© sierra college astronomy department1 telescopes portals of discovery

© Sierra College Astronomy Department 1

TELESCOPESTELESCOPES

Portals of Discovery


Telescopes: Portals of Discovery

The Eye: The Everyday Light Sensor

The eye is made of a lens, pupil, and a retina– The pupil allows a certain amount of

light to enter the eye. • The pupil is constricted (and lets in less

light) when it is bright and dilates (and lets in more light) when it is dim

– The lens focuses light to point on the retina


Telescopes: Portals to Discovery

Reflection and Refraction

Light travels in a straight line as long as it remains in the same medium (i.e., the material that transmits light).

Reflection is the redirecting of light off a surface– Incident angle = reflection angle

Refraction is the bending of light as it crosses the boundary between two materials in which it travels at different speeds.




The amount of refraction is determined by two factors:– Relative speeds of light in the two materials

(e.g., air and glass)• The ratio of the speed light in vacuum to its speed in

some material is called the index of refraction

– The angle between rays of light and the surface (i.e., the smaller the angle between the ray of light and the surface, the more the light bends on passing through the surface).




Different colored light beams refract at slightly different angles.

Dispersion is the separation of light into its various wavelengths upon refraction (it’s what a prism does).

This effect can seen when light is refracted (and reflected) in water droplets, producing a rainbow



Refraction and Image Formation

Focal point (of a converging lens or mirror) is the point at which light from a very distant object converges after being refracted or reflected.

Focal length (F) is the distance from the center of a lens or a mirror to its focal point.

Image is the visual counterpoint of an object, formed by refraction or reflection of light from the object.

Focal plane is where the image focuses



Refraction and Image Formation

While dispersion can be useful in examining the colors coming from an object, it also introduces an inherent problem

Chromatic aberration is a defect of optical systems that results in light of different colors being focused at different places. The resulting image will be fuzzy at the edges.



Basic Refracting Telescopes

The objective is the main light-gathering element - lens or mirror - of a telescope. It is also called the primary. It is characterized by its diameter (D).– An example of a basic refracting telescope in

nature: the human eye An eyepiece (which may be a combination

of lenses) added just beyond the focal point of the telescope’s objective acts as a magnifier to enlarge the image.



The Powers of a Telescope

Angular size of an object is the angle between two lines drawn from the viewer to opposite sides of the object.

Magnifying power (or magnification) is the ratio of the angular size of an object when it is seen through the instrument to its angular size when seen with the naked eye.

objective eyepieceM F F




Long focal length eyepieces (>25 mm) produce less magnification; short focal length eyepieces (<10 mm) produce more magnification.

Field of view is the actual angular width of the scene viewed by an optical instrument.– As magnification increases the field of

view decreases.




Light-gathering power is a measure of the amount of light collected by an optical instrument (the area of the objective lens or mirror).

Light-gathering power is related to the size of the objective, which is usually given as a diameter. [Remember that the area of a circle is proportional to the (diameter)²].

2

2

...o

o

d

DPGL


Diffraction is the spreading of light upon passing the edge of an object. This also depends on the color of light

Resolving power (or resolution or diffraction limit) is the smallest angular separation detectable with an instrument. It is a measure of an instrument’s ability to see detail

Resolving Power (or diffraction limit) is in seconds of arc and and D must be in the same units



wavelength of lightResolving power or diffraction limit = 250,000

diameter of telescopein arcseconds



The Powers of a Telescope Resolution Details

– The best human eyes can resolve is about 1 arcminute or 1/60 of a degree.

– A 15-cm (6-inch) telescope has a maximum resolving power of 1 arcsec or 1/3600°.

– Because of atmospheric turbulence (which causes the stars to twinkle), even the largest Earth-based telescopes have a practical resolution of between 1 and ½ arcsec.

– Operating above the atmosphere, Hubble Space Telescope has a resolving power of 0.1 arcsec or better.

Two distant objects can resolved if the telescope’s resolution (may not be the diffraction limit) is smaller than the angular separation of the two objects.– Recall that angular separation is determined by the small

angle formula:360

angular separation = physical separation x 2 x distance



Basic Reflecting Telescopes

An inwardly curved - or concave – primary mirror can bring incoming light rays to a focus and is used to construct reflecting telescopes.

The reflecting telescope was invented by Isaac Newton, who also used a small flat secondary mirror placed in front of the objective mirror to deflect light rays out to the eyepiece.



Basic Reflecting Telescopes

A Cassegrain focus reflecting telescope has a secondary convex mirror that reflects the light back through a hole in the center of the primary mirror.

A Newtonian focus reflecting telescope has a plane mirror mounted along the axis of the telescope so that the mirror intercepts the light from the objective mirror and reflects it to the side.

A Nasmyth/Coude focus reflecting telescope uses a third mirror to reflect light out the side but lower down than the Newtonian design



Reflectors vs Refractors

Reflectors can be made larger (and less expensively) than refractors because:– There are fewer surfaces to grind, polish, and

configure correctly

– Reflecting mirrors do not exhibit chromatic aberration as do lenses

– Light does not transmit through a mirror so imperfections in the glass are not critical

– Mirrors can be supported on their backs; lenses must be supported along their rims


Lecture 6: Telescopes: Portals of Discovery

What to do with a Telescope?

So we have a telescope! What do we do with it?

Most telescopes are set up to do imagery We must consider the location of

telescope Telescopes may also do timing and

spectroscopy Telescopes not restricted to the optical



Techniques and Instrumentation

Imaging– Camera with photographic film or plates.

