Photography and its Limits

While the eye is sensitive only to the light perceived instantly, the silver medium (photo plate and films) that was still used until the 1980s can record the light received during a long exposure. A camera equipped with an ordinary emulsion can detect very weak objects that the eye as exercised will never see, even by means of a telescope and will offer invaluable information. But this time during which we developed hypersensitization of films and other “cold camera” is over with the revolution of digital sensors.

Thanks to the progress made in electronics, in the late 1980s the first CCD cameras and the APN in the 1990s were offered to the public, revolutionizing especially astrophotography. These devices are constantly gaining new followers, complemented by HD webcams with amazing performance.

The combined use of the telescope and a digital sensor makes it possible to use this optical system as a conventional telephoto lens but with unparalleled resolution and sensitivity. Indeed, a digital sensor is at least 20000 times more sensitive than a film and it is possible by stacking images to increase the signal/noise ratio. This optical combination may be relatively sophisticated and at the time of the Internet some do not hesitate to robotize their installation and remote control to analyze the brightness of stars and galaxies from the comfort of their office. The resolution is ultimately defined by the collector area and the wavelength of the photosensitive receptors.

But the observation in white light at its limits. We know that the atmosphere diffuses heat, creating the inevitable atmospheric turbulence that degrades images. We have seen that this turbulence can be erased by using adaptive optics that cancel atmospheric turbulence by creating an artificial star whose brightness will be subtracted from that of the target object. This advanced technique uses a laser beam and can only be used in an ionizable atmosphere. It goes without saying that all the high places of astronomy of Hawaii, Chile or Pic-du-Midi use this technique as well as some amateurs.

Turbulence also creates a bright screen called the clarity of the sky, whose magnitude oscillates between +23 and +18 in a spectrum between 300 and 900 nm. Add to this the clarity of the night gleams and sometimes the auroras when it is not mist or clouds. These phenomena veil the sky and limit the capacity of optical instruments. One can just reduce or delay the effect of turbulence by making multiple short exposures or by increasing the sensitivity and resolution of the receptors, but it can not be eliminated.

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