Astronomical instrumentation

Abstract:

The Gemini-North Multiobject Spectrograph (CMOS) includes a powerful capability for integral field spectroscopy - the first to be installed and used on an 8-10m telescope. CMOS is switched to this mode by the remote insertion of an integral field unit (IFU) into the focal plane in place of the masks used for multiobject spectroscopy. With 1500 lenslet-coupled fibres, it provides a total field of view exceeding 50 square arcseconds, including a separate field dedicated to background subtraction. We describe the design, construction and testing of the IFU and present performance results obtained during commissioning.

Abstract:

The Gemini Multiobject Spectrograph (GMOS) installed on the Gemini-North telescope has a facility for integral field spectroscopy over the wavelength range 0.4-1.0 μm. GMOS is converted to this mode by the remote insertion of an integral field unit (IFU) into the beam in place of the masks used for the multiobject mode. With the IFU deployed, integral field spectroscopy is available over a fully filled contiguous field of 5″ × 7″ with a sampling of 0″.2. A separate field of half the area, but otherwise identical, is also provided to improve background subtraction. The IFU contains 1500 lenslet-coupled fibers and is the first facility of any type for integral field spectroscopy employed on an 8-10 m telescope. We describe the design, construction, and testing of the GMOS IFU and present measurements of the throughput both in the laboratory and at the telescope. We compare these with a theoretical prediction made before construction started. All are in good agreement with each other, with the on-telescope throughput exceeding 60% (averaged over wavelength). A second paper will verify the scientific performance by comparison with existing one- and two-dimensional data sets.

Abstract:

We describe MUSE (Multi Unit Spectroscopic Explorer), a second-generation integral-field spectrograph for the VLT, operating in the visible and near IR wavelength range. It combines a 1' × 1' Field of View with the improved spatial resolution (0.2″) provided by adaptive optics and covers a large simultaneous spectral range (0.48-1 μm). With this unique combination of capabilities, MUSE has a wide domain of application, and a large discovery potential. It will provide ultra deep fields with a limiting magnitude for spectroscopy of R = 28. After a brief presentation of the scientific case and the derived instrument requirements, we will focus on the MUSE optical design, including the overall architecture, the major trade-off that were conducted in order to optimize the cost and performance, and a provisional implementation scheme of the instrument on the VLT Nasmyth platform. Then the most important optical subsystems (as the 3 × 8 Field-splitter, the Image Slicers and the Spectrometers) are described. One of MUSE special feature is the impressive number of Image Slicer and Spectrometer modules which must be manufactured, that is 24. The realization of such series has been studied in collaboration with an industrial company. Finally, a preliminary estimation of the expected performance and a technological development program in order to secure the realization of the critical optical subsystems will be presented.

Abstract:

We present a comparison of how well the large-scale structure of the universe is traced by clusters from the Abell catalog and from the Automated Plate Measuring Facility (APM). We investigate selection functions for both cluster catalogs, using samples of all clusters (including clusters with estimated redshifts) and samples of clusters with measured redshifts. We present a catalog of superclusters of galaxies, based on APM clusters up to a redshift Zlim = 0.13. We find that the distribution of rich superclusters, defined by all Abell and APM clusters, is similar in the volume covered by both cluster samples. We calculate the correlation function for Abell and APM cluster samples. We show that the supercluster-void network can be traced with both cluster samples; the network has a period of ∼ 120 h^-1 Mpc. However, the APM cluster sample with measured redshifts covers a small volume, which contains only a few very rich superclusters. These superclusters surround one void and have exceptionally large mutual separations. Because of this property, the secondary maximum of the correlation function of APM clusters with measured velocities is located at larger scales than the corresponding feature in the correlation function of Abell clusters. We conclude that the APM sample is not representative of the large-scale structure as a whole because of the small volume covered. The Abell cluster catalog is presently the best sample to investigate the large-scale distribution of high-density regions in the universe.