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Archive for September, 2018

For near 400 years after Galilee and Newton invented initial telescopes, the image quality was improve through better optical quality of lenses and mirror, better polishing and coating techniques, and advanced optical design. For instance none of the present day professional telescopes are either Newtonian or Galilean. The root mean square (rms) fluctuation of the deviation in the optical surface compared to what it was supposed to be is of the order of a small fraction of the optical wavelength, say 20 nm or so. Achieving such a fine optical surface is challenging by itself. Nonetheless, the final spatial resolution of large ground-based telescopes are ALWAYS governed by the turbulence in the atmosphere, known as seeing.

What is seeing?

Seeing is the collective effect of distortions in the wavefront passing through the earth atmosphere. It causes blurring, image motion, … resulting in a smeared spatial resolution of the order of one or two arc second (1 radian = 206265 arc second). The physical mechanism behind seeing is the turbulence in the atmosphere driven by the temperature gradient which generates the convection cells. There are turbulence in day and night, and in low altitudes and high altitudes. A good fraction of the seeing is due to ground-layer turbulence, which plays role of the boundary condition to the atmosphere. It means the first say 100 m above the ground generates a significant fraction of the seeing. The famous blinking of stars at night sky is solely due to seeing: the stellar size is way much smaller than the seeing limit, therefore the intensity fluctuates. In contrast most of the planets have a large angular diameter of ten or more arc seconds and do not twinkle.

The theoretical resolution of a telescope is estimated by the Rayleigh criteria, and is about 1.22 λ/D, where λ is the wavelength and D the diameter of the telescope objective (both in meter). For a two meter telescope, the theoretical resolution is about 0.07 arc second, way smaller than say one arc second, the seeing limit for a lucky observer. The seeing frustrated many astronomers through decades. Even amateur astronomers experienced a watershed effect when they observe the Moon on high magnification with small telescopes. It is like watching the Moon through a layer of water.

 

A passive approach is to build telescopes at high altitudes to skip a significant fraction of the earth atmosphere, like the Keck or VLT telescopes. At a height of 5000m above the see level, about half of the atmosphere is “gone”. The atmosphere, however, extends over a hundred kilometer or so. To completely eliminate this effect, one has to launch the telescope to the low-earth orbit like the famous Hubble space telescope or the upcoming James Webb telescope.

 

The Adaptive Optics

A breakthrough emerged in 80s when a correlation tracker was first employed in astronomical telescopes. Although it did not sharpen the unshared images, it did fix the location of stars in the focal plane. The correlation tracker used the cross-correlation of the current image of a lock point (a bright star used as a target) with the image it has recorded just a millisecond earlier. The difference was then converted to a voltage following the calibration scheme. A tip/tilt mirror then apply the correction in a closed-loop system such then before the seeing modifies the location of the star, the image was displaced to the “correct” position. To achieve this operation, the correction speed should exceed the seeing frequency which is about 100 Hz. As a result, kHz systems were used in correlation trackers.

The adaptive optics is the natural successor of the correlation trackers. It employed a deformable mirror to correct the optical aberration of the wavefront. A costly deformable mirror is like a flat mirror in the first glance. It consists of several ten or hundred small mirrors, each controlled via a few actuators from behind. The joint action of all the small mirrors is to form the deformable mirror to correct the applied aberration to the wavefront and “undo” all those perturbations. It is a challenging task both from manufacturing and from computation point of view. One needs a dedicated mid-size machine to close the loop at a frequency much higher than the seeing frequency. The adaptive optics can correct several ten or more modes of aberration like defocus, coma, and astigmatism. As a result, the AO-corrected images gain a lot of sharpness and contrast compared to a standard telescope without AO.

You can imagine that the AO business is not in the realm of amateur astronomy. There are, however, tip/tipt systems to fix the image movement which can be purchased like the one SBIG offers. I do not think that anytime soon an AO can be realized in a mid-size amateur telescope. Their implementation for the large professional telescope is a must, I would say. The multi million Euro cost of the AO systems impeded their installation on many aging and brand-new telescopes.

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