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The Fast Two-Axis Secondary Imager (1Jun93)
(from KPNO, NOAO Newsletter No. 34, 1 June 1993)

FTAS (Fast Two-Axis Secondary) is a small R&D prototype infrared imager
designed for fast correction of atmospheric wavefront tilt. It was undertaken
in the KPNO Infrared program, in coordination with members of the Gemini
instrument group. An existing dewar, NEWT, was modified to carry one of the
256 x 256 PtSi arrays, with an image scale of 0.06 arcsec/pixel. A fast CCD
camera is employed to monitor the instantaneous position of a reference star,
and to generate an error signal for closed loop servo control of the image
position.

FTAS is currently configured to operate at the 2.1-m telescope, where the
image motion is removed by tilting the telescope secondary. In several
telescope tests, FTAS has removed 80-90% of the tilt power.  Under conditions
of good seeing (uncorrected FWHM at 2.2 æm of about 0.7 arcsec) the corrected
image has achieved a FWHM of 0.3-0.35 arcsec.  Under those conditions the image
quality is limited by residual optical aberrations in the telescope.

FTAS may evolve in the future through further image improvements at the
2.1-m telescope, use with the Phoenix IR spectrograph, and modification for
operation at the 4-m telescope.

A report describing the project and initial telescope tests is available from
the undersigned.

Steve Ridgway

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