The Astrophysical Journal Letters, 749:L17 (5pp), 2012 April 20 doi:10.1088/2041-8205/749/2/L17
C
2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
EARLY SCIENCE WITH SOFIA, THE STRATOSPHERIC OBSERVATORY FOR INFRARED ASTRONOMY
E. T. Young
1
, E. E. Becklin
1,2
, P. M. Marcum
3
, T. L. Roellig
3
,J.M.DeBuizer
1
, T. L. Herter
4
,R.G
¨
usten
5
,
E. W. Dunham
6
,P.Temi
3
, B.-G. Andersson
1
, D. Backman
7,8
, M. Burgdorf
7,9
, L. J. Caroff
10,18
,S.C.Casey
1
,
J. A. Davidson
11
, E. F. Erickson
10,18
,R.D.Gehrz
12
, D. A. Harper
13
, P. M. Harvey
14
, L. A. Helton
1
, S. D. Horner
3
,
C. D. Howard
1
, R. Klein
1
, A. Krabbe
9
, I. S. McLean
2
, A. W. Meyer
1
,J.W.Miles
1
, M. R. Morris
2
, W. T. Reach
1
, J. Rho
7,8
,
M. J. Richter
15
, H.-P. Roeser
16
, G. Sandell
1
, R. Sankrit
1
, M. L. Savage
1
,E.C.Smith
3
,R.Y.Shuping
1,17
, W. D. Vacca
1
,
J. E. Vaillancourt
1
,J.Wolf
7,9
, and H. Zinnecker
7,9
1
SOFIA Science Center, Universities Space Research Association, NASA Ames Research Center, MS 232, Moffett Field, CA 94035, USA
2
Department of Physics and Astronomy, University of California Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095, USA
3
NASA Ames Research Center, MS 232, Moffett Field, CA 94035, USA
4
Astronomy Department, 202 Space Sciences Building, Cornell University, Ithaca, NY 14853-6801, USA
5
Max-Planck Institut f
¨
ur Radioastronomie, Auf dem H
¨
ugel 69, Bonn, Germany
6
Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff AZ 86001, USA
7
SOFIA Science Center, NASA Ames Research Center, MS 211-1, Moffett Field, CA 94035, USA
8
SETI Institute, 515 North Whisman Road, Mountain View, CA 94043, USA
9
Deutsches SOFIA Institut, Universit
¨
at Stuttgart, Pfaffenwaldring 31, D-70569 Stuttgart, Germany
10
NASA Ames Research Center, Moffett Field, CA 94035, USA
11
School of Physics, The University of Western Australia (M013), 35 Stirling Highway, Crawley WA 6009, Australia
12
Minnesota Institute for Astrophysics, School of Physics and Astronomy, 116 Church Street, S. E., University of Minnesota, Minneapolis, MN 55455, USA
13
Yerkes Observatory, University of Chicago, 373 W. Geneva St., Williams Bay, WI, USA
14
Astronomy Department, University of Texas at Austin, 1 University Station C1400, Austin, TX 78712-0259, USA
15
Department of Physics, University of California at Davis, CA 95616, USA
16
Institut f
¨
ur Raumfahrtsysteme, Universit
¨
at Stuttgart, Pfaffenwaldring 31, D-70569 Stuttgart, Germany
17
Space Science Institute, 4750 Walnut Street, Boulder, CO 80301, USA
Received 2012 January 3; accepted 2012 January 24; published 2012 March 29
ABSTRACT
The Stratospheric Observatory For Infrared Astronomy (SOFIA) is an airborne observatory consisting of a specially
modified Boeing 747SP with a 2.7 m telescope, flying at altitudes as high as 13.7 km (45,000 ft). Designed to observe
at wavelengths from 0.3 μm to 1.6 mm, SOFIA operates above 99.8% of the water vapor that obscures much of
the infrared and submillimeter. SOFIA has seven science instruments under development, including an occultation
photometer, near-, mid-, and far-infrared cameras, infrared spectrometers, and heterodyne receivers. SOFIA, a joint
project between NASA and the German Aerospace Center Deutsches Zentrum f
¨
ur Luft und-Raumfahrt, began initial
science flights in 2010 December, and has conducted 30 science flights in the subsequent year. During this early
science period three instruments have flown: the mid-infrared camera FORCAST, the heterodyne spectrometer
GREAT, and the occultation photometer HIPO. This Letter provides an overview of the observatory and its early
performance.
