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NOAO Newsletter - CTIO Operations - March 2000 - Number 61


A Tip/Tilt System for the Blanco Telescope

P. Bouchet (CTIO, pbouchet@noao.edu), S. Lawrence (Columbia University), Crotts (Columbia University), and S. Heathcote (CTIO, sheathcote@noao.edu)

The Tip/Tilt System at the CTIO Blanco 4-m telescope offers first-order wavefront correction for high spatial resolution near-infrared imaging at the f/14 focus. The system is very user friendly, and doesn't require special technical skills from the observer to be operated efficiently. Advance preparation by the astronomer consists of identifying suitable guide stars at V = 8-16 magnitude within 2.5' radius of the science target. The target itself can be used if it is pointlike and sufficiently bright at visible wavelengths. At the telescope, initial acquisition and setup is performed by the operator. The observer can interact with the guider and telescope to move the science FOV while guiding, disable guiding to go off to sky, move the guider to reacquire a guide star, etc.

The Tip/Tilt System is used in conjunction with CTIO's infrared instruments: CIRIM, OSIRIS, and IRS. It couples an optical sensor and active secondary to the instrument; a guider box ahead of the instrument provides guiding signals to the telescope at a slow rate (once every few seconds) and to the steerable f/14 secondary at a high rate (10-100 Hz).

The system can in principle operate under very poor seeing conditions; the maximum amplitude and frequency of the system are 3" at 230 Hz. However, rarely are these limits pushed: typical displacements are < 1" at lower frequencies (the system is not designed for "chopping," or oscillations with amplitudes of many arcseconds). The optical quality of the mirror surfaces is the ultimate limit on system performance --- this is currently being evaluated by wavefront curvature and Hartmann tests.

A complete description of the Tip/Tilt System, together with technical information, can be found in the Users Manual (P. Bouchet et al., 1999) included under the CTIO web page (ftp://www.ctio.noao.edu/ctio/tiptilt/).

As an example of the capabilities of the Tip/Tilt System, we present a series of near-infrared images taken during our campaign to monitor the evolution of SN 1987A as its ejecta impact with the circumstellar rings and give birth to a supernova remnant. For the observations reported here, we closed the loop on a nearby star, (Star 2, 3" to the NW of SN 1987A, V ~ 14.5) with a frame rate of 50 ms. The resulting cutoff frequency for the uncorrected vs. Tip/Tilt corrected power spectra is ~ 1 Hz. This allowed us to obtain images with FWHM (prior to deconvolution) of 0.3" - 0.5" with seeing of 0.6"-0.9" (see the Highlights Section for further discussion of these observations).


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