The FIRE infrared spectrometer at Magellan: construction
and commissioning
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Simcoe, Robert A. et al. “The FIRE Infrared Spectrometer at
Magellan: Construction and Commissioning.” Proceedings of
SPIE--the International Society for Optical Engineering; v.7735
(2010): 773514–773514–12. © SPIE 2010
As Published
http://dx.doi.org/10.1117/12.856088
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SPIE
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Final published version
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Thu May 26 05:02:42 EDT 2016
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The FIRE Infrared Spectrometer at Magellan: Construction
and Commissioning
Robert A. Simcoea , Adam J. Burgasserb , John J. Bochanskia , Paul L. Schechtera , Rebecca A.
Bernsteinc , Bruce C. Bigelowd , Judith L. Piphere , William Forreste , Craig McMurtrye ,
Matthew J. Smithf , Jason Fishnera
a
e
Massachusetts Institute of Technology, Cambridge, MA, USA 02139;
b University of California, San Diego
c UCO/Lick Observatories, Santa Cruz, CA, USA 95064
d UC Observatories, CFAO 109, UC Santa Cruz, Santa Cruz, CA, USA 95064
Dept of Physics and Astronomy, University of Rochester, Rochester, NY USA 14627
f Smithsonian Astrophysical Observatory, Cambridge, MA USA 02138
ABSTRACT
We describe the construction and commissioning of FIRE, a new 0.8-2.5µm echelle spectrometer for the Magellan/Baade 6.5 meter telescope. FIRE delivers continuous spectra over its full bandpass with nominal spectral
resolution R = 6000. Additionally it offers a longslit mode dispersed by the prisms alone, covering the full z to
K bands at R ∼ 350. FIRE was installed at Magellan in March 2010 and is now performing shared-risk science
observations. It is delivering sharp image quality and its throughput is sufficient to allow early observations of
high redshift quasars and faint brown dwarfs. This paper outlines several of the new or unique design choices
we employed in FIRE’s construction, as well as early returns from its on-sky performance.
1. INTRODUCTION
FIRE (the Folded-port InfraRed Echellette) is a new medium-resolution near infrared spectrometer, designed
specifically for studies of quasar absorption lines at high redshift and radial velocity studies of low luminosity
brown dwarfs. It delivers R = 6000 spectra over a range of 21 orders, covering the full z to K bands in a single
exposure. It is therefore suitable for a wide variety of general purpose observations, and will be available as a
facility instrument for the Magellan community starting in 2010B.
FIRE mounts on the center f/11 auxiliary Nasmyth port of the Magellan I (Baade) telescope, and achieved
first light in March 2010. Operating at Δv = 50 km/s for a slit matched to the median Magellan seeing (0.6 ), it
resolves out much of the terrestrial OH background, leaving ∼ 80% of each spectrum free of OH contamination
and permitting sensitive observations in the dark inter-line continuum regions.
In the best seeing conditions, a narrow slit may be used to achieve spectral resolution as high as R = 10, 000.
Magellan’s other infrared spectrograph (MMIRS1 ) is a multi-object instrument that is available during campaigns
with the wide field F/5 secondary. The user community indicated additional demand for low-resolution, high
throughput single order IR spectroscopy during periods when MMIRS is not on the telescope. To meet this need,
FIRE may be reconfigured by replacing its echelle grating with a flat mirror, yielding longslit R ∼ 300 spectra
dispersed with the prisms alone.
Details regarding the FIRE’s design philosophy and preliminary layout are described in a previous SPIE
proceedings document.2 The present contribution presents a full discussion of the instrument’s construction, user
interface, and on-sky performance. Section 2 describes the instrument subsystems; Section 3 describes software
design and data reduction methods, and 4 describes preliminary measurements of instrument performance made
during commissioning observations.
Ground-based and Airborne Instrumentation for Astronomy III, edited by Ian S. McLean,
Suzanne K. Ramsay, Hideki Takami, Proc. of SPIE Vol. 7735, 773514 · © 2010
SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.856088
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Figure 1. Left: The FIRE instrument dewar located on its handling cart. The optical bench is vertical, shown facing the
camera, with subassemblies and light baffles attached. Vacuum hardware is at top right and the CTI1050 single stage
cooler is seen at top. Right: FIRE mounted on the Baade Telescope’s center folded port. A large serpentine cable wrap
mounts underneath the instrument, and the electronics rack hangs on board.
