An X-ray Interferometry Technology Roadmap

Keith Gendreau

NASA/GSFC

Webster Cash

U. Colorado

MAXIM Requirements Flowdown

SEU Science Objective

MAXIM Approach

Measurement Requirement

Key Technologies

Understand the ultimate endpoint of matter.

To explore the ultimate limits of gravity and energy in the universe.

Make a “movie” of a black hole, its accretion disk, and its jets.

Map doppler and gravitational redshifts of important lines in the vicinity of a black hole.

•Angular Resolution: 0.3 asrs~ 2M8/D -

6M8/D•Time Resolution ~ 1 hour

2Rs/c~10 hours•Bandpass: 0.1-10 keV

K-lines from Carbon to Iron

•E/50ASCA, Chandra, and XMM obs

• Area >1000 cm2

~10Photons/frame(~10 Photons/pixel/frame)

•Diffraction limited optics • >Flat• long and skinny

• Thermal /Mechanical Stability

•CTE ~< 10-7/K• Precision Formation Flying• X-ray CCDs

• Larger Arrays of ~< 10 micron pixels• Fast Readout (msec)

•~0.1 as Line-of-Sight alignment knowledge.

•100,000 finer than HST

Basic MAXIM Design

•Each Channel Consists of 2 flats•Primary mirrors determine baseline•Secondary mirrors combine channels at detector.

To implement this basic design, you choose how to group the mirrors.

Fringes Form HereBaseline

Original MAXIM Implementations

MAXIM Pathfinder

Full MAXIM- the black hole imager

•“Easy” Formation Flying

•Optics in 1 s/c act like a thin lens

•Nanometer formation flying

•Primaries must point to milliarcseconds

Improved MAXIM Implementation

Group and package Primary and Secondary Mirrors as “Periscope” Pairs

•“Easy” Formation Flying (microns)

•All s/c act like thin lenses- Higher Robustness

•Possibility to introduce phase control within one space craft- an x-ray delay line- More Flexibility

•Possibility for more optimal UV-Plane coverage- Less dependence on Detector Energy Resolution

•Each Module, self contained- Lower Risk.

An Alternate MAXIM Approach: Normal incidence, multilayer

coated, aspheric mirrors

• Optics demonstrated today with 1-2 Angstrom figure

• Multilayer Coatings yield narrow bandpass images in the 19-34 Angstrom range

• Could be useful as elements of the prime interferometer or for alignment

• Offer focusing and magnification to design• May require tighter individual element alignments

and stiffer structures.

Technologies: Status, Metrics, Mutual Needs

Technology

Has there been a demo?

MetricsWill we test on ground?

Current Status Vs. Requirement

Is it a big leap?

Related Missions

Mirrors Yes >/100 Yes Need longer, skinnier to maximize area/mass

No SI, TPF*, NGST, SIM

Thermal control

Yes Maintain >/100.

TBD from LOS

Yes Mission Specific No SIM*, SI, TPF, NGST, SPECS, LISA*,…

Formation Flying

(a system requiring many components)

Yes, but…

m formation flying control

Partial -Autonomy required.

-Need higher precision

-Metrology exists at component level

-Must work at longer distances

Yes SI*, TPF*, LISA, XEUS, Grace, MMS, Starlight, Darwin,…

Line-Of-Sight

(LOS)

No See Other Slide

Partial See Other Slide Yes All as missions:

SI, iARISE, & TPF, GP-B

Technical Components: Mirror Modules

•Grazing Incidence Mirrors•Grazing Incidence loosens our surface quality and figure requirements by 1/sin

•Flatness > “Simple” shapes like spheres and flats can be made perfect enough

At grazing angles, mirrors that are diffraction limited at UV are also diffraction limited at X-ray wavelengths

•Long and Skinny

•Bundled in Pairs to act as “Thin Lens”

•Thermal/mechanical Stability appropriate to >

What would one of these modules look like?

m

msin(g)

m/6

Gap~msin(g)

Pitch Control

msin(g)

3/2m

+d

m/3

+ m

sin(

g)

2(w+gap)+msin(g)By2(w+gap)+msin(g)+m/3+actuator+encoder

ASSUME: w+gap~5 cmEncoder+encoder~5cmSin(g)~1/30

-->(10cm+m/30)x(15cm+m/3+m/30)-->m=30cm-> 13cmx26cm

Technical Components: Arrays of Optics

• Baselines of > 100 m required for angular resolution.

• Formation flying a must for distance >~20 m.

• Miniaturization of ALL satellite subsystems to ease access to space.

• S/C Control to 10 m- using “periscope” configuration (metrology to better than 1 m).– A system spanning from metrology to propulsion

• Individual optic modules are thin lenses with HUGE fields of view

Technical Components: The detector

• In Silicon, the minimum X-ray event size is ~1 m• Large CCD arrays possible with fast readout of small

regions.• Pixel size determines the focal length of the interferometer

F~s/res

– 10 m pixels -> Focal lengths of 100s to 1000s of km.

