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6 March, 2023

First On-Sky Demonstration of Tomographic Scintillation Correction

High precision ground-based photometry is vital for a range of studies that look for small intrinsic variations in the intensity of astronomical sources. However, the effects of the Earth’s atmosphere can significantly limit such observations. As the light from an astronomical source passes through the atmosphere, regions of optical turbulence at high altitudes produce intensity fluctuations which change over time as the turbulence moves with the wind. This results in photometric noise known as scintillation, an effect which can also be seen with the naked eye as the twinkling of stars.

Scintillation noise can be the dominant noise source for photometric measurements of bright targets when observing with ground-based telescopes; no matter how large the telescope aperture, the effect enforces a photometric precision limit. It is unique to the conditions of the exact time of exposure, and is unpredictable a priori.

If one could compensate for the scintillation noise, ground-based telescopes would effectively reach the same precision limit as telescopes in space. However, this is a significant challenge because scintillation is produced by high altitude turbulence and therefore the range of angles over which it is correlated is very small. Hence, it cannot be corrected with standard differential photometry.

In 2016 James Osborn (Durham University) proposed a scintillation correction technique for large telescopes. If the relative heights and strengths of the turbulence profile above the telescope are known, then Wavefront Sensor (WFS) data from several reference stars located near to the astronomical target can be combined using a tomographic algorithm.

The result is a 3D estimate for the instantaneous optical phase aberrations above the telescope. From this 3D model, the estimated intensity fluctuations at the ground are calculated. This estimated scintillation pattern can then be used to normalise the measured photometric data. A key benefit to this method is that, so long as the WFS telemetry data is recorded, the numerical scintillation correction can be applied in post-processing analyses, and need not be applied in real time.

An experiment was designed by Hartley and collaborators to test this technique using the Isaac Newton Telescope (INT) in September 2021. The proof of concept experiment used a single WFS and a SCIDAR turbulence profiler attached to the INT. A reflecting prism was used to alternate between the SCIDAR measurements and WFS data measurements as shown in the accompanying photograph.

A photo of the instruments connected to the INT. Label A shows the prism that is used to direct the light to one instrument or the other, label B shows the SCIDAR instrument and label C shows the WFS optics and detector. Large format: PNG.

Hartley and collaborators chose the Orion Trapezium Cluster as an ideal target as it contains three bright stars within 21.5” of each other and was visible at this time of year. The brightest star in the field was selected as a target star to correct for scintillation noise. Data packets of 50 WFS frames with an exposure time of 0.1s were collected. The WFS was used to measure the wavefront aberrations for all three of the stars and to perform photometry.

The SCIDAR measurements were used to produce an estimate of the turbulence profile above the telescope. A tomographic reconstruction matrix based on this profile was produced and applied to the WFS data. A 3D model of the optical phase aberrations was made and the resulting scintillation pattern summed to produce the reconstructed intensity, which was then compared to the measured photometry of the target star. The best performing light curve is shown in the figure.

The measured normalised intensity and the normalised tomographically reconstructed intensity for the best performing data packet. The correlation coefficient between both is 0.86. Large format: PNG.

The strong correlation between the measured and reconstructed light curves shows that the tomographic reconstruction is correctly estimating the low frequency intensity variations. Normalising the measured photometry of this data packet with the reconstructed intensity reduced the scintillation index, or the variance of the relative intensity fluctuations, by a factor of 3.41, corresponding to a reduction in the scintillation RMS noise by a factor of 1.85. For all data packets the scintillation index was reduced on average by a factor of 1.9.

The team was therefore able to reduce the impact of scintillation noise with a method that is theoretically transferable to any detector mounted on any large telescope.


About the Isaac Newton Telescope

Based on observations made with the Isaac Newton Telescope (INT) operated on the island of La Palma by the Isaac Newton Group of Telescopes (ING) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC). The ING is funded by the Science and Technology Facilities Council (STFC-UKRI) of the United Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands, and the IAC in Spain. IAC's contribution to ING is funded by the Spanish Ministry of Science, Innovation and Universities.


Journal article

Kathryn E. Hartley, Oliver J. D. Farley, Matthew J. Townson, James Osborn, Richard W. Wilson, 2023, "First on-sky demonstration of a scintillation correction technique using tomographic wavefront sensing", MNRAS, 520, 4134. Paper: DOI 10.1093.

Contacts

Kathryn E Hartley (Centre for Advanced Instrumentation, University of Durham)
kathryn.e.hartleydurham.ac.uk

Javier Méndez (ING PR Officer)
outreaching.iac.es


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Last modified: 06 March 2023