The Role of Interferometers in Future Space Astrophysics Missions

1. Astrophysics 2020: Large Space Missions Beyond the Next Decade The Role of Interferometers in Future Space Astrophysics Missions Ron Allen Space Telescope Science Institute Synthesis Imaging Scientist Space Interferometry Mission

2. Why interferometers…I • Some recognized advantages: • Precision astrometry (SIM): • precision  Baseline length x Collector diameter • Ultra-high resolution imaging of bright targets: • Only way to get the required resolution. • Allows decoupling of the resolution and S/N • Simple, bright targets using a few baselines (VLBI, SIM) and limited observing time. • More complex targets using many baselines (VLA, SI) and/or lots of observing time. Ron Allen - Astrophysics 2020

3. Why interferometers… II • Some less-well-recognized advantages to interferometric/synthetic imaging with constellations of collectors: • Superior control of the PSF; • Suppression of “scattered light” halo around the PSF; and, • Opportunities to increase mission robustness. Ron Allen - Astrophysics 2020

4. Superior control of the PSF … • PSF is defined in a computer only at the moment when the image is calculated: • Can make use of various redundancies (e.g. closure phase) in the data coming from the constellation’s interferometers. • Allows resolution to be improved, even incrementally. • Offers opportunity for improvements in image quality with better computer algorithms. • Computer power required is modest. • Your laptop may be enough Ron Allen - Astrophysics 2020

5. Suppressing the PSF “Halo” … • Residual polishing errors on filled aperture surfaces result in a “halo” of scattered light around bright stars. This halo: • increases the “inner working angle” for coronagraphy, and; • reduces the dynamic range for photometry in the immediate vicinity of bright stars. Ron Allen - Astrophysics 2020

6. Example from HST/ACS … J. Krist 2004, SPIE 5487, 1284 Ron Allen - Astrophysics 2020

7. Ron Allen - Astrophysics 2020

8. Array PSFs can look awful … Machalek, Waghorn, & Allen in prep. Ron Allen - Astrophysics 2020

9. But still be clean underneath … Ron Allen - Astrophysics 2020

10. Why does this happen ? • This is a different kind of improvement, and not simply a result of using a computer to image. • It happens as long as the dominant sources of error in fringe amplitude and phase at different U,V sample points are independent of each other. • It’s as if the equivalent aperture had a purely random polishing error at every scale size included in the image. • This will be the case for many sources of instability in each interferometer (e.g. delay line jitter, etc.). • It is not true for station-keeping errors, but these are expected to be relatively slow and may be eliminated by using closure phase imaging. Ron Allen - Astrophysics 2020

11. Comparison with HST/ACS … Ron Allen - Astrophysics 2020

12. SIM photon and phase noise … Ron Allen - Astrophysics 2020

13. Mission robustness … • Reduce cost and single points of failure • Most elements can be identical • Loss of elements not crippling for science • Permit upgrades to array over time • Array can continue to operate • Upgrades incremental with improved elements • Hitch-hike on freight missions • Small elements can be “stowaways” • Carried on supply missions to lunar base Ron Allen - Astrophysics 2020

14. Summary • The physics of image formation and restoration using interferometers is well understood and applicable to UV/OIR space interferometers. The required computing power is modest. • Constellations of collectors as imagers decouple resolution from sensitivity, permit better PSF control, and offer new flexibility in mission design, construction, and repair. • Some of the outstanding technical challenges presently include space demonstrations of feasibility, station-keeping, beam combination, and wide-field imaging. Ron Allen - Astrophysics 2020