• Formation Flying Necessary– Huge Depth of focus loosens longitudinal control (meters)– Large array sizes loosen lateral control (inches).– High angular resolution requirement to resolve a black hole: The

Line-Of-Sight Requirement.

Technical Components: Line-of-Sight

• We must know where this telescope points to 10s-100s of nanoarcseconds– Required for ALL microarcsecond imagers

• The individual components need an ACS system good to only arcseconds (they are thin lenses)

• We only ask for relative stability of the LOS- not absolute astrometry

• This is the largest technical hurdle for MAXIM- particularly as the formation flying tolerance has been increased to microns

dX

d

Using a “Super Startracker” to image reference stars and a laser beacon.

Super Star Tracker Sees bothReference stars and the beacon of the other space craft.It should be able to track relative drift between the reference and the beacon to 0.1 microarcseconds.

o

•Both the optics spacecraft and the detector spacecraft can rotate to arcseconds- they are “thin lenses”

•Imaging problems occur when one of these translates off the line of sight

•We need to KNOW dx/F to 0.1 microarcseconds.

•AND We need to know a reference direction to the same level

•The CONTROL of the Line-of-Sight is driven by the detector size.Beacon

F

Options to Determine Line-Of-Sight

• All options require beacons and beacon trackers to know where one s/c is relative to another.

• OPTION 1: Track on guide stars– Use a good wavelength (radio, optical, x-ray)

– Use a good telescope or an interferometer

• OPTION 2: Use an inertial reference – Use a VERY good gyroscope or accelerometer

– GP-B

Summary of Key Technical Challenges

• The mirrors and their associated thermal control are not a tremendous leap away.

• “Periscope” implementation loosens formation flying tolerance from nm to m. This makes formation flying our second most challenging requirement.

• Determination of the line-of-sight alignment of multiple spacecraft with our target is the most serious challenge- and MAXIM is not alone with this.

Using Stars as a Stable Reference• A diffraction limited telescope will have a PSF ~ /D• If you get N photons, you can centroid a position to

/D / N1/2

• Nearby stars have as and mas structure• Stars “move” so you need VERY accurate Gimbals

– Parallax (stars @500 pc can move up to 40 as in a day)– Aberration of Light (as big as 40 as in a minute)– Stellar orbits, wobble due to planets– Other effects…

An Optical Star Tracker

• A “reasonable” size telescope (<1m diam.) @ optical wavelengths will require 1012 photons to centroid to 0.1 as.

• Practical limits on centroiding (1/1000) will need large F numbers

• Lack of bright stars requires complicated gimbals to find guide stars

• HST would barely squeak by with 15th mag stars

An 100 as X-ray Star Tracker

• A 1 m diffraction limited X-ray telescope (probably an interferometer) would need only 106 photons to centroid to 0.1 as

• A 1000 cm2 telescope would get ~ 100 photons/sec from reasonable targets.– 104 second integration times needed to get

enough photons– This is too big…. And even then, there are not

that many targets

An 10 as X-ray Star Tracker

• A 10 m baseline X-ray interferometer would need only 104 photons to centroid to 0.1 as

• A 1000 cm2 telescope would get ~ 100 photons/sec from reasonable targets.– 100 second integration times– This is too big….possibly…– And even then, there are not that many targets

An Optical Interferometer

• Eg. SIM

• Metrology at picometers demonstrated in lab

• OPD control to nanometers

• Expensive?

Local Inertial References

• Superconducting Gyroscopes– Eg. GP-B Gyros will have drift < 1/3 as /day

• Superconducting Accelerometers– Eg. UMD accelerometer sensitive to 10-15 m/s2

• Kilometric Optical Gyroscope– Eg. Explored for “Starlight”- a BIG laser ring gyroscope

• Atomic Interferometer Gyroscopes– Like a LRG, but with MUCH smaller wavelengths– Laboratory models ~10 as/sec drifts– ESA proposed “Hyper” mission

Superconducting Gyroscopes• Capitalize on GP-B technology

– Of all our options, this one has had a CDR

• Improve with better squids– Readout is white noise limited

• Improve by requiring only hours-days of stability at a time – Make the rotor have a larger moment- easier to read, but less

stable over long times

• Use NGST/ConX Cryocoolers to replace cryogen– Get rid of “Lead bags”– Make lighter

• No need to find stars (no Gimbals)

Superconducting Accelerometers

• 10-15m/s2 sensitivity exist now

• Need integrators

• Need higher sensitivity, unless used with other things

Kilometric Optical Gyroscopes

• A Laser-Ring-Gyroscope with BIG area/perimeter ratio– Resolution ~ /(area/perimeter)– Use area bounded within space between

multiple spacecraft

• Proposed for Starlight- but rejected in the end– “cost” and “technical” reasons

Atomic Interferometer Gyroscopes

• Same principle as a LRG, but use matter waves to make many orders of magnitude smaller

• Benchtop demonstrations in lab are as good as best LRGs (10 as/sec)- but should be much better

• ESA proposed mission “Hyper” based on these to do GR physics.