What is an astronomical refracting telescope made up of?
Two converging lenses.
What are the two lenses in an astronomical refracting telescope called?
The objective lens and the eye lens.
What does it mean if a telescope is in normal adjustment?
The principle focus of the objective lens is in the same position as the principle focus of the eye lens.
How do you draw a ray diagram for an astronomical refracting telescope?
- Draw non-axial ray passing through centre of objective lens and ends at eye lens axis
- Draw non-axial rays either side of this ray parallel to it and ending at the eye lens
- Draw dotted line from point where rays cross through centre of eye lens
- Rays refracted through eye lens parallel to this dotted line
What is angular magnification?
The angle subtended by the image at the eye, divided by the angle subtended by the object at the unaided eye.
What is the length of the telescope equal to?
The sum of the focal lengths of the lenses.
What is the Cassegrain arrangement in a reflecting telescope?
Parabolic concave mirror reflects light onto convex secondary mirror which goes through hold in middle of concave mirror and then passes through an eye lens like a refracting telescope.
What is a CCD?
Charge-coupled Devices. Sensitive light detectors used to capture images digitally.
What are 4 comparison points between the human eye and CCDs?
- CCDs detect much more light (80% quantum efficiency compared with 1% for eye)
- CCDs detect a wider spectrum of light
- CCDs have a smaller spatial resolution and can therefore capture fine detail
- CCDs less convenient to use but produce digital images which can be shared
What does the resolving power of an instrument depend on?
The minimum angular resolution which is the smallest angular separation at which the instrument can distinguish two points.
What happens if a beam of light passes through a circular aperture? What is the central circle called?
A diffraction pattern of bright maxima and dark minima is formed. Central circle is called the Airy disc.
What is the Rayleigh Criterion?
Two light sources can be distinguished if the centre of the Airy disc from one source is at least as far away as the first minimum of the other source.
What do the symbols and letters mean in the Rayleigh Criterion equation?
Theta = minimum angular resolution in radians Lambda = wavelength in metres D = diameter of objective.
What are 2 advantages and 2 disadvantages of refracting telescopes?
Advantages
-Objective does not tarnish and does not need to be re-silvered
-Does not suffer from spherical aberration
Disadvantages
-Secondary can reduce total incident light on the primary
-Secondary mount can cause diffraction patterns
What are 4 advantages of reflecting telescopes?
- Light does not pass through the glass and therefore not affected by defects within glass
- Objective can be supported over entire surface area - does not sag and gives more stability
- Does not suffer from chromatic aberration
- Active optics (movement of objective to maintain perfect shape), and adaptive optics (movement of secondary to correct for atmospheric distortions) possible
What is chromatic aberration?
Refractive index of glass for blue light is slightly different to red light so blue refracts more than red causing distortion.
How can chromatic aberration be solved?
Can be (partially) corrected with coated optics or an achromatic lens
What is spherical aberration?
Occurs in mirrors which spherical cross section - rays that strike close to centre of mirror have larger focal length than rays striking edge of mirror so there is no focal point.
How can you solve spherical aberration?
Objective made in the shape of a parabola.
How does a radio telescope work?
- Parabolic dish reflects EM radiation and focuses it at antenna (antenna is the principle focus)
- Signals amplified twice and then put through tuner to select wavelengths you want
- Final signals go to computer to create a false-colour image
What is the main problem with a radio telescope and why?
It has a very low resolving power as the wavelength of radio waves is much larger than the wavelengths of light.
How do radio astronomers get around the problem with radio telescopes?
They link lots of telescopes together and using some computer programs combine their data to form an image - equivalent of one huge dish.
What are the main benefits of radio telescopes?
- Construction much easier and cheaper as dish is made of wire mesh
- The longer the wavelength of radiation, the less it’s affected by imperfections in the shape of the dish
How do I-R and U-V telescopes work?
- Use same parabolic mirror set up to focus radiation onto detector
- CCDs used as radiation detectors
