In the beginning, all that was available to humans to observe the Cosmos was the human eye. However, despite its usefulness
on Earth, the human eye as an astronomical detector has a number of limitations.
Overcoming these limitations will require building both a telescope and a detector that is capable of gathering and storing light.
Among the limitations of the eye are the following:
The eye has limited frequency response, since it can only see electromagnetic radiation
in the visible wavelengths; most of the interesting physics in the Universe radiates at other wavelengths.
The eye distinguishes a new image multiple times a second (about 20-30 times a second), so it cannot be
used to accumulate light over a long period in order to intensify a faint image (e.g. each ‘exposure’ of the eye-brain
only lasts 1/30th to 1/20th of a second) --the eye doesn't integrate. Photons are destroyed upon impact with
your retina and that information is transmitted electro-chemically to your brain for processing.
In fact it’s a general rule of physics that information transmission in the universe involves particle destruction.
The eye cannot store an image for future reference like a photographic plate can -- your brain is an inefficient archive.
The eye is not a linear detector; that is, a light source which is intrinsically twice as bright as another one, will not appear to your eye-brain system as being twice as bright. Instead it appears to be about 30% brighter as human senses are logarithmic in nature (and the log of 2 is 0.3).
Because of the small aperture of your eye your eye can not see faint stars. Hence on a very clear moonless night, the human eye Universe consists only of about 3000 – 4000 individual stars, even though billions of stars intrinsically exist. Hence the Universe is a much, much bigger place than the human eye can actually sense.
To do better than the eye, we need to build a light bucket (e.g. a telescope). The term light bucket, however, is the appropriate analogy because one can think of incoming light (e.g. photons) as rain drops – the bigger bucket you have to collect the rain, the more drops in the bucket. Hence the bigger the telescope the larger its light gathering power becomes. However, observing through our atmosphere puts somewhat of a limitation on our ability to securely detect faint objects. This atmospheric effect will become clear in the simulation to come.
For telescopes there are two basic designs. The initial telescope built by Galileo is a
refracting telescope meaning that it consists of two lenses. The first lens refracts the light so that
it passes through a second lens that focuses it. The focal length of this telescope is essentially the
distance between the two lenses. While this kind of telescope is the simplest to actually design and
construct it has serious limitations in that one can not make lenses arbitrarily big before they just collapse under their own weight.
In practical terms, the largest refractors in the world are 30-40 inches – since we now have telescopes of diameter 10-meters,
there clearly must be another way to build a telescope!
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The alternative to the refracting telescope is the reflecting telescope which was originally conceived by Newton. In this case, a parabolic mirror is figured (and the hard part of this process is figuring the mirror correctly) so that the incoming light is reflected to a focus point where one can either place another mirror to deflect the focus to the outside of the telescope or put an actual detector at this point (called the prime focus). The great advantage of this telescope design is that the parabolic mirror can be made very large. In some cases, like the Hale 200 inch telescope, it’s a single piece of glass 200 inches in diameters (about 5-meters). For the modern large scopes, the mirror itself consists of many small segments, which act, en masse, as one large parabolic surface. The individual segments are usually hexagonal in nature as shown in the image below which represents a prototype of the Next Generation Space Telescope, currently under construction by NASA.
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But a telescope by itself is not an astronomical instrument. To compliment the telescope we must build detectors and place them at the focal point. The kinds of detectors available to us to place at the focal point are:
Photographic media (glass plates, film) (1900-1981);this means you can now use the telescope just like a long focal length lens on a camera – open the shutter and collect the light. Due to the need to maintain a sharp focus and the limitations of precise tracking by large telescopes, exposure times are generally limited to an hour or so before there is too much drift in the field of view to compromise the image
Digital detectors (just like your digital camera) (1981 - present) ;We will discuss the importance of digital detectors and their properties soon.
Image Courtesy of NASA
In addition to single telescopes, we can now produce pairs (or more) of telescopes that can synchronize together to
form an observing network. An example is the twin 10 Meter telescopes of the NASA Keck Observatory on the top of Mauna Kea.
Each telescope can be used separately or the light from both telescopes can be combine to effective have an aperture of 1.41 (square
root of 2) times
10 meters.
While on the Earth we do build telescopes on mountaintops to elevate ourselves as far away from the atmospheric turbulence as possible, this is not very successful. In practice, both image resolution and the ability to detect faint objects are greatly hindered by observations made through the atmosphere. Therefore, detailed, precise measurements can not be made from the ground. This is the principle value of the Hubble Space Telescope. Even though is mirror is only 2-meters in diameter, its ability to image the Universe without atmospheric smearing allows for very high resolution images to be made. An example image of a supernova remnant (Supernovas are discussed later on in this course) shows the exquisite level of detail which the Hubble Space Telescope (HST) can achieve. Overall, HST sees details at a level of 10-20 times finer than ground based imaging telescopes.
Image Courtesy of NASA
Another limitation on ground based observations is imposed by the increasing brightness of the night sky. This brightness is increasing because of global aerosol pollution associated with an industrialized planet. These aerosol particles reach the stratosphere and serve to increase the amount of light that is scattered. Thus light from cities is being increasing scattered in the atmosphere such that the overall level of the night sky brightness is increasing. A brighter background makes the detection of faint objects all the more difficult; all of you have experienced and noticed this whenever you might look up into the night sky during a Full Moon – not many stars are visible due to the elevated brightness of the entire sky.
Finally the evolution of detector technology is very important. Traditional photographic media is very inefficient. That is, photographic media detects only about 1% of the light that is incident upon them; for every 100 photons that strike the media, only one of them is recorded meaning that you’re throwing away 99% of the light. This clearly limits your ability to detect faint objects. In contrast, digital detectors (initially called CCDs for charged coupled devices) are about 80% efficient. In simple terms, that means that the conversion of photographic media to digital detectors would allow objects 80 times fainter to be now detected on that telescope. This was clearly a huge breakthrough in detector technology.
In 1905 Einstein showed that incident light upon certain metals would produce a current flowing through the material. Thus light energy can be converted to electrical energy by some mechanism.
Imagine the surface of our digital detector is like the graph paper shown above. Each square (pixel) can contain a certain amount of information -that is, receive a certain amount of incident light.
The more light that is received, the more electrical charge that is built up on that square. That charge is converted to a number depending upon the amount of charge there. More charge gives you higher numbers. So array of numbers might look like this.
Once we have numbers to represent the light that was incident on each pixel element in the detector we can reconstruct a picture on the computer screen by assigning a value of 0 to complete black, a value of 255 to completely white and any combination of black and white would be a level of gray (e.g. 128 would be half black and half white). The result of that process would appear as below.
Note that because the light sensitive elements are so small we usually don't see the individual picture elements ("pixels"). Reduced in size, we see what this star looks like at the usual level of detail. Individual pixels can't be seen by the eye at this scale, but they're there.
And Voila, we have Digital Imaging:
