Solar Adaptive Optics

Abstract

Adaptive optics (AO) has become an indispensable tool at ground-based solar telescopes. AO enables the ground-based observer to overcome the adverse effects of atmospheric seeing and obtain diffraction limited observations. Over the last decade adaptive optics systems have been deployed at major ground-based solar telescopes and revitalized ground-based solar astronomy. The relatively small aperture of solar telescopes and the bright source make solar AO possible for visible wavelengths where the majority of solar observations are still performed. Solar AO systems enable diffraction limited observations of the Sun for a significant fraction of the available observing time at ground-based solar telescopes, which often have a larger aperture than equivalent space based observatories, such as HINODE. New ground breaking scientific results have been achieved with solar adaptive optics and this trend continues. New large aperture telescopes are currently being deployed or are under construction. With the aid of solar AO these telescopes will obtain observations of the highly structured and dynamic solar atmosphere with unprecedented resolution. This paper reviews solar adaptive optics techniques and summarizes the recent progress in the field of solar adaptive optics. An outlook to future solar AO developments, including a discussion of Multi-Conjugate AO (MCAO) and Ground-Layer AO (GLAO) will be given.

Electronic supplementary material: Supplementary material is available for this article at 10.12942/lrsp-2011-2.

Figure 1:

Sunspot image obtained with the DST adaptive optics system and post-facto speckle reconstruction. This diffraction limited image was taken at at a wavelength of 430 nm (courtesy of F. Wöger, NSO).

Figure 2:

Left: Sunspot region recorded with the Swedish 1 m Solar Telescope using adaptive optics and after post-facto processing using phase-diversity. A g-band filter centred on 430.5 nm was used. Tick marks are 1000 km on the Sun. Penumbral filaments with dark cores are seen protruding into the umbra. Right: Close-up of several penumbral filaments with dark cores. Tick marks are 100 km (from Scharmer et al., 2002).

Figure 3:

Simulated long exposure observations of granular convection and associated magnetic structure (courtesy of Stein, Nordlund, Keller). The impact of the achieved long exposure AO Strehl ratio is visualized by convolving the simulated solar data with the long exposure adaptive optics PSF of a 4 m telescope. The AO system is assumed to provide partial correction quantified by the number of corrected modes and the resulting Strehl ratio. Shown are intensity images (upper panel) and line-of-sight magnetograms (lower panel). The assumed Fried parameter in this simulation is r₀ = 5 cm. The long exposure Strehl after fully correcting 0, 100, 400, 600, 1000, 2000 modes is S(0) = 0.001 (no AO case), S(100) = 0.002, S(400) = 0.1, S(600) = 0.2, S(1000) = 0.35, and S(2000) = 0.554 (images 1 - 6, left to right and top to bottom). The two images on the lower right in each of the panels show the input data convolved with the ideal 4 m telescope PSF (image 7) and the input data (image 8).

Figure 4:

Left: long exposure Point Spread Function (PSF) of the turbulent atmosphere (red,dashed), the perfect, diffraction limited telescope (black, solid) and a typical partially AO corrected PSF (blue, dotted). The corresponding modulation transfer functions (MTF) are shown on the right.

Figure 5:

Principle of adaptive optics. The main adaptive optics components are the deformable mirror, the wavefront sensor and a control system that includes a wavefront reconstructor. A beam splitter sends a small fraction of the light to the wavefront sensor while most of the light is distributed to the science instrument(s) (courtesy of Claire Max, Center for Adaptive Optics, UC Santa Cruz).

Figure 6:

Fried parameter as a function of time as measured at the DST. The seeing during the daytime can fluctuate significantly and with short time scales (from Marino et al., 2004).

Figure 7:

Corrected and uncorrected modal PSD for Zernikes (Noll, 1976) Z4 and Z24 derived from the NSO low order AO system. Z24 is not corrected with this system due to its low order of correction of about 20 modes. The seeing contains frequencies well above 100 Hz (from Rimmele, 2000).

