Lab Manual - Radford University

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CHAPTER B. COMPUTER LABORATORIES (CLEA) 63 Name: Section: Date: B.1 Astrometry of Asteroids I. Introduction Astrometry is one of the fundamental tools of astronomers. Astrometry is the technique of precisely measuring the positions of stars and other objects in the sky. By doing so, astronomers can make charts of objects in the sky, assigning them coordinates (right ascension and declination). Astrometry is useful in that it also helps us to measure parallax (Lab A.1) that occurs over the period of a year due to the orbit of the Earth around the Sun. By using those same techniques used in that lab, astronomers can measure the distances to nearby stars. In addition we can measure the proper motion of nearby objects. Proper motion is the drifting of a celestial object against the background sky due to the actual motion of the object itself. By using computers and images obtained via CCD cameras, astronomers can determine the coordinates of an object to high precision. Even the relatively simple program used in this lab can pinpoint objects to within less than 0.1 arcseconds (the size of a dime viewed at a distance of 20 km). In recent years, astronomers and the general public have shown increased interest in mapping the orbits of asteroids. The reason is to find all near-Earth objects which could conceivably collide with the Earth, causing untold damage to society as we know it. To map the orbit of asteroids (or comets), astronomers require precise observation of the object’s location over an extended period of time. To accurately measure the position of an asteroid, or another object such as a planet, which has no fixed position in the sky we need to measure its position relative to other objects whose positions are known. For example, look at the image below. It shows two stars whose positions are known, A and B, and a third object whose position is not known. Imagine that star A is known to lie at 13 h 0 m 0 s right ascension and 32 ◦ 0’ 0” declination while B lies at 14 h 0 m 0 s right ascension and 29 ◦ 0’ 0” declination. A 12 X B 8 10 18 Figure B.1: Determining the coordinates of object X
64 CHAPTER B. COMPUTER LABORATORIES (CLEA) In this case the unknown object’s coordinates are easy to determine since object X is at the midpoint of the line connecting stars A and B. Look at the table below. Table B.1: Coordinate data for stars A and B and the unknown object X. Object Right Ascension Declination X Position on Image Y Position on Image Star A 13 h 0 m 0 s 32 ◦ 0’ 0” 10 12 Star B 14 h 0 m 0 s 29 ◦ 0’ 0” 18 8 Object X ? ? 14 10 It will have coordinates that are then the values at the midpoint, 13 h 30 m 0 s right ascension and 30 ◦ 30’ 0” declination. In general of course, things will not be this simple! In addition to the ideal situation above being unlikely, the equatorial coordinate system is composed of curves of right ascension and declination, not straight lines. Still, a simple coordinate conversion can be done to find the coordinates of the unknown object. The positions of the known objects will be obtained using a guide catalog, in this case the Hubble Guide Star Catalog. This catalog is a compilation of the positions of about 20 million stars (almost all the stars greater than 16th magnitude). The program will allow you to identify stars in this catalog and then do the coordinate transformation from the object’s coordinates in the image to the equatorial coordinates in the sky. In this lab you will make simulated observations of the sky, identify an asteroid in pairs of CCD images and use measurements of the parallax (by comparing images from two different observatories on either side of the United States) to calculate the asteroid’s distance. II. Reference • CLEA Astrometry of Asteroids Lab, http://www.gettysburg.edu/academics/physics/clea/CLEAhome.html • Astronomy Lab A.1 III. Materials Used • CLEA Astrometry of Asteroids program • calculator IV. Observations The observations you will be making will be simulated using the CLEA program. You will do the following things in this observation: • learn to display CCD images of the sky using an astronomical display program • blink pairs of images and learn to identify objects which have moved from the time of one image to the next • call up reference star charts from the Hubble Guide Star Catalog (GSC) • recognize and match star patterns on the GSC charts against the stars in your image • measure the coordinates of unknown objects on your images using the GSC reference stars • measure the parallax of an asteroid and use that to find its distance
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