# Presentation: 246641

## 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 as
- rs~ 2M8/D  -    		6M8/D
- Time Resolution ~ 1 hour
- 2Rs/c~10 hours
- Bandpass: 0.1-10  keV
- K-lines from Carbon to Iron
- E/50
- ASCA, Chandra, and XMM obs
-  Area >1000 cm2
- ~10Photons/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 Here

- Baseline

## 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? | Metrics | Will 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 + msin(g)

- 2(w+gap)+msin(g)
- By
- 2(w+gap)+msin(g)+m/3+actuator+encoder
- ASSUME:
-  w+gap~5 cm
- Encoder+encoder~5cm
- Sin(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 both
- Reference 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.