. 5 m telescope

The ongoing development of large infrared array detectors has enabled wide field, deep surveys to be undertaken. There are, however, a number of challenges in building an infrared instrument which has both excellent optical quality and high sensitivity over a wide field. We discuss these problems in the context of building a wide field imaging camera for the 3.5m telescope at Calar Alto with the new 2K×2K HgCdTe HAWAII-2 focal plane array. Our final design is a prime focus camera with a 15 ′ field-of-view, called Omega 2000. To achieve excellent optical quality over the whole field, we have had to dispense with the reimaging optics and cold Lyot stop. We show that creative baffling schemes, including the use of undersized baffles, can compensate for the lost K band sensitivity. A moving baffle will be employed in Omega 2000 to allow full transmission in the non-thermal J and H bands. 1. SCIENTIFIC MOTIVATION There are numerous scientific projects which would benefit from large area infrared surveys. Most fit into the category of discovering and characterising new objects. An example is a survey for very low mass stars, brown dwarfs and free floating giant planets in open clusters and star forming regions. All of these objects are cool (T eff < ∼ 3000) and have a significantly larger J than R or I band flux. They can thus be detected with their optical–infrared colour, or even their Z–J colours obtainable on the same HgCdTe detector. Other areas of science which would benefit from surveys include: the initial mass function of star forming regions; the dark matter content and the age of the Galaxy from cool white dwarfs in the Galactic disk and halo; Galactic structure traced via K and M giants (which, due to the lower extinction in the near infrared, can be traced to larger distances); galaxy surveys at low Galactic latitudes; quasar surveys; star formation history and damped Lyman alpha galaxies; high redshift galaxies; gravitational lenses and the cosmological constant. To serve these scientific goals, we plan to build a wide field near infrared (0.8–2.5µm) imaging camera for doing large area surveys. Given the nature of the Calar Alto Observatory as a resource for German and Spanish astronomers, this camera (Omega 2000) is intended for use in common-user mode rather than undertaking pre-defined surveys. Some projects are less concerned with area coverage than with volume, in which case deep …