VXMS: the VISTA extreme multiplex spectrograph

A study for a spectrograph delivering at least 10000 slits for galaxies and 20000 for stars over a 2.5 deg² field have been completed as an answer to the call for proposal for future VISTA MOS instrumentation. In a single night, 65000 galaxy redshifts can be measured to z~0.7 and beyond for measuring the Baryon Acoustic Oscillation (BAO) scale and many other science goals. The design features ten cloned spectrographs which give a smaller total weight and length than a unique spectrograph to make it placable in the space envelope of the Cassegrain 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. Two bands cover the 550nm to 900nm wavelength range at resolution of 1100 for blue end and 3000 for red end while another cover the Calcium triplet at 5000. An optional box does imaging but we studied different innovative methods for acquisition without imaging. A new 2.3° corrector was designed that places the pupil before and relatively near the focal plane which permits to give more space at the back of the spectrographs by placing them in a hedgehog configuration. An offaxis field lens in each spectrograph permits to control the pupil position. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.