Astrophysics optics Flashcards
what are the two types of lenses
Convex
Concave
what is a convex lens and focal length
- In a convex lens, parallel rays of light are brought to a focus
- This point is called the principal focus
- This lens is sometimes referred to as a converging lens
- The distance from the lens to the principal focus is called the focal length
- This depends on how curved the lens is
- The more curved the lens, the shorter the focal length
what is a concave lens
-In a concave lens, parallel rays of light are made to diverge (spread out) from a point
- This lens is sometimes referred to as a diverging lens
- The principal focus is now the point from which the rays appear to diverge from
Draw a diagram for a convex and concave lens
what are the two types of images produced by a lens
A real image
A virtual image
real image def
An image that is formed when the light rays from an object converge and meet each other and can be projected onto a screen
virtual image def
An image that is formed when the light rays from an object do not meet but appear to meet behind the lens and cannot be projected onto a screen
describe a real image
A real image is one produced by the convergence of light towards a focus
Real images are always inverted
Real images can be projected onto pieces of paper or screens
An example of a real image is the image formed on a cinema screen
describe a virtual image
A virtual image is formed by the divergence of light away from a point
Virtual images are always upright
Virtual images cannot be projected onto a piece of paper or a screen
An example of a virtual image is a person’s reflection in a mirror
Virtual images are where two dashed lines, or one dashed and one solid line crosses in ray diagrams
what colours in the electromagnetic spectrum have the longest or shortest wavelengths
Red has the longest wavelength (and the lowest frequency and energy)
Violet has the shortest wavelength (and the highest frequency and energy)
What kind of images does chromatic aberration create?
Images with coloured edges
what does it mean that telescopes magnify the angular size of distant objects.
- The telescope produces an image which subtends a larger angle than the object
- When viewed by the naked eye, the angle subtended by the object alpha is much less than the angle subtended by the image beta when viewed through a telescope
How does a refracting telescope work
Describe the use of angular magnification and its equation
what is the most common type of reflecting telescope
Cassegrain telescope
Equation for angular magnification relating f0,fe, beta and alpha angles
Describe how reflecting telescopes work through the law of reflection
Describe and draw a diagram of how a cassegrain telescope works
What are the two types of aberration that affect the quality of images produced by refractors and reflectors
- chromatic aberration
- spherical aberration
What happens in chromatic aberration (only in refracting telescopes)
Different wavelengths of light are refracted by different amounts causing the edges of an image to appear clouded
– this is due to the fact that blue light has a shorter wavelength and red light, meaning blue light is refracted more by a lens than red light. This is because blue light has a bigger refractive index
– consequently, different colours of water to focus at different points. For example, blue light focuses closer to the lens than red light, because of this greater rarefraction
Why does chromatic aberration not happen in reflecting telescopes
Because mirrors can only reflect, not refract
How can chromatic aberration be reduced using a second diverging lens
How is spherical aberration fixed
Describe how spherical aberration can be reduced/ eliminated in a refracting and reflecting telescope
What is spherical aberration
Spherical Aberration is an optical problem that occurs when all incoming light rays end up focusing at different points after passing through a spherical surface
Advantages of a refracting telescope
- Refractors require less maintenance than reflectors
- Refractors are not as sensitive to temperature changes as reflectors
Disadvantages of a refracting telescope
- It is difficult to make large-diameter glass lenses which are completely free from defects. Large magnifications require large objective lenses and very long focal lengths
- Large-diameter lenses are heavy and tend to distort under their own weight
- Refractors are heavy and difficult to manoeuvre so they have a slower response to astronomical events
- Lenses can only be mounted and supported around their edges however, this is where they are thinnest and weakest, so it’s difficult to construct
- Refractors suffer from both chromatic and spherical aberration
- Refractors are only able to observe wavelengths of visible light (can’t see non-visible wavelengths)
advantages of reflecting telescopes
- The diameter of a mirror can be much larger than that of a lens so greater magnifications can be achieved
- Large single mirrors can be made, which are light and easily supportable from behind
- Reflectors are lighter which allows for a more rapid response to astronomical events
- Mirrors only use the front surface for reflection, which eliminates many of the problems associated with lenses
- Mirror surfaces can be made very thin (a few nm) which allows for greater image detail
- Mirrors cannot produce chromatic aberration
- Reflectors do not suffer from spherical aberration use if parabolic mirrors are used
- Reflectors can be designed to observe wavelengths of light outside of the visible spectrum
- Reflectors can be sent into space which eliminates light absorption due to the atmosphere
Disadvantages of reflecting telescopes
- The secondary mirror has the disadvantage of blocking some light from entering the primary mirror
- The secondary mirror and its supports will cause some diffraction which can affect the clarity of the image
- Mirrors in a reflecting telescope are exposed to air so they require regular maintenance
- Light is refracted in the eyepiece lens and therefore some chromatic aberration may be introduced at this stage
describe how an ‘airy disk’ is formed
- A circular aperture, such as a lens in a telescope, is designed so that a cone of light can enter into a region behind it
