Night-time calibrations

Some observing modes require attached calibration to be taken during the night. Here we detail which ones are mandatory and will be charged to the user observing time, and which ones are part of the calibrations provided by the observatory.

Whenever a coronagraph is used, the SPHERE calibration plan recommends including Flux-F and Star Center - C observation types in observing templates both before and after the science observing template. The purpose of the first calibration (Flux - F) is to measure the unsaturated peak flux of the star by offsetting it from the coronagraph spot, in order to ensure accurate relative photometry, and thus contrast calibration. The second calibration (Star Center - C) applies a waffle pattern on the deformable mirror to produce echoes of the PSF hidden behind the coronagraphic mask at very specific locations. The goal of (Star Center - C) is to accurately measure the position of the star behind the coronagraph, which is critical for astrometric measurements of off-axis sources.

Night-time calibrations provided by the observatory
The SPHERE calibration plan foresees taking photometric standard stars for all imaging modes using broadband filters, and polarimetric standard stars for all polarimetric modes (polarized and zero-polarization standard stars). The calibration plan also foresees observing reference astrometric fields (binary systems or star clusters with high precision astrometry available) without coronagraph (including ND filters if needed to avoid detector saturation) on a regular basis (frequency ~1 week).

Optional night-time calibrations
Photometric standard stars in narrow band filters, or wavelength calibration for IRDIS_LSS, must be requested by the user and is charged to the observer. No spectro-photometric standards or telluric standard will be taken unless explicitly requested as special night-time calibrations. The time required to do so will be charged to the observer.

Day-time calibrations

In addition, all necessary day-time calibrations will be acquired by the observatory staff during the day following the night of the observations (or within the validity period indicated in the tables below). In the following tables, "Modes" refers to the instrument mode among IRDIFS, IRDIFS_EXT, IRDIS_DBI, IRDIS_CI, IRDIS_LSS, and ZIMPOL_I, ZIMPOL_P1, ZIMPOL_P2, when applicable. "Matching parameters" describes the most critical parameter for generating the automatic calibration. For instance, darks/backgrounds will have the same DIT as the science, flats will be taken in the same filter as the science, etc. On the other hand, ZIMPOL polarimetric calibrations will be done in corresponding broadband filters (see filter correspondence table below).

Daytime calibrations for IRDIS

Daytime calibrations for IFS

Daytime calibrations for ZIMPOL

NOTE: ZIMPOL distortion maps are taken weekly in V, N_R, N_I for monitoring purposes.

Filter correspondance table for ZIMPOL calibrations