Initially, the 1.5 m GREGOR and the 1.6 m NST will be outfitted with high order conventional AO systems. The main parameters that describe these systems are listed in Table 2.
|GREGOR||1.5 m||196||156||10 cm||24 × 24||130 Hz|
|NST||1.6 m||357||308||8.4 cm||20 × 20||130 Hz|
Both systems plan to operate with a correlating SHWFS and use stacked actuator, continuous facesheet DMs. The NST system is designed as a scaled up version of the DST AO76 and utilizes a new generation of DSPs (TIGERSHARC) while the GREGOR is based on the KAOS heritage and uses general purpose processing.
Predictions of the Strehl ratio achieved in the detector plane of post-focus instruments of the NST can be derived from detailed performance modeling as described in Section 6. The analysis uses the ATST site survey data and is partially based on the extensive error budget modeling that was done for the ATST since both NST and ATST are similar off-axis designs with ventilated enclosures and face similar non-common path issues. The results are shown in Figure 38 in the form of histograms of the predicted Strehl at visible and NIR wavelengths. This figure compares the Strehl performance of the AO76 currently operating at the NST with the predicted performance of the high order 357 actuator system. The conclusion is that with the NST and AO76 diffraction limited observations of reasonable Strehl at visible wavelengths are only possible for the most extraordinary seeing conditions. In comparison, the high order 357 actuator system is expected to provide reasonable Strehl in the visible for a significant fraction of the observing time. Figure 38 clearly demonstrates the need for an upgrade to a high order AO system for the NST. GREGOR will have a similar high order system. These systems will be essential to realize full scientific benefit from these 1.5 m class aperture telescopes.
These conventional high order AO systems should be regarded as intermediate steps toward the implementation of MCAO systems at both GREGOR and NST (see Section 9.2).
As an example of an extremely high order solar AO system this section summarizes the design characteristics of the ATST conventional AO system, which is the centerpiece of the ATST wavefront correction system (Rimmele et al., 2006a). The AO system planned for the 4 m EST will be discussed in Section 9.2 in the broader context of MCAO since both ATST and EST plan to implement MCAO systems as soon as the technology has been demonstrated at the 1.5 m class solar telescopes.
The ATST wavefront correction system is required to achieve the high Strehl requirements at visible and infrared wavelengths called for in the Science Requirements Document (Rimmele, 2005).
The ATST uses a comprehensive wavefront correction strategy with several correctors and wavefront sensors including:
The HOAO system of the ATST has been integrated into the ATST’s optical path from the start and will correct atmospheric seeing and any internal seeing and optical aberrations along the optical path to the WFS. The top-level science requirements that drive the ATST AO design are:
The original baseline design used a 1313 actuator DM and a correlating SHWFS with 1280 subapertures operating at > 2 kHz rates. It is interesting to note that with the ATST entering construction and further refinement of error budgets the system size has now been increased to about 1700 actuators in order to accommodate pressures on the error budget exerted by various subsystems including instrumentation (Richards et al., 2010). This clearly demonstrated the importance of careful and continuously updated error budget analysis and systems engineering approach in general. To further illustrate this point Figure 39 plots predicted Strehl histograms for the ATST baseline HOAO design at visible and near infrared wavelengths. Considering the atmospheric fitting error only, which unfortunately sometimes is done to provide crude performance predictions, results in an overly optimistic performance estimate (visible histogram, dashed line). The dotted line is produced by including atmospheric fitting error, aliasing error, bandwidth error, and WFS measurement error, which sometimes are referred to as the AO system errors. However, a realistic error budget has to include all possible error sources (visible histogram, solid line). The science instrument, for example a spectrograph, introduces wavefront errors that are not sensed by the AO wavefront sensor and, therefore, are not corrected. It is not always possible to calibrate out these non-common path errors. In particular, in a multi-instrument experiment calibration of non-common path wavefront errors of one instrument results in adding these errors to all other instruments involved in the experiment. Multi-instrument experiments are becoming the norm in solar observations. The imaging performance of AO instrumentation, in particular in the visible, is driven to much higher imaging quality standards than used to be the case for seeing limited instruments. The NIR (1.6 µm) wavelengths Strehl histogram indicates that with the ATST Strehl ratios of S 0.6 might be achieved for most of the clear time, i.e., space quality observations with a resolution of 0.1” will be possible for the majority of the available observing hours.
