Unformatted text preview: Astronomy Chapter 6
Astronomical Instruments
Telescopes, Visible-Light Detectors and Instruments
Radio Telescopes, Observations outside Earth’s atmosphere,
The Future of Large Telescopes There may seem an overwhelming number of stars in the sky, but really, only about 9000 are visible.
Light from other stars in our solar system is sometimes so weak that by the time the light reaches Earth, it
cannot be detected by the human eye.
Radiation and Spectra has made it possible to detect electromagnetic radiation at all wavelengths, from
gamma rays to radio waves. The different wavelengths carry different kinds of information, and the
appearance of any given object often depends on the wavelength at which the observations are made.
There are three basic components of a modern system for measuring radiation from astronomical sources.
First there is a telescope which serves as a bucket for collecting visible light. Larger telescopes can
capture more light than your eye can. Second, there is an instrument attached to a telescope that sorts the
incoming radiation by wavelength. Third we need a detector, a device that senses the radiation
wavelength regions we have chosen and permanently records the observations.
The three basic components are; the telescopes, the wavelength-sorting device, and the detectors.
Many ancient cultures build special sites called observatories. In these observatories, they could measure
positions of celestial objects, mostly to keep track of time and date. Many of these observatories had
religious and ritual functions as well.
The eye was the only device available to gather light, and the only permanent records of the observations
made were written down or sketched.
Hans Lippershey, Zaccharias Janssen, and Jacob Metius are all credited with the invention of the
telescope around 1608, all applying for patents within weeks of eachother. However, Galileo in 1610
invented his “spyglass” to observe the sky and gather more light than eyes could. This revolutionized idea
about the nature of planets and the position of the Earth.
The most important functions of a telescope are 1) to collect the faint light from an astronomical source
and 2) to focus all the light into a point or an image.
Most objects that interest astronomers are extremely faint, so the more light we can collect, the better we
can study these objects.
Although we are focused on visible light, other telescopes can collect visible radiation using a mirror lens.
In all telescopes, the light-gathering ability is determined by the areoa of the device acting as the lightgathering “bucket”.
Since most telescopes have mirrors or lenses, we can compare their light-gathering power by comparing
the apertures, or diameters of the opening through which light travels or reflects. The amount of light a telescope can collect increases with the size of the aperture, or diameter, of the
opening through which light travels or reflects.
A telescope with a mirror that is 4 meters in diameter can collect 16 times as much light as a telescope
that is 1 meter in diameter.
πd2/4 the diameter is squared because the area of a circle equals πd2/4 where d is the diameter of the
circle.
Once telescopes form an image, we need a way to record it. Before the 19 th century, astronomers were
only able to write down a description of what they saw. Then in the 19 th century, photographs became
widespread.
Similar to photographs these days that are taken digitally, astronomers use the photographs to analyze
collected data and rarely actually look through these large telescopes.
A transparent piece of material that bends the rays of light passing through it takes the parallel lines of
light and focuses them to one point called the focus. The distance from the lens to the location where the
light rays focus behind the lens is called the focal length of the lens.
Stars and astronomical objects are extremely far away, by the time the fews rays of light pointed towards
us actually arrive at Earth, they are parallel to each other.
To view these transformed images by the lens of a telescope, we use an additional lens called an eyepiece.
The eyepiece focuses an image at a distance that is either directly viewable by a human or at a convenient
place for a detector.
Using different eyepieces, we can change the magnification of the image and also redirect the light to a
more accessible location.
Stars look like points of light and magnifying them makes little difference. But the image of a planet or
galaxy that has structure, benefits from being magnified.
When thinking of a telescope, its easy to imagine a long tube with a large glass lens at one end. This
design, which uses a lens as its main optical element to form an image, is known as a refractor. A
telescope based on this design is called a refracting telescope. Galileo’s telescopes were refractors, as are
today’s binoculars and field glasses. There are limits to refracting telescopes. The largest one ever built
was a 49-inch refractor built for the Paris 1900 Exposition, and was dismantled after its exposition. The
largest current refractor is the 40-inch refractor at Yerkes Observatory in Wisconsin.
One problem with refracting telescope is that the light must pass through the lens of the refractor,
meaning the glass must be perfect all the way through, and it has proven difficult to make large pieces of
glass without flaws and bubbles in them.
Optical properties of transparent materials change with wavelengths or colors of light so there is
additional distortion, known as chromatic aberration.
Telescopes designed with mirrors avoid problems of refracting telescopes.
Most astronomical telescopes today, use a mirror rather than a lens to form an image. This type of
telescope is called a reflecting telescope. The first successful reflecting telescope was built in 1668 by
Isaac Newton. In reflecting telescopes, the concave mirror is placed at the bottom of a tube or open framework. The
mirror reflects light back up the tube to form an image near the front end at a location called the prime
focus.
Since an astronomer at the prime focus can block much light coming into the main mirror, the use of a
small secondary mirror allows more light to get through the system.
Prime focus. Newtonian focus. Cassegrain focus.
Active control is a process used to correct sag of mirrors that reflect errors.
