Speaker
Description
DESI-II is the planned second phase of the Dark Energy Spectroscopic Instrument, with survey operations expected from 2029 and science goals spanning dark energy, inflation, and dark matter. A key requirement for the high-redshift component of DESI-II is new medium-band imaging able to identify Ly$\alpha$ emitters and Lyman-break galaxies at $z \gtrsim 2$ with much higher efficiency and photometric-redshift precision than is possible from existing broad-band data alone. Within the wider DESI-II imaging effort, we are developing a VST/OmegaCAM Large Programme designed to provide an independent and complementary data set over about 1000 deg$^2$ of the equatorial footprint using a custom 26-nm-wide M5168 filter centered at 516.9 nm.
The proposed survey will deliver six-pass imaging to a stacked depth of about 25 AB mag, matching the DESI-II imaging baseline while taking advantage of VST's wide field and flexible scheduling across a broad right-ascension range. In addition to strengthening target selection for the DESI-II high-redshift survey, the VST data will provide an important cross-check on photometric systematics, calibration stability, and field-to-field uniformity relative to the DECam, CFHT, and Subaru imaging programmes. The project also offers a strong legacy component for the VST community: the VST observations would be released as part of the wider DESI-II medium-band imaging effort, alongside the ongoing CFHT and Subaru programmes, following the successful model established by the public DECaLS releases. We will present the science case and applications, survey design, observing strategy, current instrument requirements for installing the new filter in OmegaCAM, and the role of VST within the broader international DESI-II imaging programme.