Edward Cheng et al- Illuminating Dark Energy with the Joint Efficient Dark-energy Investigation (JEDI)

8/3/2019 Edward Cheng et al- Illuminating Dark Energy with the Joint Efficient Dark-energy Investigation (JEDI)

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Illuminating Dark Energy with the Joint Efficient Dark-energy

Investigation (JEDI)

Edward Cheng!a, Yun Wangb, Edward Baronb, David Branchb, Stefano Casertanoc, Arlin Crottsd,

Helmuth Drosdate, Luke Dubord

f, Robert Egerman

g, Peter Garnavich

h, David Gulbransen

i,

Alexander Kutyrevj, John W. MacKentyc, John W. Milese, Leonidas Moustakasf, Mark Phillipsk,Thomas Roellig

l, Robert Silverberg

j, Gordon Squires

m, J. Craig Wheeler

n, Edward L. Wright

o, for

the JEDI Team

aConceptual Analytics LLC, 8209 Woburn Abbey Road, Glenn Dale, MD USA 20769;b

The University of Oklahoma, Department of Physics and Astronomy, Norman, OK 73019;cSpace Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218;

dColumbia Univ., Columbia Astrophysics Laboratory, 550 West 120

thStreet, New York, NY 10027;

eLockheed Martin Advanced Technology Center, 3251 Hanover Street, Palo Alto, CA 94304;

fJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109;gITT Space Systems Division LLC, 800 Lee Road, Rochester, NY 14650;

hUniversity of Notre Dame, Physics Department, Notre Dame, IN 46556;iRockwell Scientific Company, 5212 Verdugo Way, Camarillo, CA 93012;

jNASA Goddard Space Flight Center, Code 665, Greenbelt, MD 20771;

kLas Campanas Observatory, Carnegie Institution of Washington, Casilla 601, La Serena, Chile;

lNASA Ames Research Center, MS 245-6, Moffett Field, CA 94035;

mSpitzer Science Center, California Institute of Technology, Pasadena, CA 91125;

nDepartment of Astronomy, University of Texas, Austin, Texas 78712;

oUniversity of California Los Angeles, Astronomy Department, P.O. Box 951547, Los Angeles, CA

90095

ABSTRACT

The Universe appears to be expanding at an accelerating rate, driven by a mechanism called Dark Energy. The nature of

Dark Energy is largely unknown and needs to be derived from observation of its effects. JEDI (Joint Efficient Dark-

energy Investigation) is a candidate implementation of the NASA-DOE Joint Dark Energy Mission (JDEM). It will

probe the effects of Dark Energy in three independent ways: (1) using Type Ia supernovae as cosmological standard

candles over a range of d istances, (2) using baryon acoustic oscillations as a cosmological standard ruler over a range of

cosmic epochs, and (3) mapping the weak gravitational lensing distortion by foreground galaxies of the images of

background galaxies at different distances. JEDI provides crucial systematic error checks by simultaneously applying

these three independent observational methods to derive the Dark Energy parameters. The concordance of the results

from these methods will not only provide an unprecedented understanding of Dark Energy, but also indicate the

reliability of such an understanding. JEDI will unravel the nature of Dark Energy by obtaining observations only

possible from a vantage point in space, coupled with a unique instrument design and observational strategy. Using a 2

meter-class space telescope with simultaneous wide-field imaging (~ 1 deg2, 0.8 to 4.2 m in five bands) and multi-slit

spectroscopy (minimum wavelength coverage 1 to 2 m), JEDI will efficiently execute the surveys needed to solve the

mystery of Dark Energy.

Keywords: JDEM, JEDI, Dark Energy, infrared, space telescope, spectroscopy, astronomical survey

[email protected]; phone 301-805-8618; FAX 301-860-1694; www.conceptual-analytics.com


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1. INTRODUCTION

The expansion of the Universe is accelerating1, 2, 3, 4, 5, 6

, driven by an unknown mechanism that is referred to as Dark

Energy. This mechanism is the posited driving force; it is a theoretical construct and there is no known way to directly

measure it. The observables are its effects on the geometry of the Universe as a function of cosmic time. Until the

recent SNe Ia measurements by ground and space-based observations, the expansion rate of the Universe in recent

cosmic time was thought to be decelerating (the expansion rate is slowing). What the SNe Ia measurements imply is that

the Universe was smaller in the past than expected from the expansion of the Universe as determined by generallyaccepted cosmological models. As a result, it appeared that the expansion of the Universe started accelerating sometime

in the redshift range 0 < z < 2. Understanding the origin and evolution of this acceleration is the key to unraveling the

mystery of Dark Energy.

