We summarize the operational realities of re-aluminizing 8.4-meter primary mirrors in-situ on the Large Binocular Telescope. We review the evaporative coating system design, and summarize its performance in the 16 coatings since 2005. A mostly manual system with long-handled mops and traditional chemicals is used to remove the old coating and to clean the glass surface. After cleaning, the telescope is moved to horizon-pointing orientation and the aluminizing belljar is mounted to the primary mirror cell using the overhead crane internal to the enclosure. We report on the multi-year struggle to understand variations in deposition rate among the 28 crucibles that evaporate the aluminum. We describe the challenges of making operational improvements to a system that must reliably coat one of the two primary mirrors every year, and we report on some lessons learned along the way.
We present the design for the 340 Mpixel KMTNet CCD camera comprising four newly developed e2v CCD290-99
imaging sensors mounted to a common focal plane assembly. The high performance CCDs have 9k x 9k format, 10
micron pixels, and multiple outputs for rapid readout time. The camera Dewar is cooled using closed cycle coolers and
vacuum is maintained with a cryosorption pump. The CCD controller electronics, the electronics cooling system, and the
camera control software are also described.
The Multi-Object Double Spectrographs (MODS) are two identical high-throughput optical dichroic-split double-beam
low- to medium-dispersion CCD spectrometers being deployed at the Large Binocular Telescope (LBT). They operate in
the 3200-10500Å range at a nominal resolution of λ/δλ≈2000. MODS1 saw first-light at the LBT in September 2010,
finished primary commissioning in May 2011, and began regular partner science operations in September 2011. MODS2
is being readied for delivery and installation at the end of 2012. This paper describes the on-sky performance of MODS1
and presents highlights from the first year of science operations.
The Multi-Object Double Spectrographs (MODS) are two identical high-throughput optical low- to medium-resolution
CCD spectrometers being deployed at the Large Binocular Telescope (LBT). Operating in the 340-1000nm range, they
use a large dichroic to split light into separately-optimized red and blue channels that feature reflective collimators and
decentered Maksutov-Schmidt cameras with monolithic 8×3K CCD detectors. A parallel infrared laser closed-loop
image motion compensation system nulls spectrograph flexure giving it high calibration stability. The two MODS
instruments may be operated together with digital data combination as a single instrument giving the LBT an effective
aperture of 11.8-meter, or separately configured to flexibly use the twin 8.4-meter apertures. This paper describes the
properties and performance of the completed MODS1 instrument. MODS1 was delivered to LBT in May 2010 and is
being prepared for first-light in September 2010.
As is only fitting, the largest Optical/Infrared Telescope (the Large Binocular Telescope, LBT) has the two largest
telescope-mounted spectrographs (MODS) and the MODS's have the four largest scientific CCDs. We describe herein
the design, fabrication and early use of the e2v CCD231-68 8k × 3k 15 micron back-illuminated detector designed
specifically for low and intermediate resolution multi-object spectroscopy on large telescopes. The 123 mm length of
the CCD231-68 is the largest of any scientific CCD. The device can be read out in full frame mode to cover the whole
6 arc-min slit length of MODS, in full frame mode for multi-object spectroscopy with short slits, or in split frame
transfer mode to allow readout while integrating subsequent exposures. The four very low noise (<2 e- RMS at 100
kPixels/second) outputs are located at the ends of the four 4k serial registers. Excellent CTE (five-9s5 per Pixel)
insures good photometric accuracy across the device. Backthinned red and blue optimized variants are used on the
corresponding channels of both MODS.
The recently commissioned system for aluminizing the 8.408 meter diameter Large Binocular
Telescope mirrors has a variety of unusual features. Among them are aluminizing the mirror in the
telescope, the mirror is horizon pointing when aluminized, boron nitride crucibles are used for the
sources, only 28 sources are used, the sources are powered with 280 Volts at 20 kHz, high vacuum
is produced with a LN2 cooled charcoal cryo-panel, an inflatable edge seal is used to isolate the
rough vacuum behind the mirror from the high vacuum space, and a burst disk is mounted in the
center hole to protect the mirror from overpressure. We present a description of these features.
Results from aluminizing both primary mirrors are presented.
We discuss the performance of the Image Motion Compensation System (IMCS) for the Multi-Object Double Spectrograph (MODS). The system performs closed-loop image motion compensation, actively correcting for image motion in the spectrograph's focal plane caused by large scale structural bending due to gravity as well as other effects such as temperature fluctuation and mechanism flexure within the instrument. Not only does the system control instrumental flexure to within the specifications (0.1 pixels on the science CCD, or 1.5 μm), but it also has proven to be an excellent diagnostic tool for assembling and testing the spectrograph. We describe both the final performance of the system as deployed in the spectrograph as well as the instrumental tests made possible by the IMCS.