Key words: infrared: general – instrumentation: miscellaneous – telescopes
1. INTRODUCTION
The Stratospheric Observatory For Infrared Astronomy
(SOFIA) is a joint project of the National Aeronautics and Space
Administration, USA (NASA) and the German Aerospace Cen-
ter Deutsches Zentrum f
¨
ur Luft und-Raumfahrt (DLR). The
NASA and DLR development and operations costs, as well as
the science observing time, are divided up in 80:20 propor-
tions, respectively. SOFIA consists of a 2.7 m telescope devel-
oped by DLR that resides in a highly modified Boeing 747SP
aircraft, enabling observations of a wide variety of astronom-
ical objects at wavelengths from 0.3 μm to 1.6 mm (Stutzki
2006; Becklin et al. 2007; Gehrz et al. 2009). The SOFIA tele-
scope design and its evolving instrument complement build upon
the legacy of NASA’s Kuiper Airborne Observatory (KAO),
a 0.9 m infrared telescope that flew from 1974 to 1995 in
a Lockheed C141 Starlifter aircraft (Gillespie 1981). SOFIA
flies at altitudes of up to 13.7 km (45,000 ft), which is above
99.8% of the atmospheric water (H
2
O) vapor. At SOFIA’s op-
erational altitudes, the typical precipitable H
2
O column depth
18
Retired.
is about 10 μm (roughly a hundred times lower than at good
terrestrial sites). This enables observations in large parts of
the infrared spectrum that are inaccessible from the ground.
Figure 1 compares the computed atmospheric transmission from
the operating altitude of SOFIA with that from one of the best
terrestrial sites, the 5612 m Cerro Chajnantor site in Chile. The
calculations were done using the ATRAN model (Lord 1992)
and assume a median precipitable water vapor of 0.7 mm for
Cerro Chajnantor (Radford et al. 2008). In addition to providing
access to virtually all of the infrared, SOFIA, with its projected
20 year operational lifetime, serves as a platform for the de-
velopment of new generations of instruments. Education and
public outreach are important elements of the SOFIA mission.
Consequently, the design of SOFIA included from the begin-
ning provisions for flying educators who would be able to share
the scientific experience with students.
A schematic view of the SOFIA facility is shown in Figure 2.
The telescope resides in an open cavity in the aft section of
the aircraft and views the sky through a port-side doorway. The
door has a rigid upper segment and a flexible lower segment
that can be tracked together to allow the telescope to operate,
unvignetted, over an elevation range of 23
◦
–58
◦
. The telescope is
1
The Astrophysical Journal Letters, 749:L17 (5pp), 2012 April 20 Young et al.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
10 100 1000
TRANSMISSION
WAVELENGTH ( m)
SOFIA
Chajnantor
Figure 1. Calculated atmospheric transmission for SOFIA and Cerro Chajnantor. Computed using the ATRAN program (Lord 1992) assuming 10 μm and 700 μmof
precipitable water vapor, respectively. The average SOFIA transmission between 20 μm and 1.3 mm is 80%.
Figure 2. Cutaway schematic of the SOFIA observatory. The pressurized cabin is to the left of the pressure bulkhead. The telescope looks out from the port side of
the airplane. The 10,000 kg mass is supported by a hydrostatic spherical bearing that allows an elevation range of 23
◦
–58
◦
. The light for the science instrument is fed
through the Nasmyth tube which goes through the bearing. An environmental control system prevents condensation during descent when the door is closed after a
night of observations.
moved by magnetic torquers around a 1.2 m diameter spherical
hydrostatic bearing that floats under an oil pressure of 20
atmospheres within two closely fitting spherical rings. The rings
are mounted in the 6.4 m diameter pressure bulkhead on the
axis of the Nasmyth beam. The travel of the bearing for azimuth
tracking is only ±3
◦
so the aircraft heading must be periodically
adjusted to keep the source within the telescope field of view
(FOV). The forward part of the airplane is pressurized, and
the working environment for the crew is typical of that in a
commercial airliner. This pressurized region provides access to
the science instrument during the flight. The characteristics of
the SOFIA observatory are given in Table 1.
The first SOFIA science observations were conducted in 2010
December using the FORCAST (Faint Object Infrared Camera
for the SOFIA Telescope) mid-infrared camera. In the 11 months
following this first flight, SOFIA conducted 30 science flights
using the FORCAST camera, the GREAT (German Receiver for
Astronomy at Terahertz frequencies) heterodyne submillimeter
spectrometer, and the HIPO (High-speed Imaging Photometer
for Occultations) occultation photometer. This initial phase is
known collectively as Early Science and represents a demon-
stration of initial SOFIA capabilities while the facility and its
instrumentation are still under development.