2. INSTRUMENT CONSTRUCTION
FIRE’s optical design employs a 50mm collimated beam, which was deliberately kept small in order to constrain
the total size of the cryostat. This decision was made in order to reduce complexity, cost, and build time, but
also because of stringent weight and moment restrictions for instruments at the Magellan folded ports. Another
implication of folded port operation is limited access for servicing, which drove our cooling design towards a
closed-cycle refrigerator. This required significant infrastructure development on site during installation, but
eliminates the long-term need for regular cryogen fills. By accepting the additional engineering investment for
folded port instrument development, we gain a dedicated port on the telescope. FIRE runs continuously without
instrument changes, during both dark and bright observing time. This is extremely beneficial for scheduling,
particularly for target-of opportunity science programs (e.g. gamma-ray burst follow up).
2.1 Cryostat and Optical Bench
Figure 1 shows the assembled FIRE instrument dewar. The cryostat is approximately 35 inches in diameter,
and 20 inches deep, constructed of welded aluminum by Atlas UHV (Port Townshend, WA). The optical package
resides on a 6061 aluminum optical bench, roughly 30 inches round by 2 inches thick. The bench is substantially
light weighted, and thermally stabilized by liquid nitrogen immersion and uphill quench in a high-temperature
steam bath before final machining.
The bench, whose cold mass approaches 150 pounds when fully loaded, is suspended mechanically in the
dewar using four cryogenic grade G10 flexures. The flexures mount to a warm support ring, which forms a
vacuum flange and does not deflect upon evacuation of the dewar. The ring indexes directly to the telescope
structure and may be articulated to align FIRE’s cold stop with the telescope pupil.
During normal operation, the cryocooler’s cold head (a CTI1050 single stage unit) runs at a temperature of
65K, lifting approximately 65W to maintain the bench an equilibrium of 90-100K. We included a single-layer
passive thermal blanket, constructed of aluminized Kapton dressed on a thermally isolated space frame, to reduce
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Figure 2. The pre-slit optical assembly, containing the Offner relay and slit viewing camera. The Offner’s cold stop may
be seen at top center; is primary mirror faces away from the camera at bottom. The slit viewing optics are located in
the barrel at right. All are contained in a light-tight enclosure whose small dimensions are driven by the small clearance
between the telescope’s mounting flange and f/11 focal plane.
radiative loads. The cryocooler is vibrationally isolated from the rest of the dewar via sorbothane gaskets and
vacuum bellows seals. Its cold surface couples to the optical bench via two 1 cm thick by 2.5 cm wide stacks of
OFHC copper. A large activated charcoal getter attached to the cold head cryopumps on the dewar, in concert
with a Varian StarCell 40 ion pump. We routinely achieve vacuum levels of 10−8 − 10−9 Torr. The long term
“hold time” (essentially the period of vacuum integrity) is not yet known, through our experience thus far shows
it is at least several months. We attribute part of the cleanliness to the elimination of any thermal or vacuum
greases on the dewar. Careful attention to fabrication and protection of flanges and O-rings permits high-vacuum
operation without the need for greased seals.
2.2 Optical Fabrication and Assembly
We employed a subsystems approach to the realization of FIRE’s optical train, which was designed and optimized
by R. Bernstein. The only powered optical elements in the instrument are a pre-slit Offner relay, the collimator,
and the camera. Internal alignment of these systems is challenging, but the relative alignment of the subsystems is
fairly forgiving. The sensitive internal alignment of the camera was performed and verified by Jenoptik (formerly
Coastal Optical, Jupter, FL) as part of the fabrication contract. We aligned the internal Offner relay elements
in our lab as described in Section 2.2.1. The relative alignments of all subassemblies were then verified using an
alignment telescope fixture, indexed to precision-drilled holes in the optical bench.
2.2.1 Pre-Slit Optics
FIRE employs a cold, 1:1 Offner relay in front of the slit to provide an achromatic Lyot stop. This also ensures
that diffraction effects from the spectrograph slit, which scatter light into a beam faster than the f/11 beam from
the telescope, do not introduce unbaffled thermal backgrounds.
The Offner relay is an athermal design, achieved by constructing both the mirrors and housing from 6061T6 alloy. The mirrors are diamond-turned spheres, figured on specialized RSA-6061 aluminum substrates.
