Astro - Telescopes

Question 1

What happens to the rays in a converging lens?

Answer 1

Parallel rays to the principal axis converge to the focal point.

Question 2

What is f?

Answer 2

Focal length.

Question 3

1 / f = ?

Answer 3

1 / u + 1 / v

Question 4

What is u?

Answer 4

Distance from the object to the lens.

Question 5

What is v?

Answer 5

Distance of the image to the lens.

Question 6

What are the two lenses in refracting telescopes?

Answer 6

Objective lens and the eye piece.

Question 7

How far away are the objects from a refracting telescope, what does this do to the rays?

Answer 7

Infinite, parallel.

Question 8

Magnification = ?

Answer 8

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

Question 9

What sort of telescope is a Cassegrain telescope?

Answer 9

Reflecting.

Question 10

What are the two lenses in a Cassegrain arrangement?

Answer 10

Secondary convex and parabolic concave.

Question 11

What happens in a Cassegrain arrangement?

Answer 11

Parallel rays hit concave mirror, reflect onto convex mirror, reflect to converge to a point (at eyepiece lens).

Question 12

What are the three points/problems with a refracting telescope?

Answer 12

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

Question 13

What is chromatic aberration?

Answer 13

Different colours refracted by different amounts.

Question 14

What are the two points/problems with a reflecting telescope?

Answer 14

Mirrors can be supported better, distorted less
Spherical aberration

Question 15

What is spherical aberration?

Answer 15

If mirror is not perfectly parabolic the rays may not converge to a single point causing a blurry image.

Question 16

What is the structure of a radio telescope?

Answer 16

Use parabolic mirror and antenna (eye piece equivalent) at focal point.

Question 17

What is the difference in resolving power between optical and radio telescopes?

Answer 17

Radio is worse. Radio waves have bigger wavelength.

Question 18

How do we increase the resolving power of a radio telescope?

Answer 18

Link many together, resolving power equal to resolution of the telescopes.

Question 19

Where do we place infrared, X-Ray and UV telescopes? Why?

Answer 19

High altitude because the atmosphere absorbs infrared, X-Ray and UV.

Question 20

What is the structure of infrared, X-Ray and UV telescopes?

Answer 20

Parabolic mirror and CCD to detect.

Question 21

What does CCD stand for, how does it work?

Answer 21

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.

Question 22

What are the four comparison points between the eye and a CCD?

Answer 22

Quantum efficiency
Wavelengths
Resolution
Convenience

Question 23

Explain quantum efficiency comparison of eye and CCD.

Answer 23

It is the ratio of emitted photons over incident photons.
CCD = 80%
Eye = 1%

Question 24

Explain wavelength comparison of eye and CCD.

Answer 24

CCD = infrared, visible and UV
Eye = visible only

Question 25

Explain resolution comparison of eye and CCD.

Answer 25

CCD = 50 mega pixels
Eye = 500 pixels
CCDs have better spatial resolution, better for fine detail

Question 26

Explain convenience comparison of eye and CCD.

Answer 26

No extra equipment needed for eye.

Question 27

What is minimum angular resolution?

Answer 27

Smallest angular separation for a telescope to distinguish between two points.

Question 28

What is collecting power proportional to?

Answer 28

Diameter sqaured.

Question 29

What is the Rayleigh criterion?

Answer 29

theta is around lambda over diameter of aperture (objective lens/mirror)