John B. Roll

Hectospec, the mmt's 300 optical fiber-fed spectrograph

ABSTRACT The Hectospec is a 300 optical fiber fed spectrograph commissioned at the MMT in the spring of 2004. In the configuration pioneered by the Autofib instrument at the Anglo‐Australian Telescope, Hectospec’s fiber probes are arranged in a radial “fisherman on the pond” geometry and held in position with small magnets. A pair of high‐speed, six‐axis robots move the 300 fiber buttons between observing configurations within ∼300 s, and to an accuracy of ∼25 μm. The optical fibers run for 26 m between the MMT’s focal surface and the bench spectrograph, operating at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textc...

Design of grazing-incidence multilayer supermirrors for hard-x-ray reflectors.

Extremely broadband grazing-incidence multilayers for hard-x-ray reflection can be obtained by a gradual change of the layer thicknesses down through the structure. Existing approaches for designing similar neutron optics, called supermirrors, are shown to provide respectable performance when applied to x-ray multilayers. However, none of these approaches consider the effects of imperfect layer interfaces and absorption in the overlying layers. Adaptations of neutron designs that take these effects into account are presented, and a thorough analysis of two specific applications (a single hard-x-ray reflector and a hard-x-ray telescope) shows that an improved performance can be obtained. A multilayer whose bilayer thicknesses are given by a power law expression is found to provide the best solution; however, it is only slightly better than some of the adapted neutron designs.

Kinematics of NGC 2264 : Signs of cluster formation

We present results from 1078 high-resolution spectra of 990 stars in the young open cluster NGC 2264, obtained with the Hectochelle multiobject echelle spectrograph on the 6.5 m MMT. We confirm 471 stars as members on the basis of their radial velocity and/or Hα emission. The radial velocity distribution of cluster members is non-Gaussian, with a dispersion of σ ≈ 3.5 km s-1. We find a substantial north-south velocity gradient and spatially coherent structure in the radial velocity distribution, similar to that seen in the molecular gas in the region. Our results suggest that there are at least three distinguishable subclusters in NGC 2264, correlated with similar structure seen in 13CO emission, which is likely to be a remnant of initial structure in this very young cluster. We propose that this substructure is the result of gravitational amplification of initial inhomogeneities during overall collapse to a filamentary distribution of gas and stars, as found in simulations by Burkert & Hartmann (2004).

Construction of the Hectospec: 300 optical fiber-fed spectrograph for the converted MMT

The Hectospec consists of a robotic positioner that will position 300 optical fibers at the f/5 focus of the converted MMT and a bench mounted moderate-dispersion spectrograph. Hectospec will be the first wide-field instrument to be used at the converted MMT and is now under construction at the Smithsonian Astrophysical Observatory. Commissioning at the converted MMT is scheduled for mid 1999, shortly after first light at the f/5 focus. The innovative features of the instrument are described, emphasizing recent developments.

Accretion, Kinematics, and Rotation in the Orion Nebula Cluster: Initial Results from Hectochelle*

We present results from high-resolution spectra of 237 stars in the Orion Nebula cluster (ONC) obtained during two engineering runs with the Hectochelle multifiber echelle spectrograph on the 6.5 m MMT. The ONC is the nearest populous young (age ~ 1 Myr) cluster and is therefore an important object for studies of the evolution of protoplanetary disks. Using the high spectral resolution of Hectochelle, we are able to distinguish stellar accretion and wind emission-line profiles from nebular emission lines and identify accreting members of the cluster from H? profiles with greater accuracy than previously possible. We find 15 new members on the basis of Li ?6707 absorption and H? emission. Line profiles of H? of some objects that are not too contaminated by nebular emission show features characteristic of mass inflow and ejection. We also present rotational velocities as part of an initial investigation into angular momentum evolution of very young stars, confirming a difference between classical T Tauri stars and weak-line T Tauri stars that had been found from period analysis. Finally, we present an initial study of the radial velocity dispersion of the brighter stars in the central cluster. The very small dispersion derived, ?1.8 km s-1, is in good agreement with estimates from proper motions.

