Optical Telescopes Flashcards
What is the principal axis of a lens?
An imaginary line through the centre of the lens, at 90°
What happens to all light rays incident on a converging lens, parallel to the principal axis?
The converge onto a single point
“Principal Focus”
What happens to all parallel light rays incident on a converging lens?
They converge onto a single point on the focal plane
What is the focal length, f?
The distance between the lens axis and the focal plane
What is the lens axis?
The plane of the centre of the lens
Draw a converging lens for axial rays
Draw a converging lens for non-axial rays
What are the two types of images?
Real and virtual
What defines a real image?
The light rays are actually there, and the image can be captured on a screen
How is a real image formed?
Light rays from an object are made to pass through another point in space
What defines a virtual image?
The light rays aren’t actually where the image appears to be. The image cannot be captured on a screen
How is a virtual image formed?
When light rays from an object appear to have come from another point in space
What kind of image can a converging lens form?
Both real and virtual
What determines whether a virtual or real image is formed for a converging lens?
The distance of the object from the lens
Where will the image of an object sit, if the object is on the principal axis?
On the principal axis
If an object doesn’t sit on the principle axis, how do you find where the bottom of the image will be formed?
Draw two rays from the bottom of the object as well as the top
Draw a ray diagram for light passing through a converging lens (from an object)
If the object is further than the focal length away from the converging lens, what kind of image is formed?
Real image
At what distance from a converging lens is the object, for a real image to be formed?
Further that the focal length away
If the object is closer than the focal length away from the converging lens, what kind of image is formed?
Virtual image
At what distance from a converging lens is the object, for a virtual image to be formed?
Closer than the focal length away
When you draw a ray diagram for a converging lens, what two rays do you draw?
One parallel to the principal axis (therefore refracted through the principal focus on the other side of the lens)
One passing through the lens’ centre (therefore no refraction)
On a ray diagram for a converging lens, how do you determine if the image will be virtual?
If the refracted rays on the other side of the lens will never meet
What and where is the lens equation in the formula book?
Medical Physics: 1/f = 1/u + 1/v
On a ray diagram for a converging lens, how do you determine where a real image is formed?
Where the refracted rays meet
What is a refracting telescope made up of?
Two converging lenses
Objective lens and Eye lens
When drawing a ray diagram for a converging lens, what must be true about the distance of the object from the lens?
Closer than the focal length for a Virtual image
Further than the focal length for a Real image
What does a real and virtual image look like on a ray diagram for a converging lens?
What does the eye lens do?
Acts as a ‘magnifying glass’ on the real image to form a magnified virtual image
How do you tell if the image is diminished, same size or magnified?
If the image is further from the lens axis than the object, it’s magnified.
If the image is closer from the lens axis than the object, it’s diminished.
If it’s the same distance away, it’s the the same size.
How can we assume that the real image formed by the objective telescope is formed on the focal plane?
If you assume the object is at infinity, the rays from it are parallel.
Therefore, the image must be on the focal plane
In a telescope, where is the principal focus of the objective lens set up to be?
In the same position as the principal focus of the eye lens
In a telescope, where is the principal focus of the eye lens set up to be?
In the same position as the principal focus of the objective lens
What is the distance between the object and lens axis known as?
u
What is the distance between the image and lens axis known as?
v
What if v is negative?
v is virtual if it’s negative.
v is real if it’s positive.
(Think of the sides of the x axis - virtual image is on the left side of the lens axis, which would be negative on the x axis).
What other value do v and u depend on?
Focal length, f
What does the objective lens do?
Converges the light rays from the object to form a real image
What is the magnification (M) equation relating to angles?
θᵢ / θ₀
What is θᵢ?
The angle subtended by the image at the eye
What is θ₀?
The angled subtended by the object at the eye
Why is θᵢ > θ₀ for the magnification equation?
Because it is a magnifying telescope
What is f₀?
The focal length of the objective lens
What is fₑ?
The focal length of the eye lens
What is the length of the refracting telescope?
f₀ + fₑ
How do you draw a ray diagram for a normal adjusted telescope
What is the magnification (M) equation relating to the focal length?
In the formula book: M = f₀/fₑ
Why is f₀ typically much greater than fₑ?
Large magnification is required to view objects from space, so f₀/fₑ needs to be large
What is a Cassegrain reflecting telescope made up of?
Primary mirror,
Secondary mirror,
Eye lens
What is the shape of a reflecting telescope’s primary mirror?
Parabolic concave
What is the shape of a Cassegrain reflecting telescope’s secondary mirror?
Convex
What is the main characteristic of a Cassegrain arrangement?
The presence of a convex secondary mirror
What necessitates the presence of a convex secondary mirror for a Cassegrain arrangement?
The principal focus of the primary mirror is in front of the mirror, so an observer would block out the light
What does Cassegrain telescope look like? Where is the principle focus of the primary mirror?
What is the resolving power of a telescope?
A measure of how much detail you can see
What is the resolving power for a telescope dependent on?
