Astronomy & Cosmology 5.2 || More about telescopes

Concave/ Converging mirrors

bend parallel rays of light onto a single point. like a lens, but on the same side as the approaching light

Spherical abberation

Parallel rays converge at different distances. This is caused by a blurring of the image.

How to solve spherical aberration

make the mirrors parabolic in shape

How does light get into our eyes/cameras?

Light is collected and concentrated by a primary mirror. Then extra mirrors and lenses are used to get into our eyes/cameras

Pros of reflecting telescopes

Easier to make a large mirror vs large lens

You only need to coat one surface to be shiny. Can be large

Much lighter than lenses

No chromatic aberration (All colors converge at same point)

Cons of reflecting telescopes

Contrast/quality worse than refractors

Mirror surface needs to kept clean but is delicate and easily scratched

Affected by humidity and dust (both ends of the telescope are no longer closed)

Telescope structure complicated

Refractor

More expensive

Less compact

Chromatic Aberration

Reduced light-gathering

Objective lens

gathers and concentrates light

Large diameter apertures

also increase the total amount of light let in making the image brighter

The total amount of light

let in scales as the square of the aperture size

Large diameter aperture

improves the resolving power of a telescope

With a bigger aperture

a telescope can see more detail

The light gathering power of a telescope

proportional to the square of the size of the aperture

As the aperture size increases

the resolution or resolving power (the ability to see detail) of the telescope also increases [more detail in the same image)

Large scale Terrestrial telescopes

Keck telescope, Maunakea, Hawaii

CCDs

All modern large scale telescopes or even home telescopes have camera sensors (CCDs) attached where the human eye go.

This lets us record the image for more processing

No need for human observers

Data can be stored instead of having to be drawn

Radio telescopes

Reflector telescopes

Radio waves are reflected by metal surfaces that do not need to be shiny

Principle is the same as a Cassegrain optical telescope

Radio waves bounce off a large parabolic metal dish onto a secondary/sub reflector and then into a detector

E.g VLA

Multiple small telscopes

arranged in a particular shape can act like a much larger telescope with a larger effective aperture

Atmosphere

absorbs many wavelengths of light

Observation only

possible at certain wavelengths

Atmospheric turbulence/temperature fluctuations

cause seeing

Seeing

appears as a shimmering blurriness (similar what the air above a hot road looks like)

Adaptive optics

means that the optical equipment in the telescope (i.e, the mirrors, sensors, etc) are modified in real time to compensate for the shimmering due to the atmosphere

Small parts of the can be tilted slightly

to adapt for changes in the atmosphere

Hubble space telescope (HST)

a bus sized reflecting telescope launched in 1990

orbits around earth every 96 mins at a height of 540 km above the surface of the Earth

spherical aberrations was caused by launch realizing main mirror was ground incorrectly. made more spherical in shape than parabolic

1933 servicing mission added corrective optics (i.e. glasses)

Chandra X-Ray Telescope

Orbits around earth (x-rays cannot reach the surface)

Lenses of any material absorb x-rays. X-ray telescopes cannot use lenses

Use mirrors to slowly reflect light into correct path so it behaves like a lens bending light

James Webb

Infrared space telescope

Constructed by NASA & ESA

Does not orbit the earth, orbits the Sun at a location called Lagrange Point 2 (L2)

Launched Dec 25, 2021

Vera Rubin Observatory completed in 2025

Largest terrestrial telescope

Built high in Chilean mountains to reduce atmospheric effects

Largest digital camera ever built

First images in June 2025

Still in testing phase until late 2025

Apparent magnitude/brightness

value of which star brightness is described

Apparent magnitude

measure of how bright a star appears to be for us (not how bright it actually is)

Absolute brightness/ absolute magnitude

depends on how bright the star actually is and how far away the star is from us

The lower the number

the brighter it is (it's backwards)

The naked eye

can see objects in the night sky down to an apparent magnitude (m) of about 6

apparent magnitude

how bright we see the star (or other celestial object)

absolute magnitude

a measure of the inherent brightness of a star (or other celestial object)

defined as how bright the star would appear if it were 10 parsecs away (32.6 light years away)

parsec

measure of distance