A Level Physics Telescopes

Telescopes

What is a Telescope?

2 lenses separated by the sum of their focal lengths the larger one being the objective lens and the other being the eye piece lens

How does the ray diagram look for an object perfectly aligned with principle axis?

How does the ray diagram look for an object not aligned with principle axis?

Light Rays enter parallel to each other into the objective lens

Refract towards focal point

Carry on in their directions until they reach the eyepiece lens

Refract out the eyepiece lens parallel to each other

Line that passes through the principle axis at the objective lens doesn’t change direction until it reaches the eye piece lens

How to calculate focal length?

1/f = 1/v + 1/u

v = distance to object

u = distance to image

Approximated to 1/f = 1/u at infinity

Why use MIrrors over Lenses

Mirrors are:

Cheaper to manufacture

Thinner so can be adjusted using pressure pads

No Chromatic Aberrations

Can be manufactured in segments and brought together to create larger telescopes

lighter so better for large telescopes (easier to handle)

only surface needs to be perfect

What is Magnifying Power

ratio of fo / fe or

ratio of angles subtending to the image to the object:

beta/alpha

only works for small angles in radians

What are the different types of Telescopes

Refracting Telescopes

Reflecting Telescopes (Cassegrain, Newtonian)

How does a Reflecting Telescope work?

Works with the law that angle of reflection = angle of incidence

Uses a large primary mirror to direct light traveling parallel to the principle axis towards a focal point.

-Light travelling along principle axis is reflected straight back

-Light traveling parallel to the normal is reflected to the principle focus

-Light that meets the principle axis at the mirror is reflected at its angle of incidence

Uses Spherical and Parabolic Mirrors

Why are Parabolic Mirrors used over Spherical ones?

Parabolic Mirrors solve the issue of Spherical Aberrations as all waves travelling parallel to the principle axis will meet at one point

What is Chromatic Aberration?

It is when light meets at different points causing the image to look blurry and discolored.

This is because angle of refraction is slightly dependent on wavelength causing different wavelengths of light to refract to different focal points.

How do you reduce Chromatic Aberrations?

Use light filters to allow only 1 wavelength of light

Use large lenses

Use a reflecting telescope/mirrors

!!! Issue with such methods is they reduce the amount of light entering causing reduced image quality !!!

What is a Spherical Aberration?

When using a circular/spherical mirror, light doesn’t focus at the same point although travelling parallel to the principle axis. For example, rays that are further from the principle axis will come to focus closer to the mirror than waves that hit the mirror closer to the principle axis.

How are spherical aberrations reduced?

Parabolic Mirror

Using oversized lenses

only using the center of lens by closing aperture

What is the Newtonian telescope?

A primary mirror that reflects light to a secondary plane mirror that causes light to be reflected parallel to each other through a gap so the retina can focus the light again.

What is the Cassegrain telescope?

A primary mirror that has a gap in it. Light is reflected off this mirror onto a secondary convex mirror that straightens the reflected waves through a gap in the primary mirror.

What is resolving power?

minimum angular resolution:

the ability of a telescope to resolve 2 point sources

What is collecting power?

The rate at which useful energy is received by the telescope.

dependent on the surface area of the collecting surface (d²)

It determines the brightness of an image

What is the Rayleigh Criterion?

the minimum angular separation between 2 objects so that they can just be resolved into their separate bodies.

This occurs where the central maxima of the diffraction pattern of 1 body aligns with the first minima of the other

What is the equation for 2 bodies to be resolved?

Calculated using distance between 2 objects/distance from observer to object

What is the equation for the Rayleigh Criterion of a telescope?

Maximum Resolving Power

Wavelength/Distance

Angular Separation must be greater than this value for the 2 objects to be resolved.

What is resolution?

The detail in an image that a telescope can form

What is a charged couple device?

Device used to store images.

Works similar to photoemission where photons strike the surface causing electrons to be ejected.

Charge is then stored on the surface and released linearly to form a number sequence for an encoded picture.

The charge left on the surface once the electron emitted is known as an electron hole pair

electrons released proportional to intensity of light source

What is the operating wave lengths of a CCD and why type of waves are included?

400nm-1000nm (visible light to infrared)

can see X-rays and UV radiation when using phosphorus as fluorescence can be converted into visible light.

This means it can view more objects than the naked eye and film cameras.

What is Quantum Efficiency?

it is a measure of how many incident photons are detected at the surface of the CCD.

CCD’s are 70% efficient meaning:

• takes less time to view an object

• can detect objects that are more distant and dimmer

Why are CCD’s the best for object viewing?

they have greater resolution as it has smaller pictures so the image has more quality and is sharper

can detect dimmer objects

can detect more distant objects

can observe a wider range of wavelengths and inturn more objects