The minimum angular resolution (Rayleigh Criterion with Wavelength and Diameter of Objective mirror/dish)
Quality of detector - resolving power of the detector
What effects the resolving power of the detector?
How many pixels there are on a CCD
How fine the wire mesh is
Why do UV telescopes have better resolving power that Radio telescopes
Radiation it detects has much shorter wavelength
What is the minimum angular resolution?
The smallest angular separation at which the telescope can distinguish two points
What phenomena limits resolution?
Diffraction
What is formed when a beam of light passes through a circular aperture?
A diffraction pattern of bright maxima and dark minima is formed.
What is the central circle of the diffraction pattern formed when a beam of light passes through a circular aperture?
Airy disc
What does the diffraction pattern look like?
At what distance can two light sources just be distinguished from each-other?
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 formula is used to calculate the minimum angular resolution?
Rayleigh Criterion
What is D in the Rayleigh Criterion?
The diameter of the objective lens or the objective mirror for telescopes. Can also be the diameter of the circular aperture from the diffraction pattern.
What is θ in the Rayleigh Criterion?
Minimum angular resolution
What size lenses are needed to see fine detail?
Very large
What are the problems with refracting telescopes?
Chromatic aberration
Bubbles and impurities
Distorted lenses
Need to be very large
What is chromatic aberration?
Glass refracts different colours of light to slightly different positions, causing the final image to be blurred with the edges coloured
What does Chromatic aberration look like compared to a ray which doesn’t refract?
.
Why do glass lenses have bubbles and impurities?
Good-quality glass is expensive and difficult to make
What effect do bubbles and impurities in the glass have?
They absorb some of the light, so faint objects aren’t seen
How can lenses become distorted?
Large lenses are heavy and must be supported from the edges, distorting their shape
Why do refracting telescopes need to be very large? Why is it a problem?
The objective lens must have a very long focal length for a large magnification. Means very large expensive buildings being needed to house them.
Why are are large mirrors of good quality better than large lenses?
They are cheaper to build, and can be supported from underneath so they don’t distort as much
Do lenses suffer chromatic aberration?
Yes
Why is the Cassegrain telescope almost free of chromatic aberration?
A Cassegrain telescope has mirrors that will reflect, not refract. So it can’t have chromatic aberration. However, the eye lens can have a small amount.
What other less known problems are caused by the Cassegrain telescope?
The secondary mirror in a Cassegrain telescope can also cause problems. Some light incoming will be blocked by the secondary mirror and mirror supports. Some of the light reflected from the primary mirror will diffract around the secondary mirror. Both leading to a decrease in image clarity.
(Also eye lens has small amount of chromatic aberration).
What is spherical aberration?
In mirrors that aren’t quite parabolic, parallel rays that are reflected do not converge exactly onto the same point
What does Spherical aberration look like?
What notable telescope suffered from spherical aberration?
Hubble Space Telescope
What problem did the Hubble Space Telescope suffer from?
Spherical aberration
What is a Charge-Coupled Device (CCD)?
A small silicon chip divided into a grid of millions of identical pixels
How do Charge-Coupled Devices (CCDs) work?
Arriving photons excite electrons in each silicon pixel, creating a charge that can be measured to create a digital signal
What two properties does a digital signal in a pixel in a Charge-Coupled Device (CCD) describe?
Where it is, how bright it is (intensity)
How are intensity and location of the light described
The charge on each pixel will vary depending on how many photons hit it.
What are the applications of Charge-Coupled Devices (CCDs)?
Digital cameras, barcode scanners, giant astronomical telescopes
What is quantum efficiency?
The proportion of incident photons that are detected
What is the quantum efficiency of the human eye?
Around 1%
What is the quantum efficiency of a Charge-Coupled Device (CCD)? So what can a CCD detect more of
80% +. It detects far more of the light that falls on them than the eye does.
What is the detectable light spectrum of the human eye?
Only visible light
What is the detectable light spectrum of a Charge-Coupled Device (CCD)?
Infrared, visible, UV
If you were to project the whole visual field of an eye onto a screen, how many megapixels would you need for the eye not to see any pixilation?
500 megapixels
What is the resolution of the human eye?
500 megapixels
What is the resolution of a Charge-Coupled Device (CCD)?
50 megapixels
If a sensor has more megapixels, what can it do? What else does it depend on?
Capture more details, also depends on spatial resolution
What is spatial resolution? What do we use to measure this?
How far apart different parts of the object being viewed need to be in order for them to be distinguishable. Minimum resolvable distance
What is the minimum resolvable distance of the human eye?
100 μm
What is the minimum resolvable distance of a Charge-Coupled Device (CCD)?
10 μm
Which is better at capturing fine detail - the human eye or Charge-Coupled Devices (CCDs)?
Charge-Coupled Devices (CCDs)?
Which is more convenient to set up; a telescope or a Charge-Coupled Device (CCD)?
Telescope
Why might a Charge-Coupled Device (CCD) be more convenient than a telescope?
Images are digital, so they can be stored and copied