In this Letter, we provide an overview of the telescope and
observatory and describe its performance during the Early Sci-
Tab le 1
SOFIA Observatory Characteristics
Characteristic Value
Operational wavelength 0.3–1600 μm
Clear aperture diameter 2.5 m
Telescope elevation range (unvignetted) 23
◦
–58
◦
Unvignetted FOV 8
Telescope optics image quality at 0.6 μm
a
1.
6 (80% encircled energy)
Diffraction-limited image size (FWHM)
b
0.0825 × λ(μm) (
)
Chopper frequencies (2 pt square wave) 1–20 Hz
Maximum chop throw (unvignetted) ±5
Maximum flight altitude 13.7 km (45,000 ft)
Typical flight duration 10 hr
Flight time above 12.5 km (41,000 ft) 6 hr
Operational lifetime 20 yr
Notes.
a
Does not include the effects of seeing or pointing stability, which dominate at
the shorter wavelengths.
b
The goal at full operational capability is diffraction-limited imaging for
λ>15 μm.
ence period. We set the context for the rest of the articles in
this special issue, which present Early Science results based
on observations using the FORCAST mid-infrared instrument
2
The Astrophysical Journal Letters, 749:L17 (5pp), 2012 April 20 Young et al.
Figure 3. Schematic of the optical system of the SOFIA telescope (left) and a model of the SOFIA telescope assembly (right). Everything left of the bulkhead is
contained in the forward pressurized crew cabin, while everything to the right is contained in the open telescope cavity.
during the first three flights. An additional 10 flights with
FORCAST were conducted for the general astronomical
community, a period called Basic Science. Those results will be
described elsewhere. Observations with HIPO during the Early
Science period included an occultation of a star by Pluto on 2011
June 23, during which the observatory was flown through the
central chord of the shadow cast by Pluto (Person et al. 2011).
The GREAT instrument was flown on 16 scientific flights dur-
ing Early Science. The first results from those observations will
appear in parallel in a special issue of Astronomy and Astro-
physics.
2. SOFIA TELESCOPE AND OBSERVATORY OVERVIEW
The SOFIA telescope, supplied by DLR as part of the
German contribution to SOFIA, is a bent classical Cassegrain
with a 2.690 m diameter parabolic primary mirror, and a
0.352 m hyperbolic secondary mirror. The secondary mirror
is deliberately undersized and illuminates a 2.5 m diameter
effective aperture to enable chopping without spilling the beam
onto the warm region surrounding the primary mirror. The
secondary mirror is attached to a chopping mechanism providing
amplitudes of ±4
at chop frequencies up to 20 Hz. The
unvignetted FOV is 8
.Thef/19.6 Nasmyth infrared focus is fed
by a 45
◦
gold-coated dichroic mirror (Figure 3). This dichroic
mirror separates the light into a reflected infrared beam that
is sent to the science instrument and a visible light beam that
is sent to a CCD guide camera, the Focal Plane Imager (FPI).
Two other imaging/guiding cameras, the Wide Field Imager
(WFI) and the Fine Field Imager (FFI), are used as part of the
acquisition system. They are attached to the front ring of the
telescope. Table 2 gives some of the key optical parameters
of the SOFIA telescope and guide cameras. A more detailed
description of the telescope has been given by Krabbe (2000).
The pointing of the telescope is performed more like a space
telescope than a ground-based telescope. The primary reference
frame for SOFIA is a set of fiber optic gyroscopes that maintain
an inertial reference frame. Once a target is acquired on the sky,
its position is stabilized by these gyroscopes with occasional
updates by the guide cameras.
The telescope optics are designed to provide 1.
1 FWHM
images on-axis at 0.6 μm with diffraction-limited performance
at wavelengths longer than 15 μm. However, the telescope is
subject to various vibrations as well as variable wind loads in
flight, which affect the telescope pointing stability and hence the
Tab le 2
SOFIA Telescope Parameters
Parameter Value
Optical configuration Classical Bent Cassegrain
Primary mirror diameter 2.69 m
Primary mirror material Zerodur
Primary conic constant −1
Primary focal length 3200 mm
Secondary mirror diameter 352 mm
Secondary mirror material Silicon carbide
Secondary radius of curvature 954.13 mm
Secondary conic constant −1.2980
Effective entrance pupil diameter 2.5 m
Nominal focal length 49141 mm
Nominal system f-ratio f/19.6
Wide Field Imager FOV 6
◦
× 6
◦
Fine Field Imager FOV 70
× 70
Focal Plane Imager FOV 8
× 8
delivered image quality. SOFIA has active and passive damping
systems designed to mitigate some of these effects. In the Early
Science flights, after limited tuning of the damping systems, the
telescope produced an image quality of 3.
