokes parameters in a single image adquisition.

 
GRASSP instrument

 

In addition, GRASSP incorporates a very high sensibility scene camera (CCD SONY 1/3'' with 440000 pixels and 7 μm/pixel) able to detect a minimum luminance of 0.0001 lux within a field of view (FOV) of 19º in diameter.

The spectrograph uses a 1440 lines/mm grating working in the visible - near infrared (700 - 800 nm) spectral range. The detector is a 1360 x 1024 pixel, frame-transfer and back-illuminated ICCD camera (6.4 μm/pixel, 90 QE) with peltier cooling. The field of view (FOV) of the GRASSP spectrograph is 4º x 0.03º. The entrance slit (~ 100 microns) is horizontally oriented to optimize the likelihood of TLE ocurrence during observation. GRASSP can detect TLEs for elevations between 10º and 35º over the horizon.

The polarimeter covers the spectral range 550 - 750 nm and incorporates a 2K x 2K (15 μm/pixel) CCD detector with a circular field of view (FOV) of 5º.

GRASSP is located in the exterior of the 2.2 m telescope dome of the Calar Alto (CAHA) observatory in Spain from where it can cover the atmospheric electrical activity occurring in an area (37º - 45º N; 2º W - 14º E) of the west Mediterranean Sea where most TLE take place in Europe.

 

 

The first light of the GRASSP spectrograph was May 2013.

The GRASSP polarimeter is still under development.

The GRASSP team is part of the scientific team of the ASIM and TARANIS space missions.