Telescopes Flashcards
What are the two basic type of optical lenses?
Concave(divergent) and convex(convergent)
Which lens spreads the incident beam of light and which focuses incident beam of light
Convex focuses, concave spreads
What is the name of the distance between the principle focus and the centre of the lens
Focal length
What are the three ways of describing an image on a ray diagram?
1) Real/virtual
2) Magnified/diminished
3) Right way up/inverted
When does a lens act as a magnifying glass?
When the object is close to the lens than the focal length. A magnified, virtual image is formed the right way up on the same side of the lens
What are optical telescopes?
Astronomical telescopes that receive light in the visible part of the Electromagnetic spectrum.
What are refracting telescopes?
Telescopes which focus the incident light by refraction through lenses. Bigger telescopes allow for greater magnification.
How does normal adjustment in a refracting telescope reduce eye strain?
The eyepiece acts as a magnifying glass. Observers eye doesn’t have to keep refocussing between looking at a distant object and looking through the eyepiece at the image.
What is the formula for the angular magnification/magnifying power for a refracting telescope?
M = Fo/fe - A a ratio of the two focal lengths.
What is chromatic abberation?
Refraction by lens causes white light to disperse into its component colours. Due to refractive index of lens being different for each wavelength of light. The objective lens focuses the different colours over a range of focal lengths. Produces coloured edges to the image.
What is spherical abberation?
Occurs due to the curvature of the lens. Light rays in a parallel beam are focussed at slightly different positions, resulting in image blurring.
Which light rays are deviated more during spherical abberation?
Light rays near the edge of the lens.
what is one way of accounting for abberations?
Using an achromatic doublet, which is made of two individual lenses cemented together to bring lights of 2 wavelengths into focus on the same plane. Each lens has a different dispersion rate to account for abberation of the other lens.
What are reflecting telescopes made up of?
A curved objective mirror (primary mirror) to collect light from a distant object and direct it onto a secondary mirror.
What is a common arrangement for reflecting telescopes?
The cassegrain arrangement, consisting of a parabolic prrimary reflector and a secondary convex mirror.
What are 3 pros’ of reflecting telescopes?
Large single mirrors can be made which are light and easily supportable from behind.
Reflectors can be made much larger than refractors because mirrors can be supported from behind, whereas a lens must be supported at the edge. This can lead to lens distorting under it’s own weight.
No chromatic or spherical abberation when using parabolic mirrors.
3 con’s of refracting telescopes?
Heavy and difficult to maneouvre quickly
suffer from chromatic and spherical abberation.
Large diameter lenses are heavy and tend to distort under their own weight.
How is image quality limited in a telescope?
By absorption and distortion in the visible region of the EM spectrum. Ozone, oxygen and co2 all contribute to the absorption of light. Also, dust in atmosphere absorbs and scatters light on its way to the telescope. Atmospheric turbulence reduces image quality.
How can location of telescopes help to improve image quality?
Observatories should be built in dry, pollution free areas at high altitudes.
What is atmospheric opacity?
A measure of the absorption of EM radiation by the atmosphere, as a function of wavelength. Large ranges on non-visible light wavelengths are absorbed by our atmosphere.
Which part of EM spectrum is strongly absorbed by the atmosphere?
Gamma rays, X-rays, UV rays and infrared are strongly absorbed. We need space-based observatories to observe these. Visible wavelengths and radio waves aren’t absorbed and can be observed.
What is the definition of resolving power?
A telescope’s ability to produce separate images of two closely spaced objects.
What happens to waves as they pass through the aperture of a telescope?
They interfere and diffract to produce a diffraction pattern. Star doesn’t appear as a perfect point but an airy disk is seen.
What does the size of the central maximum indicate to us?
How much blurring of the image there is. The less blurring there is, the more detail is seen.
