Effective Stray Light Correction for EUV Images

1. Effective Stray Light Correction for EUV Images Paul Shearer with Richard A. Frazin, Anna C. Gilbert, and Alfred O. Hero III

2. 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, off-limb are all dominated by stray light in EUVI

3. The Solution: Blind Deconvolution u1 u2 • Blind deconvolution (BDC) • Given observations f, find true-emission images u and PSF h. • BDC appears underdetermined, but can be solved if u and h are constrained enough. • Challenge: find model that is solvable + contains true PSF/images h f1 f2

4. Previous Work (under)estimating stray light by deconvolution • current EUVI PSFs: from SolarSoft’seuvi_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 171 Å obs deconv est. stray light %

5. 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

6. BDC on Synthetic Data • 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 • 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! Results recovered image error is at worst 5%, comparable to off-limb SNR; negligible on disk. PSF well recovered without parametric formula.

7. BDC on EUVI-B 171 Å estimated stray light % log10f log10u log10 PSF

8. BDC: Effects on Features • some faint features become up to 70% dimmer • up to double stray light removal of euvi_psf.pro observed deconvolved euvi_psf.prodeconv

9. 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

10. 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?

11. Applications • differential emission measure studies • coronal loop widths, background subtraction • ???