HAWK-I/GRAAL Science Verification

Contents

General Information

An integral part of the commissioning of a new instrument at the VLT is the Science Verification phase. SV programmes include a set of typical scientific observations that should verify and demonstrate to the community the capabilities of the new instrument in the operational framework of the VLT Observatory. In accordance with its SV Policy and Procedures  ESO encourages the community to submit also highly challenging or risky science observations that will push HAWK-I/GRAAL and the VLT to its limits in order to better understand the performance parameter space and its envelope. Such observations should focus on the instruments core modes in order to benefit a wide community.

HAWK-I is a cryogenic wide-field imager installed at the Nasmyth A focus of UT4. The on-sky field of view is 7.5'x7.5', with a cross-shaped gap of 15" between the four detectors. The pixel scale is of 0.106".  The instrument is offered with 10 observing filters placed in two filter wheels: 4 broad band filters (Y, J, H & K) and 6 narrow band filters (Br𝛾, CH4, H2, 1.061 μm, 1.187 μm & 2.090 μm).

In combination with GRAAL (the ground layer adaptive optics module of the VLT Adaptive Optics Facility) HAWK-I will offer a significantly improved image quality. GRAAL is able to compensate for the lowest layers of the atmospheric turbulence (up to ~ 300-500 m, depending on the spatial frequencies considered), carrying more than half of the turbulence variance.

The HAWK-I+GRAAL seeing enhanced mode is expected to provide:

  • deeper expositions for nearly point-source objects, or
  • deeper detection limiting magnitude
  • the operation of HAWK-I for more than 80% of the time with an equivalent K-band seeing of 0.55" (instead of 0.7" without GRAAL)
  • excellent image quality in good atmospheric conditions (0.3” for around 30% of the time).

The adaptive-optics assisted HAWK-I mode is now offered to the community for Science Verification (SV) for 4 nights in January 2018. All astronomers are invited to participate in this opportunity to obtain unique science with the HAWK-I AO-supported mode and thus to demonstrate its scientific capabilities. A call for proposals has been issued and the community is invited to submit proposals for the HAWK-I/GRAAL science verification using the simplified proposal template.

The deadline for proposal submission is 31 October 2017, 18:00 CET.

Proposals will be reviewed by an internal panel and allocated time on the basis of scientific merit and feasibility, as well as in the demonstrated ability of the Principle Investigators to deliver results on a timely basis.

The observations will be conducted during the nights of 2-5 January 2018 in Service Mode by a dedicated team of ESO astronomers. The HAWK-I/GRAAL SV team will be able to assist the successful PI’s in the preparation and optimisation of the OBs.

The HAWK-I ETC has not been updated for the new mode yet, as the commissioning is still pending. We encourage the proposers to use the current HAWK-I ETC with a slightly increased (i.e. better) image quality for the calculation of the exposure times.

The latest version of the HAWK-I data reduction pipeline will be available for reduction of the SV data. Proposers are reminded that all SV data are made public worldwide immediately after passing the usual quality control checks.

Short description of HAWK-I/GRAAL

GRAAL is based on the 4 sodium Laser Guide-Stars system of the VLT. The lasers are pointed towards areas located outside of the science field of view (7.5 arcmin), and the light re-emitted by the 80-100 km altitude Sodium layer is collected by 4 WFS (wavefront sensors) each with 40x40 subapertures. The slopes provided by the WFS are combined to provide an estimate of the WFE for the lowest layers; this shape is then removed from the actual shape of the DSM. The AO-loop is closed at a 700 to 1000 Hz frequency.

As the LGS are not useful to sense tip-tilt, an additional visible Natural guide-star WFS is embedded in GRAAL, and the tip-tilt is corrected at a loop frequency of 250 Hz. The science HAWAII 2RG infrared sensors of HAWK-I will as well be used to correct slow drifts between visible wavefront sensing and IR imaging paths, coming from flexures and uncompensated atmospheric dispersion, taking the opportunity of continuous reading of the science detectors during integration.


IMPORTANT TECHNICAL REQUIREMENT:

HAWK-I/GRAAL observations are supposed to make use of a bright nearby star to sense the tip-tilt of the wavefront. The tip-tilt star (TTS) should have a R-magnitude in the range 6-16.5 mag, and must be located at 8.2'+/-0.5' radial distance from the HAWK-I science target (see figure below).  The magnitude of the TT star must be indicated in the SV proposal. In case there is no suitable TTS, the observations can be carried out in TTS-free mode by using only the 4 LGS. This mode still provides good correction, with performances that are only few percents lower than the full AO mode (i.e. TTS and 4LGS).

file
Left: GRAAL focal plane illustration. The 4 LGS rotate with respect to the FoV. The visible TT-star is selected outside of the LGS ring. Right: GRAAL expected performance.

 

Approved programmes and Phase2

For approved proposals the Phase2 observation preparation will be carried out with the help of the Guidecam tool that allows the selection of TTS and other observational settings. A HAWK-I/GRAAL specific version of the Guidecam tool will be released to SV users as soon as the approved SV proposals have been announced.

The list of approved proposals as well as instructions regarding the Phase2 OBs preparation will be posted here on this webpage after the successful SV proposals have been selected.

Useful links