Two feasibility studies for spectrographs that can deliver at least 4000 MOS slits over a 1° field at the prime focuses of the Anglo-Australian and Calar Alto Observatories have been completed. We describe the design and science case of the Calar Alto eXtreme Multiplex Spectrograph (XMS) for which an extended study, half way between feasibility study and phase-A, was made. The optical design is quite similar than in the AAO study for the Next Generation 1 degree Field (NG1dF) but the mechanical design of XMS is quite different and much more developed. In a single night, 25000 galaxy redshifts can be measured to z~0.7 and beyond for measuring the Baryon Acoustic Oscillation (BAO) scale and many other science goals. This may provide a low-cost alternative to WFMOS for example and other large fibre spectrographs. The design features four cloned spectrographs which gives a smaller total weight and length than a unique spectrograph to makes it placable at prime focus. The clones use a transparent design including a grism in which all optics are about the size or smaller than the clone rectangular subfield so that they can be tightly packed with little gaps between subfields. Only low cost glasses are used; the variations in chromatic aberrations between bands are compensated by changing a box containing the grism and two adjacent lenses. Three bands cover the 420nm to 920nm wavelength range at 10A resolution while another cover the Calcium triplet at 3A. An optional box does imaging. We however also studied different innovative methods for acquisition without imaging. A special mask changing mechanism was also designed to compensate for the lack of space around the focal plane. Conceptual designs for larger projects (AAT 2º field, CFHT, VISTA) have also been done.

Date Published: 15 July 2010
PDF: 15 pages
Proc. SPIE 7735, Ground-based and Airborne Instrumentation for Astronomy III, 77351Q (15 July 2010); doi: 10.1117/12.857932

Published in SPIE Proceedings Vol. 7735:
Ground-based and Airborne Instrumentation for Astronomy III
Ian S. McLean; Suzanne K. Ramsay; Hideki Takami, Editor(s)