Tour of MAESTRO

Last update:  June 10, 2013

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 glued 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.

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.

schematic.jpg


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 the University of Arizona Imaging Technology Lab (ITL). It is type JPL4096, UA ITL serial number SN3464. The camera uses an ARC Gen-3 controller, run by an on-board PC and the AZCAM data acquisition software.  The user controls the CCD via an IRAF/ICE interface running on a linux box in the control room.  The CCD 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 RMS. 

GRATING AND SPECTRAL FORMAT

The grating was custom ruled by Richardson Grating Labs, in order to optimize the spectral format on the detector. It is a single plano echelle grating of zerodur with aluminum overcoat, 204mmx408mm with 60 grooves/mm and a blaze angle of 64 degrees.  The manufacturer measured the quantum efficiency which, for a typical order, is shown below.

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. The individual components were glued together with JB weld with invar reinforcements at most joints. The panel edges were sealed with carbon fiber tape which was glued on with JB weld. Bolt holes were made by gluing pre-made plastic inserts.

TARGET ACQUISITION AND GUIDING

A Steward CCD Guider camera is used as a slit viewer, and enables the target to be centered on the slit. It has a 512x1024 pixel CCD used in frame transfer mode. More information is given at http://www.itl.arizona.edu/ITLGuiders/index.html.  Guiding is done off-axis with the f/5 guider.   Eventually, wave front sensing will also be done without interrupting the observing sequence, off-axis with the f/5 WFS camera.

The rest of the tour may be found HERE.