To address this scientific need, the Joint Dark Energy Mission (JDEM) is being sponsored by NASA and DOE to

investigate the nature of Dark Energy in our Universe. The Joint Efficient Dark-energy Investigation (JEDI) mission is a

candidate implementation for JDEM. The JEDI mission is joint because of the programmatic collaboration between

NASA and DOE, and also because it exploits all of the most promising observational techniques for probing Dark

Energy; and it is efficient because the strategy behind the mission and instrument design performs all measurements

over two different but complementary observing campaigns over the three-year mission lifetime. These are the JEDI

Deep (mission year 1) and the JEDI Wide (years 2 & 3) campaigns, both of which feature simultaneous imaging and

multi-object spectroscopy. Table 1 shows the basic JEDI measurement capabilities available to both of these campaigns.

Table 1. JEDI Basic Measurement Capabilities.

Survey Mode WavelengthRange

Resolution

Photometric 0.8 to 4.2 m infive spectralbands:z(0.8 1.1 m)

j(1.1 1.5 m)h (1.5 2.0 m)

k(2.0 3.0 m)

l(3.0 4.2 m)

Diffractionlimited at 1m

Spectroscopic 1 to 2 m R = !/"! ~ 500

JEDI uses a powerful combination of the three most promising methods to derive the size of the Universe as a function

of its age during the period when Dark Energy is expected to have caused the acceleration in expansion of the Universe:

1. Type Ia Supernovae (SNe) as standard candles,

2. Cosmic shear as a tracer of Weak Lensing (WL) from intervening matter, and

3. Baryon acoustic oscillations (BAO) in the distribution of galaxies as a standard ruler in both the transverse andradial directions.

The combination of these independent observational methods provides JEDI with the advantage that the individual

systematic errors of each method can be verified, understood, and likely corrected. At a minimum, any irreconcilable

differences among these methods would at least point out an inconsistency that needs to be further explored. The ability

to provide this level of confidence is essential, given the very challenging nature of these measurements.

The goal of the JEDI Deep campaign is to acquire time-dependent infrared light curves to discover SNe, and to providemagnitude-limited spectroscopy. The JEDI Wide campaign maps the galaxies used for the WL and BAO surveys. The

focal plane instrumentation is built for five-band photometric imaging, with simultaneous multi-object spectroscopy in

an adjoining field, as described in Section 3.2. The observing strategy is described in Section 3.3.

The JEDI implementation takes advantage of flight and design heritage subsystems to support imaging and multi-object

spectroscopy efficiently and simultaneously. Several key hardware subsystems for JEDI are being or have been

developed for other programs: JEDI uses a 1.5 m telescope similar to the NextView Earth observing system; the multi-

object spectroscopy is implemented using the exact microshutter array being developed for the James Webb Space


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Telescope (JWST) NIRSpec instrument; the infrared focal planes are based on mosaics of the HAWAII-2RG

multiplexer-based midwave HgCdTe detectors, as developed for JWST and other missions; and the mildly cryogenic

requirements on the telescope and focal planes are easily met with a spacecraft design that is similar to Spitzer. This

extensive reuse of key designs provides a proof-of-concept of the proposed implementation, and is important in reliably

assessing the cost and risk of the mission. The JEDI Team includes the original developers of these elements, which

provides the experience base needed to properly execute the mission.

The JEDI Science Team is constructed to provide the expertise needed for understanding and implementing the threeobservational methods, as well as to provide experience in executing space missions of comparable complexity. It is

supported by a technical and management team from the NASA Jet Propulsion Laboratory (JPL), Lockheed Martin

(LM), ITT Space Systems Division, Rockwell Scientific Company (RSC), and the Infrared Processing and Analysis

Center (IPAC).

2. SCIENTIFIC MOTIVATIONWithin the framework of Einsteins General Relativity, there are two classes of possibilities for the nature of Dark

Energy. It could be in the form of a static cosmological constant with energy density, !X,constant in time, or it could

be a dynamical quantity with density !X(z) that varies with cosmic time z. The equation of state for these types of Dark

Energy can be parameterized as w(z) = w0 + w! zfor redshiftz< 2, and assumed to be constant forz! 2. The energy

density is then given by !X(z), where d[ln(!X(z)]/d[ln(1+z)] = 3[1+w(z)]. For the cosmological constant case,

!X(z)=!X(0), so w0="1 and w#=0. Alternatively, Dark Energy could be the consequence of deviations from GeneralRelativity (hence not an energy but a kind of modified gravity). Determining which, if any, of these possibilities is

correct is one of the great challenges for cosmology today.

