Telescopes Flashcards
What is the lens formula?
1/u + 1/v = 1/f
u is the distance from lens to object, v is the distance from the lens to image, f is the focal length
What is the formula for power of a lens?
P = 1/u + 1/v = 1/f
What is the power of a lens?
A measure of how closely a lens can focus a beam. It is equal to 1/focal length and measured in Dioptres (D).
What does a refracting telescope consist of?
Two converging lenses: the objective lens and the eyepiece lens.
How is magnification of a telescope calculated (angles)?
M = β/α
α is angle subtended by unaided eye and object, β is angle subtended by image at the eye.
How can magnification of a telescope be found when the angle subtended by the object and image are small (<10°)?
M = fₒ/fₑ
What is chromatic aberration?
When different wavelengths of light refract by varying amounts when passing through a lens.
Blue refracts more than red.
What does chromatic aberration look like?
An image with coloured fringing.
What are the mirrors in a reflecting telescope (Cassegrain) made of?
Thin layer of aluminium and silver atoms. This makes the surface smooth and reduces distortion.
What is spherical aberration?
Where rays of light at the edge of a lens are focused in a different position. Leading to image blurring and distortion.
How can spherical and chromatic aberration be reduced?
Using an achromatic doublet. This consists of a convex lens (crown glass) and concave lens (flint) cemented together. This helps bring all rays of light into focus at the same position.
What are the five disadvantages to refracting telescopes?
Glass must be pure (free from defects).
Large lenses bend and distort under their own weight.
Chromatic and spherical aberration both affect lenses.
Heavy and therefore hard to manoeuvre.
Large magnifications require very large lenses.
What are 5 advantages of reflecting telescopes?
Mirrors unaffected by chromatic aberration, spherical aberration can be minimised by parabolic mirrors.
Mirrors are lighter than lenses ∴ easier to manoeuvre.
Chromatic aberration on the eyepiece can be resolved with achromatic doublet.
Describe the basics of how a radio telescope functions?
Radio waves from space are not absorbed by the atmosphere and are incident on the disc.
The telescope’s disc then focuses these waves into a detector, the waves are then amplified and traced.
What is the collecting power of a telescope?
A measure of a mirror or lenses ability to collect EM radiation.
