Compared to optical telescopes, IACTs have to make due with very sparse data. A typical high-energy gamma-ray shower will be a very short and weak flash of light, lasting only a few nanoseconds, and only consisting of around one hundred Cherenkov photons as an image. After applying selection criteria, that image corresponds to a single initial gamma ray at the top of the atmosphere.
Above you can see an optical image of the Crab Nebula in the constellation Taurus, remnants of a supernova which could be observed in China in 1054. Visible is a structured cloud of hydrogen gas, about 6000 lightyears from our solar system, and today some 10 lightyears across. The picture was taken by the 3.5 m WIYN telescope on Kitt Peak, Arizona. A typical exposure time is on the order of minutes.
In the gas cloud of the Crab Nebula, a source has been observed (by satellite experiments) that emits pulsed X-ray radiation (keV to MeV range), at a frequency of 30 Hz. This radiation originates from a pulsar, a spinning neutron star, in the center of the nebula. The Crab Nebula also emits VHE gamma radiation, at a fairly constant rate. This radiation has been observed by all existing IACTs, including HEGRA (image to the right).
The observation time is typically several nights. The source of this radiation (gamma rays of more than 100 GeV) is thought to be due to shock wave acceleration in the nebula. The gamma-ray image can not compete in resolution with optical observations, but the existence of VHE gamma rays, and their intensity, allows the development of models of both the source and the propagation of its emission through space.
The recording of images in a Cherenkov telescope has quite different constraints from what optical telescopes require. The phenomenon to be studied, the electromagnetic shower, is comparatively large. The extraordinary resolution of CCDs used in most optical telescopes is not required, nor is the ultimate precision in the mirror surface. Instead, sensitivity to single photons and the best possible time resolution are important, because the signal is weak. For the discrimination against non-electromagnetic showers, determining the precise arrival times is necessary. High-quality photomultipliers are used, their size is matched to the required resolution of the shower image. The pictures below show the camera of one of the HEGRA telescopes (left), and the light collecting surface in front of the camera (right), specifically developed to ensure optimal entrance angles for all incoming photons (so-called Winston cones).