The nature of Dark Energy can be probed with several combinations of cosmological parameters: the cosmic expansion

historyH(z) (the expansion rate of the Universe as a function of redshiftz); luminosity and angular-diameter-distances asfunctions ofz, dL(z) and dA(z), respectively; and the linear growth factor of cosmic large scale structure, G(z) (the growth

rate of matter density perturbations in the Universe relative to that of a flat universe without Dark Energy) . The JEDI

mission concept provides measurements over a large redshift range, 0 < z< 2 where the Dark Energy mechanism is

expected to manifest itself. It implements three powerful and complementary observational surveys that probe the

cosmological parameters. Through the combination of a Deep and a Wide campaign, we survey a) Type Ia supernovae

(SNe) as standard candles (dL(z)); b) cosmic shear as a tracer of weak lensing (WL) from intervening matter, which gives

dA(z1)/dA(z2) ratios and the cosmic structure growth factorG(z); and c) baryon acoustic oscillations (BAO) in the galaxy

distribution as a standard ruler in both the transverse and radial directions (dA(z) and H(z)), and as a probe ofG(z).

Though all of these measures are formally related for a given cosmological model, they all suffer from differentsystematic uncertainties. The JEDI mission concept is designed to have the complementarity and redundancy that is

essential to achieve not only extremely small stochastic errors, but also to identify and account for the systematic errors

to which each method is susceptible. This strategy minimizes the scientific risk for the mission.

2.1 The Advantage of Combining Three Observational MethodsThe challenge to the Dark Energy parameter measurements will not be with precision (from statistics), but in

determining the accuracy, which requires understanding and adequate modeling of systematic uncertainties. Multiple

observational methods with different systematic susceptibilities resolve this problem, and drive the entire JEDI mission

rationale. Figure 1 illustrates the advantage of the JEDI mission approach. Measurement-error ellipses from each of

three techniques are tilted in different directions; this is because the three JEDI methods probe Dark Energy in different

ways, resulting in the combined error ellipse shown (see also Table 2). If unaccounted-for systematic effects dominate,

the error ellipses may be offset from each other, and the goodness-of-fit of the results would be poor. With the JEDI

mission redundancy, these systematic effects can be identified, analyzed, and removed. This capability should drive anyconcerted effort to definitively determine the nature of Dark Energy.

Figure 1 also shows that the JEDI mission will measure H(z) accurately for 0 < z< 2 using both SNe and BAO, thus

enabling model-independent constraints on the time dependence of Dark Energy7. The WL survey will provide

independentH(z) measurements8, and serve as a cross-check of the SNe and BAO results. Finally, the JEDI mission will

measure G(z), providing a new test of gravity over cosmological scales8. The JEDI missions measurement ofH(z) and

G(z) as free functions of cosmic time (for 0 < z< 2) is key to the scientific success of JDEM. This capability removes

the dependence of the Dark Energy results on the assumption of simplified parameterizations.


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Figure 1. The left panel shows the Dark Energy parameter uncertainties expected from the JEDI baseline mission (68.3% or

1! confidence region contours). Each error ellipse becomes offset from the true model (!m=0.3, w0=-1, w"=0) when

systematic errors dominate. Ancillary results from a successful Plank mission are assumed in this analysis. The rightpanel shows the cosmic expansion historyH(z) / H(z=0) expected from SNe and BAO (1! errors).

Table 2. Marginalized 68.3% confidence level errors expected for the JEDI baseline mission.*

Survey Redshift Key Assumptions # (w0) # (w") R!SNe only

[~4000 SNe Ia]0


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Recent work has shown that in addition to light-curve shape, SN Ia spectral ratios (measurable at the JEDI resolution,

Table 1) can be used as complementary luminosity indicators15

. The spectroscopy also allows weak lensing effects to be

minimized by flux-averaging over large numbers of SNe Ia at high-z16, systematic correlations between luminosity and

spectral properties to be searched for, and the diagnostic data needed to identify SN Iasub-types to be gathered17.

The JEDI SNe survey is executed during JEDI Deep Campaign, which consists of repeated visits to the same 4 deg2

field, and will reach a magnitude limit ofHAB=26.5 at a signal-to-noise ratio (S/N) of 10 for each visit. This is sensitive

enough for observing SNe Ia at z = 3 (if they exist at such high z). Our preliminary simulations indicate that while wewill have very high quality SN Ia spectra at low and intermediate redshifts, we will have good quality SNe Ia spectra

(S/N! 10) atz= 1.6 and beyond, which will be improved by co-adding the spectra from successive visits18.

