9 research outputs found
SHIMM: A Low-Cost Portable Seeing Monitor for Astronomical Observing Sites
Characterisation of atmospheric seeing and optical turbulence is crucial for the design and operation of modern ground-based optical telescopes. With a new generation of extremely large telescopes being proposed and constructed, new obstacles will be faced with regards to imaging through our turbulent atmosphere. The Differential Image Motion Monitor (DIMM) has been a commonly employed seeing monitor at astronomical observing sites across the world. For decades it has inexpensively provided sites with measurements of the Fried parameter (r0). In this thesis a variation on the current DIMM design will be presented. The Shack-Hartmann Image Motion Monitor (SHIMM) employs a low order Shack-Hartman (SH) lenslet array instead of the two hole aperture mask utilised by the DIMM. The SHIMM is a low-cost, portable instrument, comprised of off-the-shelf components, making it easily duplicated and therefore ideal for comparisons of atmospheric conditions at large observing sites.
In this thesis the four key advantages of the SHIMM will be addressed. By
utilising a SH lenslet array the SHIMM can employ methods for estimating the value of r0, independent of noise; estimate the atmospheric coherence time; correct for the effect of scintillation on the measurement of r0; and produce a low-resolution fixed three layer turbulence profile. On-sky results of each feature will be presented in this thesis
SHIMM as an atmospheric profiler on the Nickel Telescope
Optimal atmospheric conditions are beneficial for detecting exoplanets via
high contrast imaging (HCI), as speckles from adaptive optics' (AO's) residuals
can make it difficult to identify exoplanets. While AO systems greatly improve
our image quality, having access to real-time estimates of atmospheric
conditions could also help astronomers use their telescope time more
efficiently in the search for exoplanets as well as aid in the data reduction
process. The Shack-Hartmann Imaging Motion Monitor (SHIMM) is an atmospheric
profiler that utilizes a Shack-Hartmann wavefront sensor to create spot images
of a single star in order to reconstruct important atmospheric parameters such
as the Fried parameter (), profile and coherence time. Due to its
simplicity, the SHIMM can be directly used on a telescope to get in situ
measurements while observing. We present our implementation of the Nickel-SHIMM
design for the one meter Nickel Telescope at Lick Observatory. We utilize an
HCIPy simulation of turbulence propagating across a telescope aperture to
verify the SHIMM data reduction pipeline as we begin on-sky testing. We also
used on-sky data from the AO system on the Shane Telescope to further validate
our analysis, finding that both our simulation and data reduction pipeline are
consistent with previously determined results for the Fried parameter at the
Lick Observatory. Finally, we present first light results from commissioning of
the Nickel-SHIMM.Comment: Conference Proceedings for 2023 SPIE Optics and Photonics, Techniques
and Instrumentation for Detection of Exoplanets X
GPI 2.0: Performance Evaluation of the Wavefront Sensor's EMCCD
The Gemini Planet Imager (GPI) is a high contrast imaging instrument that
aims to detect and characterize extrasolar planets. GPI is being upgraded to
GPI 2.0, with several subsystems receiving a re-design to improve the
instrument's contrast. To enable observations on fainter targets and increase
stability on brighter ones, one of the upgrades is to the adaptive optics
system. The current Shack-Hartmann wavefront sensor (WFS) is being replaced by
a pyramid WFS with an low-noise electron multiplying CCD (EMCCD). EMCCDs are
detectors capable of counting single photon events at high speed and high
sensitivity. In this work, we characterize the performance of the HN\"u 240
EMCCD from N\"uv\"u Cameras, which was custom-built for GPI 2.0. The HN\"u 240
EMCCD's characteristics make it well suited for extreme AO: it has low dark
current ( 0.01 e-/pix/fr), low readout noise (0.1 e-/pix/fr at a gain of
5000), high quantum efficiency ( 90% at wavelengths from 600-800 nm; 70% from
800-900 nm), and fast readout (up to 3000 fps full frame). Here we present test
results on the EMCCD's noise contributors, such as the readout noise,
pixel-to-pixel variability and CCD bias. We also tested the linearity and EM
gain calibration of the detector. All camera tests were conducted before its
integration into the GPI 2.0 PWFS system.Comment: 16 pages, 14 figures. Conference Proceedings for AO4ELT7, held in
June 2023 in Avignon, Franc
Observations of the dynamic turbulence above La Palma using Stereo-SCIDAR
