Astrophysics lenses Flashcards
What shape are converging lenses?
Convex
What do converging lenses do to parallel light rays?
Bring them together and focus them on the principle focus point.
What are axil rays?
Rays parallel to the principle axis.
What is the criteria for a real image formed by a lens?
If the lens was put infant of a screen, the image formed could be projected onto the screen.
What is the criteria for a virtual image?
When the light rays from an object seem to have come from a different position than they truly do.
What is the lens equation?
1/(focal length) = 1/(distance between object and lens) + 1/(distance between image and lens)
What is the set up of a refracting telescope?
Two converging lenses focus and then magnify light rays from very distant objects?
What can we assume about the angle of incidence of light rays from stars and why?
They are parallel to each other. They are so far away.
What is normal adjustment of a refracting telescope?
The focal point of both lenses (objective and eye) are the same point.
What does being in normal adjustment mean for the image formed by a refracting telescope?
The rays come out of the eye lens parallel and the magnified image appears to be at infinity.
What is the set up of a cassegrain reflecting telescope?
Two parabolicconcave mirrors and one convex mirror with an eyepiece lens.
What are charge coupled devices?
Silicon chips that are covered in millions of pixels that collect signals from electromagnetic rays and can create digital images of objects.
How do the pixels on a CCD work?
When photons hit the silicon in the pixel, they cause the photoelectric effect to occur and so freed electrons build up in a potential well beneath. This charge build up can be measured and used to build up a digital image.
What is quantum efficiency and how does the quantum efficiency of a CCD differ to that of the human eye?
QE - the proportion of incident photon that are detected by a light detector. Human eye is 1% and a CCD is 80%
What are the positives of observing space using a CCD instead of our eyes?
- Quantum efficiency of 80% rather than 1%
- CCDs can detect a wider spectrum of light - Infrared, Visible and UV
- CCDs have a better spatial resolution - can distinguish points 10 um apart whereas the human eye can only get down to 100um
What is resolving power?
The smallest angular resolution that a telescope can distinguish.
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”
Describe the diffraction pattern when a beam of light passes through a circular apeture.
A circular diffraction pattern of bright maxima and dark minima, with a central maxima called the Airy Disk.
What is chromatic aberration?
Glass refracts the different wavelengths of light by different amounts and so the image for each is in a slightly different position. This blurs the image.
What are the cons of refracting telescopes?
- Chromatic abberation occurs.
- It is difficult and expensive to build sufficiently high quality as impurities and bubbles would scatter light.
- Lens can only be supported from the edges of the shape may become distorted.
- Focal length must be very long - very large telescopes.
What is spherical aberration and how can it be prevented?
If the shape of the mirror isn’t exactly parabolic, parallel rays do not converge onto the exact same point. So, the mirrors must be exactly parabolic.
What are the pros and cons of reflecting mirrors?
- Much cheaper to build
- Can be supported from beneath so don’t distort.
- No chromatic aberration as the mirrors are front silvered.
- Spherical abberation.
What is the set up of radio telescopes?
A very large parabolic mesh dish reflects and focusses light and is collected at the principle focus.
What is the max size of the mesh gaps in a radio telescope?
wavelength / 20
