Ground-based coronagraphy: Performance of a four-quadrant phase mask

Detection of faint companions near bright stars requires the usage of high dynamic range instrumentation. The four quadrant phase mask is a quite efficient nulling device for the light of on-axis stars as shown by simulations. (1,2) We conducted a test of the true performance of this concept starting with the manufacturing of the optical element, continuing with the installation in the telescope and the usage of the Adaptive Optics system (essential for this coronagraphic tool). A four quadrant phase mask was installed in the 3.5m telescope on Calar Alto and several tests with both an artificial source and natural stars were conducted. Tests in order to detect faint companions around HD 140913, TRN 1 and HD 161797 were successful for the last target and also, although almost serendipitously, in the case of HD144004. The main limitations found for the phase mask cancelling effect at relatively low Strehl ratios (16%-63%) were the residual tip-tilt of our system and the control of placement of the mask in the optical train.