The RSA alloy is specifically designed to yield improved surface roughness on diamond turning,3 which we
deemed important given FIRE’s coverage in the “far-optical” z band. The delivered optics achieved surface
RMS roughness of Rq = 32, 18 Å for the primary and pupil mirrors, respectively. This yields an integrated
scatter of 0.16% and 0.08% for the two surfaces. This performance is superior to standard aluminum; it is
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Figure 3. Left: FIRE’s prism enclosure during optical installation. One infrasil prism (at left) and one ZnSe prism are
visible. Right: The spectrograph camera assembly during rough alignment on the optical bench. The A-frame and flexure
stage shown at right form the mount for the HAWAII-2RG detector.
somewhat more rough than electroless nickel coated substrates but does not suffer from any bimetallic bending
stress on cool down.
We aligned the Offner internally using a laser bore sight and knife edge. Measurements of the MTF made
using a USAF test target (at room temperature) showed contrast on spatial scales corresponding to 0.02 . Cold
performance was verified with stellar images through the telescope, which registered FWHM values better than
0.30 in good seeing. The Offner’s usable field of view is limited to ∼ 1 arcminute because of off-axis aberrations.
2.2.2 Slit Viewing Camera
Since many of FIRE’s most interesting science targets are not visible at optical wavelengths, an infrared slit
viewing capability was deemed a high priority for target acquisition. To this end we included a small, custom-built
2.75:1 reimager to project the 1 × 1 arcminute slit plane onto a subraster of an engineering grade HAWAII-2RG
sensor.
The slit viewer is a fixed filter design using an MKO-J bandpass. Designs incorporating filter wheels were
ruled out because of severe space restrictions in our dewar, owing to a small clearance between the folded port
mounting flange and the telescope’s focal plane. The optical design is all-spherical and refracting, with two
doublets of CaF2 and S-FTM-16, and an Infrasil 301 field flattener. Its lenses are radially loaded into precisionmachined cells with integrated wire-EDM flexures. This design maintains an athermal centration of the optics.
The axial lens stack up is maintained with spacers of varying materials to compensate for thermal contraction,
and loaded with leaf springs.
In practice, the slit viewer is used in “manual” mode where images are obtained individually for locating
targets and placing objects on the slit. In principle it may also be used for guiding. However, the observatory
provides separate optical pickoff mirrors in front of the instrument package for guiding and wavefront sensing.
We use the pickoff systems for guiding, and only servo on the slit view occasionally, to check for flexure between
the observatory guider and the cold mass during long integrations.
2.2.3 Spectrograph Optics
After passing through the slit, the f/11 beam is collimated by an off-axis paraboloid. The OAP is figured onto a
Zerodur substrate and coated with protected gold. It delivers a 50 mm collimated beam to the prism assembly,
which is shown in Figure 3.
Optical design of the prisms was a particular challenge because the linear dimension of the H2RG sensor
is large, the camera is relatively fast, and the chromatic dispersion of glasses in the infrared is low. Moreover
the most dispersive near-IR glass—ZnSe—is only available in limited thicknesses. Many prisms are therefore
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required to achieve the angular deviation needed to span the full detector. Our solution incorporates a train of
four prisms, two used in double-pass, for a total of six passes. Half are through ZnSe and half through Infrasil
301, which was chosen to balance the partial dispersion across the detector and minimize wavelength variation
in anamorphic compression of the beam.
FIRE’s camera consists of three 92mm diameter lenses (CaF2 , S-FTM-16, CaF2 ), and a small Infrasil 301
field flattener. The camera lenses were fabricated, assembled, and aligned into a barrel by Jenoptik (Jupiter, FL).
Two of the CaF2 surfaces are even aspheres. These were manufactured by rough turning the aspheric shapes on
a diamond lathe, and post-polishing with a magneto-rheostatic (MRF) finishing machine. All other surfaces are
conventionally polished spheres.
The lenses are individually mounted in cells using cryogenically qualified elastomeric materials. Like other
opto-mechanical elements, the lens barrel and cells were constructed of 6061-T6 aluminum, to provide a good
thermal match to the optical bench.
Great care was taken to construct both warm and cold models of the camera. The cold model was used
in design optimization, while the warm model was used for fabrication and testing. Rather than employing
the standard Zemax-supplied tools for thermal analysis, we explicitly modified the design configurations using
custom glass catalogs constructed from the CHARMS database45 and integrated thermal expansion properties
of each material at cryogenic temperatures taken from the NIST Cryogenics Database ∗ or the general literature.
For testing purposes, we generated theoretical models of the warm MTF, and a theoretical OPD diagram for
interferometric analysis of the optical assembly. Warm interferometric tests at 1064 nm were compared with the
theoretical models as a basis for acceptance of the camera package.
