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Session 9 - Instruments, Techniques.
Display session, Monday, June 09
South Main Hall,
A novel, low cost f/15 tip-tilt secondary mirror is now in operation on the Blanco 4-m telescope on Cerro Tololo. Drivers for our tip-tilt system were rapid deployment at low cost with good performance. All were achieved by limiting the functionality to small angular motions--the system does not "chop" over tens of arcseconds. We reconfigured a flip mirror assembly and Cervit blank, constructed but not deployed when the telescope was built 25 years ago. The blank was lightweighted and figured in the NOAO optical shop, and the mirror cell adapted to accept commercial piezoelectric actuators for driving the mirror. The piezos provide +/- 1.5 arcsec of throw with a natural frequency greater than 200 Hz when coupled to the mirror blank. However, no reactive mass could be easily added to the system, so low level mechanical resonances appear when the piezos are driven at full amplitude with frequencies greater than 65Hz. A guider box with an XY stage having +/- 2.5 arcminutes of travel has been built for use with our near-IR instruments. A standard CTIO CCDTV camera, modified for high speed readout of a small subraster, views the tiptilt reference star through a dichroic mirror and provides feedback for the secondary mirror. The current CCD TV system can sample images at greater than 250Hz on stars as bright at 6th magnitude. At this speed, all image motion at frequencies lower than 25-30Hz is removed and the residual image motion has an RMS of less than 0.02 arcseconds. During very limited testing at the telescope the FWHM of K band images decreased by 0.2" arcseconds and the intensity of the brightest pixel increased by 60The system appears very robust and performance is rather insensitive to the exact tuning of the servo feedback loop. Currently we can operate with stars as faint at 16th magnitude with camera sampling speeds of greater than 20Hz. We plan to upgrade the CCD during the next few months which should lead to an additional gain in sensitivity of 1.5-2 magnitudes.
Program listing for Monday