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Problem: ShaneAO NGS Dithering
Manifestation:
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Waffle patterns appear on tweeter display in closed loop when
dithering in NGS mode with scales larger than 2.5 arcseconds
using the conventional bxy script executed via the
scriptproc.tcl GUI.
16x Mode Solution/Recovery:
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Use the following temporary dithering script:
saoBxy4tmpNGS
Located at:
user@shimmy:observers/lgs/scripts/
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Syntax:
saoBxy4tmpNGS <Dither_Scale> <Number_Exposures>
Where <Dither_Scale> is an integer number of arcseconds.
As of 2016-09-14 the value can only be 1, 2, 3 or 4 (the only calibrated
distances as of 2016-09-14) and
<Number_Exposures> is the integer number of exposures at
each dither position.
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Example: To obtain 2 exposures at each position of the 4 point dither
pattern, with a dither scale of 4 arcseconds:
user@shred:observers/lgs/scripts/saoBxy4tmpNGS 4 2
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On completion of the script, before acquiring a new target, use the
following command to drive motors back on-axis:
on-axis
Located at:
user@shimmy:observers/lgs/scripts/
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WARNING: Object MUST start on-axis. Re-centering with
Move Tel in sharcsdisp will be undone by this
as they are absolute positions.
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WARNING: This script does NOT take any exposures at the central
position. Take on-axis/central position exposures before executing
the script to ensure exposure parameters are correct.
8x Mode Solution/Recovery:
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Use the following temporary 4 or 5-point dithering script:
saoBxy4tmp8xNGS
Located at:
user@shimmy:observers/lgs/scripts/
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Syntax:
saoBxytmp8xNGS <Number_Exposures> <center>
<Number_Exposures> is the integer number of exposures at
each dither position.
center is an optional addition, enabling a set of
expoures in the center of the dither pattern.
As of 2018-10-27 the only dither scale which has been calibrated is 5
arcseconds. Hence, the dither scale is coded into the script.
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Example: To obtain 2 exposures at each position of a 5 point dither
pattern, with a (default) dither scale of 5 arcseconds:
user@shimmy:observers/lgs/scripts/saoBxytmp8xNGS 2 center
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On completion of the script, before acquiring a new target, use the
following command to drive motors back on-axis:
on-axis8
Located at:
user@shimmy:observers/lgs/scripts/
Cause:
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Following July/August 2016 intervention to replace motor(s),
WFS Steering Mirror 2 stage did not go back in to the same position,
thus compromising NGS field.
Conventional bxy script with 4 arcsecond dithers works for
the first 2 off-axis positions, but the last two positions do not
work because about 40 per cent of WFS subapertures are not illuminated.
Associated Issues:
Change Log:
- 2018-10-27: Updated (Added 8x Mode).
- 2017-08-06: Updated.
- 2016-11-20: Updated.
- 2016-09-14: Updated.
- 2014-05-09: First version.
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