IRIS, Infrared Image Sensor
The VLTI Infrared Image Sensor (IRIS) is a tilt sensor located in the VLTI interferometric laboratory. IRIS measures the tilt of up to 4 stellar beams simultaneously, in 3 possible spectral bands, J or H or K. The role of IRIS is to monitor the image drift introduced inside the VLTI between the Coudé focus of each telescope (AT or UT) and the VLTI laboratory. These drifts are indeed not corrected by STRAP/MACAO. In addition, IRIS allows a potentially better correction for atmospheric lateral dispersion effects thanks to its sensing band closer to the science band.
Iris is designed to achieve an image centroiding accuracy of 3 arcsec/lab RMS (equivalent to 6.8 marcsec/sky RMS for the UTs, resp. 30 marcsec/sky RMS for the ATs). This applies for a star magnitude up to 14 (resp. 10.7) in any of the IRIS sensing bands (J, H and K) on the UTs (resp. on the ATs).
The tilt actuator of IRIS is currently in the X-Y table of STRAP/MACAO and IRIS delivers tilt error vectors at a rate of up to 10Hz. Provision has been taken to possibly deliver tilt error vectors at a faster rate when a faster actuator will be available.
In addition to the tilt sensing mode, IRIS includes a PSF mode which allows to inspect of the image quality (PSF) of a single VLTI beam.
IRIS is operational with AMBER (3 beams) and MIDI.
More information: publication SPIE-5491-103, Glasgow June 2004