2.3 Weak Lensing (WL) Method The Advantage of JEDI Unbiased Photometric RedshiftsAs light travels toward us from distant galaxies, its path is gravitationally influenced by the intervening matter

distribution. In the strong limit of high mass column density in the lens and good line-of-sight alignment, one can get

high magnifications and multiple images through gravitational lensing19. In the more typical weak limit the distant

galaxies will be distorted according to the distribution and concentration of the intervening matter20, 21. Measuring this

shear requires an accurate and unbiased determination of the apparent shapes of many distant galaxies22. If systematic

errors are understood and controlled, and if the redshifts of the background galaxies are known, several distinct and

complementary probes of Dark Energy become possible.

In the age of accurate, large, high-resolution N-body simulations, it is possible to characterize the evolution of the

growth of cosmic large-scale structure. Comparing such predictions against the measured cosmic shear correlations iscalled lensing tomography and can be used to measure G(z)23, 24, 25. G(z) depends on the equation of state of Dark

Energy, since different expansion histories will result in different relations between the growth of gravitationally-

collapsed structures and cosmic time.

A more elegant application of WL is possible when the redshifts of the background galaxies are known, as is the case

with the JEDI mission. If for the same foreground screen of mass, one measures the WL signal for galaxies at different

redshifts (i.e. distances), the relative shear WL signal effectively gives the ratio of the angular diameter distances

between those redshifts, dA(z1) / dA(z2). This WL signal is called cross-correlation cosmography10, 26

, and has the

advantages of (1) not requiring detailed knowledge of the distribution of the foreground lensing masses, (2) not being

limited by the completeness of the galaxies used to determine the shear signal, and (3) being most sensitive as a

measurement ofH(z).

The JEDI mission design exploits all of these techniques, thanks to its unsurpassed combination of high spatial

resolution imaging and stability of the telescope PSF. The JEDI data are uniquely suited to this measurement because ofthe millions of spectroscopic redshifts that will be collected during the course of the mission.

The JEDI Wide campaign will produce a near-infrared photometric and spectroscopic study of 1000 deg2

to magnitude

HAB ~ 25.5 at S/N~ 20 and a resolved galaxy number density greater than 60 to 100 arcmin-2

(2 to 3 times that of a

typical ground-based survey). The PSF stability is ensured by the mission design, mostly by careful attention to

maintaining thermal stability. The spectroscopic redshifts from the JEDI Wide campaign will calibrate the photometric

redshifts of the resolved galaxies to unprecedented accuracy. The JEDI WL surveys depth at infrared wavelengths

results in a higher mean redshift distribution (and therefore gain in expected shear) relative to ground based surveys or

space based surveys that image at shorter wavelengths. Both higher redshift distribution and higher resolved galaxy

density result in more stringent constraints on Dark Energy for a given survey area.

2.4 Baryon Acoustic Oscillations (BAO) Method The Advantage of Using the Standard Ruler Over 0.5 < z < 2.0In the early Universe, baryonic matter and radiation were tightly coupled in a hot plasma due to the interplay of gravity

and radiation pressure, and sound waves traveling at more than half the speed of light rang through the Universe. When

the Universe was a few hundred thousand years old (corresponding toz~1000), electrons combined with protons to form

hydrogen atoms in the event known as recombination. The sound speed subsequently dropped to zero, freezing each

sound-waves crest and trough in their places. These baryon acoustic oscillations (BAO) are imprinted in the large-

scale distribution of matter as well as in the Cosmic Microwave Background. Up until the epoch of recombination, the

waves had time to travel about 150 comoving Mpc across space; this distance is the length of the BAO cosmic ruler.

This physical scale will have different apparent sizes on the sky at different redshifts in different cosmologies.


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Assuming that galaxies trace matter well, the BAO signature is measured through the spatial correlation (or power

spectrum) of galaxies.

The BAO cosmic ruler can be mapped out in both the transverse and the radial direction. Comparing the apparent size of

this scale in each direction across different epochs (e.g., compared to the signal at the epoch of recombination) leads to

measurements ofdA(z) and H(z) respectively27, 28, 29

. G(z) can also be measured, since the signature of BAO is sensitive

to the growth of cosmic large scale structure.

