Astrophysics Flashcards
What are the parts and functions of a refracting lens?
A converging objective lens: produces a real image of a very distant object.
A converging eye piece lens: which magnifies the image.
[diagram needed]
Normal adjustment?
Light from the object forms a real image between the lenses.
If the telescope is in normal adjustment, the light emerges in parallel lines and the angle between the emerging light and optical axis is bigger than the angle subtended by the object to the unaided eye.
Because the light is parallel to itself, extrapolating it back would result in an image appearing at infinity.
This reduces eye strain as they don’t need to refocus between looking at a distant object and through an eyepiece.
What is an object’s angular size?
The angle between the lines of sights of it’s two opposite ends.
How big the object appears to be to the unaided eye.
What is a reflecting telescope?
A telescope which uses a curved primary mirror called the objective mirror to direct light onto a secondary mirror.
[diagram needed]
What are the types of aberrations?
Chromatic and spherical.
What is chromatic aberration?
Aberrations caused by lenses focusing light over a range of focal lengths due to dispersion of different wavelengths in glass.
It forms images with coloured edges and can be corrected by using high quality optic materials.
[diagram needed]
What is spherical aberration?
The curved lenses focussing parallel beams of light at slightly different positions.
[diagram needed]
The closer to the edge of the lease and the larger it’s diameter, the greater the dispersion.
The effect can be minimised by ensuring both surfaces of the lens contribute equally to the ray deviation.
What is an achromatic doublet?
Two lens elements cemented together. Each lens is made of glass with different dispersion so the chromatic aberration of one lens is compensated by that of the other so light of many wavelengths is focused at the same place.
What are the optical and mechanical advantages of reflecting telescopes?
Optical:
Large single mirrors can be made.
Mirror surfaces can be made just a few nanometers thick giving excellent image properties.
No chromatic aberration and no spherical aberration when using parabolic mirrors.
Mechanical:
The mirrors are light and easily supported so response time to astronomical events is small.
Larger composite objective mirrors can be made from smaller segmented mirrors.
What are the advantage and disadvantages of refracting telescopes?
Advantage: slightly cheaper.
Disadvantages:
The lenses can only be mounted at their edges.
The glass needs to be free of defects and of sufficient clarity and purity.
Large diameter lenses are heavy and can deform under their own weight.
Heavy and difficult to manoeuvre quickly.
Large magnifications require large objective lenses and very long focal lengths.
They suffer from both chromatic and spherical aberration.
What is resolving power?
The ability to produce separate images of closely spaced objects.
What is the Rayleigh Criterion?
The Raleigh criterion is used to find the minimum separation of two objects with the prerequisite that they can be resolved.
Theta is the minimum angular resolution of the instrument at a fixed wavelength.
Radio telescopes: structure and advantages and disadvantages?
A large parabolic dish which focuses incoming waves onto a receiver which produces a signal which can be traced and electronically amplified.
-can operate at both day and night.
-needs to be placed away from radio transmitters.
-range is between 30-60 GHz as below the ionosphere absorbs the signal and above which the atmospheric water absorbs the signal.
-used to see the distribution of hydrogen.
Infrared telescopes: structure, uses, range?
Cassegrain assembly - like optical telescopes.
To be able to detect the minute fluctuations in temperature caused by IR absorption, they are usually made with semiconductor and superconductor devices.
It must also be cooled by cryogenic fluids.
They are used to observe cooler regions of space like interstellar gases, cooler stars, star formation regions, active galaxies, and the large scale structure of the universe.
It’s range is 0.7-450 micrometers and are usually in space as atmospheric gases absorb IR radiation.
Ultraviolet telescopes: structure, uses, range?
Cassegrain arrangement, post detection by the solid-state devices at the focus, the photoelectric effect is used to convert the UV photons into electrons.
Used to observe: young massive stars, very old stars, white dwarf stars, active galaxies, quasars/
The range is 400-10 nm and are is space as the atmosphere blocks all wavelengths shorter than 300 nm.
X-ray telescopes: structure, uses, range?
X-rays have such high energies that reflecting mirrors cannot be used as they would be penetrated.
The mirrors used are extremely smooth and are parabolic and hyperbolic in combination so the x-rays skim off the surface of the mirrors (grazing incidence).
Charge-coupled devices (CCDs) bring the x-rays into focus.
Used to observe: active galaxies, galaxy clusters, supernova remnants, pulsars, neutron stars, black holes, interacting binary stars.
Range of 10-0.01 nm and are space based as these wavelengths reflect off of the atmosphere.
Gamma telescopes: structure, uses, range?
They don’t use mirrors, instead they have special detectors to measure the energy and direction of gamma rays.
Used to observe: solar flares, pulsars, quasars, active galaxies, supernova remnants.
Range: o.o1 nm and below. Space based.
What is collecting power (or light gathering power (LGP))?
A measure of a telescope’s ability to collect incident electromagnetic radiation.
It is directly proportional to the square of the diameter of its objective lens.
What are the benefits and negatives of larger telescopes?
Benefits:
-reduce angular resolution making the image clearer (Rayleigh criterion and resolving power)
-increase collecting power.
Negatives:
-the mass of larger telescopes cause them to deform under their own weight.
-producing large mirrors out of single pieces of glass is expensive.
To overcome negative 2, we use segmented mirror telescopes.
What is very large baseline interferometry (VLBI)?
Interferometry uses identical parabolic dishes placed a distance of L meters apart to decrease the angular resolution as waves arrive in/out of phase to the two sources producing interference patterns.
Angular resolution is approximately equal to wavelength / L
(Because angular resolution of the telescope is the angular distance between successive maxima.)
What are charge coupled devices and the quantum efficiency?
A semiconductor device in which light is converted directly into digital information.
When light strikes the CCD, electric charge is accumulated in the pixels and the amount of charge accumulated is proportional to the brightness of the specific point.
The image is produced and stored digitally as a file that can be image-processed, transmitted to research centres around the world and archived for easy retrieval. A process especially useful for space based telescopes, where the whole process needs to be automated.
To measure sensitivity of a photon detecter: Quantum efficiency (QE) = number of photons detected / number of photons incident x 100
A high QE means they need shorter exposure time.
To improve images produced by CCD, use a greater number of smaller pixels. The smaller, the better the resolution so the clearer the image.
What is brightness?
The intensity of the light of a star from a distance.