The real time control (RTC) system of the HOAO system is modeled after the AO76. A top level block diagram that details the functionality was described by (Rimmele et al., 2006c; Richards et al., 2010). This unit performs the cross correlations of the 1232 subapertures as well as the reconstruction. Servo algorithms for the DM and tip/tilt mirrors are also implemented on the RTC unit. Telemetry data is collected and streamed to a local disk via the auxiliary PCs. The size and complexity of the significantly higher order ATST HOAO RTC is not much increased compared to the RTC of the AO76. This is due to advances in processing hardware (Moore’s law) (Rimmele et al., 2006c).
Figure 40 shows the layout of the ATST HOAO system in the ATST coude instrument lab (Richards et al., 2010). The HOAO is highly integrated with the instrumentation and feeds a number of first light facility instruments that will be used from visible to infrared wavelengths. The long instrument feed optical paths can result in significant non-common path local seeing errors (Biérent et al., 2008). The ATST mitigates this by careful thermal control of the entire instrument laboratory (Phelps et al., 2010).
With a 4 m collecting area thermal control of the corrective optical elements such as the DM and the tip/tilt devices becomes essential. These devices are typically kept small for performance (bandwidth) and optical packaging reasons. With a diameter of the device of 200 mm and a FOV of 2.83’ circular the absorbed heat flux is about 100 W/m2 assuming a high reflection silver coating. Without active cooling the surface of the DM or tip/tilt device could heat up by tens of degrees C above ambient causing serious local seeing degradation. Active cooling is required to keep the mirror surface temperature with 0 ℃ to –2 ℃, which is the allowable range where local seeing can be avoided. Air cross flows could also be used to mitigate image degradation (Hubbard et al., 2006). However, actuator drifts, time dependent changes of actuator characteristics, and thermal deformations of the assembly due to a changing heat load are problems that also need to be dealt with if no thermal control is applied. Figure 41 shows, as an example for the potential added complexity of thermal control, an air cooled tip/tilt device envisioned for implementation at the ATST.
In addition to the technical challenges involved in building high order solar AO systems for 4 m class telescopes anisoplanatism poses a much more serious challenge at these larger telescopes. As was shown in Section 6.1.3 the residual wavefront errors introduced by anisopanatism limit the size of the AO corrected FOV. For current small solar telescopes and under certain conditions can be small and acceptable AO performance can be achieved over a FOV of tens of arcsec (see Figure 20). Reasonable or useful Strehl ratios (in terms of being able to apply post-facto reconstruction methods) can be achieved over even larger FOVs. For large aperture telescopes, however, even during excellent seeing conditions is generally large also for the upper atmosphere. Figure 42 (Marino and Rimmele, 2011) shows the result of repeating the simulations presented in Section 6.1.3 but for a 4 m aperture telescope with an AO system that has about 1300 actuators. These parameters closely match the ATST AO system and, therefore, predict anisoplanatic behavior expected for the ATST.
The isoplanatic angle for a best case scenario of vertical pointing and r0 of 20 cm is now only about 10 arcsec for the Haleakala profile (Figure 42, right) and 2 – 3 arcsec for the Mt. Graham profile (Figure 43, right). The Strehl drops quickly away from the lock center and for near horizon pointing. The Haleakala profile yields an isoplanatic angle of about 3” and about 10” away from the lock center the Strehl is virtually zero. The Mt. Graham profile does not yield satisfactory Strehl for near horizon pointing even if excellent seeing (r0 = 20 cm) is assumed. For more common seeing conditions (r0 = 10 cm) only the Haleakala profile leads to good Strehl performance and reasonable corrected FOV.
The impact of using an extended FOV of 8” × 8”, that is required for the correlating Shack–Hartmann wavefront sensor, is clearly revealed in Figure 43. Even for vertical pointing the directional averaging effect is significant as can be inferred by comparing Figures 42 (left) and 43 (left). For near horizon pointing averaging wavefront sensor information from different directions is devastating and leads to substantial Strehl reduction. The early morning hours are, of course, exactly when conditions for solar observations are considered best because the strong ground layer turbulence has not formed yet. Hence, high resolution solar observations are generally performed at fairly high zenith angles. These results emphasize the relative importance of MCAO for large aperture telescopes in comparison to the small aperture current solar telescopes. It is important to note that the negative impact of anisoplanatism makes post-facto processing of conventional solar AO imagery a necessity for many applications. As a side note it should be mentioned that the dominance of the ground layer seeing during the daytime might make Ground-Layer AO (GLAO) an attractive option for solar telescopes and for some scientific applications.
