Telescopes Flashcards
Draw a ray diagram to show a converging lens producing a real inverted & diminished image
Draw a ray diagram to show a converging lens producing a magnified and virtual image
What are the main components of a refracting telescope
Objective & eyepiece lens
Explain what is meant by the term normal adjustment for a refracting telescope
The distance between the object lens and the eye piece lens is equal to the sum of their focal lengths
d = F(o) + F(e)
Draw a ray diagram for a refracting telescope in normal adjustment
1) F(o) > F(e) in a telescope
Describe the image produced my a refracting telescope
A Magnified and virtual image
image is seen at infinity
Describe the main components of a Cassegrain telescope
Concave Primary Mirror with a slit in the centre
Small Convex mirror
Eyepiece lens
Draw a ray diagram to show the path of two light rays,
. Parallel to the principle axis
. Through a Cassegrain telescope
primary concave mirror must be 1 mirror with a slit in the middle not two mirrors
State the Advantages of a reflecting telescope compared to refracting telescopes (4)
. Mirrors don’t suffer from Chromatic aberration as much
. Mirrors can be larger and therefore have greater collecting powers & smaller minimum angle resolution
.Parabolic mirrors are easier to manufacture then parabolic lens’
.Reflecting telescopes are more compact for the same effective focal length
State the disadvantages of a reflecting telescope in comparison to refracting telescopes
.Secondary Mirror blocks some light so less light hits the primary mirror
. Light is diffracted as it passes the secondary mirror affecting the image
Draw a ray diagram to show how spherical aberration can occur in a reflecting telescope
State how the problem of spherical aberration can be eliminated
Using perfectly parabolic mirrors for reflecting telescope and perfectly parabolic lenses from refracting telescopes
What is Chromatic aberration
Image distortion (image is tinged with colour) due to light of different wavelengths focussing at different points along an optical axis
Draw a diagram to show how a converging lens can produce chromatic aberration
Explain what is meant by the term Airy Disc
The Smallest point to which a beam of light can be focussed using a lens
What is meant by the Rayleigh Criterion
- identifies the minimum subtended angle between objects whose images can be resolved
State 2 advantages of using telescopes with a larger diameter objective (lens or mirror)
Increased collecting power (light collected is proportional to the area of the objective)
Light Collected is proportional to 1/4 * Pi * Diameter^2
Increased Resolving power —> Can resolve smaller angles
Size of angle resolved is inversely proportional to diameter
Describe the Structure of a CCD (3)
P type silicon doped in boron gas (more conductive)
Divided into an array of the picture elements/ pixels
Each pixel has 3 metal electrodes embedded in an insulating layer of silicon dioxide
Describe how a CCD works to produce an image (5)
Photons are incident of the CCD
Electrons are liberated from the surfaces of the electrodes within the pixels
Number of liberated electrons is proportional to the number of incident photons (brightness)
The Liberated electrons are trapped in a Potential Well within the Silicon layer
After Exposure the Charge on each pixel is measured and an image is formed
State Advantages of using CCD’s for Astronomical Observations
Higher Quantum Efficiency
Long exposure times to capture faint images
Device can be directly linked to a computer for capture and analysis
What is meant by Quantum efficiency
% of incident phoned on a photoreceptors that are detected
Number of photons producing an electron / total number of incident photons * 100
What is the Principle Focus (3)
A point on the axis
which is the same distance from the lens (optical Centre) as the focal length
Where light rays traveling parallel to the principle axis prior to refraction converge
Define Focal Length
Distance Between the centre of the lens and the principle focus
In the lens Equation
1/v + 1/u = 1/f
1) What does u represent?
2) What does v represent?
3) What does f represent?
1) Distance between the object and the centre of the lens (always positive)
2)Distance between image and the centre of the lens ( +ve for real images , -ve for virtual images)
3) Focal Length of lens (+ve for converging lens, -ve for diverging lens)
Give both formulas for angular magnification in Normal Adjustment
-> indication of widely an image takes up your field of vision
M = Angle subtended by image at eye / angle subtended by object at an unaided eye.
M = Focal length of objective / focal length of eyepiece
(using small angle approximations only)
How does an Astronomical Refracting telescope work (4)
- Two converging lens’ (eyepeice and objective)
- Objective lens role is to collect light and create a real image of a distant object
-Eye piece lens role is to magnify the image created by the objective lens
-Eye piece lens produces virtual image formed at infinity for the user to observe,
How does a Cassegrain telescope work (4)
-large concave primary mirror with a slit in the centre
Smaller convex secondary mirror placed in front of the slit
-Primary mirror collect light and focus it onto the secondary mirror
-Light then is reflected into an eye piece lens producing a virtual image forming at infinity
What is Spherical aberration
-When light is focused at different points along the principle axis
-due to a non-perfectly parabolic curvature of a lens or mirror
-Causing a blurred image
Draw a lens Diagram representing spherical aberration in a lens
What happens when you increase the size of the objective mirror/lens
Increasing diameter means you can observe fainter objects, as resolving power increases. meaning a smaller angle between two images can be resolved
Collecting power us proportional to (objective Diameter)^2
produces brighter images
Explain the Structure ,Positioning and uses of a single dish radio telescope.
Structure : Large parabolic dish that focus’ em radiation onto a receiver
Positioning: Can be ground based but must be in isolating locations with not a lot of interference
uses: observing things such as galaxies, stars and black holes.
Why do radio telescopes need tho be larger then optical telescopes?
Radio waves have a larger wavelength then visible waves
in order to achieve the same resolving power the objective diameter must be larger in accordance to (angle) = wavelength/ Diameter
Explain the Structure ,Positioning and uses of an infrared telescope
Structure: Large Concave mirror focusing light onto a detector, must be cooled with cryogenic fluids to avoid interference
positioning: must be in space as infrared light is blocked by the atmosphere
uses: observing coolers regions of space
Explain the Structure ,Positioning and uses of an ultraviolet telescope
Structure: Cassegrain configuration that focus’ radiation onto solid state devices
Positioning: must be in space as uv is blocked by the ozone layer
uses: observing the interstellar mediums and star formation regions
Explain the Structure ,Positioning and uses of an x-ray telescope
Structure: Combination of hyperbolic and parabolic mirrors to focus radiation onto a CCD
Positioning: must be in space as x-rays are blocked by the atmosphere
Uses: Observing high-energy events and areas such as active galaxies, black holes and neutron stars
Explain the Structure ,Positioning and uses of a Gamma Telescope
Structure: No mirrors , radiation passes through a. detector made of layers of pixels
Positioning: must be in space as gamma rays are blocked by the atmosphere
uses: observing gamma ray bursts, quasars, black hole and solar flares
Write the equation that is used in the “Rayleigh Criterion’
Smallest angle that can be resolved = wavelength/ diameter on objective lens
What is Collecting power
Rate at which useful energy is collected by a telescope
proportional to the diameter^2 of the objective or aperture
proportional to the area of the aperture
Compare the Resolution of a CCD to the human eye
CCD has a spacial resolution of 10 micrometers, human eye has 100 micrometers
CCD can resolve a smaller angle then the human eye
So the image has a higher resolution
What is Spacial Resolution
Minimum distance two objects must be apart to distinguish them
Smaller the distance, smaller the angle the telescope can resolve
Explain the advantages of a Parabolic Dish Shape over a Spherical dish shape
No Spherical Aberration