The best observatory sites are on mountains, far from the lights and pollution of cities.
Large observatories today, require supporting staff of 20-100 people in addition to the astronomers on
site.
Earth’s atmosphere presents challenges on astronomical observations. Weather conditions, such as clouds,
wind and rain affect observations, so the best sites have clear weather as much as 75 percent of the time.
Even on a clear night, the atmosphere filters out certain amounts of starlight, especially in infrared.
Therefore, dry sites are preferred, generally at higher altitudes.
Skies above the telescopes should be dark. Near cities, the air glares from lights producing illumination
that hides the faintest stars and limits distances that can be seen. Astronomers call this light pollution.
Observatories are best located at least 100 miles from the nearest large city.
Finally, air is unsteady. Light passing though disturbs the images, resulting in blurred observations.
Astronomers call this effect bad seeing. When seeing is bad, images of celestial objects are distorted by
the constant twisting and bending of light rays in turbulent air.
The best observatories are high, dark and dry.
In addition to gathering light, astronomers want to have the clearest picture possible. Resolution refers to
the precision of detail in an image, Making the smallest features distinguishable.
A factor that determines resolution of a telescope is the sized of the telescope. Larger apertures (openings)
produce sharper images.
Twinkling stars are an example of atmospheric interruptions. As the star twinkles, its light is passing
through air particles that bend and distort the light you see, making the star appear to twinkle. In space,
however, the light from stars is steady.
Adaptive Optics beat Earth’s atmosphere at its game of blurring. This technique makes use of small
flexible mirrors placed in the beam of a telescope. Sensors measure how much the atmosphere distorts
and image and as often as 500 times per second, it sends corrections on how to fix the shape in order to
compensate for distortions that are produced by the atmosphere.
It is a common misconception that astronomers spend every moment in observatories, but the reality is
that they spend little time there gathering the data and the rest of their time at their professional university
or elsewhere analyzing the data collected. Many astronomers use radio telescopes for space experiments
which work equally as well during daylight hours.
Even when astronomers are working with telescopes, they rarely peer through them. Observations are
made remotely, with the astronomer sitting at a computer thousands of miles away from the telescope. After telescopes collect radiation from an astronomical source, the radiation must be detected and
measured. The first detector was the human eye.
Photography and modern electric detectors have eliminated quirks of human memory by making
permanent records of the information from the cosmos.
Eyes suffer from short integration time this means that you blink and the information gets sent to the
brain. Modern detectors however, collect light from astronomical objects over longer periods of time; this
technique is called taking long exposure.
Several hours of exposure is required to detect faint objects in the cosmos.
Before light reaches the detector, astronomer’s usually use an instrument to sort out the light according to
wavelength. The instrument can be as simple as colored filters.
After light passes through a filter, it forms an image that astronomers can then use to measure the
apparent brightness and color of objects.
The instrument between a telescope and detector may be one of several devices that spread the light out
into its full rainbow of colors so that astronomers can measure individual lines in the spectrum. This
instrument is called a spectrometer. It allows astronomers to measure (to meter) the spectrum of a source
of radiation.
Photographic film or glass plates served as the prime astronomical detectors, whether for photographing
spectra or direct images of celestial objects.
On these photographic plates, a light-sensitive chemical coating is applied to a piece of glass that, when
developed, provides a permanent record of the image. Observatories around the world collections of these
photographs have recorded what the sky has looked like during the past 100 years.
Astronomers today used charge-coupled devices which are similar to detectors used in video camcorders
or in digital cameras. In a CCD photons of radiation hitting any part of the detector generate a stream of
charged particles (electrons) that are stored and counted at the end of the exposure. Each place where the
radiation is counted is called a pixel (picture element), and modern detectors can count the photons in
millions of pixels (megapixels or MPs)
CCDs provide more accurate measurments of the brightness of astronomical objects than photography,
and their output is digital, in the forms that can go directly to computers for analysis. CCDs record as
much as 60-70 percent of all photons that strike them, and the best silicon and infrared CCDs exceed 90
percent sensitivity which can detect fainter objects like small moons around outer planets or icy dwarf
planets beyond pluto.
Infrared is “heat radiation”. The main challenge using infrared is to distinguish between the tiny amount
of heat radiation that reaches earth from stars and galaxies, and the much greater heat radiated by the
telescope itself and our planets atmosphere.
According to Wien’s law, the telescope, the observatory, and even the sky are radiating infrared energy
with a peak wavelength of about 10 micrometers. To infrared eyes, everything on Earth is brightly aglowincluding the telescope and camera. The challenge is to detect faint cosmic sources against this sea of
infrared light.
To solve this, astronomers protect the infrared detector from nearby radiation, just as you would shield
photographic film from bright daylight. Anything warm radiates infrared energy so detectors must be isolated in very cold surroundings, often
times, near absolute zero, by immersing it in liquid helium.
Spectroscopy is one of the astronomers most powerful tools, used to provide information about
composition, temperature, motion, and other characteristics of celestial objects. ...
View
Full Document