2.2.4 Detectors and SIDECAR Readout
FIRE contains two 2.5 micron cutoff HAWAII-2RG arrays: one science-grade device for the spectrograph, and one
engineering-grade device for the slit viewing camera. Both are read out using the Teledyne SIDECAR ASIC and
associated cryogenic development kit. We performed a full set of test procedures for each array in the University
of Rochester’s detector lab. Each array was tested first using a set of standard array control electronics with
known low noise characteristics, to provide a baseline. Then, the same sensors were coupled to the SIDECAR to
assess the difference in performance and explore techniques to bring the two measurements into agreement (i.e.
achieving detector-limited performance).
The detectors themselves are of excellent quality. After correcting for inter-pixel capacitance effects, we
measure a quantum efficiency of 88-95% across the J/H/K bands. Their dark current signal is negligible, on
order of a few counts per pixel per hour. These characteristics all exceed the original goals laid out for the
project, with a direct positive impact on FIRE’s sensitivity.
Read noise is the limiting factor in FIRE’s detector performance (excluding the sky). To match the SIDECAR
noise performance with that of the control electronics, we experimented extensively with schemes to isolate power
inputs to the SIDECAR system and JADE2 USB controller. Our final solution incorporates a commercial USBto-fiberoptic converter, powered from the same supply as the JADE2 analog power input. The input power was
heavily filtered to reduce common mode noise before being fed to the signal chain. Then, the signal path itself
was completely isolated from the dewar ground using nylon or G10 standoffs, all the way to the HAWAII-2RG
sensor. With these adjustments, the SIDECAR performed as well as in the Rochester test dewar, and in its
highest preamp gain setting, even slightly better. In the lab, we achieved optimal noise performance of 16-18 e−
RMS per CDS read; on the telescope the noise levels are 10-20% higher. We are continuing to work on grounding
schemes to bring the noise back to the baseline lab level.
3. SOFTWARE IMPLEMENTATION
The FIRE software suite contains three principal modules. The first module is dedicated to instrument control,
the second controls the guider and communicates with the TCS, and the third encompasses data reduction tools.
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Figure 4. Screen shot of the FIRE control software suite. A simple set of control commands are issues through the pull
down menus and entry fields at upper left. Other windows are used for pressure and temperature telemetry, as well as
data viewing.
3.1 Instrument Control
FIRE’s instrument control software is a multi-threaded application written in C. It is intended for use in a
Macintosh OSX environment, and uses the Cocoa graphics package to realize windows and controls.
The vast majority of the code base was written by Christoph Birk of Carnegie Observatories, in support of
the FourStar6 project. He kindly provided versions of his code in alpha and beta forms, and helped guide us in
implementing tasks specific to FIRE. Given FIRE’s limited options for user control, the interface is quite simple.
Observers use drop down menus to select between echelle and prism modes, select slits and detector gain, and
run calibration sequences.
Telemetry with remote devices (e.g. temperature controllers, ion pumps, motor controllers) is achieved using
simple serial commands. Communication with the detector hosts is more complex. For low level control of
the detector, we use the HxRG IDL package supplied by Teledyne with the SIDECAR development kit. This
software runs only in a Windows environment, in conflict with observatory standards for mac-based computing.
As a workaround, Teledyne incorporated a simple socket server capability into the HxRG code. Our control
software opens a client-side thread to the HxRG and sets detector parameters and initiates exposures through
this link.
3.2 Slit Viewing Software
The slit viewer is controlled by an IDL widget, descended directly from the public “atv” package developed by A.
Barth.7 The standard atv release includes tools for image display, scaling, zooming, and normal manipulations,
as well as simple photometry and image quality analysis.
∗
http://cryogenics.nist.gov/MPropsMAY/material properties.htm
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Figure 5. Screen shot of the FIRE slit viewer, derived from the atv7 routines distributed by Aaron Barth. We have added
capabilities for communicating with the data acquisition computer and TCS, and a simple timer. The panels at left
display strip charts of seeing and count rates. A set of interactive mouse clicks is used to locate objects on the slit.
Using IDL’s TCP/IP socket client interface, we built an additional set of procedures into the code to handle
communications with both the TCS, and the HxRG acquisition detector controller. The new modifications
included a set of timer threads and communications processes to coordinate guider loops and fetch data over
the network from the guide readout. A WCS is patched onto guider images for orientation purposes, and also
so that saved images may be used after the run for rudimentary analysis. Simple interactive routines are used
to place objects on the slit and dither in A/B/B/A sequences.
