Abstract
The explorations of exoplanets and the massive black hole by SPHERE and GRAVITY attest to our final triumph over the blurred image. They have succeeded in going beyond the seeing limit and reaching the limits imposed by diffraction when observing at the VLT with infrared light. However, this is not quite the last word on blur. Hoping that my story has not seemed too long, let us take one last stroll around the summit of Paranal, where each night we gather so many new images full of myriad details.
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Notes
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The reader can download The Adaptive Optics Facility Booklet from the ESO website at https://eso.org/sci/facilities/develop/ao/images/AOF_Booklet.pdf.
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One of the many existing versions of adaptive optics in 2019 is ground layer adaptive optics (GLAO). The name refers to the fact that, in Paranal, most of the turbulence responsible for degrading our images occurs in a layer of the atmosphere about a hundred meters thick, just above ground level. This is due to the local wind systems. The proximity of the turbulence means that light rays coming from objects separated on the sky by a few tens of minutes of arc, or even more, will cross the same layer of the atmosphere and thus be simultaneously straightened out using the same reference source or artificial star. The deblurred field of observation is thus significantly extended.
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The person in charge of its construction, who also coordinated the various contributing research groups, was Bruno Lopez, who originally got involved with interferometry in Antoine Labeyrie’s group in Nice, then worked on MIDI, one of the first instruments for measuring fringes at the VLT.
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These long-haul journeys from Europe, contributing as they do to the accumulation of CO2 in the atmosphere and the threat of climate change, are becoming less and less necessary now with all the improvements in Internet communications. It is often possible today to carry out observations at the VLT from a research center in Europe.
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In the spring of 2019, the ESO organised a workshop, similar to those held over previous decades, in order to plan for Paranal in 2030, when the E-ELT will be up and running. Early in 2020, a proposed upgrade of GRAVITY, named GravityPlus, was given top priority by the ESO’s Scientific and Technical Committee.
- 10.
With the dual field method, we discussed the possibility of differential measurements, which do not produce images in the normal sense. Their astrometric accuracy in measuring the separation between two objects can go well beyond the diffraction limit, but there is no contradiction here with this limit affecting the resolution of an image. As noted in Chap. 7 , physics can provide many examples of extremely precise differential measurements, such as those used to detect gravitational waves.
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These ideal conditions are hard to achieve on Earth, where there are many sources of background noise, even though our photoelectric visible and infrared light detectors (CCDs or equivalent) are today almost perfect.
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Among the various instruments at the focal point of the JWST will be MIRI, which includes a coronagraph, proposed by Daniel Rouan. The Paris Observatory in Meudon and the French atomic energy authority, the CEA in Saclay, are in charge of that. In this NASA mission, French teams maintain their presence.
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Media interest cannot be ignored since this is a subject that sells extremely well. However, we should be cautious of overselling. That could be counterproductive in the long run, because no one is in a position to say what will become of this conjecture!
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Recall that the number of baselines for six telescopes which can all be combined pairwise is 6 × 5∕2 = 15.
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Resolution without sensitivity is useless if the target objects are not bright enough, as we saw above. Any instrument project must be clear about this point. Likewise, a large number of telescopes sharing the light in order to operate simultaneously ends up reducing the sensitivity, as we saw for OVLA.
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As discussed in Chap. 4 , with the Infrared Spatial Interferometer (ISI) on Mount Wilson, Townes and his team in Berkeley used a method analogous to the one applied in radiointerferometry, as regards the light detection technique. The fringes are formed at a very precisely specified wavelength, the reference being supplied by a laser. The measurement is then easier, but it limits the sensitivity of the instrument.
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The research center in Meudon has often been mentioned in this story. Although the Paris Observatory has seven different research centers called ‘departments’, the one I have been mentioning, to which I still belong, is the Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), which is the largest.
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These ideas are still alive and well: Defrère et al. (2018).
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Léna, P. (2020). The Future Lies in the Details. In: Astronomy’s Quest for Sharp Images. Astronomers' Universe. Springer, Cham. https://doi.org/10.1007/978-3-030-55811-6_8
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