The Redshift of the Optical Transient Associated with GRB 010222

The gamma-ray burst (GRB) 010222 is the brightest GRB detected to date by the BeppoSAX satellite. Prompt identification of the associated optical transient (OT) allowed for spectroscopy with the Tillinghast 1.5 m telescope at F. L. Whipple Observatory while the source was still relatively bright (R 18.6 mag), within 5 hr of the burst. The OT shows a blue continuum with many superposed absorption features corresponding to metal lines at z = 1.477, 1.157, and possibly also 0.928. The redshift of GRB 010222 is therefore unambiguously placed at z ? 1.477. The high number of Mg II absorbers and especially the large equivalent widths of the Mg II, Mg I, and Fe II absorption lines in the z = 1.477 system further argue either for a very small impact parameter or that the z = 1.477 system is the GRB host galaxy itself. The spectral index of the OT is relatively steep, F? ?-0.89?0.03, and this cannot be caused by dust with a standard Galactic extinction law in the z = 1.477 absorption system. This spectroscopic identification of the redshift of GRB 010222 shows that prompt and well-coordinated follow-up of bright GRBs can be successful even with telescopes of modest aperture.

Hectochelle: a multiobject echelle spectrograph for the converted MMT

The Hectochelle will be a fiber-fed, multi-object spectrograph for the post-conversion MMT which will take 255 simultaneous spectra at a resolution of 32,000 - 40,000. The absolute efficiency, including optical fiber losses, is predicted to be 6% - 10%, depending on the position of a line within a diffractive order. In one hour, features with 60 mangstrom should be resolved in mR equals 18 stars with a signal to noise of 10.

Targeting and sequencing algorithms for the Hectospec's optical fiber robotic positioner

The Hectospec is a moderate dispersion spectrograph fed by 300 optical fibers. Hectospecs pair of five-axis robots will position fibers at the 1 degree diameter f/5 focus of the converted MMT, allowing efficient multi-object spectroscopy. We discuss algorithms that we have developed to match the optical fibers to celestial objects and then to compute the appropriate sequence of robotic positioner moves to reconfigure the fibers between successive observations. Both algorithms require essentially no user interaction, consume only modest computer resources and allow effective deployment of the Hectospecs 300 fibers. The target-to-fiber matching algorithm is a recursive procedure which allows simultaneous optimization of the multiple observations that are required to complete a large survey. The robotic motion sequence algorithm allows the two Hectospec robots to work together efficiently to move fibers directly between observing configurations.

A prototype phasing camera for the Giant Magellan Telescope

Achieving the diffraction limit with the adaptive optics system of the 25m Giant Magellan Telescope will require that the 7 pairs of mirror segments be in phase. Phasing the GMT is made difficult because of the 30-40cm gaps between the primary mirror segments. These large gaps result in atmospheric induced phase errors making optical phasing difficult at visible wavelengths. The large gaps between the borosilicate mirror segments also make an edge sensing system prone to thermally induced instability. We describe an optical method that uses twelve 1.5-m square subapertures that span the segment boundaries. The light from each subaperture is mapped onto a MEMS mirror segment and then a lenslet array which are used to stabilize the atmospherically induced image motion. Centroids for stabilization are measured at 700nm. The piston error is measured from the fringes visible in each of the 12 stabilized images at 2.2 microns. By dispersing the fringes we can resolve 2π phase ambiguities. We are constructing a prototype camera to be deployed at the 6.5m Magellan Clay telescope.

The Giant Magellan Telescope active optics system

The Giant Magellan Telescope active optics system is required to maintain image quality across a 20 arcminute diameter field of view. To do so, it must control the positions of the primary mirror and secondary mirror segments, and the figures of the primary mirror segments. When operating with its adaptive secondary mirror, the figure of the secondary is also controlled. Wavefront and fast-guiding measurements are made using a set of four probes deployed around the field of view. Through a set of simulations we have determined a set of modes that will be used to control fielddependent aberrations without degeneracies.