Effective Stray Light Correction for EUV Images

Effective Stray Light Correction for EUV ImagesPaul Shearerwith 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, off-limb are all dominated by stray light in EUVI

For this talk, we will refer to the long-range scattering component of the PSF as stray light. Stray light is caused primarily by mirror microroughness and entrance aperture diffraction. Coronal holes, prominence cavities, and the off-limb are dominated by stray light in EUVI, which is the instrument we focused on in this work. AIA is likely to be similar and we will address it in the future.2The Solution: Blind Deconvolution

u1h

f1u2

f2Blind 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

To remove stray light, we assume that each observed image is the convolution of a unknown point spread function with an unknown true image. In EUV instruments, the point spread function does not change over time, so in each band, many images are blurred by a common PSF. The problem of determining the PSF and true-emission images is known as blind deconvolution. Blind deconvolution is typically an underdetermined problem; there are many ways to decompose the observation into a convolution of two components. However, if you have enough additional constraints on the PSF (h) and true-emission images (u), you can solve this problem. The major challenge is to mathematically articulate the conditions in a solvable model. 3Previous Work (under)estimating stray light by deconvolution

est. straylight %deconvobs 171 current EUVI PSFs: from SolarSofts euvi_psf.pro utility

suggests large corrections to faint regions

doesnt remove all stray light (evidence Moon transit)

Simple parametric PSF model cannot account for complex mirror scatter

The only publically available PSFs for EUVI come from SolarSofts euvi_psf.pro utility. They suggest that faint regions have a true emission much less than their apparent emission up to 40% less in the prominence cavity on the left. However, we know that the true emission must be still less than it appears from this deconvolution. The reason is that we know the Moons true emissions are zero, so deconvolution with a correct PSF should remove all the Moons apparent emissions. In fact it removes less than half.

When the SolarSoft PSF is used to deconvolve Moon transit images, the deconvolved images are not consistent with our background knowledge that the Moons true4Our BDC Model

PSF model

h must be: positivenormalized to 1roughly monotonic

Image model

each image ui must be:positivezero on any known transit objectzero in vignetted corners (?)

inward gradient modelOur blind deconvolution model imposes some simple assumptions on the image and PSF. Of course, when they are convolved together, they must agree with observation. For the image, it must be positive, and zero on the support of any transit object. We also set the image to be zero in the vignetted corners, which may not be quite right, but it is a simple and fairly well-supported way to handle the lack of accurate counts in the vignetted regions. The PSF must be positive everywhere, normalized to 1, and roughly monotonic. The rough monotonic condition is mathematically articulated by requiring that the gradient of the PSF function point towards the origin, within a tolerance of 45 degrees.5BDC 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 0h = power-law decaying PSF, roughly approximates 171 mirror scatteringf = convolve u and h, add poisson noise

Resultsrecovered imageerror is at worst 5%,comparable tooff-limb SNR; negligible on disk.PSF well recoveredwithout parametricformula.6BDC on EUVI-B 171

log10 PSFlog10 flog10 uestimatedstray light %BDC: Effects on Features

observeddeconvolvedsome faint features become up to 70% dimmerup to double stray light removal of euvi_psf.pro

euvi_psf.pro deconvConclusions and Ongoing WorkOur PSF removes almost all stray light from EUVI-B 171 time series except far off limb must fix boundary conditionsNext steps:refine PSF wings by improving boundary conditions (how?); work on full imagesestimate cores for high res work (e.g. loops)estimate uncertainty with cross-validation and independent modeling approachesrepeat for EUVI A/B, all 4 bandsapply to AIA, which has much better transitsall PSFs will be publically available for correction useQuestionsAre any model assumptions incorrect?Are there any additional constraints we could incorporate on images or PSF? is there any help from experimental data?

Applicationsdifferential emission measure studiescoronal loop widths, background subtraction???