2 FWHM at 19.7 μm
with an rms pointing stability of 1.
7. While these values are large
enough to affect image quality at the shorter wavelength end of
the SOFIA 0.3–1600 μm observing range, the observatory was
nearly diffraction limited at λ>38 μm (i.e., over the large
majority of the SOFIA operating wavelength range) just after
First Light.
One of the larger causes of variable image quality is high-
velocity turbulent airflow across the cavity, which drives vibra-
tions and leads to elongation of images in the cross-elevation
direction. Early tests showed that the magnitude of this effect
might also depend on the telescope elevation. Therefore, because
of variables like the amount of turbulence and cross-elevation
vibration, the point-spread function (PSF) was not stable in the
mid-infrared for Early Science from observation to observation.
Further mitigation of the rms pointing jitter through tuning of
active control systems and the addition of active mass dampers
were implemented in late 2011. These dampers are installed at
various locations on the primary and secondary mirror support
structures. In the future, after optimization of the damping
systems, the rms pointing stability is expected to improve to 0.
5,
which will deliver an image quality of 2.
1 FWHM at 19.7 μm.
3
The Astrophysical Journal Letters, 749:L17 (5pp), 2012 April 20 Young et al.
Tab le 3
SOFIA First Generation Instruments
Name Description PI Institution Wavelengths Spectral
(μm) Resolution
FORCAST Mid-infrared Camera and Grism Spectrometer T. Herter Cornell 5–40 200
GREAT Heterodyne Spectrometer R. G
¨
usten MPIfR 60–240 10
6
–10
8
FLITECAM Near-infrared Camera and Grism Spectrometer I. McLean UCLA 1–5 2000
HIPO CCD Occultation Photometer T. Dunham Lowell Obs 0.3–1.1
EXES Mid-infrared Spectrometer M. Richter UC Davis 5–28 3000, 10
4
, 10
5
HAWC Far-infrared Camera D. A. Harper U Chicago 50–240
FIFI-LS Integral Field Far-infrared Spectrometer A. Krabbe U Stuttgart 42–210 1000–3750
Note.
a
Details available at http://www.sofia.usra.edu/Science/instruments
At wavelengths shorter than 10 μm, another significant con-
tributor to the PSF width is the seeing due to turbulence in the
shear layer in the vicinity of the airplane. The shear layer seeing
is wavelength dependent and increases at shorter wavelengths.
Below roughly 5 μm, the shear layer seeing becomes the domi-
nant image size contributor.
The secondary mirror is capable of chopping in any direction
from center with a maximum amplitude of 292
, with the
additional constraint of a maximum total chop amplitude of
480
. As is the case for all Cassegrain systems, tilting the
secondary mirror produces coma. For the SOFIA telescope,
the coma causes PSF smearing in the direction of the chop by
2
per 1
of chop amplitude.
3. SOFIA INSTRUMENTATION
One of the strengths of the SOFIA observatory is the ability
to change instruments. Seven first-generation instruments have
been developed or are being developed for SOFIA. Collectively,
they span a large wavelength range (0.3–250 μm) and include
imagers and spectrographs. SOFIA provides several classes
of instruments. Facility class instruments are general purpose
instruments that are maintained and operated by the observatory.
Principal Investigator class instruments are maintained and
supported by the instrument teams. Special purpose instruments
are specialized capabilities also supported by the instrument
teams.
FORCAST (PI: Terry Herter, Cornell University) is a mid-
infrared camera operating between 5 and 40 μm. The instrument
is described in the following paper by Herter et al. (2012).
GREAT (PI: Rolf G
¨
usten, MPIfR; Heyminck et al. 2012)isa
heterodyne spectrometer currently operating in the 1.2–2.5 THz
frequency range. These instruments were used during Early
Science for both guaranteed time observations planned by
the instrument teams and for programs from the astronomical
community competed via open proposal calls.
