Telescopes Flashcards
What is the focal length of a lens?
Distance between focal plane and lens
(Parallel rays converge on the focal plane)
How will this be different for a stronger lens?
Focal length shorter
Why is a real image formed here?
The light rays pass through the point where the image forms
Why is a virtual image formed here?
The light rays do not pass through the point where the image forms
Why do these two object appear the same size?
The angle subtended at the eye is the same
Generally what does a telescope need to do to magnify an object?
Increase the angle subtended at the eye
Copy the diagram and label:
- Angles α and β
- Focal lengths
- Focal points
- Two lens names
Copy the diagram and label:
- Where a real image forms
- Where a virtual image forms
Draw a labelled diagram of the refracting telescope in normal adjustment
Don’t forget the dotted construction line!
What is wrong here?
Rays from eyepiece lens must be parallel to dotted construction line
How is the length of a refracting telescope calculated?
Add the two focal lengths
How is angular magnification of a telescope calculated?
- What is chromatic aberration?
- Sketch a diagram to show it
Different wavelength’s refract different amounts
Focal point different for different colours
- What is spherical aberration?
- Sketch a diagram to show it
Refraction increases towards edge of lens
Focal point different for rays passing through different parts of lens
What is the resolution of a telescope?
Ability to distinguish between two points separated by angle
The smaller the resolution the better
How many stars are seen?
2 because the stars are resolved
How many stars are seen?
1 because the stars aren’t resolved
How many times greater is this telescope’s collecting power?
4x
P proportional to SA
What does a reflecting telescope in Cassegrain Arrangement look like?
What is Spherical Aberration?
Focal length for lens or mirror is affected by distance from centre
(Forms distorted images)
How is Spherical Aberration reduced?
Using reflecting telescopes with parabolic mirrors
What are the Advantages of using Reflecting over Rafracting Telescopes?
Why are Space Based Telescopes used?
- No atmospheric or light pollution
- Can image across entire EM spectrum
What are the only EM regions visible to Ground Based Telescopes?
- Visible
- Radio
- Some IR (dry mountainous regions)
What are the disadvantages of Space Based Telescopes?
- Expensive to build and launch into orbit
- Difficult to repair ad maintain
- Weight limits to size of mirrors and power
- A lot more difficult to control remotely (communicatiuon delays)
- What is Quantum Efficiency?
- What is the Q.E. for the eye?
- What is the Q.E. for a CCD?
Q.E. eye ≈ 5%
Q.E. CCD > 70%
How does a CCD Work?
- Semiconductor chip with millions of pixels
- CCD placed on focal plane of objective lens
- Each pixel made of potential wells
- Potential wells correspond to different photons
- Photons incident on CCD release electrons
- Electrons trapped in potential wells
- Relative amounts of electrons in each well measured to produce image
What is the Rayleigh Criterion Critical Limit (point where 2 objects are only just resolved)
Central Maxima of one diffraction pattern meets the First Minima of the other