Stereo-SCIDAR is a generalised-SCIDAR instrument which is used to characterise the atmospheric optical turbu- lence in terms of strength (Cn2) and wind velocity profile using the triangulation technique and an optical binary star. Stereo-SCIDAR differs from most other SCIDAR instruments in that, instead of overlapping pupil images on a single detector, the image of each star is recorded on a separate EMCCD. Separating the pupil images in this way leads to several advantages, including better signal to noise ratios, larger useable magnitude difference of the target stars and reliable automated wind velocity measurements. The data is analysed and made available to observatory systems in real-time. Here we review the Stereo-SCIDAR technique and present recent results from the instrument on the Isaac Newton Telescope, La Palma
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An Integrated MASS/DIMM Monitor Based on a Low-Noise CCD Detector
We propose a novel design for a turbulence profiler. Using a single detector, images of the pupil (scintillation) and stars (image motion) are formed in the detector plane. The instrument is called FASS (Full Aperture Scintillation Sensor), as it uses the full aperture of the telescope. Different processing strategies are evaluated, including spatial segmentation and Fourier analysis. The different approaches are tested via simulation and on-sky data from two telescopes and compared to profiles obtained with the Durham Stereo-SCIDAR monitor. Overall, simulations shows that the method is more accurate that the classical MASS configuration, but it is shown that the photon noise plays an important role in the accuracy of the method, imposing stringent requirements on the pixel size, which must be significantly smaller than the speckle size formed from turbulence close to the ground (Fresnel law for speckle size).Encouraging results have been obtained for on-sky data and compared to contemporaneous profiles obtained with a Stereo-Scidar technique
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An Integrated MASS/DIMM Monitor Based on a Low-Noise CCD Detector
We propose a novel design for a turbulence profiler. Using a single detector, images of the pupil (scintillation) and stars (image motion) are formed in the detector plane. The instrument is called FASS (Full Aperture Scintillation Sensor), as it uses the full aperture of the telescope. Different processing strategies are evaluated, including spatial segmentation and Fourier analysis. The different approaches are tested via simulation and on-sky data from two telescopes and compared to profiles obtained with the Durham Stereo-SCIDAR monitor. Overall, simulations shows that the method is more accurate that the classical MASS configuration, but it is shown that the photon noise plays an important role in the accuracy of the method, imposing stringent requirements on the pixel size, which must be significantly smaller than the speckle size formed from turbulence close to the ground (Fresnel law for speckle size).Encouraging results have been obtained for on-sky data and compared to contemporaneous profiles obtained with a Stereo-Scidar technique
SHIMM: a versatile seeing monitor for astronomy
Characterization of atmospheric optical turbulence is crucial for the design and operation of modern ground-based optical telescopes. In particular, the effective application of adaptive optics correction on large and extremely large telescopes relies on a detailed knowledge of the prevailing atmospheric conditions, including the vertical profile of the optical turbulence strength and the atmospheric coherence time-scale. The Differential Image Motion Monitor (DIMM) has been employed as a facility seeing monitor at many astronomical observing sites across the world for several decades, providing a reliable estimate of the seeing angle. Here, we present the Shack–Hartmann Image Motion Monitor (SHIMM), which is a development of the DIMM instrument, in that it exploits differential image motion measurements of bright target stars. However, the SHIMM employs a Shack–Hartmann wavefront sensor in place of the two-hole aperture mask utilized by the DIMM. This allows the SHIMM to provide an estimate of the seeing, unbiased by shot noise or scintillation effects. The SHIMM also produces a low-resolution (three-layer) measure of the vertical turbulence profile, as well as an estimate of the coherence time-scale. The SHIMM is designed as a low-cost, portable instrument. It is comprised of off-the-shelf components so that it is easy to duplicate and well suited for comparisons of atmospheric conditions within and between different observing sites. Here, the SHIMM design and methodology for estimating key atmospheric parameters will be presented, as well as initial field test results with comparisons to the Stereo-SCIntillation Detection And Ranging instrument