the role of interferometers in future space astrophysics missions ron allen space telescope science...
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The Role of Interferometers in Future Space Astrophysics
Missions
Ron AllenSpace Telescope Science Institute
Synthesis Imaging Scientist
Space Interferometry Mission
Astrophysics 2020:
Large Space Missions Beyond the Next Decade
13 November 2007 Ron Allen - Astrophysics 2020 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.
13 November 2007 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.
13 November 2007 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
13 November 2007 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.
13 November 2007 Ron Allen - Astrophysics 2020 6
Example from HST/ACS …
J. Krist 2004, SPIE 5487, 1284
13 November 2007 Ron Allen - Astrophysics 2020 8
Array PSFs can look awful …
Machalek, Waghorn, & Allen in prep.
13 November 2007 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.
13 November 2007 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
13 November 2007 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.
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