PHYSICS NOTES 4U / DETAILED TOPIC SERIES
Astrophysics
AQA 7408 | Detailed topic revision | Astrophysics option
Detailed option pack / AQA 7408 section 3.9
Use light to infer the properties of distant objects. Connect telescope design and radiation
measurements to stellar temperatures, distances, evolution and the expanding universe.
Refractor and Cassegrain ray diagrams; aperture and detectors; magnitudes and distance;
black bodies and stellar spectra; HR diagram and evolution; supernovae and remnants;
Doppler binaries; Hubble law, Big Bang evidence, quasars and exoplanets. Twenty original
tasks with worked solutions.
Find it Page
Refractor / Cassegrain / aberrations 2/3/4
Other bands / aperture / CCD 5/6/7
Magnitudes / distance / temperature / 10
Luminosity / spectra / HR / 13
Evolution / supernovae / remnants / 16
Bursts / Doppler / binaries / 19
Hubble / evidence / quasars / planets /
Practice / solutions / sources 24–28
Independent revision notes, not produced or endorsed by AQA. Original text, diagrams and tasks. Constants:
c=3.00×10⁸ m s⁻¹; G=6.67×10⁻¹¹ N m² kg⁻²; σ=5.67×10⁻⁸ W m⁻² K⁻⁴; 1 pc=3.09×10¹⁶ m; 1 ly=9.46×10¹⁵ m. Use
365 days per year in examples.
Independent revision notes • Edition 1.0 for review • October 2026 1
,PHYSICS NOTES 4U / DETAILED TOPIC SERIES
Refractor / normal adjustment
AQA 7408 | Detailed topic revision | Astrophysics option
A converging objective collects a nearly parallel bundle from one point on a distant object. It
forms a real inverted intermediate image in its focal plane. A shorter-focal-length converging
eyepiece has its front focal plane at the same position.
off-axis parallel rays
objective
eyepiece
Fo = Fe
intermediate image
parallel output
Lens separation fo + fe; schematic, not to scale.
Normal adjustment: final virtual image at infinity; emerging rays from one object point are
parallel.
Lens separation L=fo+fe.
Angular magnification magnitude M=β/α=fo/fe.
For small angles, the intermediate-image height satisfies |h|≈foα≈feβ, giving M=fo/fe. The final
image is inverted. Here M is angular magnification, not absolute magnitude.
Draw an off-axis parallel bundle. Its ray through the objective centre does not bend. Rays do not bend at the
intermediate image. Label the common focus on the axis. Never view the Sun through an unfiltered telescope.
Independent revision notes • Edition 1.0 for review • October 2026 2
, PHYSICS NOTES 4U / DETAILED TOPIC SERIES
Cassegrain / a folded path
AQA 7408 | Detailed topic revision | Astrophysics option
A concave parabolic primary reflects axial parallel rays toward its focus. Before they cross, a
small convex secondary reflects the converging beam back through a central hole in the
primary. An eyepiece views the image behind the primary.
parabolic primary
incoming light
convex secondary
image / eyepiece
central hole
Mirror backs marked with short strokes; ray directions shown by arrows.
Part Role
Parabolic primary Large collecting surface; avoids spherical aberration for axial
parallel rays.
Convex secondary Intercepts converging rays before they cross; folds the optical path
through the primary hole.
Eyepiece Views/magnifies the real image formed by the mirrors.
The central obstruction removes some incoming light, not a central patch of the image.
Each image point receives rays from many parts of the aperture.
Original schematic, not a scale optical prescription. Identify the reflecting faces and backs. The primary halves
belong to one continuous parabola with a central hole.
Independent revision notes • Edition 1.0 for review • October 2026 3
Astrophysics
AQA 7408 | Detailed topic revision | Astrophysics option
Detailed option pack / AQA 7408 section 3.9
Use light to infer the properties of distant objects. Connect telescope design and radiation
measurements to stellar temperatures, distances, evolution and the expanding universe.
Refractor and Cassegrain ray diagrams; aperture and detectors; magnitudes and distance;
black bodies and stellar spectra; HR diagram and evolution; supernovae and remnants;
Doppler binaries; Hubble law, Big Bang evidence, quasars and exoplanets. Twenty original
tasks with worked solutions.
Find it Page
Refractor / Cassegrain / aberrations 2/3/4
Other bands / aperture / CCD 5/6/7
Magnitudes / distance / temperature / 10
Luminosity / spectra / HR / 13
Evolution / supernovae / remnants / 16
Bursts / Doppler / binaries / 19
Hubble / evidence / quasars / planets /
Practice / solutions / sources 24–28
Independent revision notes, not produced or endorsed by AQA. Original text, diagrams and tasks. Constants:
c=3.00×10⁸ m s⁻¹; G=6.67×10⁻¹¹ N m² kg⁻²; σ=5.67×10⁻⁸ W m⁻² K⁻⁴; 1 pc=3.09×10¹⁶ m; 1 ly=9.46×10¹⁵ m. Use
365 days per year in examples.
Independent revision notes • Edition 1.0 for review • October 2026 1
,PHYSICS NOTES 4U / DETAILED TOPIC SERIES
Refractor / normal adjustment
AQA 7408 | Detailed topic revision | Astrophysics option
A converging objective collects a nearly parallel bundle from one point on a distant object. It
forms a real inverted intermediate image in its focal plane. A shorter-focal-length converging
eyepiece has its front focal plane at the same position.
off-axis parallel rays
objective
eyepiece
Fo = Fe
intermediate image
parallel output
Lens separation fo + fe; schematic, not to scale.
Normal adjustment: final virtual image at infinity; emerging rays from one object point are
parallel.
Lens separation L=fo+fe.
Angular magnification magnitude M=β/α=fo/fe.
For small angles, the intermediate-image height satisfies |h|≈foα≈feβ, giving M=fo/fe. The final
image is inverted. Here M is angular magnification, not absolute magnitude.
Draw an off-axis parallel bundle. Its ray through the objective centre does not bend. Rays do not bend at the
intermediate image. Label the common focus on the axis. Never view the Sun through an unfiltered telescope.
Independent revision notes • Edition 1.0 for review • October 2026 2
, PHYSICS NOTES 4U / DETAILED TOPIC SERIES
Cassegrain / a folded path
AQA 7408 | Detailed topic revision | Astrophysics option
A concave parabolic primary reflects axial parallel rays toward its focus. Before they cross, a
small convex secondary reflects the converging beam back through a central hole in the
primary. An eyepiece views the image behind the primary.
parabolic primary
incoming light
convex secondary
image / eyepiece
central hole
Mirror backs marked with short strokes; ray directions shown by arrows.
Part Role
Parabolic primary Large collecting surface; avoids spherical aberration for axial
parallel rays.
Convex secondary Intercepts converging rays before they cross; folds the optical path
through the primary hole.
Eyepiece Views/magnifies the real image formed by the mirrors.
The central obstruction removes some incoming light, not a central patch of the image.
Each image point receives rays from many parts of the aperture.
Original schematic, not a scale optical prescription. Identify the reflecting faces and backs. The primary halves
belong to one continuous parabola with a central hole.
Independent revision notes • Edition 1.0 for review • October 2026 3