3.3 Data Reduction
The data reduction pipeline for FIRE builds upon the MASE pipeline8 for the Magellan/MagE instrument.9 It
is still in beta testing stage, but now produces flux calibrated and combined spectra in both echelle and longslit
modes. For each frame, we generate a 2D wavelength map using fits of the background OH lines and/or arc
lamp spectra. These maps are in turn used to generate a 2D model of the night sky. The sky model for each
order is parameterized by a fast-varying bspline function in the wavelength direction that is also allowed to vary
slowly in the spatial direction to correct for illumination effects. We do not use√pairwise subtraction as a default;
provided that the sky subtraction code leaves small residuals, this leads to a 2 improvement in signal-to-noise
ratio.
A fast boxcar extraction is performed to estimate the signal-to-noise ratio (SNR) in each order. Then, orders
are optimally extracted in order of decreasing SNR. For orders with high SNR, the weighting is determined by
fitting a non-parametric spline to the object’s spatial profile. Intermediate SNR orders are fit with a Gaussian
function, and low SNR orders use a Gaussian as determined by fits to adjacent orders. For orders with little or
no flux, object positions are estimated by examining the slit position in adjacent orders, or using a standard star
as a crutch.
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2.5
K
H
J
Y
z
0.82
Figure 6. A 10-second spectrum of a bright standard star, illustrating FIRE’s spectral format.
After extraction, a telluric correction is applied. We have incorporated the “xtellcor” routines10 from Spextool
into the FIRE pipeline for this purpose. This software generates reddened model atmospheres of A0V stars and
allows the user to scale absorption features appropriately to calibrate out science exposures. This procedure also
provides a flux calibration accurate at the ∼ 5% level. Finally, the spectra are combined onto a single wavelength
grid and corrected for heliocentric velocity variations.
An example reduced spectrum is shown in Figure 10. Although substantial work remains to be done on the
pipeline, the software is well past the point required for assessing data quality and is in fact close to producing
science-quality output.
4. COMMISSIONING PERFORMANCE
FIRE’s first light was obtained in March 2010 and has subsequently been in use for 11 additional nights at
Magellan. Figure 5 shows an example spectrum of a bright star to illustrate the spectral layout of the orders.
We obtained a modest amount of calibration data during this run for the purpose of characterizing image quality
and instrument throughput. The following sections describe these measurements in greater detail.
4.1 Image Quality
During the third night that FIRE was on the telescope, we experienced a period of good seeing. We used this
time to sharpen the instrument’s focus with respect to the telescope. Internal focus had been verified using a
pinhole mask in the lab, indicating an instrument-induced blur of ≤ 1 pixel (0.15 ) consistent with FIRE’s design
specifications. A proper focus of with respect to the observatory’s Shack-Hartmann camera (which controls the
telescope primary and secondary) provides an end-to-end test of the full system.
Figure 7 shows the results of this test. Spatial cuts were made across the trace of a bright star (the one
shown in Figure 6), in four different locations in the focal plane. One position is at array center, one is at the
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1.2
1.0
Spatial profile of stellar image
Normalied counts
Four representative field angles
0.8
Mean FWHM = 0.29 arcseconds
0.6
0.4
0.29
0.2
0.0
-2
-1
0
1
Deviation from trace center (arcseconds)
2
Figure 7. Spatial cut through the stellar image shown in Figure 6, in four different locations on the focal plane. The effects
of differential anamorphic changes in the pixel scale have been normalized out by expressing the FWHM in arcseconds.
We measure a FWHM of 0.29 arcseconds from the array center to the most extreme field angles seen by the camera. This
meets the requirements set out for best-case natural seeing conditions at Magellan.
center of the bluest order, and two are at the extreme ends of the reddest order. These selections span the full
range of field angles seen by the camera.
We correct from pixel coordinates to arcseconds, to normalize out variations in the pixel scale arising from
chromatic dependence of the anamorphic magnification factor from the prism network. After this correction, the
implied FWHM is 0.29 across the whole focal plane, indicating a uniform, excellent image quality.
4.2 Sensitivity
We obtained spectra of the spectrophotometric standard GD71 in both longslit and echelle mode, to estimate
the instrumental throughput. Relatively few spectrophotometric standards are calibrated as far as K; a handful
are available from the HST calspec archive, and eventually the ESO X-shooter team will be releasing a new set
of southern IR secondary standards.11 In the interim we use a combination of model atmospheres for our A0V
telluric standards to achieve a low-accuracy flux calibration, bootstrapping as required from the (rare) primary
standards for more accurate calibration.
