Why are large telescopes used together with digital cameras or other electronic instruments?

And why do astronomers seldom look through such telescopes directly?

The human eyeball is a very imperfect observing instrument. Modern CCD cameras can record far fainter objects in more detail, and spectroscopes and photometers can obtain numeric data which is far easier to standardize and analyze.

The game is entirely about collecting photons. Distant objects in space are very very dim, because light from those objects has scattered as it travels the vast distance. The larger the apeture the more photons you can collect, but the density of photons is still very thin, too thin for you to see color, too thin for you to see the finer details; by using a camera you can collect photons over a large period of time, perhaps minutes, often hours, and sometimes over many months; all of which can be assembled into a single photo of remarkable detail.

It is fun to peak through a telescope and just take in the view. However, large telescopes are too expensive to use in idle pursuits, so their hours are used to maximum advantage - and, that means with electronic eyepieces and instrumentation.

Very large telescopes are not very useful to use visually. The main reason is that in order to take advantage of all that light gathering power in such a large telescope, you need a lot of magnification, and Earth's atmosphere makes using large magnifications pretty much useless. Using a more normal magnification on such large scopes wastes a lot of light and it will do little good.

A camera sensor or film plate doesn't have this problem, because light doesn't need to be returned to a collimated form for the camera to use it (like our eyes require). Additionally, cameras can take long exposures, gathering far more light than a human eye can, therefore allowing us to capture very faint light out there. Generally speaking, the human eye is not a very useful instrument for detecting things deep in space... at least not until a camera images it and presents the image to us.

Because of this, there is little benefit to direct observation when it comes to professional astronomical work. Therefore professional astronomers don't really spend time looking through telescopes as part of their job. Visual astronomy is really only good for the asthetic value of seeing some of the great wonders of the sky with one's own eyes.

Because they can collect and process more data using the pictures and electronics than they can by looking through the telescope. It is fun to look through a telescope and see things directly, but much more useful to take pictures, gather data, etc.

A camera can take long exposures which shows more detail, this is the main reason. A camera can also be made to see in wavelengths that the eye can't see.

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