Thermal Background with Tip/Tilt Correction
If the tip-tilt mirror in the AO layer is used for correction of
atmospheric turbulence, it automatically compensates a tracking error
of the telescope as well. The mirror is also used for dithering in
order to reduce overheads. Since the tip-tilt mirror in AO36 is not
exactly located at the pupil image, these cause a residual of sky
subtraction for LM-band imaging, or K-band imaging with long integration.
The figure above shows residual of background subtraction observed
in L'-band. Two successive images were taken at the different position
of the AO tip/tilt mirror, and these are subtracted to each other. The
exposure time of each image was 9 second (0.18 sec × 50 coadds). See
Oya et al. (2004), SPIE Proc., 5490. 409 for details.
For this reason, we fix the position of the tip/tilt mirror for
these observations and do not use for AO correction. The deformable
mirror usually works well to compensates this component, however,
performance of AO correction may be degraded in some cases. This
problem will be overcome in the future by use of the secondary mirror
of the telescope for tip/tilt compensation.
7 February 2007
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