• Keys: aperture (area of lens or mirror) and exposure time

– Charge-coupled device (CCD)• Electronic “camera” with an array of pixels emit electrons

when struck by incoming photons.• The data collected is formed into images by a computer.• Advantages over cameras

– More sensitive to light– Wider dynamic range– Image can be manipulated via image processing (can be used

to create “false-color” images from non-visible observations)

– Filters for selecting desired frequencies



Large Optical Telescopes

Timing– A light curve is a plot of an object’s intensity

(over some wavelength range) with respect to time.

– Photometry is the practice of creating these light curves.

• Early photometers were like a camera’s light meter• Modern photometers use a CCD for greater speed

and accuracy


Spectroscopy– A spectrometer or spectrograph is a device that

uses a diffraction grating (or other devices) to separate light into its various wavelengths for recording by some detector (e.g., a CCD).

– The recorded spectrum from a spectrometer is also know as a spectrograph or spectrogram.

– The amount of information that can be gleaned from a spectrogram is dependent on the spectrometer’s spectral resolution (the higher the resolution, the more detail can be seen).


Large Optical Telescopes



Observation Conditions

Considerations for Ground-Based Observations– Daylight and weather– Light pollution– Twinkling and atmospheric turbulence– Limited wavelength transmission

Solutions– Place telescopes on top of mountains in dry, clear climates,

and away from artificial lights.– Active/Adaptive optics is a system that monitors and changes

the shape of a telescope’s secondary (or even a third or fourth) mirror to produce the best image.

– Place telescopes in space



Radio Telescopes

Radio waves– have less intensity than visible light and have much longer

wavelengths (which leads to less resolution).– are the only other wavelengths (besides the visible band)

that can be observed from the Earth’s surface.– are not prone to atmospheric distortion, but do compete

against man-made radio pollution. To detect radio waves with meaningful resolution

– very large parabolic dishes are required.• The largest radio telescope, the Arecibo radio dish,

stretches 305 meters, but only has a resolution of about 1 arcminute at its commonly observed wavelength of 21 cm.

– several radio telescopes are linked together in an array.



Interferometry

Interferometry is a procedure that allows a number of telescopes to be used as one by taking into account the time at which individual waves from an object strike each telescope.– Interferometry is possible because extremely

accurate atomic clocks allow for precise timing between radio telescopes.

– Interferometry increases the resolution of the resulting image because the size of the objective is effectively the size of the furthest separated dishes.



Interferometry

Interferometry is a well established technique in radio astronomy– The Very Large Array (VLA) near Socorro, New Mexico, is

the most famous example• There are twenty-seven 25-m dishes which can be separated

by as much as 40 km.– The Very Large Baseline Array (VLBA) comprise of eleven

25-m dishes spread across the United States• A radio antenna has been put in space to extend the resolution

further

For smaller wavelengths, timing the signals is very difficult, but there has been some success in the infrared and visible regions (and there plans to extend the technique into the X-ray band).



Detecting Other EM Radiation Infrared Telescopes

– These telescopes collect and focus infrared much the same way as the visible light telescopes, with the practical limitation of the Earth’s atmosphere.

• Some observations can be made on the ground through limited “infrared windows” in the atmosphere

• High and dry mountain tops allows for a broader range of infrared signals to be detected (e.g., Mauna Kea in Hawaii)

• Generally, infrared observations are best done high in the atmosphere (e.g., from aircraft such as the SOFIA [Stratospheric Observatory for Infrared Astronomy], NASA’s Airborne Observatory) and in space (e.g., IRAS [Infrared Astronomical Spacecraft] and the Spitzer Space Telescope even the Hubble Space Telescope)

– Infrared telescopes must be cooled so heat (IR radiation) from the surroundings does not mask the signals received from space.

– There is also some specialization with respect to the near-, mid-, and far-infrared regions.



Detecting Other EM Radiation

Ultraviolet Telescopes– Like infrared telescopes, these telescopes

collect and focus most UV the same way as the visible light telescopes.

– Ozone is the chief absorber of wavelengths shorter than about 300 nm. Ultraviolet telescopes must be located in space.

• Three major UV telescopes: FUSE (Far Ultraviolet Spectroscopic Explorer), GALEX (Galaxy Evolution Explorer), and the Hubble Space Telescope



Detecting Other EM Radiation

X-Ray and Gamma-Ray Telescopes– Like UV telescopes, X-ray telescopes and gamma-ray

telescopes also must be placed above the atmosphere is orbiting satellites.

– X-Rays and gamma-rays penetrate many materials and make focusing them a real challenge.

• X-ray telescopes employ a grazing incidence technology– NASA’s Chandra X-Ray Observatory consists of several nested

grazing incidence mirrors and offers the best in angular resolution

– The European XMM-Newton telescope has a larger collecting area

• Gamma-ray telescopes utilize massive detectors so that the photons do not simply pass through the instrument

– 17-ton Compton Gamma Ray Observatory (1991-2000)– NASA’s Swift launched in 2004 with source direction capabilities



The Hubble Space Telescope (HST)

HST is designed to observe across the spectrum from infrared to ultraviolet.

HST sees light before it encounters the atmospheric turbulence of the Earth

HST underwent successful repair in 1993, functioned at design specifications for many years, but lack of service from the Shuttle may doom it in the near-future.

The Next Generation Space Telescope (NGST) now called the James Webb Space Telescope (JWST) with a planned launched in 2018.



Looking Beyond Light Other “cosmic messengers” to detect

– Neutrinos• Extremely light-weight and neutral atomic particles associated with

stellar nuclear reactions• “Neutrino telescopes” typically located in deep mines or under water

or ice– Cosmic rays

• Very high-energy subatomic particles with uncertain origin and composition

• Satellites and ground-based detectors are employed– Gravitational waves

• Oscillations in spacetime predicted by Einstein’s General Theory of Relativity

• Expected to be produced by exotic objects like orbiting neutron stars and black holes

• Detectors are up and running in Washington, Louisiana, Italy, and Germany