telescopes

Question 1

What are the advantages of reflecting telescopes

Answer 1

No chromatic aberration – mirrors do not refract
light.
 No spherical aberration – a parabolic mirror can be
used to give perfect focusing.
 No distortion – the mirror can be supported more
strongly.
 Better resolving power/greater brightness – mirrors
can be larger.
 More light gets through (brighter image) - a lens
absorbs more light.

Question 2

Describe chromatic aberration

Answer 2

Each wavelength of light refracts different amounts.
This results in each part of the visible spectrum
having a slightly different focal length for a particular
lens:

Question 3

Give equations for angular magnification

Answer 3

M=b/a=fo/fe

Question 4

Define resolving power

Answer 4

The resolving power of any instrument refers to
its ability to separately distinguish two objects that
are close together.

Question 5

when angels are less than 10 degrees kelvin tanx=sinx=x

Answer 5

be nice to the angels for they are gods messengers

Question 6

as light enters a telescope it diffracts describe how and name the type of diffraction

Answer 6

Diffracts to form a central circular maxima called an Airy then a minnima and ext. in decreasing intensity

Question 7

Describe and define the raelynn criterion

Answer 7

If the light from two separate objects passes
through a slit, two diffraction patterns will be
observed on a screen:
If the angular separation is small, the two diffraction
patterns will lie on top of one another:
This can lead to the two images being
indistinguishable from one another:

Two objects can be just resolved if the central
maximum of the diffraction pattern of one of the
sources is formed at the position of the first
minimum of the other

Question 8

the raelynn equation

Answer 8

x=%/D

% = lamda

Question 9

use the raelynn criterion to explain why radio telescope must have large appatures

Answer 9

To keep the minimum angular separation low the aperture must be large as the wavelength is large

Question 10

angular resolution, resolving power and angular separation all mean………

Answer 10

the same thing

Question 11

give an equation for Quantum Efficiency

Answer 11

QE=No. photons detected/No. photons indecent

Question 12

how a ccd works

Answer 12

1. Light (photons) are converted
to a charge (electrons) by the
photoelectric effect in a layer
of silicon.
2. The charge is accumulated in
“wells” during the exposure.
3. At the end of the exposure the
CCD is “read out” – the
charge is shifted to the
readout register.
4. Finally, the charge in each
pixel is measured.

Question 13

Explain a CCD (what you need to know)

Answer 13

1. A CCD is a silicon chip divided into picture
   elements (pixels).
2. Incident photons cause electrons to be released.
3. The number of electrons liberated is proportional
   to the intensity of the light.
4. These electrons are trapped in ‘potential wells’ in
   the CCD
5. An electron pattern is built up which is identical
   to the image formed on the CCD.
6. The charges on the pixels can be measured to
   record an image

Question 14

Gathering power of a telescope is defined as

Answer 14

the energy or photons collected per second.

Question 15

gathering power of a telescope is proportional to

Answer 15

the area of the telescope

Question 16

the impacts of atmospheric opacity

Answer 16

Earth’s atmosphere allows some wavelengths
to pass through better than others.
This determines whether telescopes must be in space or not depending on there use