### Effective Stray Light Correction for EUV Images

```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
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
• ???
```