In the beginning, all that was available to humans to observe the Cosmos was the human eye. However, the Universe is a much bigger place that the human eye can sense. For instance, on a very clear night, the human eye can only see 3,000 - 4,000 stars, even tho trillions exist.

In addition, as a detector, the human eye has a number of limitations:

1. The eye has limited size and therefore limited light-gathering power. this means you can't see faint stars.

2. The eye has limited frequency response, since it can only see electromagnetic radiation in the visible wavelengths. and most of the interesting physics in the Universe radiates at other wavelengths.

3. The eye distinguishes a new image multiple times a second, so it cannot be used to accumulate light over a long period in order to intensify a faint image. the eye doesn't integrate. Photons are destroyed upon impact with your retina information transmission in the universe involves particle destruction.

4. The eye cannot store an image for future reference like a photgraphic plate can your brain is an inefficient archive.

5. The eye is not a linear detector more on this later.

To do better than the eye, we need to build a light bucket (e.g. a big mirror). The more area, the more light can be collected and therefore fainter objects can be seen.

In simple terms, we construct a mirror that collects light and reflects it to a point of focus. At the focus point, we could put a detector.

These days we can make really large mirrors.

But ultimately, there are serious limitations to building telescopes on the ground (or on mountaintops).

Image resolution is limited by motions in our atmosphere. Therefore, detailed precise measurements can not be made. This is why we have the Hubble Space Telescope:


We can see fine detail (about 20 times finer) by avoiding imaging astronomical sources through an atmosphere.

Also, the night sky on the Earth is getting brighter due to aerosals and pollutants. A brighter background makes faint object detection difficult (as we will see in the Java simulations to come).

Detector Issues:

• Photographic media are only about 1% efficient. For every 100 photons that strike the detector, only 1 of them is recorded.

• Digital detectors, also called CCDs (Charge couple device) are about 80% efficient.




A Comprehensive Reference

Moving on to Digital Detectors