Reading: Diagram Label Completion: Question 9

A simple refracting telescope gathers and focuses light using nothing but a pair of lenses mounted at opposite ends of a long tube, a design essentially unchanged since amateur astronomers began building their own instruments in home workshops during the nineteenth century. A basic 900-millimetre refractor assembled by the Hallowick Astronomy Club for its outreach programme in 2019 demonstrates every part typically found in an entry-level instrument of this kind.

At the front, or "sky", end of the main tube sits the objective lens, a large glass lens whose job is to gather incoming light from a distant object and bend it inward so that the rays converge to form a small, sharp image partway down the tube. The wider the objective lens, the more light the telescope can collect, which is why serious observers judge a refractor chiefly by the diameter of this front lens rather than by its magnifying power.

Because the image formed by the objective lens does not always land at exactly the same distance down the tube, the correct focal point shifts slightly depending on what is being observed, the rear section of the telescope includes a draw tube, a narrower sliding sleeve that can be moved in and out of the main tube to bring the image into sharp focus. A focus knob mounted on the side of the draw tube turns a fine gear that moves the sleeve smoothly and precisely, allowing an observer to sharpen the image without touching, and so accidentally shifting, the rest of the telescope.

At the very end of the draw tube sits the eyepiece, a small, removable lens assembly that the observer looks through directly; it takes the small image formed by the objective lens and magnifies it further before it reaches the eye, and swapping in an eyepiece of a different focal length changes the telescope's overall magnification.

Because the main tube's narrow field of view makes it difficult to locate faint objects directly, a small finder scope is mounted alongside the main tube, giving a wider, lower-magnification view that helps the observer aim the main instrument at the right patch of sky before switching to the eyepiece. The whole assembly rests on a tripod mount, a three-legged stand with a pivoting head that lets the observer swing the tube smoothly to track a target as it moves slowly across the night sky.

The diagram below shows the layout of the Hallowick Astronomy Club's refracting telescope, from front (sky end) to rear (eyepiece end). Label the diagram below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1 ____________________. The large lens at the front, or "sky", end of the tube that gathers and bends incoming light 2 ____________________. The narrower sliding sleeve at the rear of the tube that moves in and out to focus the image 3 ____________________. The control on the side of the draw tube that turns a gear to move the sleeve smoothly 4 ____________________. The small lens assembly at the very end of the draw tube that the observer looks through 5 ____________________. The smaller, wide-view instrument mounted alongside the main tube, used for aiming at the right patch of sky 6 ____________________. The three-legged stand with a pivoting head that supports the whole assembly