During the JEDI Wide campaign more than 10 million color-selected H! emission line galaxies will be targeted

spectroscopically30

over a chosen 1000 deg2, through color-selections calibrated from ground-based surveys

31. The JEDI

BAO survey is designed for both transverse and radial measurements in a large set of independent redshift shells to z~

2. The high end of this redshift range is open to deep spectroscopic surveys only through low-background infrared

coverage, a distinct advantage of space over ground-based surveys. The peak in cosmic star formation is at z~ 1 to 2,

guaranteeing detectable emission lines in galaxies analogous to the star-forming galaxies detected in the deepest near-

infrared ground-based surveys (e.g., in the Great Observatories Origins Deep Survey, which detects star-forming

galaxies at ~60 arcmin2 to HAB = 25.5 and z~ 1.531). The JEDI mission will measure dA(z), H(z), and G(z) as free

functions of cosmic time for 0.5 < z< 2. The redshift range of 1.5 < z< 2 is expected to provide the highest sensitivity

for detecting the effects of Dark Energy32.

2.5 The Need For A Space-Based PlatformTo fully exploit each of these methods to the levels of the known astrophysical limits, the JEDI mission will need to

achieve extremely deep and uniform imaging and spectroscopy at infrared wavelengths not accessible from the ground.These requirements, combined with the need for a small and extremely stable point-spread function (PSF) drives the

need for a space-based platform. The image quality is required to support the point-source sensitivity for the JEDI SNe

survey as well as for morphological studies required by the JEDI WL survey.

Accessibility of near and mid-infrared observations made possible by a space mission provides key information from a

redshift range that is not fully accessible from the ground. In addition, the JEDI mission exploits the minimum in the

zodiacal light between 3 and 4 m to maximize survey sensitivity.

The main hurdle to accurate measurements of Dark Energy is not statistics, but the control of systematic effects. A

space-based mission reduces the systematic uncertainties in each of the three JEDI methods (SNe, WL, and BAO) for

probing Dark Energy: (1) Near and mid-infrared sensitivity allows observations of thousands of SNe Ia atz> 1 required

for quantifying systematic effects; (2) Stable and small PSF yields accurate shape measurements for WL to minimize

systematic effects; (3) Low background observations enable efficient harvesting of millions of galaxy redshifts in the

contiguous redshift range 0.5 < z < 2 (bridging the sparse optical redshift region). This provides continuousmeasurements ofH(z) and G(z) using the same galaxy population, simplifying the modeling of BAO systematic effects.

Robust and precise constraints can be placed on Dark Energy only with a firm control of systematic effects. JEDI

provides an effective way to achieve this control: by combining three observational methods, each with minimized

systematic errors made possible by the space platform.

3. MISSION DESIGN3.1 Mission RequirementsConfiguration

At the core of the JEDI mission concept is a wide field-of-view telescope that is capable of performing simultaneous

imaging and multi-object spectroscopy at near and mid-infrared wavelengths. The spacecraft design is simple and

compact and borrows heavily from Spitzer: the bus structure is wrapped around the instrument assembly, completelypassive thermal control is provided by a fixed sunshade and passive radiators, solar arrays are fixed to the sunshade and

the high gain antenna is fixed to the base of the spacecraft. This enables the JEDI observatory to be exceptionally stable,

both thermally and structurally, providing the required PSF performance.

Orbit

The JEDI mission takes advantage of an Earth-trailing orbit to provide a thermal environment that supports passive

cooling to mid-cryogenic temperatures while providing the thermal stability to ensure a stable PSF. Using an orbit very


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Type Ia Supernovae Weak Lensing

Science

Requirements

1% distance measurement for

0 < z < 2 with 4,000 Type Ia SNe

< 1% shear measurement of

galaxy shapes over 1000 square

degrees to HAB ~26 (zmedian = 1)

Wavelength (Spectroscopy) 1.0 to 2.0 m N/A

Wavelength (imaging) 0.8 to 4.2 m (z, j, h, k, l) 0.8 to 3.0 m (z, j, h, k)

Resolution !/"!~ 500 > 0.8 Strehl

Signal to Noise > 10 > 20

Multiplexing 50 spectra over 0.14 ()2

N/A

Imaging Detectors 20 2048x2048 HgCdTe 16 2048x2048 HgCdTe

Imaging Temperature < 80 K < 80 K

Spectroscopy Detectors 4 2048 x2048 HgCdTe N/A

Spectroscopy Temperature < 90 K N/AAperture Size 1.5 m 1.5 m

Field-of-View 0.14 ()2 0.14 ()2

Pointing Accuracy < 0.02 arcsec

Pointing Stability < 0.05 arcsec / 100 sec

Pointing Knowledge < 1 arcsec

Does Dark Energy Modify Gravity?Is Dark Energy a Cosmological Constant?

