Astrophysics P3 Flashcards
What is the difference between apparent and absolute size?
The apparent size is the size an object appears to be and the absolute size is the true size of the object.
What are two ways we can calculate angular magnification?
- Angle subtended by image at eye / Angle subtended by object at unaided eye
- The focal length of the objective lens / The focal length of the eyepiece length (When in normal adjustment)
How do you draw a refracting telescope in normal adjustment?
- Draw the principal axis
- Draw the objective lens on the left and eyepiece lens on the right.
- Draw a diagonal ray going through the centre of the objective lens which meets at the bottom of the eyepiece lens.
- Draw two more parallel rays, one above which meets lands on the eyepiece below and vice versa.
- Draw a construction line from where the rays meet going through the centre of the eyepiece lens.
- Draw the emerging rays parallel with the construction line and draw an eye.
What criteria must be met for a refracting telescope to be in normal adjustment?
The focal length of the objective lense must be much greater than the focal length of the eyepiece lens.
What is chromatic aberration?
Different colours of light focus at different points due to them refracting by different amounts.
This leads to a blurry image.
What are 2 other issues with refracting telescopes?
Long Telescopes - Caused by the focal length of the objective lens being really long for a good magnification.
Heavy lenses - More powerful telescopes need larger lenses which end up being very heavy and distort the edges of the glass.
How do you draw a Cassgrain Reflecting Telescope?
- Draw a principal axis
- Draw a large primary, concave mirror and a smaller secondary, convex mirror to the left (With a gap in the centre of the primary mirror)
- Draw two parallel rays above and below the secondary mirror.
- These rays reflect off the primary mirror, towards a single focal point behind the secondary mirror.
- The rays then reflect back towards the gap in the primary mirror and hit a lens which aligns them for an eye.
What are the three advantages of reflecting telescopes over refracting telescopes?
Mirrors do not suffer from chromatic aberration.
Mirros are less heavy than lenses.
Reflecting telescopes are shorter than refracting telescopes.
What is spherical aberration?
Where parallel rays of light reflect to different focal points leading to a blurry image.
It can be avoided by using mirrors with a parabolic shape.
What are the three key issues with reflecting telescopes?
Spherical Aberration.
The secondary mirror blocking incoming light and diffracting incoming light.
What is the minimum angular resolution of the human eye?
0.02 degrees (3.2 x 10^-4 rad)
How can the Rayleigh Criterion be used to compare telescopes resolving power?
The Rayleigh Criterion will give the smallest angle which can be resolved by the telescopes.
A telescope that can resolve a smaller angle will have a greater resolving power.
What is the approximate quantum efficiency of the human eye?
1%
What are 4 advantages of using CCD over a human eye?
- Greater quantum efficiency
- Can detect a wider region of the EM spectrum
- Better resolution
- Images can be stored, edited and shared
What is collecting power?
The amount of light a telescope can collect. The greater the collecting power, the brighter the image.
What is resolving power?
A telescopes’s ability to resolve fine details
What is the minimum angular resolution?
The smallest angle two objects can make with our eye or a telescope such that we can still distinguish between them
In what scenario can we ‘just’ distinguish the difference between two stars?
When the central maximum of one star’s diffraction pattern coincides with the first minimum of the others. (Angular separation = Rayleigh’s Criterion)
What makes a good telescope?
High collecting and resolving power.
This can be achieved through a large objective lens diameter, using CCD’s and a low Rayleigh Criterion.
What are 2 similarities between radio telescopes and optical reflecting telescopes?
They both have a primary reflector which collects the radiation that is parabolic to avoid spherical aberration.
What are 2 differences between radio telescopes and optical reflecting telescopes?
The radio telescopes dish is made from wire mesh, not glass and they have an antenna in place of a secondary mirror.
Why do radio telescopes typically have a greater collecting power but a worse resolving power than optical telescopes?
The wavelengths that radio telescopes observe are 1 million times larger than optical telescopes therefore their diameter must be larger which increases their collecting power as it is proportional to the cross sectional area. This means the resolving power will be less as the diameter would have to be insanely large to match the power.
Why do optical telescopes have to be placed at high altitudes and away from cities?
In order to minimise light distortion when passing through the atmosphere and away from cities to reduce light pollution.
In what areas do radio waves have to be positioned?
In radio-quiet areas.
Why do optical telescopes have a better resolving power than IR telescopes?
Since they have similar diameters and IR telescopes detect longer wavelengths, the Rayleigh Criterion of IR telescopes is greater therefore it has a worse resolving power.
Why do UV telescopes have a greater resolving power than optical telescopes?
Since they have similar diameters and UV telescopes receievs radiation of shorter wavelengths, their Rayleigh Criterion is lower therefore it has a better resolving power.
Why are X-Ray telescopes not structured using the cassegrain arrangement?
X-Ray radiation will only reflect at really high angles of incidence, this is why grazing structures are used.