Two other instruments, HIPO (PI: Edward Dunham, Lowell
Observatory; Dunham et al. 2004) and FLITECAM (First Light
Infrared Test Experiment Camera; PI: Ian McLean, UCLA;
McLean et al. 2006), were used on development flights on
SOFIA. HIPO consists of a pair of high-speed CCD detectors
specifically configured to be used for occultation and other
high-speed imaging applications. FLITECAM is a near-infrared
camera operating between 1 and 5 μm. The instrument also has
grisms installed that provide moderate resolution spectroscopy
at these wavelengths. All four instruments will be offered to
the astronomy community for the first annual observing cycle
(Cycle 1).
The remaining three instruments, EXES (Echelon-Cross-
Echelle Spectrograph; PI: Matt Richter, UC Davis; Richter et al.
2003), FIFI-LS (Field Imaging Far-infrared Line Spectrometer;
PI: Alfred Krabbe, University of Stuttgart; Klein et al. 2010),
and HAWC (High-resolution Airborne Wideband Camera; PI:
Doyal Harper, U. Chicago; Harper et al. 2004), are expected
to be offered in a second annual Call for Proposals (Cycle 2).
Table 3 summarizes the characteristics of the First Generation
SOFIA Instruments. The SOFIA program also plans to introduce
new instruments in future years.
4. SOFIA OPERATIONS
SOFIA is based at the NASA Dryden Aircraft Operations
Facility (DAOF) in Palmdale, California. The DAOF is a
multi-user NASA facility that supports a number of scientific
aircraft. The SOFIA Operations Center (SOC) is located at
the DAOF and performs the mission operations and laboratory
support for the observatory. The SOFIA Science Center (SSC)
is located at the NASA Ames Research Center in Moffett Field,
California. Collectively, the SSC and the SOC combine to
make up SOFIA Science Mission Operations (SMO), which
is responsible for the conduct of the science on SOFIA.
The overall SOFIA mission development and operations are
managed by NASA’s SOFIA Program Office, currently located
at the DAOF. The SOFIA aircraft operations are managed
by NASA’s Dryden Flight Research Center (DFRC), which
provides the aircraft maintenance staff, aircraft safety personnel,
the pilots, navigators, and other aircraft flight crew members.
The SMO is jointly managed by the Universities Space Research
Association (USRA) for NASA and by the Deutsches SOFIA
Institut (DSI), in Stuttgart, Germany for DLR. Science support
for the user community will be provided by the SMO and
the DSI.
Typical crew members during the Early Science period
consisted of the flight deck (pilot, co-pilot, navigator, and flight
engineer), mission operations (mission director, science flight
planner, telescope operators), instrument team, engineering
support (safety technicians, telescope engineers, and water
vapor monitor engineer), and General Investigators (GIs).
SOFIA will mainly operate from its home base at DAOF but
will also be deployed to operate from other bases in various parts
of the world. In particular, regular deployments to the Southern
Hemisphere are planned to provide access to targets that cannot
be observed from the Palmdale base.
To maximize the operational efficiency of SOFIA, observa-
tions are all queue scheduled. Starting in Cycle 1, the bulk of
SOFIA observing time will be awarded on a yearly basis via
open, peer-reviewed proposal calls, and observations will be
awarded in units of time. Thus, there will be a process of flight
planning for every observing cycle which will be very much like
planning year-long observations for a space mission. For SOFIA
4
The Astrophysical Journal Letters, 749:L17 (5pp), 2012 April 20 Young et al.
Figure 4. Representative SOFIA flight plan for the FORCAST Basic Science
period. Targets came from the pool of selected GI proposals. Total flight duration
for this plan is 9.5 hr.
there will be the added constraints imposed by the limited sky
visibility determined by aircraft heading. Also, sensitive obser-
vations requiring the driest conditions need to be scheduled for
the highest possible aircraft altitudes. After takeoff, the initial
aircraft cruising altitude is 11.6–11.9 km (38,000–39,000 ft). As
fuel is burned off, the aircraft can fly higher. The performance
of the aircraft allows for 6 hr of observing time above 12.5 km
(41,000 ft) of which 4 hr can be above 13.1 km (43,000 ft). The
maximum flight time under routine operations is 10 hr.
The flight planning process involves selecting suitable targets
from a larger pool that satisfy requirements such as correct
azimuth, necessary altitude, and instrument configurations.
