TY - JOUR
T1 - Measurements of speckle lifetimes in near-infrared extreme adaptive optics images for optimizing focal plane wavefront control
AU - Goebel, Sean B.
AU - Guyon, Olivier
AU - Hall, Donald N.B.
AU - Jovanovic, Nemanja
AU - Lozi, Julien
AU - Martinache, Frantz
N1 - Publisher Copyright:
© 2018. The Astronomical Society of the Pacific. All rights reserved.
PY - 2018/10
Y1 - 2018/10
N2 - Although extreme adaptive optics (ExAO) systems can greatly reduce the effects of atmospheric turbulence and deliver diffraction-limited images, our ability to observe faint objects such as extrasolar planets or debris disks at small angular separations is greatly limited by the presence of a speckle halo caused by imperfect wavefront corrections. These speckles change with a variety of timescales, from milliseconds to many hours, and various techniques have been developed to mitigate them during observations and during data reduction. Detection limits improve with increased speckle reduction, so an understanding of how speckles evolve (particularly at near-infrared wavelengths, which is where most adaptive optics science instruments operate) is of distinct interest. We used a SAPHIRA detector behind Subaru Telescope’s SCExAO instrument to collect H-band images of the ExAO-corrected point-spread function (PSF) at a frame rate of 1.68 kHz. We analyzed these images using two techniques to measure the timescales over which the speckles evolved. In the first technique, we analyzed the images in a manner applicable to predicting performance of real-time speckle-nulling loops. We repeated this analysis using data from several nights to account for varying weather and AO conditions. In our second analysis, which follows the techniques employed by Milli et al. (2016) but using data with three orders of magnitude better temporal resolution, we identified a new regime of speckle behavior that occurs at timescales of milliseconds. It is not purely an instrument effect and likely is an atmospheric timescale filtered by the ExAO response. We also observed an exponential decay in the Pearson’s correlation coefficients (which we employed to quantify the change in speckles) on timescales of seconds and a linear decay on timescales of minutes, which is in agreement with the behavior observed by Milli et al. For both of our analyses, we also collected similar data sets using SCExAO’s internal light source to separate atmospheric effects from instrumental effects.
AB - Although extreme adaptive optics (ExAO) systems can greatly reduce the effects of atmospheric turbulence and deliver diffraction-limited images, our ability to observe faint objects such as extrasolar planets or debris disks at small angular separations is greatly limited by the presence of a speckle halo caused by imperfect wavefront corrections. These speckles change with a variety of timescales, from milliseconds to many hours, and various techniques have been developed to mitigate them during observations and during data reduction. Detection limits improve with increased speckle reduction, so an understanding of how speckles evolve (particularly at near-infrared wavelengths, which is where most adaptive optics science instruments operate) is of distinct interest. We used a SAPHIRA detector behind Subaru Telescope’s SCExAO instrument to collect H-band images of the ExAO-corrected point-spread function (PSF) at a frame rate of 1.68 kHz. We analyzed these images using two techniques to measure the timescales over which the speckles evolved. In the first technique, we analyzed the images in a manner applicable to predicting performance of real-time speckle-nulling loops. We repeated this analysis using data from several nights to account for varying weather and AO conditions. In our second analysis, which follows the techniques employed by Milli et al. (2016) but using data with three orders of magnitude better temporal resolution, we identified a new regime of speckle behavior that occurs at timescales of milliseconds. It is not purely an instrument effect and likely is an atmospheric timescale filtered by the ExAO response. We also observed an exponential decay in the Pearson’s correlation coefficients (which we employed to quantify the change in speckles) on timescales of seconds and a linear decay on timescales of minutes, which is in agreement with the behavior observed by Milli et al. For both of our analyses, we also collected similar data sets using SCExAO’s internal light source to separate atmospheric effects from instrumental effects.
KW - Adaptive optics-instrumentation
KW - Atmospheric effects-instrumentation
KW - High angular resolution
KW - High angular resolution-methods
KW - Statistical-techniques
UR - http://www.scopus.com/inward/record.url?scp=85055673126&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85055673126&partnerID=8YFLogxK
U2 - 10.1088/1538-3873/aad8ed
DO - 10.1088/1538-3873/aad8ed
M3 - Article
AN - SCOPUS:85055673126
SN - 0004-6280
VL - 130
JO - Publications of the Astronomical Society of the Pacific
JF - Publications of the Astronomical Society of the Pacific
IS - 992
M1 - 104502
ER -