Determine the Nature of the Mysterious Dark Energy

Pulling the Universe Apart

5000 galaxy spectra over 0.14 ()2> 5

!/"! ~ 500

Baryonic Oscillation

1.0 to 2.0 m

Redshifts from 0.5 < z< 2

z /(1+z)=0.001 of 10 million H

galaxies over 1000 square degrees

Instrument

Requ

irements

Pointing

< 1 arcsec

1.5 m

0.14 ()2

< 0.02 arcsec

Parameter

Mesurement

Requirements

16 2048x2048 HgCdTe

< 80 K

4 2048 x2048 HgCdTe

< 90 K

0.8 to 3.0 m (z, j, h, k)

similar to Spitzer to provide thermal stability and a clear view of the North Ecliptic Pole, the spacecraft will use a direct-

ascent injection launch into an Earth-trailing orbit in the ecliptic. The spacecraft performs no trajectory correction

maneuvers and the resulting Earth-JEDI range will be ~0.3 AU after the 3-year mission duration. Unlike a low-Earth

orbit or geosynchronous orbit, the Earth-trailing orbit provides a thermal environment that allows passive cooling of the

telescope to the level necessary to enable observations in the near and mid-infrared wavelengths (0.8 to 4.2 m).

Requirements Summary

The JEDI mission design follows directly from the scientific needs discussed in Section 2. The key driving requirements

are summarized in Table 3. Note that these requirements are categorized by the scientific survey needs in order to

understand which observations drive certain performance areas.

These are the baseline JEDI performance requirements. In the course of refining the JEDI mission requirements, we

have started examining the cost vs. science capability trade space. It appears that the incremental cost of doubling, or

even tripling the survey areas may be quite modest. The result is a scaleable mission design that can readily be tailored

for a cost-capped mission implementation.

Table 3. JEDI Mission Key

Scientific Requirements.

3.2 Hardware ComponentsSpacecraft and Telescope

The efficiency of the JEDI mission results from a scientific payload that is capable of performing simultaneous imaging

and programmable multi-slit spectroscopy in the near infrared. A single wide-field Cassegrain is coupled to two

integrated but functionally distinct channels: one for imaging and a second for simultaneous spectroscopy. The optical

design is almost entirely reflective in order to minimize mass, fabrication costs, and chromatic aberration while

improving throughput. There are well-defined structural, electrical and thermal interfaces between the scientific payload

and the spacecraft bus that allow stream-lined assembly flow and minimize program technical, schedule and cost risk.

The JEDI spacecraft will leverage existing hardware and designs from Spitzer, Mars Reconnaissance Orbiter (MRO),

IKONOS, and NextView.

A proof-of-concept point design for the JEDI payload (telescope and instruments) has been completed, demonstrating

that the required imaging and spectrographic performance can be obtained in a compact, packageable design. The total


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instrument and telescope package has a mass of ~ 750 kg, requires 100 W (for thermal control and electronics), and is

passively cooled to 100 K.

Block Diagram

A conceptual design of the JEDI instrumentation is illustrated in the scientific data-flow block diagram in Figure 2. The

imaging and spectroscopic channels share a common telescope assembly, but are otherwise distinct. They observe

adjacent but separate fields-of-view that are designed to execute the survey strategies described in Section 3.3.Instrument Overview

The JEDI instrument consists of a 0.8 to 4.2 m wide-field imager and a 1 to 2 m programmable slit multi-object

spectrograph. Both the imaging and spectrographic focal planes use the RSC molecular-beam epitaxy HAWAII-2RG

2048 x 2048 pixel Mercury-Cadmium-Telluride (HgCdTe) detectors33. These detectors are bandgap-engineered to

provide tailored long-wavelength response to accommodate the JEDI spectral bands, balancing between the number of

cutoff wavelength variants in the focal plane and the dark current in the shorter wavelength bands. Our point design

relies on the 2.5 m and 5.0 m technologies developed for JWST that have demonstrated performance that meets JEDI

requirements. These detectors have an extensive experience base and production history that is unrivalled by shorter or

longer wavelength variants of this technology, resulting in reduced mission risk and cost.

The HAWAII-2RG detectors are read and commanded by the RSC SIDECAR (System for Image, Digitizing,

Enhancing, Controlling and Retrieving) Application Specific Integrated Circuits (ASIC)34. These ASICs are integrated

into the cold focal plane subsystem and control all aspects of the detectors including the 16-bit analog-to-digitalconversion. These ASICs are also capable of performing co-addition of frames, measuring and identifying guide star

data, and interacting directly with the payload digital processing unit further simplifying the cold focal plane electronics.

Imaging Channel

The imaging channel provides diffraction-limited performance over a 0.14 deg2

field-of-view between 0.8 and 4.2 m.

The imaging focal plane has 20 HAWAII-2RG detectors, arranged in a compact 4 column by 5 row arrangement. Each

row of detectors will have optical bandpass filters integrated into the shadow mask to create the spectral bands of

interest.

