THE IRTF TIP-TILT SYSTEM
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 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.
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
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.
Back to IRTF Homepage
Last modified 2001 April 4