PATENTS: Optical engineering advances for astronomy

  

Innovations in astronomy have been applied to fields ranging from medicine to military operations. For example, to compensate for image distortions caused by atmospheric effects, adaptive optics techniques have been developed by astronomers and optical engineers. Elements of this technology have found their way into ophthalmic instruments designed to image the retina of the eye. The images are of such high quality that aberrations of the eye can be measured with very high precision. With this instrumentation, it is possible to specify precise parameters for making corrective lenses or performing corrective surgery. Various eye diseases can also be diagnosed with greater confidence. This advancement in ophthalmology is only one of many examples of the technological progeny born from the synergy of optical engineering and astronomy. Five recent U. S. Patent applications exemplify the important contributions from the astronomy field.

Optical detectors for sensing and imaging light sources in multiple spectral bands are required by applications ranging from astronomy to medicine. Such detectors are particularly useful to astronomy since different regions of the electromagnetic spectrum reveal different types of phenomena in the universe. Often, complete understanding is possible only if the entire spectrum is examined. US patent application US 2006/02141113 A1 discloses a design for an optical detector of infrared, sub-millimeter and high energy radiation. In the art of light detection, detectors with the highest sensitivity are, essentially, a type of calorimeter. As light shines on the detecting medium, photons are absorbed and the energy of absorption goes into heating the material. The resulting rise in temperature can be translated into (voltage) light intensity or even photon counts. For the absorption of photons having energies of around 1 KeV, the temperature rise is sufficient to detect a single photon.

The sensitivity of calorimetric detectors depends strongly on the heat capacity (thermal mass) of the detecting medium. The lower the heat capacity, the better the sensitivity. One method of reducing the heat capacity is to cool the detector with liquid helium. In the temperature regime of liquid helium, the heat capacity of the detecting medium is proportional to the cube of the absolute temperature. Thus for temperatures of less than 1 Kelvin, the heat capacity can become very small. For many applications, the light intensity is high enough that cryogenic cooling is not necessary. Even then, it is essential that the heat capacity of the detecting medium be minimized.

The detector design disclosed by US 2006/02141113 A1 improves the state of the art by reducing the heat capacity in a novel way. The detecting medium is filled with microscopic "photoluminescent temperature probes," each with a mass on the order of 10^-11 grams. Such a small mass aids in keeping the heat capacity low. Incident light from a source to be imaged causes the temperature probes to fluoresce. For a given intensity and wavelength of incident radiation, the generated photoluminescence is a unique indicator of temperature. The photoluminescent light can be sensed by a low noise photo-electronic device which allows an image of the source to be constructed. This detection scheme has the additional advantage of circumventing electrical noise and Joule heating.

High resolution x-ray images are critical to many fields including medicine, dentistry, airport security and astronomy. For astronomy, x-rays are of interest because they are radiated from high energy processes such as accretion disks around black holes. An invention that claims to provide high image resolution and detection efficiency of x-ray sources is described in patent application US 2006/0192129 A1. This imaging system can be used for x-rays ranging in energy from 1 to 20 KeV. It is consists of three main components: a scintillator, an optical system and a CCD camera. The principles of operation are as follows: x-rays emitted from a source cause excitations in the scintillator of a detector. The scintillating material de-excites by radiating light in the visible spectrum. A substrate is spaced between the scintillator and lens system. The lens system includes an objective lens and a tube lens. Light is imaged onto a CCD detector by the lens system. The scintillator and lenses function together as an optical system which gives rise to magnifications ranging from 500 to 1000. This level of magnification enables image resolutions on the order of 1 µm for typical CCD pixel sizes. Quantum efficiencies on the order of 95% are claimed.

The theoretical angular resolution of an optical system such as a telescope is limited by diffraction. This limit is given by the well known Rayleigh criterion,

where the angular resolution α is in radians, λ is the wavelength and d is the aperture diameter. For astronomical telescopes, this limit cannot be achieved due to atmospheric turbulence and air currents. The result is excessive blurring of images. Less than ideal angular resolution amounts to a loss of information. For example, atmospheric effects might not allow a binary star system to be resolved into two stars and so it appears as a single star-like object. Adaptive optics solutions can compensate for atmospheric effects, leading to sharper images.

In some cases, the mitigation of atmospheric effects may allow the optical system to approach its theoretical diffraction limit. By changing the effective shape of an optical surface such as a deformable or segmented mirror, wavefront distortions induced by the atmosphere are counteracted. A bright "reference beacon" (such as a star) near the object to be imaged is used to quantify the wavefront distortions. This allows "corrected shapes" to be derived in real time with subsequent rapid mechanical actuations applied to the optical surface. A novel system of actuator arrays for an adaptive optics system is disclosed in patent application US 2006/0193065 A1. The actuator arrays in this invention are intended for deformable surfaces. The actuator arrays are arranged in "braking groups" and "force-altering groups." Precise control of these groups can mitigate atmospherically induced aberrations.

Over the years, amateur astronomers have made significant contributions to astronomy including the discovery of comets, asteroids and supernovae. Patent application US 2006/0132908 A1 describes an apparatus and method used to focus and collimate small telescopes. Since portable telescopes are usually transported to the observing site, there is a risk that the optical alignment may suffer from unintended jolts. Consequently, these telescopes are in need of frequent collimation. The process of collimation can be a tedious, time consuming procedure. In addition to the necessity of precise collimation, image quality depends on proper focusing. The invention disclosed in US 2006/0132908 A1 purports to automate the focusing and/or collimation adjustments. The automation is accomplished by control electronics, motorized components, and inputs such as physical specifications of the optical elements, imaging devices such as CCD cameras and even information about a particular user.