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 Earthâs 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.