Figures 8 and 9 illustrate our preliminary sensitivity estimates for the two modes. The plots show the zero
point, defined as the AB magnitude yielding 1 count/second, as a function of wavelength. No corrections have
been applied for slit losses, telescope losses, or airmass. This therefore represents a realistic observation, obtained
in non-ideal conditions.
Nevertheless, the zero point appears to be fairly flat at 17th magnitude over most of FIRE’s range, dropping
off in the K band and the very blue. The slit was accidentally positioned far from the parallactic angle during
these observations. This fact, coupled with the J band filter in the slit viewing camera, may account for part of
the rolloff to the red. Future testing should resolve this question when FIRE completes its commissioning period.
The general zero point is consistent with our clear detection of signal in 10-15 minute exposures for sources as
faint as J = 19.7 (Vega).
The bottom panel of Figure 8 shows an estimate of the instrument efficiency, where we have removed the
effects of the three telescope mirrors and slit losses (though no airmass correction is applied). Over most of its
bandpass, FIRE’s efficiency varies in the 20 − 25% range.
Figure 9 shows a preliminary zero point calculation for the prism-dispersed longslit mode. The zero point
extends 3-3.5 magnitudes deeper in this configuration, consistent with its lower resolution, plus a correction of
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Figure 8. Top:Preliminary zero point for the FIRE spectrograph in R = 6000 echelle mode. No corrections have been
applied for airmass, slit, loss, or telescope losses, representing a realistic observing condition. However the slit was not
oriented at the parallactic angle, which leads to extra losses in K (since the slit viewer uses at J filter). The zero point
is expressed as the AB magnitude that yields one count per second. Corrections must be applied for objects measured in
the Vega system. Bottom Estimate of instrument efficiency, correcting for telescope and slit losses.
10 − 20% for the increased efficiency of the mirror relative to the echelle grating. This mode suffers however from
a substantially increased sky background because of blending among the OH forest. During commissioning, we
are exploring the tradeoff in ultimate sensitivity for observations made in both modes.
5. CONCLUSIONS
We have described the construction and commissioning program for FIRE, a new facility infrared spectrograph
for Magellan. FIRE’s design philosophy is discussed in detail in an earlier companion paper,2 the present
contribution follows up on this work and brings the project to conclusion. Although FIRE was originally designed
for observations of high redshift QSOs and local brown dwarfs, it has already been scheduled for ∼ 50 nights
in 2010B, for observations ranging from supernovae, to gamma-ray bursts, high redshift Lyman alpha emitters
and clusters, near earth asteroids, main belt asteroids, exoplanetary atmospheres, high redshift clusters, X-ray
binaries, and more. As a facility instrument, FIRE is available to the Magellan community and (through the
TSIP program) to the larger US community as well.
ACKNOWLEDGMENTS
It is a great pleasure to thank the outstanding staff of the Magellan Observatory, in Pasadena, La Serena,
and Las Campanas, for their tremendous support. In particular we thank Frank Perez, Alan Uomoto, Mark
Phillips, Dave Osip, and Povilas Palunas, all of whom endured many hours of teleconferencing and travel. We
also thank the members of external review panels who made valuable contributions to FIRE’s development.
During development, we enjoyed particularly close working relationships with Richard Blank from Teledyne and
Jay Kumler from Jenoptik, and wish to acknowledge their special contributions to the success of the project.
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Figure 9. Zero point for prism-dispersed longslit mode, defined as above for Echelle mode. The fainter zero point is
consistent with the lower resolution of this mode R ∼ 350, but this operation incurs a background penalty because of
blended OH lines.
We benefited from useful discussions on visits to the IR labs at the University of Virginia and UCLA. Finally,
the FourStar team, especially Eric Persson and Christoph Birk, were extremely generous with their time and
resources in helping us develop our instrument and software designs. FIRE’s construction was supported by
the NSF MRI program, under award number AST-0649190. We gratefully acknowledge additional support from
the Curtis Marble Instrumentation Development Fund, MIT-Kavli Instrument Development Fund, and the MIT
Department of Physics.
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Figure 10. Spectrum of a recently discovered J = 16.5 T dwarf, taken in R = 6000 echelle mode. No telluric correction is
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