Using Astronomical Site Monitors (ASM) to Assess Science Image Quality at the VLT Observatory

Thursday, 19 December 2013 1 p.m. — 2 p.m. MST

AURA Lecture Hall

NOIRLab South Colloquia
MARC SARAZIN (ESO and NOAO South viviting Astronomer)

It is well known that a seeing monitor (DIMM) at a few meter above ground is often overestimating the actual observing conditions at large telescopes. Several avenues re explored to improve the accuracy of the information available in the control room for a more efficient queue scheduling at Paranal.

  • The DIMM tower can be elevated to the level of the entrance slit of the large telescope (e.g. from 6m to 15m above ground), as planned for the upgrade of the Paranal ASM in 2014 .
  • The large telescope itself can be turned into a site monitor monitoring the active optics  wavefront sensor image when available: a one year database has been gathered of the VLT active optics Shack-Hartmann wavefront sensor images taken at each operating focus about every 30 seconds. A dedicated code insensitive to atmospheric dispersion was developed to accurately extract the median full width at half maximum (FWHM) of the unvignetted S-H spots.
  • The DIMM data can be corrected to the height of a large telescope when information on the turbulence in the surface layer is known, either from a SLODAR profiler, or from the difference between MASS profiler and DIMM.