The imaging channel also performs fine guidance using the positions of known stars to stabilize the telescope as well as

controlling the fine-pointing motions such as dithering. This ability of the imaging focal plane to simultaneously

perform scientific integrations and provide high-speed fine guidance contributes to survey efficiency because of reduced

telescope settling times. The implementation of the focal plane with RSC HAWAII-2RG-type multiplexers and the

SIDECAR ASIC controller provides a low-risk path for achieving these goals since they are being developed for theJWST Fine Guidance Sensor for a similar purpose.

Spectroscopic Channel

The spectrographic channel provides multi-slit spectroscopy over a 0.2 deg2 field-of-view between 1 and 2 m. The

telescope assembly delivers a focused flat-field image to a microshutter assembly that provides selectable slits for the

spectrograph. In order to minimize cost and risk, the JEDI design directly reuses the JWST/NIRSpec-developed

microshutter assemblies without modification. The image is then relayed through a small internal three-mirror

anastigmat telescope that is used in double pass to first deliver collimated light to a dispersing prism and then deliver

focused spectra to the spectrograph focal plane. Mechanical complexity is greatly simplified by using the anastigmat

telescope in double pass, reducing fabrication, assembly and test costs. The spectroscopic focal plane uses four

HAWAII-2RG detectors.

Thermal Design

The thermal environment of the optical assembly will be cold biased so that positive thermal control can be maintained

through the use of trim heaters. The telescope assembly will be designed to operate at ~100 K, the imaging focal plane at

~70 K, and the spectroscopy focal plane at ~80 K to minimize dark current from the detectors. A passive radiator

mounted on the cold, anti-Sun, side of spacecraft will provide the thermal sink required to maintain all the focal planes at


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Figure 2. Main scientific data paths for JEDI and key implementation features. The main body of the figure shows the data

paths from the telescope through the ground processing. See Section 3.2 for further discussion. (a) shows the flight

segment mounted in the fairing of a Delta-IV 4-m configuration. (b) shows the fields-of-view of the imaging andspectroscopic channels projected onto the sky. (c) shows a preliminary optical point design that demonstrates that the

desired functions are packageable. (d) shows an exploded view of the JWST/NIRSpec microshutter array. This exact

hardware is baselined for JEDI. Practical packaging constraints for this hardware cause the small horizontal gap

between the two spectroscopic fields-of-view in panel (b). (e) shows a mechanical mockup of a 5x7 focal plane array

built by RSC to demonstrate fabrication and alignment processes. (f) shows a single hybrid detector based on the

HAWAII-2RG design that is being produced for three instruments on JWST.


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the required operating temperatures. With an orbit very similar to that proposed here, Spitzerhas demonstrated outer

shell temperatures of less than 40 K, confirming that our passive cooling requirements for the detectors can be met.

3.3 Survey Strategy

The three-year JEDI mission will include two campaigns. The JEDI Deep will continuously monitor a 4 deg2 field at the

North Ecliptic Pole (NEP) during the first year with a cadence of 7 days. The JEDI Wide over 1000 deg2

is executed

over years 2 and 3. Both campaigns feature simultaneous imaging and targeted spectroscopy. The imaging is split into2 or 4 exposures, with dithers between them (achieved through a spacecraft motion) that are much smaller than the

length of the microshutter slits. Then the spacecraft shifts to the next imaging tile position, while the microshutters are

programmed to stay with their original targets, and so on, until the full allotted exposure time is completed. The key

survey parameters are provided in Table 4.

Table 4. JEDI Key Survey Parameters.

Year CampaignArea

in(deg)

2

Imagingexp. timeper obs.

Galaxyimagingdepth atS/N~20

Imagingdata / day

Spectro.exp.time

Spectro.data/day

Sciencedownlink

rate

1 JEDI Deep 4 4 x 10 min(5 bands)

HAB =28.1 20 GB 400min 99 MB 12 Mbps

2 + 3 JEDI Wide 103

2 x 500s

(4 bands)

HAB =25.5 20 GB 10200s 234 MB 12 Mbps

As shown in Figure 2, the imaging and spectroscopic fields-of-view are offset in a specific way. As a scan strip is

imaged by contiguous, slightly-overlapping steps, the strip adjacent is visible to the spectrograph, as illustrated in

Figure 3 (left) for the JEDI Deep. At the completion of each strip, the spacecraft shifts by one imaging field-of-view so

that spectroscopy is possible on the just-completed imaging strip. With this sequence, the very first strip does not have

targeted spectroscopy, and the very last strip will have imaging only. A similar arrangement is planned for the 24-month

JEDI Wide campaign shown in Figure 3 (right). The NEP field will be segmented into separately-completed quadrants

for optimal coverage. In both campaigns, the spacecraft rolls 90 every three months so that the cold side of the

telescope is maintained in a nearly anti-Sun attitude.