As pointed out in a previous section at near infrared wavelengths high Strehl AO performance can be expected for a considerable fraction of the available observing time. In addition, the isoplanatic angle increases with and, thus, one might expect roughly a factor of four increase in isoplanatic patch size when near infrared instead of visible observations are performed. Figures 44 and 45 show the equivalent plots to what was shown in Figures 42 and 43 but for 1.6 micron instead of visible. These plots confirm that the isoplanatic angle (patch) is indeed about 4 times larger at near infrared wavelengths compared to visible wavelengths. The Haleakala profile yields an isoplanatic angle of about 17 arcsec (isoplanatic patch = 34 arcsec) for r0 = 10 cm and zenith angle of 45°, i.e., the isoplanatic patch covers the size of a small sunspot. These results indicate that with the development of 4 m solar telescopes infrared observations are likely to gain in importance. A 4 m aperture will provide better than 0.1 arcsec diffraction limited resolution at a wavelength of 1.6 microns.
Another form of anisoplanatism that effects and limits solar AO performance is chromatic anisoplanatism. A detailed discussion of chromatic anisoplanatism is given by Hardy (1998, Section 9.3). Due to atmospheric dispersion light of different wavelength propagates through different parts of the atmosphere and, thus, samples different turbulence volumes. Atmospheric dispersion becomes a limiting factor for large zenith angle observations, which solar observations usually are.
The most significant error caused by chromatic anisoplanatism is the multi-spectral error. The vast majority solar observations are multi-spectral typically covering wavelengths that range from UV to near infrared. The wavefront sensor of existing solar AO systems operates at a particular visible wavelengths with a fairly narrow passband. For example, the atmospheric dispersion between 430 nm, the observing wavelength of the science camera, and 500 nm, the wavefront sensor wavelength, is about 1 arcsec. For early morning observations with the Sun at 20° elevation (zenith angle = 80°) and with a single turbulence layer at 10 km the 430 nm phase screen that needs to be corrected by the AO is shifted by about 0.14 m with respect to the 500 nm phase screen that is actually measured by the AO wavefront sensor. This simple estimate gives the order of magnitude of the effect. The exact equations for calculating the ray displacement can again be found in Hardy (1998, Section 9.3).
This misregistration of the science beam wavefront and the sensor beam wavefront can be of order r0, which results in significant reduction of Strehl when compared to an AO system that senses at the same wavelength as the science detector. This is demonstrated with Figure 46, which plots the Strehl ratio as a function of elevation for a number of wavelengths. This simulation again models the four layer Haleakala turbulence profile and the ATST 1300 actuator AO system. The r0 is assumed to be 10 cm in all cases. The dotted, solid, dashed, and dashed-dotted lines represent the expected Strehl performance for the ideal mono-chromatic AO system, which senses at 430 nm, 500 nm, 630 nm, and 1600 nm, respectively, and observations are performed at these same wavelengths. The same lines with over-plotted symbols show the performance of the multi-spectral AO, which senses the wavefront errors at 500 nm but the correction is evaluated at 430 nm (diamonds), 630 nm (stars), and 1600 nm (circles), respectively. For elevation angles smaller than 30° (zenith angle 70°) the curves diverge with significantly reduced Strehl for the multi-spectral AO. Figure 46 suggests that solar multi-spectral observations could gain form careful selection of the WFS wavelength taking into account the primary wavelengths, science priorities of the multi-spectral experiment and the possibility of performing post-facto reconstruction of some instruments that might not exist for other participating instruments. For example, post-facto reconstruction is more difficult for spectrograph instruments than it is for imaging devices. One might, therefore, in some cases, decide to optimize the AO performance for 630.2 nm polarimetry and accept a performance hit with the secondary instrument, the g-band imager, since the g-band images will be post-fact reconstructed (e.g., speckle). A WFS that can operate at different, user-selectable wavelengths would be of advantage.
Solar AO has the luxury of using a relatively narrow bandpass WFS. Science observations are usually performed within a very limited wavelengths range as well. In this way other error sources that would have to be considered for broad-band WFS and broad-band science observations, such as the angular dispersion error and the dispersion displacement error can be largely neglected. It should be noted that solar astronomers often use the term broad-band to describe filtergraph observations with passbands of 10 nm or even less. In the context of atmospheric dispersion such passbands would still be considered narrow-band.
Living Rev. Solar Phys. 8, (2011), 2
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