Astrophysics Flashcards
What are refracting telescopes made of?
Two converging lenses: the objective lens and the eyepiece lens
What does the objective lens of a refracting telescope do?
Collect light and create a real image of a very distant object. The lens should have a long focal length and be large so as to collect as much light as possible. The collecting power of a telescope is directly proportional to the square of the radius of the objective lens.
What does the eyepiece lens of a refracting telescope do?
Magnifies the image produced by the objective lens so that the observer can see it. This lens produces a virtual image at infinity since the light rays are parallel. This reduces eye strain for the observer as they do not have to refocus every time they look between the telescope image and the object in the sky.
When is a refracting telescope in normal adjustment?
When the distance between the objective and eyepiece lenses is the sum of their focal lengths. This means that the principal focus for these two lenses is in the same place.
How do you calculate magnifying power / angular magnification, M?
angle subtended by the object at the unaided eye
When both angles are less than 10 degrees, you can say
M = alpha/beta = f0/fe
What is chromatic aberration?
Light of different wavelengths refracted to different foci.
For a given lens, the focal length of red light is greater than that of blue light, which means they are focused at different point (since blue is refracted more than red). This can cause a white object to produce an image with coloured fringing, with the effect being most noticeable for light passing the edges of the lens. It only occurs in the eyepiece lens.
What is spherical aberration?
Different focal points for rays at different distances from axis.
The curvature of a lens or mirror can cause rays of light at the edge to be focused in a different position to those near the centre, leading to image blurring and distortion. This effect is most pronounced in lenses with a large diameter, and can be avoided completely by using parabolic objective mirrors in reflecting telescopes.
How is material a disadvantage for refracting telescopes and an advantage for reflecting telescopes?
Glass must be pure and free from defects. Achieving this for a large diameter lens is very difficult.
Mirrors that are just a few nanometres thick can be made and these give excellent image quality.
How is aberration a disadvantage for refracting telescopes and an advantage for reflecting telescopes?
Chromatic and spherical aberration both affect lenses.
Mirrors are unaffected by chromatic aberration, and spherical aberration can be solved by using parabolic mirrors. Though chromatic aberration can affect the eyepiece lens, this can be solved by using an achromatic doublet.
How is size a disadvantage for refracting telescopes and an advantage for reflecting telescopes?
Large lenses can bend and distort under their own weight due to how heavy they are. They are incredibly heavy and therefore can be difficult to manoeuvre.
Mirrors are not as heavy as lenses, so they are easier to handle and manoeuvre to follow astronomical objects/events.
How is support a disadvantage for refracting telescopes and an advantage for reflecting telescopes?
Lenses can only be supported from the edges, which can be an issue when they are large and heavy.
Large primary mirrors are easy to support from behind since you do not need to be able to see through them.
What is an achromatic doublet?
A way of minimising spherical and chromatic aberration. It consists of a convex lens made of crown glass and a concave lens made of flint glass cemented together in order to bring all rays into focus in the same position.
