Letts Astronomize: Ch. 5 Sections 1-3

5.1- Optical Telescopes

Telescope = 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.

Optical telescopes are designed to collect wavelengths visible to the human eye.
History since Galileo in 17th centuries.

Lens thought of as series of prisms combined in a way so that all light rays arriving parallel to its axis (imaginary line through center of lens) are refracted to pass through a single point called the focus. Distance between primary mirror and focus is focal length.

Reflecting telescopes use curved mirrors instead of lenses to focus the incoming light.

The mirror that collects the incoming light is called the primary mirror, as telescopes often have more than one mirror. Focus of the primary mirror is called prime focus.

Prime-focus images are often magnified with a lens called eyepiece before being observed by eye, or recorded as a photograph/digital image.
Reflecting telescopes tend to be favored over refracting ones:

The fact that light must pass through the lens is a disadvantage of refracting telescopes. Tends to focus red and blue light differently.
When light passes through lens, glass absorbs some of it. This is a problem for infrared and ultraviolet observations because glass blocks most of the radiation in those regions.
Large lenses are heavy, so they deform under own weight. Meanwhile, a mirror doesn't have this problem because it is supported over it's entire back surface.
A lens has two surfaces that must be accurately machined and polished, while a mirror only has one.

In Newtonian telescope, the light is intercepted before it reaches the prime focus and then is deflected by 90 degrees, usually to an eyepiece at the side of the instrument. Uncommon in larger instruments but popular for smaller more common ones.
In Cassegrain telescope, incoming light hits the primary mirror and then is reflected upward toward the prime focus, where a secondary mirror reflects the light back down through a small hole in the main mirror into a detector or eyepiece. Has rear platform.
In Nasmyth/coudé focus, starlight is reflected by several mirrors- by primary mirror toward prime focus, down the tube by a secondary mirror, and then a third, smaller mirror reflects light out of the telescope where the beam may be analyzed by a detector mounted alongside, at the Nasmyth focus, or via a series of further mirrors into an environmentally controlled laboratory "coudé" room. This lab room is separate from the telescope itself, allowing astronomers to use heavy and finely tuned equipment that cannot be placed at any other foci.

5.2- Telescope Size

Development of astronomical instruments has led to an increase in size for 2 reasons: The amount of light a telescope can collect- light-gathering power.Amount of detail to be seen- resolving power.
Light-gathering power

Larger telescopes have greater collecting area- total area capable of gathering radiation.
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. (eg. a 5-m telescope will produce an image 25 times as bright as a 1-m instrument.) Relationship also in terms of time required for a telescope to collect enough energy to create a recognizable image on a photographic plate.

Larger telescopes have finer angular resolution- Ability of a telescope to distinguish between adjacent objects in the sky.
Diffraction and light bending around corners limits resolution. When rays spread out and are not focused to a sharp point, fuzziness ensues.
The amount of diffraction is proportional to the wavelength of the radiation and inversely proportional to the diameter of the telescope mirror.
For a circular mirror and "perfect" optics we can write: angular resolution (arcsec) = 0.25(wavelength(in micrometers)/diameter(m)) when one micron is 10(^-6) m
Diffraction-limited resolution- Theoretical resolution that a telescope can have to to diffraction of light at the telescope's aperture. Depends on the wavelength of radiation and the diameter of the telescope's mirror.
Larger telescopes produce less diffraction than small ones.

5.3- Images and Detectors

Many different detectors and devices to study radiation are placed at various points along light path outside the telescope.
Computers play a vital role in observational astronomy.

Electronic detectors called charge-coupled devices (CCDs) send output directly to a computer. Composed of many tiny pixels, each of which records a buildup of charge to measure the amount of light striking it.
The amount of charge is directly proportional to the number of photos striking each pixel, or the intensity of the light at that point. Buildup of charge is monitored electronically.
Advantages of CCDs include:

they are more efficient than photographic plates, recording as many as 90 percent of the photons striking them, compared with less than 5 percent for photographic methods. thus shows objects 10 to 20 times fainter. does this in less than a tenth of the time of photographic techniques.
CCDs produce a faithful representation of an image in digital format that can be placed directly on magnetic tape or disk or across a computer network to an observer's home institution.

Computers used to reduce background noise of astronomical images. Noise corrupts integrity of messages
Large reflectors are good at forming images of narrow fields of view, wherein all the light that strikes the mirror surface moves almost parallel to the axis of the instrument.
As angle at which light enters increases, accuracy of the focus decreases. Effect is called coma, which worsens as we move farther from the center of the field of view.
Photometry- Measurement of brightness of star. Astronomers often combine photometric measurements using colored filters to limit the wavelengths they measure.
When highly accurate and rapid measurements of light intensity are required, a specialized device known as a photometer is used, to measure the total amount of light received in all or part of the field of view.
When astronomers want to study the spectrum of incoming light, large spectrometers work in tandem with optical telescopes.

Letts Astronomize