Effective Stray Light Correction for EUV Images

Report
Effective Stray Light Correction
for EUV Images
Paul Shearer
with Richard A. Frazin, Anna C.
Gilbert, and Alfred O. Hero III
The Stray Light Problem
• Stray light (for this talk):
the long range scattering
component of the PSF
• Stray light makes bright
features dimmer, dim
features MUCH brighter,
hindering quantitative
science
• coronal holes,
prominence cavities, offlimb are all dominated by
stray light in EUVI
The Solution: Blind Deconvolution
u1
u2
h
Blind deconvolution (BDC)
Given observations f, find trueemission images u and PSF h.
• BDC appears underdetermined,
but can be solved if u and h are
constrained enough.
f1
f2
• Challenge: find model that is
solvable + contains true PSF/images
Previous Work
est. stray
light %
deconv
obs
171 Å
(under)estimating stray light by deconvolution
• current EUVI PSFs: from
SolarSoft’s euvi_psf.pro utility
• suggests large corrections to
faint regions
• doesn’t remove all stray light
(evidence – Moon transit)
• Simple parametric PSF model
cannot account for complex
mirror scatter
Our BDC Model
Image model
each image ui must be:
• positive
• zero on any known transit object
• zero in vignetted corners (?)
PSF model
h must be:
• positive
• normalized to 1
• roughly monotonic
inward gradient model
BDC on Synthetic Data
• Problem: such an unusually flexible
model may overfit and/or be
impossible to solve in practical
problems.
• Question: can it work on realistic
images? Our experiments say yes!
Synthetic Data Experiment
• u = series of EUVI obs. with Moon
and off-limb set to 0
• h = power-law decaying PSF, roughly
approximates 171 Å mirror scattering
• f = convolve u and h, add poisson
noise
Results
recovered image
error is at worst 5%,
comparable to
off-limb SNR;
negligible on disk.
PSF well recovered
without parametric
formula.
BDC on EUVI-B 171 Å
log10 f
log10 u
estimated
stray light %
log10 PSF
BDC: Effects on Features
• some faint features become up to 70% dimmer
• up to double stray light removal of euvi_psf.pro
o
b
s
e
r
deconvolved
v
e
d
euvi_psf.pro deconv
Conclusions and Ongoing Work
• Our PSF removes almost all stray light from EUVI-B 171
time series
– except far off limb – must fix boundary conditions
• Next steps:
– refine PSF wings by improving boundary conditions
(how?); work on full images
– estimate cores for high res work (e.g. loops)
– estimate uncertainty with cross-validation and
independent modeling approaches
– repeat for EUVI A/B, all 4 bands
– apply to AIA, which has much better transits
– all PSFs will be publically available for correction use
Questions
• Are any model assumptions incorrect?
• Are there any additional constraints we could
incorporate on images or PSF? is there any
help from experimental data?
Applications
• differential emission measure studies
• coronal loop widths, background subtraction
• ???

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