What is the source of straylight in SST/CRISP data?

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Wendy Lambert
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1 What is the source of straylight in SST/CRISP data? G.B. Scharmer* with Mats Löfdahl, Dan Kiselman, Marco Stangalini Based on: Scharmer et al., A&A 521, A68 (2010) Löfdahl & Scharmer, A&A 537, A80 (2012) + simulations by Marco Stangalini + work in progress (Scharmer, de la Cruz Rodriguez et al.) *Institute for Solar Physics, Stockholm University

much better than scale of granulation Granulation contrast not reduced because of lack

2 Fundamental problem Min. umbra intensity ~15% Granulation contrast only 9% but should be ~17% Spatial resolution much better than scale of granulation Granulation contrast not reduced because of lack of resolution Must be straylight Straylight PSF must be narrow Movie by Henriques Dust gives large-angle scattering => Main suspect: Small-scale aberrations

3 Small-scale aberrations (seeing/telescope) High-order (small-scale) aberrations from seeing not correctable by AO High-order aberrations from seeing not corrected by MOMFBD High-order AO wavefront sensor calibration errors and noise Small-scale polishing errors (from one or many mirrors) not correctable by AO

4 The AO-halo (From Adaptive optics for Astronomical Telescopes by John Hardy) * Core FWHM given by λ/d * Core peak given by Strehl (~0.5 for SST) * Halo FWHM given by λ/d (d<<d), where d ~ actuator pitch

5 The MFBD-halo FOV shown is 4.2x4.2 arcsec Circle outlines 90% of energy * Core FWHM given by λ/d * Core peak given by Strehl * Halo FWHM given by λ/d (d<<d), where d ~ typical scale of KL aberrations not corrected by MFBD (From Scharmer et al. A&A 521, A68, 2010 )

6 The MFBD-halo FOV shown is 4.2x4.2 arcsec, logarithmic intensity scaling Circle outlines 90% of energy. * Core FWHM given by λ/d * Core peak given by Strehl * Halo FWHM given by λ/d (d<<d) (From Scharmer et al. A&A 521, A68, 2010, Fig. )

7 The MFBD-halo MTF squared * Core FWHM given by λ/d * Core peak given by Strehl * Halo FWHM given by λ/d (d<<d) * Accounting for 90% encircled energy when r0=15 cm requires a straylight PSF with at least 1.8 diameter (Figures from Scharmer et al. 2010, A&A 521, A68)

8 Problem? RMS contrast seems to saturate in excellent seeing 26 June 2009 data. Blue: 538 nm, red: 630 nm. 37-electrode AO. Seeing data from wide-field wavefront sensor (WFWFS) Note: Contrasts at 630 nm multiplied with factor 1.22

9 SST optics Straylight target

10 Straylight target setup

11 Straylight target images (logarithmic scaling) From biggest (1 mm) pinhole => conventional straylight

12 Observed MFBD 231 KL s MFBD 36 KL s (FOV: ~ 2 x2 ) Note: Results with previous 37- electrode DM (now replaced by 85- electrode DM)

13 Conclusions from straylight target tests (with old 37-electrode AO) Conventional straylight over ~10 low (~0.3%) Additive straylight ~2% from ghost images Strehl drops to 74-77% from small-scale aberrations Print-through of electrode pattern indicates that AO mirror dominates wavefront errors

New SST AO Electrode and microlens (WFS) layout Microlenses used to measure seeing (from differential image motion) 50 mm monomorph (CILAS) Less print-through than with bimorph mirrors Excellent

14 New SST AO Electrode and microlens (WFS) layout Microlenses used to measure seeing (from differential image motion) 50 mm monomorph (CILAS) Less print-through than with bimorph mirrors Excellent optical quality 85 electrodes, 85 microlenses 24x24 pixel (12 x12 ) cross correlations Pupil diameter (34 mm) 2 khz update rate Real-time seeing monitor (2 sec averages obtained every second using 20 sec reference for calculating variances)

15 26 June 2009 data. Blue: 538 nm, red: 630 nm. Dotted: theoretical but with RMS contrast divided by electrode AO. Seeing data from wide-field wavefront sensor (WFWFS) Note: Contrasts at 630 nm multiplied with factor 1.22

16 Better, but RMS contrast still saturates in excellent seeing 85-electrode AO. Seeing from AO wavefront sensor (2 sec averages).

17 Better, but RMS contrast still saturates in excellent seeing 85-electrode AO. Seeing from AO wavefront sensor (2 sec averages).

18 Solar limb data with new AO Data recorded with the AO mirror flattened and then switched off

24 Conclusions from tests with new AO (RMS granulation contrast and limb data) Conventional straylight over ~10-20 low Additive straylight ~1% far outside limb?? Granulation contrast higher but still saturates in excellent seeing But: Large FOV of wavefront sensor blind to high-altitude seeing (good!!). Solution: Add seeing measurements with 8x8 pixel (4 x4 ) cross correlations to improve seeing characterization (implemented but not yet tested against science data)

25 What about high-altitude seeing? (SST simulation by Marco Stangalini) Single seeing layer at 8 km above telescope 60 deg zenith distance r0 = 40 cm at zenith short exposures λ = 500 nm variable wavefront sensor (WFS) FOV

27 Conclusions from simulation of SST with high-altitude seeing Off-axis seeing made worse by AO FWHM degraded by low-order aberrations (but will frequently be diffraction limited which is enough for MOMFBD) But: Off-axis degradation does not match SST science images and that the AO seeing monitor reports very large r0 values (~1m!) in the morning. => r0 = 40 cm likely pessimistic?

28 Summary and conclusions Main source of SST straylight must be from small-scale aberrations, from the telescope and/or seeing Conventional AO makes off-axis image quality from high-altitude seeing worse even with 12 x12 FOV WFS Effects of high-altitude seeing under investigation Anything resembling accurate photometry requires understanding of telescope aberrations and continuous monitoring of seeing up to at least the tropopause SST likely gives higher RMS granulation than any other solar telescope. But that is not good enough. Comment added: the WFS will alias smallscale aberrations (unresolved by WFS) into lower-order aberrations that the AO will falsely compensate for, making things worse!

Capabilities of SST* and CHROMIS

Capabilities of SST* and CHROMIS Göran Scharmer Institute for Solar Physics Stockholm University *Swedish 1-m Solar Telescope Hinode 9, Belfast, 16 September 2015 Strengths of SST Outstanding image quality