The coatings of the FORS2 linear atmospheric dispersion corrector (LADC) prisms have degraded, so it was decided to replace them with the uncoated prisms of the twin FORS1 LADC. The exchange took place on 10 November 2014 and the performance of the prisms has been extensively tested.
Exchange of FORS2 Atmospheric Dispersion Corrector
Published: 16 Dec 2014
The throughput of FORS2 has been found to have increased by 0.1 mag in the blue and 0.05 mag in the red, most likely because of scattering by the damaged coating of the former prisms. Systematics seen in time-resolved light curves with FORS2 should now have been reduced, again allowing study of transmission spectra of exoplanets. As a demonstration, on 15–16 November 2014 one transit of WASP-19b was observed under thin cirrus with the LADC fixed in the beam (i.e., not actively correcting the atmospheric dispersion), as usually done for such observations. These data show that the light curve of the transit could be easily detrended with a second-order degree polynomial.
The observations of WASP-19b are publicly available from the ESO Science Archive: the data were obtained between 2014-11-16 05:16 UT to 08:49 UT under Prog. ID: 60.A-9203(F).