NEAR Science Demonstration
ESO, in collaboration with the Breakthrough Initiatives, has modified the Very Large Telescope mid-IR imager (VISIR) to greatly enhance its ability to search for potentially habitable planets around both components of the binary Alpha Centauri, part of the closest stellar system to the Earth. Apart from this specific goal, a large variety of science cases will benefit from the enhanced sensitivity and PSF contrast.
The NEAR (New Earths in the Alpha Centauri Region) concept combines adaptive optics using the deformable secondary mirror at Unit Telescope 4, annular groove phase mask (AGPM) coronagraphy optimized for the most sensitive spectral bandpass in the N-band, and novel fast (up to 10 Hz) chopping techniques using either the DSM or an internal chopper based on a concept for longer wavelengths invented by the microwave pioneer Robert Dicke. The internal chopper alternates between sky and an internal blackbody adjusted in temperature to match the sky brightness.
With AO and fast chopping NEAR increase the sensitivity by about a factor 3 with respect to classical VISIR observations in all N-band filters. The sensitivity in broad-band filters like B10.7 or the new NEAR filter (10-12.5 μm) is better than 1 mJy [10 σ/hr].
The imaging contrast is enhanced by AO in combination with an AGPM coronagraph or a Shaped Pupil mask. With the AGPM, the inner working angle of ~1 λ/D corresponds to ~300 mas in N-band, and the raw PSF contrast is around 10-5 at separations larger than 1”. The shaped pupil mask produces a PSF with high contrast (~10-5) regions between 0.8” and 1.5” and 3” to 7” from the central source. The sensitivity with the AGPM coronagraph is 25% times higher than with the shaped pupil mask, and the images are dimmed, so they do not suffer from electronics ghosts produced by bright sources. The shaped pupil mask, instead, does not leak extended emission into a ring-like structure and has a spatially invariant PSF facilitating de-convolution and PSF subtraction techniques. It is therefore an interesting option for the observation of extended emission around stars like circumstellar disks or shells at later evolution stages.
Observations of extended objects can be achieved through chopping with the internal chopper (Dicke switch). This is useful for the observation where classical chopping would lead to self-subtraction of the target. The blackbody cannot perfectly mimic the spatial flux distribution of the sky, such that additional chopped sky frames must also be recorded in order to flatten the image in a scheme similar to classical nodding. The sensitivity with internal chopping is lower than with DSM chopping by a factor 2.
The new capabilities of NEAR will be offered for science demonstration for two runs: in September 2019 4 first halves of nights and 8 nights in December 2019. ESO encourages the community to submit challenging and/or risky science observations that will push NEAR and the VLT to its limits in order to better understand the performance parameter space and its envelope. We wish to demonstrate the superior capabilities of NEAR for N-band observations requiring high sensitivity and PSF contrast.
VISIR N-band spectroscopy with AO is also possible. Spectroscopic observations must be compatible with pupil-stabilized tracking, i.e., the field of view rotates with respect to the slit.
The following constraints apply for observations with NEAR, which does not serve all VISIR observing modes:
- NEAR offers VISIR N-band imaging in all filters listed in the user manual, and the new NEAR 10-12.5μm broad band filter.
- The AO guide star requires a brightness I≤8.5 magnitude, which has to be located close to center field. A field-selector can be used to offset the guide star by up to 2”, e.g. to well center it on the coronagraph. NEAR observations are performed in pupil stabilized imaging, so the field of view rotates during an observation depending on where the telescope points. The Detector Integration Times (DITs) of 5-10 ms are short enough to avoid smearing in the FoV. Exposures need to be de-rotated before stacking.
- Chopping with the DSM is offered in a 45” imaging field and can be done with chop throws between 3-5.5”. Chopping can be performed between two components of a binary star (AO loop closed during both half cycles) or using a single star (AO loop closed while on the star and suspended while on sky). Target acquisition in this mode takes 10 minutes.
- Chopping with the internal chopper (Dicke switch) is accomplished in a reduced field-of-view of 20”x40”, and a factor 2 lower sensitivity than with DSM chopping. Standard chop-nod observations introduce unavoidable self-subtraction of the signal when applied on large crowded fields such as star forming regions. The Dicke switch technique avoids such systematic errors. However, the internal blackbody cannot perfectly mimic the spatial flux distribution of the sky, such that chopped sky frames must also be recorded in order to flatten the image in a scheme similar to classical nodding. Target acquisition in this mode takes 15 min.
- For integration time estimates the VISIR exposure time calculator (www.eso.org/observing/etc/) may be used only when correcting to the increased NEAR sensitivity performances, which is a factor 3 when using DSM chopping, (hence a factor 9 in integration time), and a factor 1.5 in Dicke switch (hence factor 2.3 in integration time).
More technical details can be found in the NEAR commissioning report.
VISIR N-band spectroscopy with AO is also possible. Spectroscopic observations must be compatible with pupil-stabilized tracking, i.e. the field of view rotates with respect to the slit.
The community is invited to participate in this opportunity to obtain unique science with NEAR and thus to demonstrate its scientific capabilities. A call for proposals has been issued and the community is invited to submit proposals for the NEAR science demonstration using the new Phase 1 web interface. Please use "103 VISIR-NEAR" for the proposal cycle. The The new proposal submission system was announced in a recent Messenger article (Messenger 175, 63) . A detailed description is available on the P1 Help Page.
The deadline for proposal submission is 7 August 2019, 12:00 CEST.
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 in Service Mode by a dedicated team of ESO astronomers. The NEAR SV team will be available to assist the successful PI’s in the preparation and optimisation of the OBs on a best effort basis. The collected data will be made available to the whole ESO user community.
All (raw) data and calibrations are public immediately after the observations. There is no proprietary period nor early data release to the PIs. See the VLT Science Verification Policy and Procedures for more details.