Robert G. Fata

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...

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.

Mounting large lenses in wide-field instruments for the converted MMT

We describe the techniques that we have used to mount large optics in three wide-field instruments for the converted MMT: the wide-field corrector uses to provide a 1 degree diameter field at the f/5 focus of the converted MMT, the Hectospec bench spectrograph fed by 300 optical fibers and the wide- field dual-beam Binospec spectrograph. These optics are primarily refractive elements with diameters between 0.2 and 0.8 m that must be mounted from their edges, although we also describe mounts for two large mirrors in the Hectospec bench spectrograph. Both the wide-field corrector and Binospec mounts must perform under varying gravity loads: the corrector is fixed to the converted MMTs primary mirror cell and is tilted from zenith to horizon while Binospec is mounted at the converted MMTs Cassegrain focus. Furthermore, the optics mounts for both instruments must fit within tight space constraints. The Hectospec spectrograph is mounted in the MMTs rotating building and experiences a constant gravity vector. In all cases, the mounts must perform over a wide temperature range, -20 to 20 degrees Celsius, so the issue of differential thermal expansion between the mounts and optics must be carefully considered. As a result, the mounts we discuss include either RTV elastomeric or flexural elements.

Design and support of the 1.7-m f/5 secondary mirror for the MMT conversion

The f/5 focus of the converted Multiple Mirror Telescope (MMT) has been designed for optical fiber spectroscopy over a 1 degree(s) diameter field as well as superlative imaging over a 0.5 degree(s) diameter field. The secondary mirror required for this purpose is nearly as large as one of the existing MMT primary mirrors (1.7 m). To attain the specified performance of the telescope support structure, this secondary must be made quite light (approximately 300 kg), without excessively compromising its stiffness. Our design calls for removing excess weight by machining hexagonal cells into a near zero-expansion materials such as ULE or Zerodur. The completed blank will be approximately 20 cm thick at the center, tapering to approximately 13 cm at the edge. The support of a secondary of this size must be considered as part of the blank design. We describe two possible axial support techniques: a vacuum support or a multiple actuator support similar to that adopted for the primary mirror of the converted MMT. Tangent flexures at the edge of the blank appear to be the most attractive radial support option.

The F/5 instrumentation suite for the Clay Telescope

The f/5 instrumentation suite for the Clay telescope was developed to provide the Magellan Consortium observer community with wide field optical imaging and multislit NIR spectroscopy capability. The instrument suite consists of several major subsystems including two focal plane instruments. These instruments are Megacam and MMIRS. Megacam is a panoramic, square format CCD mosaic imager, 0.4° on a side. It is instrumented with a full set of Sloan filters. MMIRS is a multislit NIR spectrograph that operates in Y through K band and has long slit and imaging capability as well. These two instruments can operate both at Magellan and the MMT. Megacam requires a wide field refractive corrector and a Topbox to support shutter and filter selection functions, as well as to perform wavefront sensing for primary mirror figure correction. Both the corrector and Topbox designs were modeled on previous designs for MMT, however features of the Magellan telescope required considerable revision of these designs. In this paper we discuss the optomechanical, electrical, software and structural design of these subsystems, as well as operational considerations that attended delivery of the instrument suite to first light.

Flexure mounts for high performance astronomical lenses

We have developed practical, high performance flexure mounts for large astronomical lenses in the Binospec spectrograph. Flexure mounts are an attractive alternative to the widely used elastomeric lens mounts when high axial stiffness is a priority and coupling fluids are incompatible with elastomers. We describe coupling fluid seals for the flexure mounts.

Design of a cell for the wide-field corrector for the converted MMT

We describe an approach for mounting the approximately 0.8 m diameter optical elements in a refractive corrector for the Multiple Mirror Telescope (MMT). The optical elements are mounted on discrete pads of RTV rubber to an Invar/carbon steel cell. Following the conversion of the MMT to use a single 6.5 m primary mirror, the corrector will provide up to a 1 degree(s) diameter field-of-view for multi-object spectroscopy with optical fibers or for wide- field imaging.

Mounting large lenses for the MMT's f/5 wide-field corrector: lessons learned

At its f/5 focus, the 6.5 meter converted MMT uses a refractive corrector to produce excellent images over a field of view as large as 1° in diameter. We describe the construction challenges we encountered and the lessons we learned mounting the ~30 inch diameter lenses for this wide-field corrector. The corrector was completed in May 2002, was commissioned in May and June of 2003, and is now in regular use at the MMT for spectroscopic and imaging observations.