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LIGO Livingston Observatory NewsInterferometer Installation Continues
Summer Programs at LLO
Last month we completed the reinstallation of optics, replacement of the positioning actuators, and realignment of the output optics of the Livingston interferometer. This work, begun in May, necessitated venting the vertex and end vacuum chambers so that the optics could be accessed by LIGO staff and scientific collaborators.
Each actuator, called an "OSEM" (orientation sensor and motor), pushes slightly on a small permanent dipole magnet glued to one of four points on the circumference of the mirror or one of two locations on the cylindrical surface. A small solenoidal magnet generates a gentle force which can pull or repel the magnet relative to the mechanical support from which the mirror is suspended. A shadow sensor consisting of an LED and photodiode surrounds the magnet and provides a direct measure of the location of the mirror. During commissioning activities at Livingston and Hanford, it was observed that light scattered from the main laser beam of the interferometer was scattering into the photodiode of some of the OSEM's, and creating "noise" in the control system. This stray light and the consequent noise interfered with the operation of the OSEM by degrading the measurement of the mirror location. As a result, the angular alignment and linear positioning of the suspended optics were difficult to control with the accuracy required for proper operation of the interferometer.
Above: At left, the four white cylinders spaced along the mirror's circumference are the OSEM's. An additional OSEM is located on the side of the suspended optic to control side motion. Right, a dipole magnet glued to the cylindrical side of a suspended mirror. The suspension wire supporting the mirror and the standoff which defines the contact point between the wire and the mirror are also clearly visible.
In order to proceed with commissioning activities, the laser power was reduced to around 100 mW (instead of a few wattts) so that the scattered light power measured at the OSEM photodiode was much smaller than the light from the LED within the OSEM. The downside of this is that the signal output of the interferometer was reduced. The sensitivity of the interferometer, which should be dominated at frequencies above a few hundred Hz by the amount of light circulating in the interferometer, was instead dominated by the electronic noise in the measurement apparatus. Now that the OSEM's have been replaced with a revised version which solves the problem, this important signal/noise ratio will be substantially improved. The new OSEM's solved the problem by utilizing shorter wavelength light in the shadow sensor so that it will be less sensitive to the scattered light from the main beam. Further enhancements will be implemented later this year when electronics will be added to synchronously modulate the light source and the photodiode to reject the main laser beam scattered light.
This summer, the LIGO Livingston Observatory was host to seven participants of Caltech’s Summer Undergraduate Research Fellowship program, or "SURF." (More information on SURF can be found at http://www.ligo.caltech.edu/LIGO_web/students/undergrads.html.) This year the student participants came from Caltech (Sean Hardesky), Xavier University of New Orleans (Misty Watson), the University of Texas at Austin (Lonique Coots), Southern University of Baton Rouge (Keisha Williams), Florida Tech (Hareem Tariq), New College of Florida (Homer Wolfe), and the University of Michigan (David Leibrandt). We also had one undergraduate student from MIT (Antimony Gearhardt), two undergraduates from LaTech University (Clay Westbrook and Nathan Scott), one undergraduate student from LSU (Matt Weitz), one student from SLU (Jeremy Day), and four students from the University of Texas at Brownsville (Sean Morris, Andreas Kaden, Douglas Morrison, and Robert Johnston). On top of that, we had two high school teachers who were participants in the National Science Foundation's IRET program. This program, which LIGO is participating in for the first time, provides K-12 teachers with research experience that they can translate into enrichment activities when they return to their classrooms in the fall. We thank Southeastern Louisiana University in Hammond for providing housing again this year to our students and to some of the visiting faculty who spent their summer here at LLO.
An extensive set of summer lectures has been created for this group of visitors, and many of the lectures are available on the web. We are in the process of assembling all of the lectures so that they are available to everyone. Please see http://www.ligo-la.caltech.edu/surflect2001.php. A seminar day was planned for the week of August 6 for the students to report on the results of their investigations.
Above: Professor Joe Romano lectures to an interested audience about gravitational radiation from gamma ray bursts.
Dave Leibrandt and Homer Wolfe set up instrumentation to measure the infrared absorption properties of sapphire.
Misty Watson (left) and Keisha Williams (right) prepare to make acoustic measurements which could affect LIGO performance. Also seen in the right-hand photo, LIGO staff member and SURF mentor Szabi Marka (at computer).
From top-to-bottom and left-to-right above: Hareem Tariq, Douglass Morrisson, Lonique Coots, Matt Weitz, and Sean Hardesky spent a lot of time learning new data analysis skills.
Teachers John Thacker and Wilson Doucette developed some new exhibits for our hands-on science center in the multipurpose room at LLO. At left, John is shown with a constant period pendulum; and at right John and Wilson are testing a "tautochrone," a curve that a rolling ball will traverse at constant time independent of where it is placed on the curve.