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2. Generalities: why CCD?

The single most important challenge facing all of astronomy is how to extract

information from the little light that reaches Earth from the huge distances in the Universe.

We must appreciate the fact that all interesting objects except the Sun, the Moon, and the

planets (star clusters, gaseous nebulae, galaxies, comets, asteroids) are all very faint,

invisible to the naked eye, and very hard to see at all, even in large telescopes. Our first job

is to look around the sky with a telescope and draw a baseline of how much we can see

without the aid of sophisticated devices. A few objects may look interesting, but most will

not impress anyone.

What really makes the

difference in real astronomy is

the detector. The human eye can

collect light only for about

0.1 sec at a time. After that, our

eyes start collecting light for

another image, and so on. We

developed this sort of “short

exposure” because we need to

see things in motion. However, a

photographic film or a digital

camera can collect light for a

much longer time, so it can see

much fainter objects. In our

observatory we can take 15-60

minute long exposures at a time,

and combine a few to improve

the images even more. Most of

the deep-sky objects are simply invisible when we try to find them visually in the field of

the telescope.

The last 20 years saw a revolution in astronomical detectors. By now the only type of

detector that survived competition is the CCD chip (charge-coupled device). This detector

is the same type as those used in digital photography, only with significant differences in

quality and services.

A CCD chip is a small slab of semiconductor, divided up into pixels. When light hits a

pixel, the energy of the light kicks one electron over into the “well”. The chip is exposed to

light for some time, and at the end the chip is “read off”. The electrons are removed from

the “well” in each pixel, and their numbers are counted. These numbers, one for each pixel,

are downloaded onto a computer, which stores them. Upon the user’s request, the software

builds up an image from these numbers.

Up to this point a CCD is the same as any other digital camera. What distinguishes an

astronomical CCD is that it is cooled (to remove as much noise as possible), that it has

16 bit image depth (twice that of a regular camera, to increase the dynamical range of

Fig. 1. The ST10 CCD camera with the filter wheel.

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