The IRTF's new tip-tilt system will is available for use with NSFCAM and SpeX. Observers need to request the use of tip-tilt in their observing proposals since scheduling of the tip-tilt secondary mirror alternates with the chopping secondary mirror. There are currently no plans to offer tip-tilt for use with CSHELL. The purpose of this brief report is to help observers prepare observing proposals.

Tip-tilt Overview

Tip-tilt is just one element of a system designed to improve the overall image quality of the IRTF. The other equally important elements are telescope optical figure (mostly primary mirror support) and alignment, and telescope dome thermal control. All three elements must work well for the telescope to deliver the ultimate goal of near-diffraction-limited imaging of 0.2-0.4 arcsec at wavelengths of 1-4 microns in average seeing conditions.

Tip-tilt consists of an active secondary mirror to reduce the tip-tilt (low order) components of seeing and telescope tracking, a tip-tilt sensor package to provide optical feedback to the active secondary, and associated electronics and computers. A light-weight silicon carbide mirror is used as the tip-tilt secondary. The mirror is attached to a piezo-actuated momentum-compensated platform. Directly attached to the platform is a hexapod which provides support, focus and optical alignment. The tip-tilt sensor package contains a filter wheel, re-imaging optics, and 800x800 pixel cooled CCD array. A cold dichroic beam-splitters inside NSFCAM and SpeX reflect the f/37 optical telescope beam into the sensor package, which is mounted to the side of the science instrument. The infrared beam is transmitted by the dichroic into the instrument.

The tip-tilt system is operated from a Unix workstation with an X-Windows graphical user interface. It will be controlled by the telescope operator and observers will only need to operate the science instrument.

Current and Expected Performance

Median image quality is now typically about 0.7 arcsec at K with a strehl ratio in the range 0.05-0.1. Tip-tilt correction further improves the strehl by a factor of two provided a suitable guide star can be found. We are currently seeing little improvement in FWHM. The current performance is limited by the 5 Hz correction rate (50 Hz sampling). We are hopeful that the rate can be increased to 10 Hz, with a corresponding improvement in FWHM and strehl, in the coming months. Over the 0.9-5.5 micron range, observers can expect to see the best images in the K- and L-bands.

Observers should keep in mind the following points when preparing observing proposals:

  • Tip-tilt FOV is 80x80 arcsec, and guide stars must be found from within this field, which is centered on the science instruments field of view.

  • When using tip-tilt, NSFCAM can be used at all three image scales (0.06, 0.15 and 0.3 arcsec/pixel). NSFCAM can also be used in its grism modes. The SpeX spectrograph has a spatial scale of 0.15 arcsec/pixel and the SpeX IR imager/slit-viewer 0.12 arcsec/pixel.

  • Both NSFCAM and SpeX contain two dichroic beam-splitters with cut-ons at 1.0 micron and 0.8 micron. Both dichroics transmit out to 5.5 micron.

  • The current tip-tilt guiding limit is V<14.5 at 5 Hz correction (50 Hz sampling) with the 1.0 micron dichroic. The guiding limit with the 0.8 micron dichroic V<13.8. We recommend that observers arrange their program to use bright guide stars whenever possible. The limiting magnitude of the tip-tilt CCD is degraded by scattered light from the moon and so observers should avoid the moon unless their guide stars are bright.

  • We plan to provide for tip-tilt on non-sidereal objects. It may also be possible to guide on the bright limb of planets.

  • Tip-tilt can be used to slow guide on objects V>14.5. The slower the rate the fainter the object.

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    Last modified 2001 April 4

    Questions to Doug Toomey