- This allows light to act like a point source once it passes through
- When two point sources are placed near each other, or viewed from a large distance, they will appear to be a single unresolved source of light
For example, two distant car headlights may initially appear as a single point source until the car moves close enough for your eyes to resolve them into two individual headlights - Light from any object passing through a circular aperture, including the human eye, will diffract and create interference fringes upon the detector inside
- The pattern is circular and is an approximate pattern for a circular aperture
- The large central maximum is called an Airy disc and is twice as wide as the further maxima in the pattern
Draw a diagram of an airy disk
Draw a diagram showing how an airy disk is formed
What does the Rayleigh criterion state
Two sources will be resolved into two distinct objects if the central maximum of one diffraction pattern coincides with the first minimum of the other
How can the resolution or resolving power of a telescope be increased
- By reducing the amount the light diffracts, for example, by:
Increasing the diameter of the aperture
Operating at a shorter wavelength of light
Draw the visual diffraction pattern and the graph (Intensity vs separation) of two sources that cannot be resolved
Draw the visual diffraction pattern and the graph (Intensity vs separation) of two sources that can only just be resolved, as defined by the Rayleigh Criterion
Draw the visual diffraction pattern and the graph (Intensity vs separation) of two sources that are clearly resolved
What is the equation associated with Rayleigh’s criterion and draw a diagram for it
What is the equation which gives the angle that minima in the pattern appear as
Describe how Raleigh’s criterion can be written mathematically
collecting power definition
A measure of the amount of light energy a telescope collects per second
This is equivalent to the power per unit area, or intensity of the incident radiation collected
what does a higher collecting power create
brighter images
How does collecting power and aperture link and why
The collecting power of a telescope is directly proportional to the square of the diameter of its aperture
This is because:
Intensity is proportional to surface area
The surface area of a circular object of diameter D is equal to fraction numerator straight pi D to the power of italic 2 over denominator 4 end fraction
Why are larger aperture diameter telescopes better
- They have a greater collecting power so images are brighter
- They have a greater resolving power so images are clearer
How can the collecting power of two telescopes be calculated
How can the resolving power of two telescopes operating at the same wavelength be calculated
What is an optical telescope
a telescope that detects wavelengths of light from the visible part of the electromagnetic spectrum
What types of non-optical telescopes are there
- Radio telescopes
- Infrared (IR) telescopes
- Ultraviolet (UV) telescopes
- X-ray telescopes
What is the operating wavelength range of a ground-based telescope limited by
- the absorption of certain wavelengths by the Earth’s atmosphere
- large ranges of wavelengths are partially, or completely, absorbed by our atmosphere
What can a ground-based telescope observe
- All visible wavelengths (although there is often some distortion)
- Very narrow ranges of infrared wavelengths
- Most microwave & radio wavelengths
What can a space-based telescope observe
all wavelengths, making it possible to clearly observe:
- Gamma rays, X-rays & ultraviolet rays
- All infrared wavelengths (usually split into near-IR, mid-IR and far-IR)
What are the 3 advantages to putting telescopes into space
- There is no absorption of electromagnetic waves by the atmosphere
- No light pollution or other sources of interference at ground level
- No atmospheric effects, such as scattering or scintillation (i.e. twinkling) of light
Compare radio and optical telescopes in terms of their structure, positioning and uses
Compare radio and optical telescopes in terms of their resolving and collecting power
Compare UV and optical telescopes in terms of their structure, positioning and uses
Compare UV and optical telescopes in terms of their resolving and collecting power
Compare X-ray, gamma and optical telescopes in terms of their structure, positioning and uses
Compare X-ray, gamma and optical telescopes in terms of their resolving and collecting power
What is a charge-coupled device (CDD)
a detector which is highly sensitive to photons, making it ideal for use in the detection system of modern telescopes
How does a CDD Work
- Incident photons cause electrons to be released
- The number of electrons released is proportional to the intensity of the incident light
- An image is formed on the CCD, which can be processed electronically to give a digital image
Quantum efficiency def
The percentage of incident photons which cause an electron to be released
Quantum efficiency equation
Compare the quantum efficiency of a CDD and the eye
CCDs are renowned for achieving high values of quantum efficiency, generally upwards of 80%, whereas a human eye is only capable of achieving around 1%
Describe the resolution of a CDD
- The resolution of a CCD is related to the total number of pixels per unit area, and their size
- The smaller the size of the pixel, the better the resolution, hence the clearer the image will be
- In most cases, the overall resolution of a telescope is limited by the diameter of the aperture
- Hence, the resolution of the CCD (or the eye) is not likely to make a difference to the final image observed
What are the benefits of a CDD
- The number of images captured in a time period and exposure time can be easily adjusted
- The information stored on a CCD can be accessed remotely
- The generated images can be stored and analysed digitally
- They can detect a larger range of wavelengths, including beyond the visible spectrum
Compare the properties of a CDD with those of the human eye.
Is a higher or lower angular resolution better
A lower angular resolution is better because it means the closer the measurement points are resulting in “seeing” smaller objects.