Figure 3. JEDI Deep (left) and Wide (right) survey strategies. See Section 3.3 for explanation.


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The critical element of the JEDI observing strategy is the need to determine and communicate to the spacecraft the

coordinates of the spectroscopic targets reliably, and to have failsafe backup target selections. The intent is to process

and analyze the down-linked imaging data in ~ 5 days, in time to identify supernovae and all galaxies (in JEDI Deep),

and color-selected emission-line galaxies (in JEDI Wide), and to uplink the spectroscopic target information before the

relevant footprint is viewed by the spectrograph. This strategy requires fast image processing and source detection, with

automated and pre-tested validation procedures.

3.4 Data RateThe JEDI mission fully uses the Deep Space Network (DSN) 12 Mbps Ka band capability developed for MRO. After

lossless compression onboard, the data produced by the detector arrays are collected on a 512 Gbit solid-state recorder

and downlinked daily for two hours. The recorder can accommodate up to two days of data in the event of a missed DSN

pass.

The JEDI data volume is large, but within current capabilities to transfer, process, archive, and analyze. Uncompressed,

a single imaging exposure generates 168 MB of data from all 20 arrays, while the four spectroscopic detectors generate

33.6 MB per exposure. At our imaging rate, the downlink data volume is approximately 20 GB/day (see Table 4).

Assuming a factor-of-two compression onboard, this is less than the capacity of the daily two-hour downlink at 12Mbps. This is within current DSN capabilities in the Ka band.

The efficiency of the JEDI observing strategy also makes it a data-rate limited mission. The baseline configurationassumes only the currently available data rates. As these improve, JEDI is well positioned to take advantage of the new

capabilities to increase scientific return.

4. COSTThe baseline JEDI mission concept described here has been found to be feasible within the current JDEM cost cap of

$600M (FY06). This is achieved through extensive use of heritage designs, as well as leveraging technologies currently

being developed for other missions, thus avoiding some significant non-recurring engineering costs.

The JEDI mission concept is scaleable to increase the survey areas and corresponding scientific return. We have preliminarily examined modifications such as 1) increasing imaging focal plane detector area, 2) increasing

spectrographic sky coverage area, 3) increased data downlink and processing rates, and 4) increased mission lifetime.

The initial conclusion is that the survey areas can be increased up to a factor of 10 for costs on the order of a few $100M.

5. THE JEDI TEAMThe JEDI Mission is led by Yun Wang of the University of Oklahoma. The science team members with special

leadership responsibilities include: Edward Cheng (Implementation and Technical Lead), Arlin Crotts, Tom Roellig, Ned Wright (Interdisciplinary Scientists), Ian DellAntonio (WL Lead), Peter Garnavich (SNe Ia Lead), LeonidasMoustakas (BAO Lead), Jason Surace (Calibration Lead), Gordon Squires (Data Processing Lead). Science teammembers include: Edward Baron, David Branch, Stefano Casertano, Salman Habib, Katrin Heitmann, Alexander

Kutyrev, Mark Phillips, Judith Pipher, Robert Silverberg, Volker Springel, Craig Wheeler, William Forrest, Thomas

Hale, John MacKenty, Craig McMurtry, Casey Papovich, William Priedhorsky, and Max Tegmark.

JEDI benefits from an outstanding technical and management team that has helped to create the current implementation

concept: Luke Dubord (JPL Mission System Engineer), Robert Egerman (ITT Lead), Helmuth Drosdat (Lockheed

Martin Lead), John Miles (LM System Engineer), and David Gulbransen (Rockwell Scientific Lead).

6. CONCLUSIONSA preliminary concept has been developed by the JEDI Team for an implementation of the JDEM mission that providesa robust set of measurements to understand the nature of Dark Energy. The JEDI mission design uses three independent

observational methods to derive the Dark Energy parameters to reduce the risk of contaminating the results with

systematic errors. The mission implementation takes full advantage of technologies already developed or in the process

of being developed for other missions to minimize mission risk. An extremely capable mission that meets the JEDI

baseline requirements is feasible under the current JDEM mission cost cap of $600M (FY06). The JEDI mission design

is also scaleable and can increase survey areas by factors of up to 10 for a modest increase in cost.


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Documents

Edward Cheng et al- Illuminating Dark Energy with the Joint Efficient Dark-energy Investigation (JEDI)