Non-Optical Telescopes — Quick Summary
Mr Toogood's Physics · Astrophysics
AQA 3.9.1.3 / 3.9.1.4
θ ≈ λ/D
Rayleigh criterion
Power ∝ D²
Collecting power
gap < λ/20
Mesh reflects, not diffracts
Resolution ∝ baseline
Interferometry
Radio telescopes
Mesh gaps < λ/20 reflect the signal; larger gaps cause diffraction instead.
A large parabolic dish (solid or wire mesh) reflects incoming signals to an aerial at the focus. A mesh dish is lighter and its exact shape is less critical, provided gaps stay below λ/20.
- Observe 30 MHz–600 GHz (λ = 10 m to 0.5 mm); can run 24 hours, through cloud.
- Poor resolving power — long λ means huge D is needed (e.g. Lovell telescope, D=76 m).
- Collecting power ∝ D², same relationship as optical reflectors.
Radio telescope arrays (interferometry)
Signals from separated dishes are combined to act like one huge-diameter telescope.
Resolution set by baseline (distance between dishes), not the sum of dish areas. E.g. the Very Large Array (27 × 25 m dishes) achieves an effective diameter over 30 km.
Across the EM spectrum
| Type | Where observed from | Key notes |
|---|---|---|
| Radio | Ground (always) | Parabolic dish/mesh; poor resolution due to long λ; interferometry improves this. |
| Infrared | Ground (high/dry) or orbit | Reflecting design; telescope itself must be cooled to avoid emitting its own IR; sees cool objects & redshifted early-universe light. |
| Ultraviolet | Orbit only | Reflecting; mirrors need even higher precision (shorter λ); sees hot, energetic objects. |
| X-ray | Orbit only | Grazing-incidence mirrors, several m long; low collecting power; hot gas & supernova remnants. |
| Gamma-ray | Orbit (some ground) | Can't be focused — collimators/coded aperture masks instead; pulsars, black holes, GRBs. |
The general trend
Shorter λ: better resolving power for a given D, but far more atmospheric absorption — UV, X-ray and gamma telescopes must be in orbit. Longer λ (radio): can stay ground-based, but need a huge D (or an array) for useful resolution.
Water vapour absorbs IR and UV; X-rays and gamma rays are absorbed even more strongly, so only radio (and some IR/optical) astronomy is possible from the ground.
Exam essentials
Key equations
- θ≈λ/D
- Collecting power ∝ D²
- Mesh reflects if gap < λ/20
Ground vs. orbit
- Ground OK: radio (always); IR/optical at high, dry sites.
- Orbit only: UV, X-ray, gamma — absorbed by the atmosphere.
Common slips
- "Higher resolving power" means a smaller θ — don't mix up the direction.
- Radio telescopes need huge D specifically to compensate for their long λ.
- Interferometry increases resolution via baseline separation, not by adding up collecting areas.