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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 Monday 24 February 14 Fundamental problem • • Min. umbra intensity ~15% • Spatial resolution much better than scale of granulation • Granulation contrast not reduced because of lack of resolution • • Must be straylight Granulation contrast only 9% but should be ~17% Straylight PSF must be narrow Dust gives large-angle scattering => Main suspect: Small-scale aberrations Movie by Henriques Monday 24 February 14 Small-scale aberrations (seeing/telescope) Monday 24 February 14 • 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 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 Monday 24 February 14 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 Monday 24 February 14 (From Scharmer et al. A&A 521, A68, 2010 ) 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) Monday 24 February 14 (From Scharmer et al. A&A 521, A68, 2010, Fig. ) 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) Monday 24 February 14 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 Monday 24 February 14 SST optics Straylight target Monday 24 February 14 Straylight target setup Monday 24 February 14 Straylight target images (logarithmic scaling) From biggest (1 mm) pinhole => conventional straylight Monday 24 February 14 Observed MFBD 231 KL’s MFBD 36 KL’s (FOV: ~ 2”x2”) Note: Results with previous 37electrode DM (now replaced by 85electrode DM) Monday 24 February 14 Conclusions from straylight target tests (with “old” 37-electrode AO) Monday 24 February 14 • “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 Microlenses used to measure seeing (from differential image motion) Electrode and microlens (WFS) layout Pupil diameter (34 mm) Monday 24 February 14 • • 50 mm monomorph (CILAS) • • Excellent optical quality • • 2 kHz update rate Less print-through than with bimorph mirrors 85 electrodes, 85 microlenses 24x24 pixel (12”x12”) cross correlations Real-time seeing monitor (2 sec averages obtained every second using 20 sec “reference” for calculating variances) 26 June 2009 data. Blue: 538 nm, red: 630 nm. Dotted: theoretical but with RMS contrast divided by 1.85. 37-electrode AO. Seeing data from wide-field wavefront sensor (WFWFS) Note: Contrasts at 630 nm multiplied with factor 1.22 Monday 24 February 14 Better, but RMS contrast still saturates in excellent seeing 85-electrode AO. Seeing from AO wavefront sensor (2 sec averages). Monday 24 February 14 Better, but RMS contrast still saturates in excellent seeing 85-electrode AO. Seeing from AO wavefront sensor (2 sec averages). Monday 24 February 14 Solar limb data with new AO Data recorded with the AO mirror “flattened” and then switched off Monday 24 February 14 Monday 24 February 14 Monday 24 February 14 Monday 24 February 14 Monday 24 February 14 Monday 24 February 14 Conclusions from tests with new AO (RMS granulation contrast and limb data) Monday 24 February 14 • • • “Conventional” straylight over ~10-20” low • 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) Additive straylight ~1% far outside limb?? Granulation contrast higher but still saturates in excellent seeing What about high-altitude seeing? (SST simulation by Marco Stangalini) Monday 24 February 14 • • • • • • 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 Monday 24 February 14 Conclusions from simulation of SST with high-altitude seeing Monday 24 February 14 • • Off-axis seeing made worse by AO • 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? FWHM degraded by low-order aberrations (but will frequently be diffraction limited which is enough for MOMFBD) 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! Monday 24 February 14