All devices we use to see the sky

For instance, telescopes can be on land or in orbit.

Telescopes can help us focus on objects that emit visual light,

And also other types of light.

telescope

Instrument used to capture as many photons as possible from a given region of the sky and concentrate them into a focused beam for analysis.

a "light bucket" whose primary function is to capture as many photons as possible from a given region of the sky and concentrate them into a focused beam for analysis.

reflecting telescope A telescope which uses a mirror to gather and focus light from a distant object.

refracting telescope A telescope which uses a lens to gather and focus light from a distant object.

The mirror is constructed so that all light rays arriving parallel to its axis, are reflected to pass through a single point, called the focus. regardless of their distance from that axis.

This is usually called the primary mirror because telescopes often contain more than one mirror,

A refracting telescope uses a lens to focus the incoming light.

Refraction is the bending of a beam of light as it passes from one transparent medium (for example, air) into another (such as glass).

i.e. a pencil half immersed in a glass of water looks bent.

the light by which we see it is bent—refracted—as that light leaves the water and enters the air.

Refraction by a prism changes the direction of a light ray by an amount that depends on the angle between the faces of the prism.

A lens can be thought of as a series of prisms.

A light ray traveling along the axis of a lens is unrefracted as it passes through the lens.

Parallel rays arriving at progressively greater distances from the axis are refracted by increasing amounts, in such a way that all are focused to a single point.

Astronomical telescopes are often used to make images of their field of view. illustrates how this is accomplished, in this case by the mirror in a reflecting telescope.

Any ray of light entering the instrument parallel to the telescope's axis strikes the mirror and is reflected through the prime focus. Light coming from a slightly different direction—inclined slightly to the axis—is focused to a slightly different point. In this way, an image is formed near the prime focus. Each point on the image corresponds to a different point in the field of view.

Light from a distant object (in this case, a comet) reaches us as parallel, or very nearly parallel, rays.

The prime-focus images produced by large telescopes are actually quite small—the image of the entire field of view may be as little as 1 cm across.

Often, the image is magnified with a lens known as an eyepiece before being observed by eye or, more likely, recorded as a photograph or digital image.

Figure (a) shows the basic design of a simple reflecting telescope, illustrating how a small secondary mirror and eyepiece are used to view the image. Figure (b) shows how a refracting telescope accomplishes the same function.

Both types are used to gather and focus cosmic radiation—to be observed by human eyes or recorded on photographs or in computers. In both cases the image formed at the focus is viewed with a small magnifying lens called an eyepiece.

The two telescope designs shown achieve the same result—light from a distant object is captured and focused to form an image.

It might appear that there is little to choose between the two in deciding which type to buy or build.

However, as telescope size has steadily increased over the years, a number of important factors have tended to favor reflecting instruments over refractors:

1. The fact that light must pass through the lens is a major disadvantage of refracting telescopes.

Large lenses cannot be constructed in such a way that light passes through them uniformly.

Just as a prism disperses white light into its component colors, the lens in a refracting telescope focuses red and blue light differently.

This deficiency is known as chromatic aberration.

2. As light passes through the lens, some of it is absorbed by the glass.

This absorption is a relatively minor problem for visible radiation, but it can be severe for infrared and ultraviolet observations because glass blocks most of the radiation coming from those regions of the electromagnetic spectrum.

This problem obviously does not affect mirrors. 

3. A large lens can be quite heavy.

Because it can be supported only around its edge (so as not to block the incoming radiation), the lens tends to deform under its own weight.

A mirror does not have this drawback because it can be supported over its entire back surface.  

4. A lens has two surfaces that must be accurately machined and polished, which can be very difficult, but a mirror has only one.

A prism bends blue light more than it bends red light, so the blue component of light passing through a lens is focused slightly closer to the lens than is the red component. As a result, the image of an object acquires a colored "halo," no matter where we place our detector.

lensBlue focus

Red focus

White Light

Green focus

This shows the world's largest refractor, installed in 1897 at the Yerkes Observatory in Wisconsin and still in use today.

It has a lens diameter of 1 m (about 40 inches). By contrast, some new reflecting telescopes have mirror diameters in the 10 m range, and larger instruments are on the way.

Four reflecting telescope designs: (a) prime focus,

(b) Newtonian focus,

(c) Cassegrain focus, and

(d) coudé focus.

Each uses a primary mirror at the bottom of the telescope to capture radiation, which is then directed along different paths for analysis. Notice that the secondary mirrors shown in (c) and (d) are actually slightly diverging, so that they move the focus outside the telescope.

Astronomers generally prefer large telescopes over small ones, for two main reasons.

1. light-gathering power.

2. resolving power.

One reason for using a larger telescope is simply that it has a greater collecting area—the total area of a telescope capable of capturing radiation

collecting area

The total area of a telescope that is capable of capturing incoming radiation. The larger the telescope, the greater its collecting area, and the fainter the objects it can detect.

