Astro - Telescopes Flashcards
What happens to the rays in a converging lens?
Parallel rays to the principal axis converge to the focal point.
What is f?
Focal length.
1 / f = ?
1 / u + 1 / v
What is u?
Distance from the object to the lens.
What is v?
Distance of the image to the lens.
What are the two lenses in refracting telescopes?
Objective lens and the eye piece.
How far away are the objects from a refracting telescope, what does this do to the rays?
Infinite, parallel.
Magnification = ?
Angle subtended by image / angle subtended by object
theta i / theta o
focal length of the objective lens / focal length of the eyepeice
fo / fe
What sort of telescope is a Cassegrain telescope?
Reflecting.
What are the two lenses in a Cassegrain arrangement?
Secondary convex and parabolic concave.
What happens in a Cassegrain arrangement?
Parallel rays hit concave mirror, reflect onto convex mirror, reflect to converge to a point (at eyepiece lens).
What are the three points/problems with a refracting telescope?
Chromatic aberration
Good quality lenses hard to make, large lenses get distorted by weight, held up by edges
Magnification is proportional to focal length, large M large telescope
What is chromatic aberration?
Different colours refracted by different amounts.
What are the two points/problems with a reflecting telescope?
Mirrors can be supported better, distorted less
Spherical aberration
What is spherical aberration?
If mirror is not perfectly parabolic the rays may not converge to a single point causing a blurry image.
What is the structure of a radio telescope?
Use parabolic mirror and antenna (eye piece equivalent) at focal point.
What is the difference in resolving power between optical and radio telescopes?
Radio is worse. Radio waves have bigger wavelength.
How do we increase the resolving power of a radio telescope?
Link many together, resolving power equal to resolution of the telescopes.
Where do we place infrared, X-Ray and UV telescopes? Why?
High altitude because the atmosphere absorbs infrared, X-Ray and UV.
What is the structure of infrared, X-Ray and UV telescopes?
Parabolic mirror and CCD to detect.
What does CCD stand for, how does it work?
Charged couple device.
CCD is divided into identical pixels. When photons hit the CCD (photoelectric effect), e- is emitted creating a digital signal. Position and intensity measured as charge varies with number of photons hitting the CCD.
What are the four comparison points between the eye and a CCD?
Quantum efficiency
Wavelengths
Resolution
Convenience
Explain quantum efficiency comparison of eye and CCD.
It is the ratio of emitted photons over incident photons.
CCD = 80%
Eye = 1%
Explain wavelength comparison of eye and CCD.
CCD = infrared, visible and UV
Eye = visible only
Explain resolution comparison of eye and CCD.
CCD = 50 mega pixels
Eye = 500 pixels
CCDs have better spatial resolution, better for fine detail
Explain convenience comparison of eye and CCD.
No extra equipment needed for eye.
What is minimum angular resolution?
Smallest angular separation for a telescope to distinguish between two points.
What is collecting power proportional to?
Diameter sqaured.
What is the Rayleigh criterion?
theta is around lambda over diameter of aperture (objective lens/mirror)