Figure 8:

mpg-Movie (229.625 KB) Still from a movie showing Focault knife-edge wavefront sensor applied to the planet Venus. A beam splitter arrangement images four images of Venus onto pairs of orthogonal knife edges. This knife edge configuration encodes wavefront gradients as intensity fluctuations in the pupil plane. The movie shows the temporal evolution of these patterns and clearly shows how wavefront aberrations are carried across the telescope aperture by the wind. (For video see appendix)

Figure 9:

Focal plane derivative mask for granulation (see von der Lühe, 1988).

Figure 10:

The 19 element Lockheed AO system. Shown are the segmented deformable mirror and the Shack-Hartmann wavefront sensor. Both subsystems were custom built. The images of a small sunspot recorded with and without AO correction demonstrate the systems ability to partially correct seeing aberrations (from Acton and Smithson, 1992).

Figure 11:

Diffraction limited long exposure (18 s) images of a small sunspot collected at the DST with the NSO low order solar AO system. Upper left: narrow-band image at 630 nm. Upper right: corresponding line-of-sight magnetogram. Lower left: narrow-band image at 557.6 nm. Lower right: corresponding velocity map (see Rimmele, 2004b).

Figure 12:

Left: Diffraction limited long exposure (30 s) granulation image recorded with the NSO low order AO at 630 nm. Tick marks are 1”. Right: Granule with surrounding bright point structure (500 nm, exposure: several seconds).

Figure 13:

Strehl as function of the number of corrected modes and with r0 as a parameter. A low order system such as the NSO LOAO, which corrected between 15–24 modes (vertical lines on the left), produces high Strehl only for the best seeing conditions. Fluctuations of the Fried parameter result in large variations of the Strehl. A high order system such as the DST AO76, which corrects up to about 75 modes (vertical line on the right), can significantly reduce but not entirely eliminate Strehl fluctuations. These Strehl calculations are theoretical but assume realistic conditions and, as will be shown later, actual Strehl measurements closely match the modeled Strehl predictions.

Figure 14:

mpg-Movie (782.873046875 KB) Still from a movie showing Principle of correlating Shack-Hartmann wavefront sensor. Crosscorrelation techniques are used to track the low contrast granulation images or any other extended object of sufficient contrast (Rimmele and Radick, 1998). The movie shows a time sequence of wavefront sensor camera images with 12 subapertures across the pupil of the DST. The cross-correlation functions of the subaperture images of granulation are shown on the right. (For video see appendix)

Figure 15:

mpg-Movie (5807.69238281 KB) Still from a movie showing Long exposure (3 s) granulation image recorded with AO76 (top) and with just tip/tilt correction applied (bottom). The movie shows the real time video sequence obtained during first light with AO76. The system is locked on a small pore. The AO is turned off several time during the sequence to show the uncorrected image quality delivered by the DST. (For video see appendix)

Figure 16:

Implementation of AO76 system at the DST.

Figure 17:

Schematic implementation of a SHWFS. Adjustable components are motorized to automate alignment and calibration procedures (from Richards et al., 2010).

Figure 18:

Left: functional block diagram of AO76 DSP based real time control (RTC) system. Right: image of RTC (see Rimmele et al. (2004) for details).

Figure 19:

Visualizing the isoplanatic patch. These long exposure (11 s) granulation images were obtained with the DST AO76 system locked at the center of the FOV. The image on the left was recorded in bad seeing conditions with a significant fraction of the seeing located at higher altitudes due to the jet stream. The isoplanatic patch over which the AO corrects optimally is rather small (circle, about 10” diameter). The long exposure image on the right was recorded under good seeing conditions and the jet stream not passing right over the telescope site, i.e., a larger fraction of the turbulence is located at low altitudes resulting in a larger isoplanatic patch.

Figure 20:

Strehl ratio as function of field position (zero = AO lock center) and elevation (90°-zenith angle) of the Sun in the sky. An AO system with 76 subapertures and 97 actuators was modeled using the Haleakala atmospheric model of Table 1, which simulates a case of very low high altitude turbulence. The FOV of the WFS is 10”. Two seeing cases were modeled using an overall Fried parameter of r₀ = 10 cm and r₀ = 20 cm, respectively (from Marino and Rimmele, 2011).

