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W. M. KECK OBSERVATORY
CALIFORNIA ASSOCIATION FOR RESEARCH IN ASTRONOMY
Keck Engineering Time Proposal
Title:
Date of Observation:
Lead:
K1 TSS field curvature measurements
3/11/09
Thomas Stalcup
Other persons involved:
Telescope(s)
Instrument(s)
Sky time Requested
Date(s) to avoid
Required conditions
K1
SHARC
3 hours
Purpose (Please describe succinctly the main purpose of the proposal: development project, general telescope engineering,
instrument commissioning, problem trouble shooting, data quality, etc.):
K1 TSS characterization
The focal plane at the TSS is curved so when operating with off-axis guide stars a focus
adjustment must be applied. Previous work with the K2 system showed that the curvature as
measured during the day with the fiber source was different than that measured at night using
stars, so nighttime calibrations are essential to properly calibrate the K1 TSS system.
Description (Please describe your night-time engineering plan; provide justification for the time request, and include figures,
ECR description and other attachments if necessary):
A suitable open cluster will be acquired and the AO loops closed on a central member of the cluster
using the normal NGS mode. This will ensure stable focus and pointing during the test. A target
away from transit is desirable to minimize the field rotation during the test. Since there are two
components of the field curvature, one from the telescope itself and one from the AO bench, it is
desirable to take a complete set of data at two or preferably three rotator positions.
The TSS will then be commanded to move to each star in the field that is bright enough to give
reasonable measurements. At each point the signal from the STRAP will be used to fine tune the
TSS position prior to recording at least three wavefront measurements using the LBWFS.
If the cluster does not provide adequate sampling of the field, the telescope can be offset to lock
the NGS loop onto one of the other members of the cluster. This will change the field positions of
all of the cluster members without altering the test setup.
Plan for data reduction, analysis, and reporting:
The focus measured by the LBWFS will be fit to a surface that describes the focal plane at the
TSS. The surface parameters will then be used to set up the TSS focus compensation.
Since the AO bench component of the field curvature is fixed relative to the TSS while the
telescope component is rotator dependent, the two different terms can be found if the calibration is
completed for two different rotator positions assuming that they are not both rotationally symmetric.
If successfully computed from these measurements, the magnitude of the telescope component
will determine if the rotator angle must be taken into account when calculating the TSS focus
compensation.
Other resources needed:
Status and progress reports from previous use of engineering time: