J/A+A/649/A158 CANARY ELT-elongated LGS AO telemetry (Bardou+, 2021)
ELT-scale elongated LGS wavefront sensing: on-sky results.
Bardou L., Gendron E., Rousset G., Gratadour D., Basden A.,
Bonaccini Calia D., Buey T., Centrone M., Chemla F., Gach J.-L., Geng D.,
Hubert Z., Laidlaw D.J., Morris, T.J., Myers R.M., Osborn J., Reeves A.P.,
Townson M.J., Vidal F.
<Astron. Astrophys. 649, A158 (2021)>
=2021A&A...649A.158B 2021A&A...649A.158B (SIMBAD/NED BibCode)
ADC_Keywords: Optical
Keywords: instrumentation: adaptive optics - methods: observational -
telescopes - atmospheric effects
Abstract:
Laser Guide Stars (LGS) allow Adaptive Optics (AO) systems to reach
higher sky coverage and correct the atmospheric turbulence on wider
field of views. However LGS suffer from limitations, among which is
their apparent elongation which can reach 20 arcseconds when observed
with large aperture telescopes such as the European Southern
Observatory's 39m telescope. The consequences of these extreme
elongations have been studied in simulations and lab experiments, but
never on-sky. Yet understanding and mitigating those effects is key to
taking full advantage of the Extremely Large Telescope (ELT) six LGS.
In this paper, we study the impact of wavefront sensing with an
ELT-scale elongated LGS using data obtained on-sky with the AO
demonstrator CANARY on the William Herschel telescope (WHT) and ESO's
Wendelstein LGS unit. CANARY observed simultaneously a natural guide
star and a superimposed LGS launched from a telescope placed 40 m away
from the WHT pupil.
Comparison of the wavefronts measured with each guide star allows to
build an error breakdown of the elongated LGS wavefront sensing. With
this error breakdown, we isolate the contribution of the LGS
elongation and study its impact. We also investigate the effects of
truncating or undersampling the LGS spots.
We successfully used the elongated LGS wavefront sensor (WFS) to drive
the AO loop during on-sky operations, but it necessitated regular
calibrations of the non-common path aberrations on the LGS WFS arm. In
the off-line processing of the data collected on-sky, we separate the
error term encapsulating the impact of LGS elongation in a dynamic and
quasi-static component. We measure errors varying from 0 to 160nm rms
for the dynamic error and are able to link it with turbulence strength
and spot elongation. The quasi-static errors are significant and vary
between 20 to 200nm rms depending on the conditions. They also
increase by as much as 70nm in the course of 10 min. We do not
observe any impact when undersampling the spots with pixel scales as
large as 1.95 arcseconds but significant errors appear when truncating
the spots. These errors appear for field of views smaller than 10.4 to
15.6 arcseconds, depending on the spots elongations. Translated to the
ELT observing at zenith, elongations as long as 23.5 arcseconds must
be accommodated, corresponding to a field of view of 16.3 arcseconds
if the most elongated spots are put in the diagonal of the
subaperture.
Description:
The previous results were obtained from 259 datasets containing each
2500 to 5000 frames of data. Each of these datasets is recorded in a
different fits file. Each fits file contains cubes of the two
wavefront sensor images with flat and dark corrected, Deformable
Mirror (DM) commands, the corresponding timestamps and the necessary
data to reprocess the WFS images: subapertures coordinates, DM
interaction matrix with each WFS and NGS WFS reference slopes.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 92 259 List of fits files
fits/* . 259 Individual fits datacubes
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Right Ascension of center (J2000)
4- 5 I2 min RAm Right Ascension of center (J2000)
7- 10 F4.1 s RAs Right Ascension of center (J2000)
12 A1 --- DE- Declination of center sign (J2000)
13- 14 I2 deg DEd Declination of center (J2000)
16- 17 I2 arcmin DEm Declination of center (J2000)
19- 22 F4.1 arcsec DEs Declination of center (J2000)
24- 26 I3 --- Nx Number of pixels along X-axis
28- 30 I3 --- Ny Number of pixels along Y-axis
32- 35 I4 --- Nz Number of slices
37- 55 A19 "datime" Obs.date Observation date
57- 58 I2 --- Seq [1/15] Sequence to which the dataset
belongs (1)
60- 66 I7 Kibyte size Size of FITS file
68- 92 A25 --- FileName Name of FITS file, in subdirectory fits
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Note (1): Sequence indexes as follows:
Table 3: Observation conditions for each sequence of selected data.
-------------------------------------------------------------------------------
Seq Asterism Beginning Dist Zenith LGS elongation LGS LGS Flux r0
name of sequence LLT-WHT angle above 20% FWHM
Obs for 39m
(UTC) (m) (deg) (") (") (") (103e-) (cm)
-------------------------------------------------------------------------------
1 A349 28/09 05h51m 36.1 6 15.9 16.9 2.0 1.2 11.3
2 A34 28/09 21h22m 37.8 33 16.9 20.8 2.0 1.5 10.6
3 A34 28/09 21h49m 38.8 39 16.4 22.4 2.0 1.3 10.8
4 A53 28/09 22h51m 31.8 18 15.1 19.7 1.9 1.5 11.7
5 A53 28/09 23h13m 33.3 15 14.5 18.2 1.9 1.5 12.0
6 A53 29/09 02h37m 39.0 35 12.3 14.7 2.0 1.2 13.4
7 A53 29/09 03h02m 39.0 40 12.2 16.0 2.2 1.1 12.5
8 A53 29/09 03h26m 38.8 45 11.2 16.1 2.1 0.9 14.7
9 A349 29/09 04h43m 32.1 20 13.1 16.9 2.0 1.7 12.4
10 A349 29/09 05h05m 33.7 15 14.0 16.7 2.1 1.7 10.5
11 A349 29/09 05h27m 35.2 10 15.8 17.6 2.1 1.9 10.8
12 A349 29/09 05h50m 36.2 6 16.5 18.1 2.1 2.0 10.0
13 G413 02/10 01h24m 33.3 20 13.8 16.8 2.1 1.0 10.8
14 G413 02/10 01h44m 34.4 19 14.3 17.3 1.9 1.2 14.8
15 G413 02/10 02h07m 35.5 19 14.2 16.3 1.7 1.2 22.1
-------------------------------------------------------------------------------
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Description of fits files:
HEADER
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Card name Units Explanations
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DATE-OBS --- Date of observation with format YYYY-MM-DD HH:mm:ss
TELESCOP --- Telescope name
TELDIAM meter Telescope diameter
TELOBS --- Telescope central obscuration ratio
INSTRUME --- Instrument name
RA --- Telescope right ascension with format "hh mm ss"
DEC --- Telescope declination with format "+dd mm ss"
NA_BASE meter Distance between telescope and laser launch telescope
perpendicular to the pointing direction
AIRMASS --- Airmass - inverse of cosine of zenith angle
LS_DIST meter Altitude to which the LGS WFS is conjugated
GS_NAME --- Name of guide star
AST_NAME --- Name of asterism (CANARY convention)
SEQUENCE --- Index of sequence to which the dataset belongs
GAINDM --- Gain of main loop driving the 52-actuator deformable
and Tip-Tilt mirror
GSTEER --- Gain of loop driving steering mirror
DITH_FR Hz Frequency of tip-tilt mirror modulation
FREQ Hz Loop frequency
NFRAMES --- Number of frames
LS_NXSUB --- Number of subapertures across the pupil for the LGS WFS
TS_NXSUB --- Number of subapertures across the pupil for the NGS WFS
LS_PXARC arcsec/pix Pixel scale of the LGS WFS
TS_PXARC arcsec/pix Pixel scale of the NGS WFS
LS_PITCH --- Number of pixel across one subaperture for the LGS WFS
TS_PITCH --- Number of pixel across one subaperture for the LGS WFS
LS_LAT frame Latency between LGS WFS and DM commands
TS_LAT frame Latency between NGS WFS and DM commands
LSCAMG ADU/e- Gain of LGS WFS camera
TSCAMG ADU/e- Gain of NGS WFS camera
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EXTENSIONS
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Ext Dimension Units Explanations
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1 264 x 242 x NFRAMES ADU LGS WFS images compensated for dark and flat
2 128 x 128 x NFRAMES ADU NGS WFS images compensated for dark and flat
3 56 x NFRAMES volt Actuators commands (2)
4 NFRAMES s Timestamps for the first 3 extensions
5 72 x 56 pix/volt NGS WFS interaction matrix
6 72 x 56 pix/volt LGS WFS interaction matrix
7 72 pix TS reference slopes
8 36 x 2 pix TS subaperture center position (3)
9 36 x 2 pix LS subaperture center position (3)
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Note (2): Actuators 1-52: DM - actuators 53-54: tip-tilt mirror -
actuators 55-56: steering mirror
Note (3): x-axis positions are in the first column, y-axis position on the
second.
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Additional notes:
The LGS WFS must be rotated by 90 degrees towards the right to share
the on-sky orientation of the NGS WFS images. The interaction matrices
and subaperture positions correspond to the native orientation of
their respective WFS. Whenever all slopes are in one vector (e.g. the
NGS WFS reference slopes), all slopes along the x-axis are followed by
all slopes on the y-axis.
Acknowledgements:
Lisa Bardou, lisa.f.bardou(at)durham.ac.uk
(End) Patricia Vannier [CDS] 01-Mar-2021