First results using PRIMA FSU as a fringe tracker for MIDI
Ancla Müller, Jörg‐Uwe Pott, S. Morel, R. Abuter, Gerald van Belle, Roy van Boekel, L. Burtscher, F. Delplancke, Th. Henning, W. Jaffe, Ch. Leinert, B. López, A. Matter, K. Meisenheimer, Christian Schmid, Konrad R. W. Tristram, Arnoud P. Verhoeff · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 2010
We report first results obtained from observations using a PRIMA FSU (Fringe Sensor Unit) as a fringe tracker for MIDI on the VLTI when operating with the 1.8-m ATs. Interferometric observations require the correction of the disturbance in the optical path induced by atmospheric turbulence ("piston"). The PRIMA FSU is able to compensate for such disturbances in real-time which makes it a suitable facility to stabilize the fringe signal for other VLTI instruments, like AMBER, MIDI or later MATISSE. Currently, the atmospheric coherence time in the N band (8 to 13 μm) observed by MIDI, as well as the thermal background in this band, require a minimum target flux of 20 Jy and a correlated flux of 10 Jy (in PRISM/HIGH SENSE mode and using the ATs under standard conditions) to allow self-fringe-tracking and data reduction. However, we show that if the fringes are stabilized by the FSU, coherent integration allows a reliable data reduction even for the observation of faint targets (Fcorr <10 Jy) with MIDI at standard detector exposure times. We were able to measure the correlated flux of a 0.5 Jy source, which pushes the current limits of MIDI down to regions where numerous new targets become accessible on ATs. For faint object observations we will discuss the usage of VISIR photometry for calibration purposes. The observational tests done so far and the obtained results represent a first step towards Phase Referenced Imaging with the VLTI in the mid-infrared.