• The observed brightness of an astronomical object is directly proportional to the area of our telescope's mirror and therefore to the square of the mirror diameter.

• Thus, a 5-m telescope will produce an image 25 times as bright as a 1-m instrument because a 5-m mirror has 52=25 times the collecting area of a 1-m mirror.

• We can also think of this relationship in terms of the length of time required for a telescope to collect enough energy to create a recognizable image on a photographic plate.

• A 5-m telescope will produce an image 25 times faster than the 1 m device because it gathers energy at a rate 25 times greater.

• Expressed in another way, a 1-hour time exposure with a 1-m telescope is roughly equivalent to a 2.4-minute time exposure with a 5-m instrument.

angular resolution

The ability of a telescope to distinguish between adjacent objects in the sky.

•Two comparably bright light sources become progressively clearer when viewed at finer and finer angular resolution. •When the angular resolution is much poorer than the separation of the objects, as at the top, the objects appear as a single fuzzy "blob.”•As the resolution improves, the two sources become discernible as separate objects.

•In general, resolution refers to the ability of any device, such as a camera or a telescope, to form distinct, separate images of objects lying close together in the field of view.

•The finer the resolution, the better we can distinguish the objects and the more detail we can see

•Detail becomes clearer in the Andromeda Galaxy as the angular resolution is improved some 600 times, from (a) 10', to (b) 1', (c) 5", and (d) 1".

d

a b

c

•What limits a telescope's resolution? One important factor is diffraction, the tendency of light, and all other waves for that matter, to bend around corners

•Because of diffraction, when a parallel beam of light enters a telescope, the rays spread out slightly, making it impossible to focus the beam to a sharp point, even with a perfectly constructed mirror

•The degree of fuzziness—the minimum angular separation that can be distinguished—determines the angular resolution of the telescope. The amount of diffraction is proportional to the wavelength of the radiation divided by the diameter of the telescope mirror. As a result we can write, in convenient units,

•(a) The world's highest ground-based observatory, at Mauna Kea, Hawaii, is perched atop an extinct volcano more than 4 km above sea level. •Among the domes visible in the picture are those that house the Canada—France—Hawaii 3.6-m telescope, the 2.2-m telescope of the University of Hawaii, Britain's 3.8-m infrared facility, and the twin 10-m Keck telescopes.

•To the right of the twin Kecks is the Japanese 8.3-m Subaru telescope, still under construction. The thin air at this high-altitude site guarantees less atmospheric absorption of incoming radiation and hence a clearer view than at sea level, but the air is so thin that astronomers must occasionally wear oxygen masks while working

•The 10-m mirror in the first Keck telescope. Note the technician in orange coveralls at center.

Astronomers use the term seeing to describe the effects of atmospheric turbulence. The circle over which a star's light (or the light from any other astronomical source) is spread is called the seeing disk.

*In fact, for a large instrument—more than about 1 m in diameter—the situation is more complicated, because rays striking different parts of the mirror have actually passed through different turbulent atmospheric regions. The end result is still a seeing disk, however.

•Individual photons from a distant star strike the detector in a telescope at slightly different locations because of turbulence in Earth's atmosphere. Over time, the individual photons cover a roughly circular region on the detector, and even the point-like image of a star is recorded as a small disk, called the seeing disk.

To achieve the best possible seeing, telescopes are sited on mountaintops (to get above as much of the atmosphere as possible) in regions of the world where the atmosphere is known to be fairly stable and relatively free of dust, moisture, and light pollution from cities.

In the continental United States, these sites tend to be in the desert Southwest. The U.S. National Observatory for optical astronomy in the Northern Hemisphere, completed in 1973, is located high on Kitt Peak near Tucson, Arizona.

That site was chosen because of its many dry, clear nights. Seeing of 1" from such a location is regarded as good, and seeing of a few arc seconds is tolerable for many purposes.

Even better conditions are found on Mauna Kea, Hawaii, and at Cerro Tololo and La Silla in the Andes Mountains of Chile (image)—which is why many large telescopes have recently been constructed at those two exceptionally clear locations.

Without atmospheric blurring, extremely fine resolution—close to the diffraction limit—can be achieved, subject only to the engineering restrictions of building or placing large structures in space. The Hubble Space Telescope (HST) was launched into Earth orbit by NASA's space shuttle Discovery in 1990.

2.4-m mirror, with a diffraction limit of only 0.05", giving astronomers a view of the universe as much as 20 times sharper than that normally available from even much larger ground-based instruments.

It is becoming rare for photographic equipment to be used as the primary means of data acquisition at large observatories. Instead, electronic detectors known as charge-coupled devices, or CCDs, are in widespread use. Their output goes directly to a computer

hundreds of thousands, or even millions, of tiny light-sensitive cells, or pixels, usually arranged in a square array. Light striking a pixel causes an electrical charge to build up on it. By electronically reading out the charge on each pixel, a computer can reconstruct the pattern of light—the image—falling on the chip.