Ohio State is building two identical Multi-Object Double Spectrographs (MODS), one for each of the f/15 Gregorian foci of the Large Binocular Telescope (LBT). Each MODS is a high-throughput optical low- to medium-resolution CCD spectrometer operating in the 320-1000nm range with a 6.5-arcminute field-of-view. A dichroic distributes the science beam into separately-optimized red and blue channels that provide for direct imaging and up to 3 spectroscopic modes per channel. The identical MODS instruments may be operated together with digital data combination as a single instrument giving the LBT an effective aperture of 11.8-meter, or separately configured to flexibly use the twin 8.4-meter apertures. This paper describes progress on the integration and testing of MODS1, and plans for the deployment of MODS2 by the end of 2008 at the LBT.
We are building a Multi-Object Double Spectrograph for the Large Binocular Telescope. The instrument is designed to have high throughput from 320 to 1000 nm, spectral resolutions of 1,000-10,000, and multi-object capability over a 6 arcminute field. The design incorporates a dichroic and splits the science beam into a blue and a red channel, each of which can illuminate an 8,192 pixel long detector (with 15 micron pixels) with good image quality. The highly modular design can hold up to three gratings and an imaging flat and a selection of filters in each channel, all of which are quickly accessible; this allows for substantial observing flexibility. Progress on the construction of the instrument and future plans will be described.
We describe progress on a closed-loop image motion compensation system (IMCS) for the Multi-Object Double Spectrograph (MODS). The IMCS actively compensates for image motion in the focal plane within the instrument caused by temperature fluctuation, mechanism flexure, and large scale structural bending due to gravity. The system utilizes an infrared laser as a reference beam that shares a light path with the science beam and is detected by an infrared reference detector adjacent to the science detector. The reference detector is read out frequently and detects any image motion in the focal plane. The IMCS compensates for this motion during a science exposure by adjusting the tip and tilt angles of the collimator mirror. A working lab prototype meets specifications and is described.
We describe a closed-loop image motion compensation system (IMCS) for the Multi-Object Double Spectrograph (MODS). The IMCS compensates for structural bending due to gravity and eliminates image motion from temperature fluctuation and mechanism flexure within the instrument during an observing period. The system makes use of an infrared laser source at the telescope focal plane, which produces reference spots in the science detector plane. Movement of these spots accurately tracks science image motion, since the two beams share a common optical path. Small real-time adjustments to the position of the MODS collimator mirror compensate for the image motions.
We describe an instrument that is capable of taking simultaneous images at one optical (UBVRI) and one near-infrared (JHK) wavelength. The instrument uses relatively simple optics and a dichroic to image the same field on to an optical CCD and an HgCdTe array. The mechanical and thermal design is similar to previous instruments built by our group and the array controllers are based on the same architecture. The instrument has been in use for the past four years on the CTIO/Yale 1m telescope in Chile and has an excellent operational/reliability record. A number of notable science results have been obtained with the instrument; especially interesting are several photometric monitoring projects that have been possible, since the instrument is available every night on the telescope.
Lens mounts for cryogenic service have many requirements: mitigation of thermal shock on the lens, maintenance of lens centering and spacing, control of mechanical stress on the lens from the cell, reliable connection of the lens to the cell, and applicability to a wide variety of lens materials. This paper describes in detail a lens mounting system successfully used in several cryogenic instruments.
The MODS optical spectrograph uses a de-centered Maksutov-Schmidt camera with a clear aperture of ~300mm. This large camera has two widely spaced elements, the corrector and the camera mirror, and a field flattener near the focal plane. This paper describes the truss system that supports the optical elements very rigidly, uses adjustable length links to provide a deterministic method for alignment of the optical elements, and uses material combinations which result in a camera with nearly zero focus shift due to changes in temperature. A novel joint design for terminating the truss links is described that has excellent stiffness and enhances ease of assembly and alignment.
We are building a Multi-Object Double Spectrograph for the Large Binocular Telescope. The main themes of our planned research with the instrument are the formation and evolution of galaxies and their nuclei and the evolution of large- scale structure in the universe, although we expect that the spectrograph will be used for many other varieties of programs as well. The science goals for the instrument dictate that it have the highest possible throughput form 320 to 1000 nm, spectral resolutions of 103 to 104, and multi-object capability over an approximately 6 foot field. Our design is highly modular, so future upgrades should be straightforward.
The optimal operating temperature range for Indium Antimonide detectors is typically near 35 Kelvin. Commercially available miniature split-Stirling cycle cryocoolers present an attractive approach to detector cooling. These units offer stand-alone operation, small size, light weight, low power input, low vibration, moderate cost, and reasonable lifetime. However, currently available units have inadequate cooling capacity at 35 Kelvin when operated in a normal manner. We have substantially increased the low temperature cooling capacity of commercial cryocoolers by utilizing the 77 Kelvin intermediate temperature available in liquid nitrogen cooled instruments. We thermally connect the liquid nitrogen cold sink to the middle of the cryocooler coldfinger, shunting heat from the coldfinger to the LN2. The resulting performance improvements and careful thermal design of the detector mount to minimize parasitic heat loads a low miniature split-Stirling cycle cryocoolers to provide adequate cooling of large format Indium Antimonide focal plane arrays.
The MDM/Ohio State/ALADDIN IR Camera (MOSAIC) is a general purpose near IR imaging camera and medium-resolution long- slit spectrometer in use on the MDM 1.3-m and 2.4-m telescopes and the Kitt Peak 2.1-m and 4-m telescopes. In cooperation with NOAO and USNO, MOSAIC is one of the first general-purpose near-IR instruments available to the astronomical community that uses a first-generation 1024 X 512 ALADDIN InSb array, with the capability to use a full 1024 X 1024 array once one becomes available. MOSAIC provides tow imaging plate scales, and a variety of long- slit grism spectroscopic modes. This paper describes the general instrument design and capabilities, and presents representative scientific results.
The ISL is a successful astronomical instrumentation program that has completed three major instruments and many smaller projects since 1987. We have developed the capabilities to perform all aspects of instrument design and construction and a range of unique skills and methods. We maintain a permanent staff that currently consists of two scientists specializing in optical design and detector systems, a seniors mechanical engineer, a programmer, an electronic engineer, a mechanical designer, two machinists, and a lab assistant. Instrumentation projects also draw upon faculty and graduate student effort.
Proc. SPIE. 3354, Infrared Astronomical Instrumentation
KEYWORDS: Signal to noise ratio, Digital signal processing, Capacitors, Sensors, Interference (communication), Amplifiers, Charge-coupled devices, Analog electronics, CCD image sensors, Signal detection
Despite the advances in digital signal processing, the analog circuits which amplify and define the bandwidth of the low level signals from CCD and IR array detectors are a critical element in obtaining the best possible signal-to- noise ratio. The choice of components and topology for these front-end circuits is discussed, including the effects of input voltage and current noise density, coupling strategies, shield driving, physical layout, and grounding. The signal chain use in Imaging Sciences Laboratory instruments is presented as an interpretation of these considerations.
The Ohio State Instrument Control and IMage ACquisition System, ICIMACS, is the computer hardware and software used by all instruments developed by the Imaging Science Laboratory (ISL) to control the detector, pre-process data, record image data on a separate computer system for data reduction and analysis, generate real time data display, control the mechanisms within an instrument, interface with the telescope controller, connect to a user interface, and perform engineering functions such as temperature or pressure logging. ICIMACS has now been used on 12 different instruments and is herein described as applied to 'MOSAIC' the near IR imager/spectrometer in use on the Kitt Peak 2.1 and 4 meter telescopes and on the MDM 2.4 and 1.3 meter telescopes.
We present a design for a near-infrared (0.9 to 5.5 micrometers ) spectrograph for use on any large telescope. For example, the instrument meets all of the scientific and technical objectives requested by the Gemini Telescope Project for their facility infrared spectrograph. The features of the instrument include a wide range of rapidly selectable spectral and spatial resolutions, full-broad-band imaging, integral field spectroscopy, and several cross-dispersed modes. Much of the instrument is based on optical, mechanical, and electronic designs currently in use. The optical design has diffraction-limited performance and no vignetting over a 150" X 150" field of view. The mechanical design draws heavily on our extensive experience with cryogenic mechanisms and uses a cassette system for selection of the large number of possible configurations. The design is very modular and allows a staged implementation of the complete set of potential operational modes.
The Ohio State Infrared Imager/Spectrometer (OSIRIS) is a general purpose near infrared (0.9 to 2.5 micrometers ) instrument that can be used at a wide variety of telescope focal planes. OSIRIS currently uses a 256 X 256 HgCdTe array detector and will accommodate larger arrays when available. OSIRIS has two modes of operation: imaging and spectroscopic. This paper describes the general instrument design and sample scientific results.
The design of aluminization systems for the MMT Conversion 6.5 m mirror and the Columbus
Project 8 m mirror has led us to reconsider many of the design issues and tradeoffs for such systems.
Coating of the large honeycomb mirrors will be done in situ on the telescope with a portable bell jar
forming the front half of a two-stage vacuum system. The mirror cell forms a "dirty" vacuum behind
the mirror to eliminate excess force on the glass. A multi-ring source geometry has been proposed to
allow a 1.0 m spacing between the mirror surface and the sources thereby minimizing the size of the
vacuum chamber. Evaporation source models have been developed to optimize the number of
sources, ring spacing, and high incidence angle emission to achieve better than 5% rms deviation in
coating thickness over the diameter. Code results are compared to empirical thickness profiles
measured at the University of Arizona's (UA) Sunnyside 2.0 m coating facility. Cryoadsorption
pumps are considered for reasons of economy, quality of vacuum, pumping speed, and reliability.
The interaction of the cryopumps and getter pumping with the pumping/cleaning/deposition cycle is
studied. Glow discharge cleaning is discussed and the results of deposition tests in 10' Torr residual
argon are given. Electrical requirements are estimated and a novel transformer design may decrease
the current entering the chamber from 12,000 A to less than 600 A.