Figure 21:

Strehl ratio as function of field position (zero = AO lock center) and elevation (90°-zenith angle) of the Sun in the sky. An AO system with 76 subapertures and 97 actuators was modeled using the Mt. Graham atmospheric model of Table 1, which simulates a case with significant high altitude turbulence. The FOV of the WFS is 10”. Two seeing cases were modeled using an overall Fried parameter of r₀ = 10 cm and r₀ = 20 cm, respectively (from Marino and Rimmele, 2011).

Figure 22:

Noise of the correlating Shack-Hartmann wavefront sensor as a function of detector well depth. The different curves are for subaperture image contrast of (top - bottom): 0.015, 0.025, 0.05, 0.1.

Figure 23:

Subaperture tilt power spectral density (PSD). Top panel: granulation excellent seeing. Subaperture tilt noise: 15 nm. Bottom panel: sunspot, good seeing. Subaperture tilt noise: 8 nm.

Figure 24:

Schematic block diagram describing the method to estimate the long exposure PSF from solar AO loop data (from Marino, 2007).

Figure 25:

Estimated PSF vs. actual PSF. The AO76 was looked on the bright star Sirius. Long exposure images of this point source directly measure the AO PSF, which can be compared to the estimated PSF provided by the AO76 telemetry (from Marino and Rimmele, 2010).

Figure 26:

Strehl vs. Fried parameter r₀. The Strehl was estimated using the AO76 telemetry data and the long exposure PSF estimation method. The AO was locked on a small sunspot. Seeing was highly variable spanning a wide range of r₀ (from Marino and Rimmele, 2010).

Figure 27:

mpg-Movie (6274.92773438 KB) Still from a movie showing Speckle reconstructed g-band movie of granulation and g-band bright points near the solar limb. The FOV is 2’ × 2’. The lower left corner zooms in on an area with magnetic bright points (courtesy of F. Wöger, NSO). (For video see appendix)

Figure 28:

mpg-Movie (13828.90625 KB) Still from a movie showing Speckle reconstructed g-band image sequence that captures the formation of a sunspot penumbra (from Schlichenmaier et al., 2010). (For video see appendix)

Figure 29:

mpg-Movie (12914.0039062 KB) Still from a movie showing MOMFBD reconstructed image and movie of chromospheric Hα fine structure (from van Noort and Rouppe van der Voort, 2006). (For video see appendix)

Figure 30:

Line-center intensity of an active region at disk center in the Ca II 854.2 nm line. The extended 240” × 240” field-of-view was constructed by mosaicing the 90” × 90” FOV of the Interferometric Bidimensional Spectrometer (IBIS). Speckle interferometry was applied to the sequences of images obtained at each of the nine mosaic positions, and the reconstructed images were stitched together to produce the final mosaic (courtesy of K. Reardon, Florence).

Figure 31:

Long exposure narrow-band images taken in the red wing of the spectral line Fe I 5576 Å. The image on the top right was recorded with a Fried parameter r₀ = 16.5 cm and a Strehl of S = 0.88. The image on the top left has a significantly lower Strehl of S = 0.46 and was recorded with r₀ = 5.4 cm. The high spatial frequency information is still present in the poor seeing image on the left but the contrast is reduced significantly. The corresponding deconvolved images are displayed at the bottom. Post processing these long exposure images can restore the contrast, i.e., the amplitudes.

Figure 32:

Radial PSDs of long exposure narrow-band images shown in Figure 31.

Figure 33:

Doppler maps obtained from long exposure filtergrams shown in Figure 31. Top: unprocessed. Bottom: dopplergram from deconvolved filtergrams.

Figure 34:

Opto-mechanical implementation of KAOS at the VTT on Tenerife (from Rimmele, 2004a).

Figure 35:

Performance of KAOS. Left: Residual wavefront errors are plotted vs. the Fried parameter r₀ (from Berkefeld, 2007). Right: Corrected and un-corrected temporal PSD of wavefront errors. The crossover occurs at a bandwidth of about 100 Hz (from Berkefeld et al., 2007).

