ESTRUCTURA DE COSTOS DE CALIDAD Y NO CALIDAD

In order to utilize the advantages of modern 8-m class large telescopes in terms of not only the light collecting power but also the spacial resolution, the adaptive optics technique has been developed. By performing a real-time compensation procedure, an AO system enable us to overcome the seeing-limited resolution imposed by the turbulent atmosphere, and to achieve higher resolution performance approaching the ideal diffraction-limited resolution. In fact, the AO is useful for compensating a wide variety of aberrations, not

4.1. Adaptive optics observations

just that imposed by the atmosphere but also one originated in optical components on the instrumental side.

The basic principle of AO is schematically shown in Fig. 4.2. An AO system consists of three principal elements, which are the crux of all modern AO systems. These are a wavefront sensor (WFS), a deformable mirror (DM), and a real-time computer unit. The whole system contains also various optical components to guide the incoming beam, as well as science instruments such as camera, which provides the output data. The light collected by the telescope is guided to the WFS experiencing reflections in the DM located in between, and the WFS measures the deviation of the wavefront from the ideal case. Utilizing the information from the WFS, the real-time control system then operates a set of actuators under the DM so as to compensate for the distortion and to flatten the wavefront interactively in the closed-loop real-time process. The most basic AO systems uses a light from ideally a point source to probe the shape of the wavefront, and the brightness of this so-called reference orguide star/source is one of the most important factors on the wavefront correction performance. As shown in the basic principle, the AO technique is achievable only if the atmospheric turbulence isfrozen in time by the speed of the real-time process. A typical time scale of the whole process (signal readout of the WFS, the calculation of the wavefront correction, and the adjustment of the actuators below the DM etc.) is a few milliseconds. As this is an order of magnitude shorter than the correlation timeτ0 (a time scale of the change of the wavefront aberration) of a few tens of milliseconds, the AO compensation system is sensitive enough and fast enough to keep up with the changing atmosphere. In other words, thisan order of milliseconds operation is the essential requirement for the realization of the AO concept and, thanks to developments of the computing technology, it became available as a strong tool in the current ground-based astronomical observations.

A type of WFS used in the NAOS, for example, is called Shack-Hartmann WFS. It consists of a lenslet array and corresponding detectors onto which beams split by the subapertures in the lenslet focus as spots. From the information of an offset of the spot position relative to the predictable position in case of the ideal wavefront on each detector, the tilt of the wavefront in the subaperture can be deduced. Then, based on the integrated information of all subapertures, the real-time computer decodes the distortion of the wavefront in the entire observing aperture, constructs appropriate movements of the actuators, and performs the deformation of the DM for the wavefront compensation in the closed-loop process. As the specifications of NAOS, it has two WFSs - one operates in the visible and one in the near-infrared - and both censors use a 14_×14 lenslet array with 144 valid subapertures as the best setup. 185 actuators at the DM are operated irrespective of the selection of the sensors.

The performance of AO system depends on many factors. From an instrumental point of view, it depends on the number of lenslets in the lenslet array, the number of actuators under the DM, and the response time of the process. In the design of an AO system, the Fried’s parameter r0 in the range of targeting wavelength is an essential input. Since this parameter basically means a seeing cell size, it practically determines, on the plane of the telescope, the number of apertures which have to be adjusted for the wavefront correction. Subsequently, configurations, such as the number of the lenslets at the WFS, the number of the actuators under the DM, determine the physical sizes of these components and other specifications. As shown in the relation r0 ∼ λ6/5, the shorter the target wavelengths is, the smaller the r0 is, yielding the complexity on the construction of the AO system due to the necessity of higher number

CHAPTER 4. OBSERVATIONS

of the controlling subapertures for achieving reasonable correction performance. Since the number of operating subapertures increases radically towards shorter wavelengths, it is still not possible to construct an AO system which has a capability of the theoretically ideal correction in the visible regime. The NAOS, for example, is designed to be able to achieve an effective wavefront correction down to around H-band, and the correction performance somewhat degrades towards shorter wavelength such as J-band and some narrow band filters.

Note that, however, the wavefront sensor does not necessarily operate at the same wavelength as the camera, which forms the image, does. Even in cases of near-infrared imaging, visible wavefront sensors have been used to date in most AO facilities in the world. This is because of the fact that currently standard infrared detectors are not sensitive enough to produce sufficient signals against generally large noises in such a short-time readout process for the wavefront correction. The visible detectors, on the other hand, are capable of detecting the flux and feeding the signals powerful enough to be applied to the wavefront correction technique. Although the coherence length in the visible regime is much smaller than in the near-infrared, i.e., the wavefront aberration due to the atmospheric turbulence shows a finer feature in the visible than in the near- infrared, the finer feature in the visible essentially matches that in near-infrared if its lower frequency component are considered. Therefore, even without the capability of the theoretically ideal correction, the best correction in the visible can reproduce the best correction for the near-infrared imaging. The NAOS, exceptionally, provides the infrared wavefront sensor as well, and this allows us to perform observations under which no visible reference source is available. This is sometimes the case in observations of highly dusty regions in the Galactic disk within which the extinction is too high for the visible light to be observable from the Sun. In the observations of the Galactic Center, for example, the infrared wavefront sensor at the NAOS enable us to use a near-infrared source at the very center as the reference source, being a strong tool.

Once an AO system is constructed, the AO performance then, of course, depend on observational conditions. From an observational point of view, the performance depends on the observing conditions such as the seeing, the brightness and morphology of the reference source, and the angular separation of the reference source and the object of interest. The seeing is primarily governed by the coherence length r0 and the coherence time, and the airmass - a measure of the height of atmosphere through which an observation is being made. The airmass is often approximated by AM = sec(ZA) where ZA is the zenith distance. The beam from the reference source should be bright enough to illuminate all the subapertures on the lenslet and to focus onto the detectors with sufficient S/N ratio in the WFS. The beam from the reference source should also be in, or as close as possible to, the direction of the object being observed in order to minimize the difference on the wavefront aberration between the both passes. In other words, both beams ideally should pass through a part of the atmosphere of approximately uniform characteristics.