MAESTRO: The MMT Advanced Echelle Spectrograph

INTRODUCTION

MAESTRO is a cross-dispersed echelle spectrograph for the MMT, funded by the National Science Foundation and Steward Observatory, with support from the MMTO. It is optimized for multi-order, simultaneous wavelength coverage, with R(theta) = 28,000, or 7.5 km/s when used with a 0.7 arcsec slit. The best possible spectral resolution, using a 0.3 arcsec slit, is Nyquist sampled with spectral resolution of R=93,000 or 3.2 km/s. The single exposure wavelength coverage is 3150 to 9850 Angstroms with small gaps redward of about 8000 A. Thus MAESTRO is complementary to the existing multi-fiber Hectochelle spectrograph at the MMT. It is optimized for high throughput in the UV, so is the instrument of choice for faint, rare objects such as quasars and metal-poor stars, where wavelength coverage is important.

OPTICAL DESIGN

MAESTRO is located at the f/5 Cassegrain focus, attached to the instrument rotator. It uses the f/5 secondary, spectroscopic corrector optics, and atmospheric dispersion prism built by Dan Fabricant's group at SAO. The light from the telescope enters the spectrograph through a small doublet (we call the injection optics) which transforms the f/5 beam to f/3. It then passes through the slit, and is turned 90 degrees by a small folding flat mirror. The collimator consists of a set of 6 lenses, and is used in double pass. The overall design is patterned after the MIKE echelle at the Magellan telescope.

The light passes through 3 singlets (BSM51Y, fused silica, PBM18Y) and then a glued triplet of BSM51Y -CaFl-BSM51Y. BSM51Y and PBM18Y are i-Line glasses made by Ohara, which have high ultraviolet transmission. The light then passes through the fused silica prism, the echelle grating, then back through the triplet and 3 singlets, and through a field flattener which is the dewar window, made of fused silica. The CCD is tilted slightly. The first of the 6 lenses (L1) is movable, for focus control. The collimated beam passes through a prism cross-disperser, and then is reflected off a custom ruled echelle grating.

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The light then passes again through the cross-disperser prism, the lens can, and onto the CCD detector. The dewar window acts as a field flattener. The CCD is tilted slightly to improve optical performance. A schematic of the optical lay-out is shown below.

MAESTRO's beam diameter is 200 mm, set by the maxiumum CaFl blank that was available. The cross-disperser is a 40 degree prism made of fused silica.

The camera-collimator lenses, dewar window and cross-disperser prism were made by Bradley J. Piccirillo, of Optical Solutions, Incorporated, in Charlestown, New Hampshire. The small injection optics were manufactured and mounted by Joe Apples at TORC, in Tucson, Arizona.

The camera-collimator contains a triplet with a large CaFl element in the center. Cargille 1160 oil is used to optically couple the three elements of the triplet. Oil was chosen over an optical glue such as Sylgard in order to allow flexibility later on, should the triplet need to be dismantled for some reason. Cargille 1160 was found to be UV transmissive, and unreactive to Sylgard and the rubber O-ring which seals the oil layers in the lens cells. At the top of the cells, a small pliable tube allows overflow relief. Tests on a small triplet showed a voltaic reaction when plumbing hardware made of brass was used, which discolored the oil severely after a few months. We therefore had to fabricate the pressure relief hardware out of aluminum and use teflon tape to isolate any non-aluminum hardware from contact with the oil.

By choosing a prism over a grating cross-disperser, we increase throughput and single exposure wavelength coverage. The cost is that order separation is small in the red. We however optimized the optical design of MAESTRO for the blue, because of the excellent blue sensitivity of the CCDs produced by ITL, and the compelling science enabled by spectroscopy in the near-UV.

CCD DETECTOR

The detector is a 4kx4k back-illuminated CCD which was thinned and AR coated to optimize UV sensitivity, by Mike Lesser's group at ITL. It is type JPL4096, UA ITL serial number SN3464. It has 15 micron pixels; one arcsecond at the slit maps to 6.5 pixels on the detector. The readnoise in the lab is 3.4 electrons

GRATING AND SPECTRAL FORMAT

The grating was custom ruled by Richardson Labs, in order to optimize the spectral format on the detector. The

OPTO-MECHANICAL DESIGN

Since the MMT does not have a Nasmyth platform, we mount MAESTRO at the Cassegrain focus. As a result, we had to worry about minimizing flecture from the changing gravity vector, while keeping the instrument within the allowed weight limit (3000 pounds) for the instrument roatator. The optics and detector are mounted in a light space frame constructed out of carbon fiber tubes and honeycomb sandwich panels. To keep costs low, we obtained the carbon fiber tubing used for boat oars, which we stress tested to insure that they were strong enough for our purposes, despite having a lower fiber density than tubing used in industrial applications.

The honeycomb sandwich panels were donated by the Hexcel Composites of Casa Grande, Arizona. They were cut using..... The individual components were glued together with JB weld with invar reinforcements at most joints. The panel edges were sealed with xxx glued on with JB weld. Bolt holes were made by gluing pre-made plastic inserts.

Precision lens cells were machined by

TARGET ACQUISITION AND GUIDING

A Steward CCD Guider camera is used as a slit viewer, and enables guiding on spilt light from the target on the slit. It has a 512x1024 pixel CCD used in frame transfer mode. More information is given at http://chinadoll.as.arizona.edu/~schmidt/guider/ and http://www.itl.arizona.edu/ITLGuiders/index.html.

-- Main.jill - 29 May 2007