Download G. Scharmer: What is the source of straylight in SST/CRISP data?

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
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Min. umbra intensity ~15%
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Spatial resolution much better
than scale of granulation
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Granulation contrast not reduced
because of lack of resolution
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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
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High-order (small-scale)
aberrations from seeing not
correctable by AO
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High-order aberrations from
seeing not corrected by MOMFBD
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High-order AO wavefront sensor
calibration errors and noise
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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
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SST optics
Straylight target
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Straylight target setup
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Straylight target images
(logarithmic scaling)
From biggest (1 mm) pinhole => conventional straylight
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Observed
MFBD 231 KL’s
MFBD 36 KL’s
(FOV: ~ 2”x2”)
Note: Results with previous 37electrode DM (now replaced by 85electrode DM)
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Conclusions from straylight target tests
(with “old” 37-electrode AO)
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“Conventional” straylight over
~10” low (~0.3%)
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Additive straylight ~2% from
ghost images
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Strehl drops to 74-77% from
small-scale aberrations
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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
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50 mm monomorph (CILAS)
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Excellent optical quality
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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
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Conclusions from tests with new AO
(RMS granulation contrast and limb data)
Monday 24 February 14
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“Conventional” straylight over ~10-20” low
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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
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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
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Off-axis seeing made worse by AO
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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
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Main source of SST straylight “must” be
from small-scale aberrations, from the
telescope and/or seeing
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Conventional AO makes off-axis image
quality from high-altitude seeing worse
even with 12”x12” FOV WFS
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Effects of high-altitude seeing under
investigation
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Anything resembling accurate photometry
requires understanding of telescope
aberrations and continuous monitoring of
seeing up to at least the tropopause
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SST likely gives higher RMS granulation
than any other solar telescope. But that is
not good enough.
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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