Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Arecibo Observatory wikipedia , lookup
Allen Telescope Array wikipedia , lookup
Lovell Telescope wikipedia , lookup
Hubble Space Telescope wikipedia , lookup
James Webb Space Telescope wikipedia , lookup
Spitzer Space Telescope wikipedia , lookup
Optical telescope wikipedia , lookup
CfA 1.2 m Millimeter-Wave Telescope wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
Demonstration of Adaptive Optics in a Laboratory Setting Jianing Yang, Elizabeth McGrath Department of Physics and Astronomy, Colby College, Waterville, ME Introduction Abstract Construction Turbulence in the Earth’s atmosphere bends and spreads the light coming from the outer space, causing a blurring effect when celestial objects are viewed through a ground-based telescope. This effect creates a blob in the image and reduces the resolution, largely limiting the performance of ground-based telescopes. On the other hand, the resolution of space telescopes is limited by their size of apertures. For a certain wavelength, the greater the aperture, the greater the angular resolution. Here is a diagram of resolutions of Hubble telescope, JWST, Hubble’s successor and a ground-based telescope without blurring. We constructed a simple adaptive optics system to demonstrate how a laser guide star coupled with a deformable mirror and wavefront sensor can be used to correct for distortions caused by turbulence in the Earth’s atmosphere. Adaptive optics (AO) systems are currently implemented at a number of national astronomical observatories, including the W. M. Keck Observatory, Gemini, and Subaru, and is a key design component for the next generation of very large (30meter-class) ground-based telescopes. Adaptive optics is crucial for improving spatial resolution of ground-based imaging in astronomy. Using AO, we are able to achieve better image quality with the largest ground-based telescopes than we can achieve with spacebased telescopes such as the Hubble Space Telescope and its successor, the James Webb Space Telescope. The laboratory setting of our device allows us to understand the principles required to correct for wavefront distortions before implementing such a system on a telescope. The system includes a mirror with a 6x6 array of actuators that can deform its shape up to 5.5 microns from its reference position. A software package receives and processes signals from the wavefront sensor and sends corrections to the mirror at a rate of 15 Hz. We will finish alignment of the optics in the coming weeks, and will begin testing by introducing artificial distortions to examine overall system performance. The process of construction mainly has two parts. The first one is building the AO system. We first put the small parts of every component together. Then we had the laser pen, optics, the deformable mirror, the beam splitter, and the wavefront sensor mounted and at certain positions relative to one another according to the instruction manual. The second part is to align the system. We carefully measured the height of each component and their distances. Then we turned on the laser for finer alignment, and made sure the beam spot was centered at the lenses, mirrors and finally the wavefront sensor. Therefore, if we can use adaptive optics to correct for the interference of the atmosphere, ground-based telescopes have huge potentials of better performance than space ones. The principles of adaptive optics (AO) are illustrated in the figures below. Image credits: Lawrence Livermore National Laboratory and NSF Center for Adaptive Optics. Third, the measurement of the wavefront sensor is sent to a computer to calculate the shape to apply to a deformable mirror. This mirror cancels out the distortion due to the turbulence. Last, light from both the guide star and the target is reflected off the deformable mirror to the main camera. The image of the target celestial object is therefore sharpened by the AO system. First, we shine a laser beam to the sky, creating an artificial star near the target of observation. Second, light from both the guide star and the target object passes through the telescope’s optics. Light from the guide star is sent to a wavefront sensor, a special high-speed camera that can measure how the star’s light is disturbed by the atmosphere at a rate of 450Hz. Below is a picture of the aligned AO system in the optics lab. Laboratory Set-up Image credits: ThorLabs AOK1 Adaptive Optics Kit User Guide DM Calibration After alignment, the next step is to use the software ThorLabs AO Kit to take in information from the wavefront sensor and control the deformable mirror (DM) to make adjustment. Before it’s possible to put adaptive optics techniques into use on Colby’s telescope, it’s necessary to test the system in a laboratory setting. The schematics of the system is shown in the figure above. The source, which is a laser beam, goes through the collimation optics, composed of two mirrors and four lenses, reflected by the deformable mirror, goes through the relay optics and goes to the Shack-Hartmann wavefront sensor. Acknowledgement Image credits: Lawrence Livermore National Laboratory and NSF Center for Adaptive Optics. Funding for this project is provided by the CARA Program. We also acknowledge Claire Max (UC Santa Cruz) for providing background information about the use of adaptive optics in astronomy. The picture above shows a screen capture of the software during the process of calibrating the DM. The left-hand side shows the shape of the wavefront measured by the wavefront sensor, along with the peak-tovalley amplitude in microns. Ideally, we want the DM to correct the wavefront to better than 0.6 microns, which is the wavelength of the laser source. The upper-right window shows the deflection of individual actuators of the DM. The lower-right window shows the higher order correction terms for the wavefront which we will use when actively correcting the wavefront.