Figure 36:

mpg-Movie (1536.15917969 KB) Still from a movie showing KAOS: uncorrected granulation movie (courtesy of T. Berkefeld, KIS). (For video see appendix)

Figure 37:

mpg-Movie (2162.17773438 KB)Still from a movie showing KAOS: corrected granulation movie (courtesy of T. Berkefeld, KIS). (For video see appendix)

Figure 38:

Histograms of expected Strehl ratios that can be obtained at the NST with the currently installed AO76 (left) and the new, high order 357 actuator (308 subaperture) AO system currently under development (right). The frequency of occurrence (normalized to maximum occurrence) of a certain Strehl (y-axis) is plotted vs. the Strehl. The Strehl distributions are derived from the r₀ distribution measured at the BBSO site. The predicted Strehl has been modeled as if it were measured at the detector plane of a science instrument, i.e., and end-to-end wavefront error budget was used. In each of the plots the dotted line is for visible (0.5 μm) and the solid line for NIR (1.6 μm) wavelengths. The AO76 performance is satisfactory only for near infrared wavelengths. The AO308 operated at visible wavelengths is expected to achieve satisfactory most common Strehl of about S = 0.35, while for the NIR the distribution is narrowly centered at high Strehls between S = 0.7 - 0.85. The upper limit of these distributions is caused by wavefront errors that are uncorrectable by the AO, such as the wavefront errors introduced by the instrument and other non-common path aberrations. For some applications it might be possible to calibrate out some of these non-common errors, i.e., shift the distribution toward higher Strehls.

Figure 39:

Histogram of expected Strehl ratio distribution. Plotted vs. the Strehl ratio (x-axis) is the number of occurance (y-axis, normalized to maximum occurance) derived from the r₀ distribution at Haleakala and the ATST comprehensive error wavefront error budget for diffraction limited observations. The predicted Strehl has been modeled for the detector plane of a ATST science instrument. Left: The visible (0.63 μm) wavelengths histograms are for atmospheric fitting error only (dashed), all AO errors, which include fitting error, aliasing error, bandwidth error, and WFS measurement error (dotted), and a realistic error budget that includes all error contributors, including the science instrument (solid). This figure demonstrates the need for performing a comprehensive systems wavefront error budget analysis. Right: The NIR (1.6 μm) wavelengths Strehl histogram derived by including all error sources indicates that with the ATST Strehl ratios of S ≥ 0.6 might be achieved for most of the clear time.

Figure 40:

Layout of the wavefront correction system, including the High Order Adaptive Optics (HOAO), of the ATST. The HOAO is integrated into the telescope optical path. DM and wavefront sensors are located in the thermally controlled coude lab on a rotating platform that serves as image de-rotator.

Figure 41:

Fast tip/tilt device of the ATST with integrated air cooling system. This example illustrates the complexity added by the thermal control requirement.

Figure 42:

Strehl ratio as function of field position (zero = AO lock center) and elevation (90°-zenith angle) of the Sun in the sky. An AO system with 1236 subapertures and 1313 actuators was modeled for a 4 m telescope using the Haleakala atmospheric model of Table 1, which simulates a case of very low high altitude turbulence. The FOV of the WFS is 8”. Two seeing cases were modeled using an overall Fried parameter of r₀ (500 nm) = 10 cm and r₀ (500 nm) = 20 cm, respectively. The observing wavelengths is 500 nm (from Marino and Rimmele, 2011).

Figure 43:

Strehl ratio as function of field position (zero = AO lock center) and elevation (90°-zenith angle) of the Sun in the sky. An AO system with 1236 subapertures and 1313 actuators was modeled for a 4 m telescope using the Mt. Graham atmospheric profile of Table 1, which simulates a case of very low high altitude turbulence. The FOV of the WFS is 8”. Two seeing cases were modeled using an overall Fried parameter of r₀ (500 nm) =10 cm and r₀ (500 nm) = 20 cm, respectively. The observing wavelengths is 500 nm (from Marino and Rimmele, 2011).

