Telescopes Flashcards
Convex/converging lens
Focuses incident light
Concave/diverging lens
Spreads out incident light
Principal axis
The line passing through the centre of the lens at 90° to its surface
Principal focus
Converging lens: point where incident beams, passing parallel to principal axis will converge
Diverging lens: point from which light rays appear to come from same distance on either side of lens
Focal length (f)
Distance between centre of lens and principal focus
The shorter the focal length, stronger the lens
fo + fe = length
Real image
Formed when light rays cross after refraction – form on screen
Virtual image
Formed on the same side of lines – light rays, don’t cross.
Can’t form on screen.
Lens/Power formula
1/u + 1/v = 1/f
u = distance of object from the centre of lens
V = distance of image from centre of lands.
F = focal length of lens.
Power of lens
Measure of how closely a lens can focus a beam- parallel to principal axis
Measured in dioptres (D)
Converging is positive.
Diverging is negative.
Astronomical telescope
Comprised of two converging lens
Objective lens
Role is to collect light and create a real image of a distant object.
long focal length and is large to collect as much light as possible
Eyepiece lens
Magnifies the image produced by objective lens.
Produces virtual image of infinity- reduce eyestrain.
Normal adjustment telescope
Angular magnification
M = a/b
When less than 10°
M= fo/fe
Reflecting telescopes
Two types: Newtonian , Cassegrain
Newtonian telescope
Newtonian telescope 2
Light is collected and focused on to an eyepiece lens, using a plane mirror
Cassegrain telescope
Cassegrain telescope 2
Convex mirror allows Cassegrain to be shorter than Newtonian
Mirrors in reflecting telescopes
-mirrors in reflecting telescope are thin coating of aluminium or silver atoms deposited onto backing material.
-allows mirrors to be as smooth as possible and minimises distortions in the image.
Chromatic aberration
– focal length of red light is greater than blue, meaning they focus on different points.
– white object produces image with coloured fringing.
– since chromatic aberration is caused by refraction. there is a little effect on reflecting telescopes as it only occurs in eyepiece
Chromatic aberration diagram
Spherical aberration
-Curvature of land is cause, raise our eyes to be focused at different points.
-Leads to image, blurring and distortion.
-Produced in lenses with large diameters.
– fixed by using parabolic objective mirrors
Spherical aberration diagram
Minimise spherical and chromatic aberration
Use achromatic doublet - consists of convex lens made of crown glass and concave lens made of flint glass cemented together - allows light to focus in the same position.
Disadvantages of refracting telescopes
-glass must be pure and free from defects ~hard to achieve in large diameters
- large lenses can bend and distort under own weight due to heaviness
-chromatic and spherical aberration both affect lenses.
-refracting telescopes are incredibly heavy and difficult to manoeuvre.
-large magnifications require large diameter objective lenses with very long focal lengths.
-lenses only supported from edges
Advantages of reflecting telescopes
-mirrors that are few nanometres thick can be made and give excellent image quality.
-mirrors unaffected by chromatic aberration and spherical aberration be solved using parabolic mirrors.
-mirrors not as heavy as lenses, so easier to handle
-large composite primary mirrors can be made from lots of smaller mirror segments
-large primary mirrors easy to support from behind since you do not need to see through them
Radio telescopes
-radio telescopes use radio waves to create images of astronomical objects.
-the atmosphere is transparent to large range of radio wavelengths (it does not absorb them) so it’s possible to build radio telescopes that are ground-based.
-they should be in isolated locations to avoid interference from nearby radio sources.
-the simplest radio telescope uses a parabolic dish to focus radio waves onto a receiver.
Similarities in radio and optical telescopes
-both telescopes function in the same way: they intercept and focus incoming radiation to detect its intensity
-both telescopes can be moved to focus on different sources of radiation, or to track moving source.
-the parabolic dish of a radio telescope is similar to the objective mirror of a optical telescope.
-both telescopes can be built on the ground since both they can pass easily through the atmosphere.
