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Solar Coronagraphs Observe off-disk coronal emissions from Sun. Dominant noise source: Diffraction of on-disk light around the occulter Vignetting on externally occulted coronagraphs Noise inversely proportional to the occulter/detector distance Better signal to noise Closer to the corona 2
Near-future state of Coronagraphs External Occulter on extended boom Deployable boom extends occulter out to a several meters Mechanical and thermal issues in alignment Formation Flying: One spacecraft serves as occulter Other spacecraft serves as detector E.g., ESA Proba-3 Top Challenges: Alignment of two spacecraft with target (i.e, formation flying Technical Programmatic technology) Perceived risk (mass, volume, cost) associated with flying two spacecraft in formation ESA Proba-3 3
Demonstration on CubeSats Purpose: Low-cost, low-risk Demonstrate novel GN&C techniques Science on CubeSats Development Path Tech Demo (VTDM, CANYVAL-X) Science Demo (TBD) Full Scale Mission (TBD) Virtual Telescope Demonstration Mission (VTDM) CubeSat Astronomy by NASA and Yonsei using Virtual Telescope Alignment Experiment (CANYVAL-X) 4
Mission Operations Considerations Relative disturbances high in Low Earth Orbit Drag, Gravity gradient dominate Complex proximity operations and station-keeping maneuvers High fuel consumption Minimal science gathering Lagrange or Drift-away orbit Eliminates issues with LEO Relative dynamics become nearly linear Power an issue: one spacecraft in shadow Deep space communications CubSats in deep space? Ideal for Science Mission The Virtual Telescope Demonstration Mission (VTDM) redezvous and proximity operations phase in LEO (Shah, N., et.al.) 5
Science Demo Mission Goals (1) Nominal FOV Image with occulter blocking Sun Image shifted in FOV (Coronagraph orientation change) Occulter shape change (Occulter SC orientation change) Occulter shifted off of Sun (relative transverse position change) Reduced image capture / Increased occulter size (decreased relative range) (converse is increased image capture due to increased range) 6
Science Demo Mission Goals (2) Observe dynamics of solar phenomena extending away from the sun. Reduce background image noise distortions Collect enough photons from 5 to 20 solar radii Keep Coronagraph camera within shadow of occulter Roughly Translates to: Image corona in visible light from 5 to 20 Solar Radii 1 arcminute pointing towards the Sun center 5 second exposure time 7
Formation Alignment Requirements Absolute Attitude Knowledge Absolute Attitude Control Transverse Relative Position Knowledge Transverse Relative Position Control Coronagrpah Spacecraft (CSC) Occulter Spacecraft (OSC) 0.5 degrees 1 arcminute ~degree ~arcminute +- 1.6 arcmin @15 m = +- 0.6 cm (1/10 of solar radii) un control available Occulter Spacecraft (OSC) Coronagraph Spacecraft (CSC) 8
How Do We Measure the Formation Axis? Solar Formation Alignment Camera (S-FAC) Single instrument to measure relative position wrt formation axis (i.e., inertial line of sight) Coronagraph Spacecraft Attitude Formation Axis Relative Transverse Position S-FAC Focal Plane S-FAC Algorithm objective: - Move dot (CSC) towards x (target) - Move x to center of focal plane - Difference in dot and x represent transverse position errors off formation axis 9
Conclusions Dominant noise source in traditional coronagraphs inversely proportional to distance between occulter and detector Formation flying of two spacecraft offers greatest opportunity to reduce these errors Formation flying complications in LEO: disturbances result in minimized science observation Lagrange or Drift away orbit ideal Various demo on CubeSats to increase TRL S-FAC Distributed Coronagraph Reduce risk for full scale mission 10