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Telescope

Bringing the distant close

Date
1608
Era
Renaissance
Rebuild stage
IV Science & industry
Difficulty

Plate

Legend
  1. 1Convex objective lens
  2. 2Main tube
  3. 3Eyepiece housing
  4. 4Concave eyepiece lens

Anatomy

Starlight: through the card, the objective and the eyepiece to the eyeMain tube: two wooden halves bound with copper wire under paper, shortened at the two breaks (1.27 m in all)Cards over the objective, holes of 10, 16, 26 and 38 mmThe tube is drawn with a broken length: about 1 m of plain main tube is left out between the break marks, and every part shown keeps its true size (the whole instrument is 1.27 m long). The convex objective bends the starlight toward a focus 1,330 mm away. The concave eyepiece catches it 94 mm before that point and sends it out parallel again, with every angle magnified 14 times, in a beam 1.8 mm wide with his card. Galileo covered his 51 mm lens down to 26 mm. A larger hole gathers more light (19 times the eye's with his card, 64 with the bare lens) and lowers the diffraction limit, but it also uses the less accurate outer zone of the lens and widens the colour fringes. A smaller hole gives a cleaner but dimmer image, and its diffraction limit grows: 14″ at 10 mm against 5.3″ at 26 mm. The field, counted where half the light still reaches the eye, hardly changes with the card (15′ with the smallest hole, 14′ with none), because the eye's own pupil sets it. The 6 mm pupil and the cards other than 26 mm are assumptions; the stand and the tray are guesses, since no mount of Galileo's survives.

A telescope gathers light from a distant object with a lens or a mirror and enlarges the image it forms. Spectacle makers in the Netherlands made the first ones in 1608, and Galileo's improved instruments showed mountains on the Moon in late 1609 and four moons circling Jupiter in January 1610.

What it is

A telescope makes distant things look nearer. Its main light-gathering element, called the objective, collects far more light than the pupil of the eye and forms an image of the distant object. In a refractor the objective is a lens at the front of the tube; in a reflector it is a curved mirror at the back; a smaller eyepiece lens then enlarges that image for the eye.

The telescope on the plate is one of the two surviving instruments made by Galileo Galilei, kept in the Museo Galileo in Florence. Its main tube is made of two wooden half-tubes held together with copper wire and covered with paper. A short housing at the front holds the objective, a convex lens 51 mm across with a focal length of 1,330 mm. Another at the back holds the eyepiece, a small concave lens. The instrument magnifies 14 times and shows a patch of sky only a quarter of a degree wide, about half the width of the full Moon.

How it works

Light from a star or a distant tower arrives as an almost parallel beam. The convex objective bends that beam so that it converges toward a point, the focus, 1,330 mm behind the lens in Galileo’s telescope. On its own, the objective would form a small, upside-down image there.

Newton’s small reflecting telescope on a ball mount, with its eyepiece at the side near the open end Newton’s reflector replaced the objective lens with a curved metal mirror at the bottom of a short tube; a small flat mirror sends the light out to the side.

In the Galilean design the concave eyepiece intercepts the converging light before it reaches the focus. It spreads the rays just enough to make them parallel again, but they now travel at steeper angles than when they arrived, so the eye sees the object larger and the right way up. The magnification is the focal length of the objective divided by that of the eyepiece, leaving aside the eyepiece’s minus sign: 1,330 mm divided by 94 mm gives about 14.

In 1611 Johannes Kepler proposed a convex eyepiece instead. The Keplerian telescope gives an upside-down image, which does not matter to an astronomer, and a much wider field of view, and it became the standard astronomical refractor. A lens also bends each colour by a slightly different amount, which surrounds bright objects with coloured fringes. Isaac Newton avoided this by gathering the light with a concave mirror, which reflects every colour alike.

How to build it

A Galilean telescope like Galileo’s needs two lenses and a tube about a metre long.

Choose the lenses. For the objective, a convex lens of about +1 dioptre (focal length 1,000 mm) and 40 to 50 mm across; a single lens works, an achromatic doublet from an optics supplier gives a sharper image. For the eyepiece, a concave lens of about −10 dioptres (focal length −100 mm) and 20 to 25 mm across. This pair magnifies 1,000 ÷ 100 = 10 times.

Make the tube. The lenses must sit about 900 mm apart: the objective’s focal length minus the eyepiece’s, counted without its minus sign. Use two cardboard or plastic tubes that slide one inside the other, so that the length can be adjusted by a few centimetres. Paint or line the inside matt black to stop stray reflections.

The Moon at first quarter in ink and wash: the lit half with craters along the ragged boundary, small bright peaks in the dark half At first quarter, peaks catching the sunlight beyond the shadow line showed Galileo that the Moon had mountains.

Mount the lenses. Hold the objective at the front end between two rings of card or wood, square to the tube. Fit the eyepiece in a short cap at the back with the hollow side toward the eye.

Focus and use. Rest the tube on something firm, such as a camera tripod or a fence, and slide the tubes until a distant chimney looks sharp. At night it shows craters on the Moon and, on a steady evening, Jupiter’s four largest moons.

Safety. Never point a telescope or any lens at the Sun, even briefly or near sunset, and never leave one where children could do it: focused sunlight can cause immediate, permanent eye damage and can set the tube on fire. Looking at the Sun through a telescope needs a filter made for that purpose that covers the whole front opening, fixed securely, checked for scratches and holes each time and used as its maker instructs. Eclipse glasses and filters that screw into the eyepiece are not safe behind a telescope.

History and evolution

On 2 October 1608 Hans Lipperhey, a spectacle maker in Middelburg, asked the States General in The Hague for a patent on an instrument for seeing far things as if they were near. Jacob Metius of Alkmaar filed a similar request a few weeks later, others claimed the idea too, and the patent was refused. No telescope from these first makers survives; Lipperhey’s probably magnified about three times.

Jupiter in a small telescope with four bright moons almost in line with it, one on the left and three on the right Jupiter and its four large moons lie almost on one line; Galileo watched them change places from night to night in January 1610.

News of the “Dutch perspective glass” spread quickly. Thomas Harriot in England was using a six-power instrument by the summer of 1609, and Galileo in Padua, who ground his own lenses, built a three-power spyglass in the summer of 1609 and showed an eight-power one to the Venetian Senate in August. By the autumn he had reached about twenty; sources differ on whether he first studied the Moon with that instrument or with a weaker one. His second surviving telescope magnifies 21 times. In March 1610 his short book Sidereus Nuncius described mountains on the Moon, countless stars in the Milky Way and four moons orbiting Jupiter. The name telescope was suggested in 1611 by Prince Federico Cesi, founder of the Accademia dei Lincei.

Astronomers then built ever longer refractors to reduce colour blur. Chester Moore Hall made achromatic lenses of two kinds of glass around 1733, and they came into general use after John Dollond began producing them in the late 1750s. Newton’s reflector, designed in 1668 and shown in a second version to the Royal Society at the turn of 1672, was the model for the mirror telescopes William Herschel built himself; with one of them he found the planet Uranus in 1781.

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