AQA A-Level Astrophysics

Telescopes Explained: Designs, Detectors, and Why Each One Matters

When students hear telescope, they often imagine a long tube pointed at the night sky. In reality, modern astronomy relies on several very different telescope designs, each optimised for a particular part of the electromagnetic spectrum.

For AQA A-Level Physics (Astrophysics option), it’s not just about naming telescope types — it’s about understanding why each design exists and what its advantages are.

---

1. Refracting Telescopes (Lens Telescopes)

How they work

Refractors use a convex objective lens to focus incoming parallel light rays to a focal point.

Advantages

• Simple optical design

• Produces sharp, high-contrast images

• Low maintenance (sealed tube)

Limitations (important for exams)

• Chromatic aberration – different wavelengths focus at different points

• Large lenses are heavy and difficult to manufacture

• Lens sag and absorption reduce effectiveness for large apertures

Exam tip

Refractors are limited by chromatic aberration and lens size, which is why professional observatories no longer use them.

---

2. Reflecting Telescopes (Mirror Telescopes)

How they work

Reflectors use a parabolic primary mirror to reflect light to a focal point, usually redirected by a secondary mirror.

Advantages

• No chromatic aberration

• Large mirrors are easier to support than lenses

• Much greater light-gathering power

Limitations

• Requires precise mirror alignment (collimation)

• Central obstruction slightly reduces contrast

Why professionals use them

Almost all major optical telescopes (e.g. Keck, VLT, JWST) are reflectors because mirrors scale efficiently to huge diameters.

---

3. Radio Telescopes

How they work

Radio telescopes use large metal dish antennas to collect long-wavelength radio signals.

Key advantages

• Can operate day and night

• Not affected by cloud cover

• Detect cold objects invisible in optical wavelengths

Resolution problem (and solution)

Because radio wavelengths are long, a single dish has poor resolution.
This is solved using interferometry — combining signals from many dishes to act like one giant telescope.

Exam language to use

“Resolution is improved by increasing effective aperture using interferometry.”

---

4. Space Telescopes

Why put telescopes in space?

Earth’s atmosphere:

• Absorbs infrared, ultraviolet, X-rays, and gamma rays

• Causes turbulence that reduces resolution

Advantages

• Access to full electromagnetic spectrum

• No atmospheric distortion

• Higher resolution images

Limitations

• Extremely expensive

• Difficult or impossible to repair

• Limited operational lifespan

AQA focus

You should be able to explain why different wavelengths require space-based telescopes.

---

5. Comparing Telescope Designs (Exam Gold)

Telescope Type	Main Advantage	Key Limitation
Refractor	Sharp images	Chromatic aberration
Reflector	Large aperture	Alignment needed
Radio	Works in all weather	Poor resolution (alone)
Space	Full spectrum access	Cost and maintenance

---

How This Appears in AQA Exams

Typical AQA questions ask students to:

• Compare refracting and reflecting telescopes

• Explain why mirrors are preferred over lenses

• Describe why radio telescopes use arrays

• Link wavelength, aperture, and resolution

If students can connect design → wavelength → advantage, they score highly.