200 Million More Things to Study

Lynn M. Savage

Astronomers associated with the Isaac Newton Telescope/Wide-Field Camera Photometric Ha Survey of the Northern Galactic Plane (IPHAS) are seeing red — lots of red.

The group has released the first comprehensive optical digital survey of a large segment of the Milky Way — the home galaxy of the Earth-Sol system. The newly released catalog includes 2.4 Tb of data with images of about 200 million individual objects, including nebulae and stars appearing in the earliest and latest stages of their life spans. The stunning crimson imagery was acquired by filtering the light emitted by hydrogen-alpha (Hα) ions and was captured with the Isaac Newton Telescope on La Palma, part of the Canary Islands.

Shown here is an offset from the center of the Rosette Nebula, highlighting dust lanes in this stellar nursery, as imaged in hydrogen-alpha emission by the Isaac Newton Telescope/Wide-Field Camera Photometric Ha Survey of the Northern Galactic Plane (IPHAS). The longer dimension in this image is ∼30 arcmin. Image courtesy of Nick J. Wright, University College London and the IPHAS consortium.

The optics of the telescope include a conventional Cassegrain system with a 2.54-m mirror and a focal length of ∼7.5 m. Images were collected at the telescope’s primary focal point with the Wide Field Camera, which comprises a set of four 2048 × 4096-pixel CCDs from EEV (now e2v Technologies Ltd. of Chelmsford, UK) that each has a pixel scale of 0.33 arcsec and a field of view of 0.29 square degrees. The CCD detectors are arranged in an L shape that is exposed twice with a small position offset in between, enabling most locations in the northern galactic plane to be imaged at least twice.

The IPHAS astronomers — more than 50 scientists representing universities and institutions in the UK, US, Spain, Austria, Australia, Thailand and the Netherlands — acquired images over 250 allocated nights spread across three years using three separate filters, including Hα and the Sloan filters r' (∼622 nm) and i' (∼763 nm). The standard exposure times per filter were 30 s for r', 10 s for the i' band and 120 s for Hα.

The group has released the survey in two forms: a complete catalog and a subset that contains only observations that meet a preferred higher and, hence, more uniform, level of quality. The data also are being made available via a new and highly versatile Virtual Observatory interface operated by the UK-based AstroGrid program.