Step by Step
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Refracting telescopes
Refractors use lenses to focus light. Their major limitation is chromatic aberration — different wavelengths of light focus at slightly different points, causing color fringing. Additionally, the glass must be flawless all the way through, and large-aperture refractors become extremely heavy and long.
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Reflecting telescopes
Invented by Newton in 1668, reflectors use a parabolic mirror that reflects all wavelengths to the same focus point, eliminating chromatic aberration entirely. Since a mirror can be supported from behind (rather than needing to be flawless all the way through, like a lens), reflectors can be built much larger.
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Types of reflecting telescope designs
Newtonian, Cassegrain (using a secondary mirror), Schmidt-Cassegrain, and Ritchey-Chrétien (the design used by both the Hubble Space Telescope and the VLT) are all common reflector configurations.
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Size records
The largest refractor ever built is the Yerkes telescope (1 meter, built in 1897) — a size limit essentially never exceeded since, due to the practical problems of scaling up lenses. The largest mirrors today include the GMT (25.4 meters) and the ELT (39 meters, currently under construction).
Applied Walkthrough
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Early telescopes, like those used by Galileo and Kepler, used lenses to focus light — but as astronomers wanted larger and larger telescopes, refractors ran into a fundamental problem: chromatic aberration, where different colors of light focus at slightly different points, blurring the image.
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Newton's 1668 invention of the reflecting telescope, using a parabolic mirror instead of a lens, elegantly solved this problem — since all wavelengths reflect to the same focus point regardless of color.
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Just as importantly, a mirror can be physically supported from behind, unlike a lens (which must remain flawless glass all the way through) — this structural advantage is exactly why reflectors could eventually be built far larger than any refractor ever could.
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This is precisely why the largest refractor ever built, the 1-meter Yerkes telescope from 1897, remains essentially the practical size limit for lens-based telescopes — while modern reflecting telescopes like the GMT (25.4 meters) and the still-under-construction ELT (39 meters) dwarf it by an enormous margin.
Exam Application
Exams test whether you understand why reflecting telescopes eventually replaced refractors for large modern instruments (specifically due to chromatic aberration and structural limitations), and whether you know the largest examples of each type.
⚠ Common Trap
The most common trap is assuming modern large telescopes still use lenses — every large modern research telescope is a reflector, using mirrors rather than lenses, precisely because mirrors avoid chromatic aberration and can be physically supported from behind, allowing much larger apertures.
✓ Quick Self-Check
1. What is chromatic aberration, and which telescope type suffers from it?
Different wavelengths focusing at different points, causing color fringing; refracting telescopes suffer from this.
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2. Who invented the reflecting telescope, and when?
Newton, in 1668.
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3. Why can reflecting telescopes be built much larger than refractors?
Because a mirror can be supported from behind, unlike a lens, which must be flawless glass all the way through.
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4. What is the largest refracting telescope ever built, and how large is it?
The Yerkes telescope (1897), at 1 meter.
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5. Name two modern reflecting telescope designs.
Any two of: Newtonian, Cassegrain, Schmidt-Cassegrain, Ritchey-Chrétien.
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