Differences between radio and optical telescopes
-radio wavelengths larger than visible wavelengths, radio telescopes have to be larger in diameter than optical telescopes to achieve same quality to have same resolving power.
-construction of radio telescopes is cheaper and simpler because wire mesh is used instead of mirror.
-radio telescopes experience man-made interference from radio transmissions, phones, microwaves etc. -
-optical telescopes experience interference from the weather conditions, light pollution etc
Infrared telescopes
-Infrared telescopes use IR to create images of astronomical objects.
-telescopes consist of large concave mirrors which focus radiation onto detector.
-all objects emit infrared radiation as heat, infrared telescopes must be cooled using cryogenic fluids (liquid nitrogen or hydrogen) to almost absolute zero.
-must be well shielded to avoid thermal contamination from nearby objects and its own infrared emissions.
- used to observe cooler regions in space.
- atmosphere absorbs most IR so telescopes must be launched into space and accessed from ground.
UV telescopes
UV telescopes use UV radiation to create images of astronomical objects.
Ozone layer blocks all UV rays that have a wavelength of less than 300nm, meaning UV telescopes need to be in space.
Telescopes use Cassegrain configuration to bring UV rays to a focus.
Rays detected by solid state devices which use photoelectric effect to convert UV photons into electrons, which then pass around circuit.
UV telescopes used to observe interstellar medium and star formation regions.
X-ray telescopes
X-ray telescopes use X-rays to create images of astronomical objects.
X-rays are absorbed by atmosphere, telescopes need to be in space to collect data.
These rays have high energy that using mirrors like optical telescope wouldn’t work as they pass through.
Means X-ray telescopes made from combination of parabolic and hyperbolic mirrors which must be smooth.
Rays enter telescope, skim off mirrors, and brought into focus on CCDs - convert light into electrical pulses.
X-rays high-energy, used to observe high-energy events and areas of space such as galaxies, black holes
Gamma telescopes
Gamma telescopes use GR to create images of astronomical objects.
Telescopes don’t use mirrors as GR have high energy so pass through.
Instead, they use detector made of layers of pixels.
As gamma photons pass through, they cause signal in each pixel
These telescopes observe things like gamma ray bursts (GRBs), black holes and solar flares.
Two types of GBRs
➔ Short-lived - last anywhere between 0.01 and 1 second, and thought to be associated with merging neutron stars or a neutron star falling into black hole.
➔ Long-lived - these can last between 10 and 1000 seconds, and they are associated with Type II supernova
Collecting power
- a measure of ability of a lens or mirror to collect incident EM radiation.
The collecting power increases with the size of the objective lens/mirror.
Its directly proportional to area of objective lens.
collecting power ∝ (objective diameter)2
The greater the collecting power, the brighter the images produced by the telescope.
Resolving power
- the ability of telescope to produce separate images of close-together objects.
For image to be resolved, angle between straight lines from Earth to each object must be at least the minimum angular resolution
Minimum angular resolution
θ = Dλ
Where λ is the wavelength of radiation and D is the diameter of the objective lens or objective mirror.
0 is in radians
Rayleigh criterion
-states that two objects will not be resolved if part of central maximum of either of images falls within first minimum diffraction ring of other.
-as light enters telescope, its diffracted in a target-like shape called an ‘airy disc’
-the central maximum is the bright white circle in the centre, and each of the dark rings around it are the minimum rings.
CCD
an array of light-sensitive pixels, which become charged when they are exposed to light by the photoelectric effect.
Quantum efficiency
the percentage of incident photons which cause an electron to be released.
Spectral range
the detectable range of wavelengths of light.
Pixel resolution
the total number of pixels used to form the image on a screen.
A lot of small pixels will be able to resolve an image more clearly than a small amount of
large pixels.
Spatial resolution
the minimum distance two objects must be apart in order to be distinguishable. This is used to observe small details.
Convenience
How easy images are to use and form
