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.
When are two stars only just resolved and what name is given to this?
When one central maximum of one airy disk coincides with the first minima of the other airy disk. This is called the rayleigh criterion.
What is the minimum angular resolution?
The mimum angle at which 2 point objects can be resolved. when angle = λ/D
What is one arcminute?
1/60 * one degree. 1 arcminute is divided into 60 seconds.
What is the collecting power of a telescope?
The measure of its ability to collect incident EM radiation. It’s directly proportional to the square of the diameter of its objective.
What is collecting power proportional to? and what does a larger light-gathering power lead to?
Proportional to the objective diameter squared. Leads to a brighter image.
What does a radio telescope consist of?
A single parabolic ‘dish’ antenna (objective) and a receiver, where radio energy is collected and brought to focus. At the receiver, radio energy is amplified and displayed on an intensity trace.
Why is a large diameter aperture required for radio telescopes?
Wavelength of radio waves are much greater than wavelength of visible light, and radio telescopes have a lower angular resolution compared to an optical telescope
What can a radio telescope do that optical telescopes can’t
Operate during the day and during the night.
Why are radio telescoped located in isolated areas.
They suffer from interference. The ionosphere at lower frequencies strongly absorbs signals and also water vapour at higher frequencies.
What are the two main advantages of large diameter telescopes?
Greater collecting power, so images appear brighter and also better reosolving power, so images are clearer (less blurry)
What are infrared telescopes designed to observe?
Cool regions such as interstellar gas, cooler stars and active galaxies. They are space-based. The detector is designed to detect small changes in temperature caused by absorption of IR radiation.
Why must the IR detector be kept very cold and kept shielded?
To avoid thermal contamination.
Why are rocket-launched satellited needed for UV astronomy?
Ozone layers block the UV rays
What system do UV telescopes use to detect UV radiation?
A cassegrain mirror system brings UV rays to focus where it’s detected by solid state devices. The detectors use the photoelectric effect to convert UV photons to electrons.
What is a CCD and how does it work simply?
A charged coupled device. It allows light to be converted directly into digital information. Light strikes the CCD and electric charge accumulates on the pixels. charge is proportional to the brightness at a particular pixel location.
What is the response of a CCD?
linear - it can be used to capture the brightest of object, unlike the eye who’s response is logarithmic.
What is one major advantage of a CCD over the eye
Image produced can be stored digitally as a file that can be transmitted to research centres. This allows for remote viewing.
What is quantum efficiency?
number of photons detected/number of photons incident * 100%. This tells us how well a detector can capture photons and make them available to further amplifcation. The percentage of incident photons which lead to successful electron liberation.
Compare Q.E of the eye and a CCD?
Human eye: 4-5% CCD: 70-80%. Therefore CCD’s require shorter exposure times. CCD also has a wider spectral range.