To ensure efficient use of observing time, the SOFIA Flight
Planning team needs to have available a pool of targets well
distributed on the sky. Figure 4 illustrates a typical flight
plan from the FORCAST Basic Science period. The flight
begins in Palmdale and the plan includes a mixture of galactic
and extragalactic targets as well as northern and southern
targets. The requirement to return to Palmdale combined with
a nominal 10 hr flight length implies a maximum leg length of
approximately 4 hr on a given source.
During the 11 months of Early Science, SOFIA conducted
30 observational flights with more than 200 hr of research time.
SOFIA is planned to ultimately fly approximately 120 flights
per year with more than 960 research hours.
5. ACCESSING EARLY SCIENCE DATA FROM
THE SOFIA ARCHIVE
Data accumulated during all flights are archived in the SOFIA
data archive. The raw data (Level 1) obtained during science
flights are ingested into the archive and made available for
download to the program GI within 24 hr. These data are
available in standard FITS format, with header information and
appropriate Flexible Image Transport System (FITS) keywords
conforming to the SOFIA FITS keyword dictionary. Raw
FORCAST data must be corrected for several instrumental
effects, including response nonlinearity, field distortion, and
image artifacts. SOFIA mission operations are responsible for
this processing of the FORCAST data via a software pipeline.
These Level 2 data (corrected for instrumental and atmospheric
effects) are scientifically valid and available for download from
the SOFIA archive within two weeks after the completion of the
flight series in which they are taken.
As a Principal Investigator class instrument, the GREAT
data follow a somewhat different path. The raw data are
archived in the SOFIA data archive, but the instrument team
provides support for the subsequent processing of the data into
scientifically useful products.
Nominally, data are accessible to the general community after
a proprietary period of 12 months, starting from the time of
ingestion into the archive. However, FORCAST Early Science
data, including those presented in this volume, have a limited
proprietary period and are now available to the community.
Readers wishing to download FORCAST Early Science data
must register an account with the SOFIA Data Cycle System
(DCS), linked to and accessible from the SOFIA World Wide
Web home page (https://dcs.sofia.usra.edu).
6. CONCLUDING REMARKS
SOFIA has completed over 30 science flights in the year
since the initial Early Science flight in 2010 December. Some
of the results of the first four flights with FORCAST are pre-
sented here in this issue, with many more results from Early
Science in preparation. Another set of papers based on GREAT
observations is being published in parallel in Astronomy and
Astrophysics. With the range of current and future instrumen-
tation, and with the projected 20 year operational lifetime, it is
expected that SOFIA observations will yield a rich collection of
scientific results of which these are the preview.
SOFIA science mission operations are conducted jointly by
the Universities Space Research Association, Inc. (USRA),
under NASA contract NAS2-97001, and the Deutsches SOFIA
Institut (DSI) under DLR contract 50 OK 0901 to the University
of Stuttgart. R.D.G. was supported by NASA and the United
States Air Force.
REFERENCES
Becklin, E. E., Tielens, A. G. G. M., Gehrz, R. D., & Callis, H. H. S. 2007, Proc.
SPIE, 6678, 66780A
Dunham, E. W., Elliot, J. L., Bida, T. A., et al. 2004, Proc. SPIE, 5492, 592
Gehrz, R. D., Becklin, E. E., de Pater, I., et al. 2009, Adv. Space Res., 44,
413
Gillespie, C. M., Jr. 1981, Proc. SPIE, 265, 1
Harper, D. A., Bartels, A. E., Casey, S. C., et al. 2004, Proc. SPIE, 5492, 1064
Herter, T., Adams, J. D., De Buizer, J. M., et al. 2012, ApJ, 749, L18
Heyminck, S., Graf, U., Gusten, R., et al. 2012, A&A, submitted
Klein, R., Poglitsch, A., Raab, W., et al. 2010, Proc. SPIE, 7735, 77351T
Krabbe, A. 2000, Proc. SPIE, 4014, 276
Lord, S. D. 1992, NASA Technical Memorandum 103957
McLean, I. S., Smith, E. C., Aliado, T., et al. 2006, Proc. SPIE, 6269, 62695B
Person, M., Dunham, E. W., Bida, T., et al. 2011, European Plan. Sci. Conf.,
Vol. 6, 1374
Radford, S. J., Giovanelli, R., Gull, G. E., & Henderson, C. P. 2008, Proc. SPIE,
7012, 70121Z
Richter, M. J., Lacy, J. H., Jaffe, D. T., et al. 2003, Proc. SPIE, 4857, 37R
Stutzki, J. 2006, Rev. Mod. Astron., 19, 293
5