Figure 44:

Strehl ratio as function of field position (zero = AO lock center) and elevation (90°-zenith angle) of the Sun in the sky. An AO system with 1236 subapertures and 1313 actuators was modeled for a 4 m telescope using the Haleakala atmospheric model of Table 1, which simulates a case of very low high altitude turbulence. The FOV of the WFS is 8”. Two seeing cases were modeled using an overall Fried parameter of r₀ = 10 cm and r₀ = 20 cm, respectively. The observing wavelength is 1600 nm (from Marino and Rimmele, 2011).

Figure 45:

Strehl ratio as function of field position (zero = AO lock center) and elevation (90°-zenith angle) of the Sun in the sky. An AO system with 1236 subapertures and 1313 actuators was modeled for a 4 m telescope using the Mt. Graham atmospheric profile of Table 1, which simulates a case of very low high altitude turbulence. The FOV of the WFS is 8”. Two seeing cases were modeled using an overall Fried parameter of r₀ = 10 cm and r₀ = 20 cm, respectively. The observing wavelength is 1600 nm (from Marino and Rimmele, 2011).

Figure 46:

Strehl ratio as function of elevation of the Sun in the sky. The same model parameters as used in the previous figures were used to compare Strehl performance between mono-chromatic wavefront sensing and the case where wavefront sensor and observing wavelengths are significantly different. The Haleakala profile was used for this simulation. The Fried parameter is 10 cm (from Marino and Rimmele, 2011).

Figure 47:

Principle of MCAO (from Rigaut et al., 2000).

Figure 48:

Strehl vs. FOV for conventional AO and a 2DM MCAO system. This figure is intended to demonstrate the principle of MCAO and does not represent a realistic performance prediction for a solar MCAO system. Realistic performance estimates of MCAO at a 4 m solar telescope will be shown later in this section (courtesy of T. Berkefeld).

Figure 49:

WFS Lenslet and subaperture image arrangement of the conventional AO stage (high order, narrow field, right) and the additional MCAO stage (low order, wide field, left) (from Berkefeld et al., 2006). The upper panel shows a top level schematic of the conventional and MCAO stages (from von der Lühe et al., 2005).

Figure 50:

Top: Results from KIS MCAO experiment at the VTT, Tenerife. Shown is the field dependence of the generalized Fried parameter ρ₀ derived from speckle interferometry. Without adaptive optics (left) ρ₀ equals r₀ and amounts to about 7 cm at 430 nm and is uniformly distributed across the field. Conventional adaptive optics (middle) corrects a FOV of a few arcseconds where ρ₀ is about 10.5 cm. The correction extends over a much larger area with MCAO (right). The data for MCAO and without AO were taken about a minute apart (from von der Lühe et al., 2005). Bottom: Results from the DST MCAO experiment. Shown are maps of residual image motion measured with conventional AO (left) and the MCAO (right). Dark blue areas in these images indicate good correction. This example was obtained with the five guide region “asterism”. The square FOV is about 45” × 45”. The MCAO corrects to a best level of 0.01” rms and over a FOV of about 40–45” compared to typically less than 10” of the conventional AO (from Rimmele et al., 2010c).

Figure 51:

Optical implementation of MCAO for GREGOR (from Berkefeld, 2007)

Figure 52:

EST MCAO linear optical arrangement. M1 and M2 constitute the main telescope. M5 and M8 are the collimator and camera optics of the conventional AO. The tip/tilt mirror is M6; the ground layer (AO) DM is M7. The four MCAO DMs M9-M12 are at conjugate heights of 30, 15, 9, and 5 km (from Berkefeld et al., 2010).

Figure 53:

Left: Strehl as a function of the field angle and zenith angles = 0° (upper curve), 30° (middle curve), 60° (lower curve). Even with 5 DMs the performance is unsatisfactory for large zenith angles. Right: Strehl as a function of the field angle and zenith angle 45° for a corrected FOV of 60” and 30”, respectively, and modified MCAO DM conjugated heights (from Berkefeld et al., 2010).

Figure 54:

Maps of the variance of residual image motion measured with GLAO mode at the DST.