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Making a reflective telescope

Replica of the first reflective telescope built by Sir Isaac NewtonNowadays, professional astronomers use another type oftelescope that is different to the first telescope invented (therefractory telescope.) This other kind of telescope can be over10m in diameter! It is called a reflective telescope. It wasinvented by Isaac Newton around 1670.The main difference is that it uses curved mirrors instead oflenses to collect the light.

Why is the reflective telescope the one most astronomersprefer to use?The universe is huge and full of stars and galaxies that are sofar away they hardly seem to shine, or we cant see them atall. The best way to look at those faraway stars is not tomagnify the sky with a refractory telescope, but to gather asmuch light as possible. If you have ever held a mirror outside in

the Sun, you will have noticed that mirrors collect and reflectlight very well!

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(Source: Caltech archives)

Building the 5 m mirror for the Palomar Observatory, Pasadena

Mirrors have other advantages too. Unlike lenses, the light justhas to reflect off one side and not shine through them. Thismeans that they dont have to be supported just around theedges (like a pair of glasses). They can be made really big andstill remain steady.

The mirror at the Gran Telescopio Canarias, Observatorio Roque de losMuchachos,

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Making a reflective telescope

GOAL: To build a simple reflective telescope and note its use

of mirrors. To imagine how a professional telescope works.MATERIALS: a concave mirror (with some magnification), likethe ones you have in the bathroom.

- a small magnifying glass like the ones used by stampcollectors.

- a thick cardboard tube, equal to the focal length of themirror

- 2 smaller tubes about 10 cm in length and 2 to 3cm indiameter.

- a 2 cm x 2 cm flat mirror- a thin wooden dowel (not rounded!) or chopstick with edges- a tape-measure- double-sided tape- gaffa tape ( strong)- masking tape (easy to use on cardboard)- scissors- cardboard cutter

AGE GROUP: Supervised, indoor/outdoor demonstration and

group activity for 7 years and up.

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(source: Caltech archives)

Polishing the 5 m mirror destined for Paloma Observatory. Opticians removed over4,500 kilos of glass before they achieved the perfect form.

*A quick word about mirrorsThe quality of reflective telescopes depends a lot on theperfection of the mirror surface.Technicians spend hours and hours ofprecise and painstakingwork grinding and polishing the mirrors to achieve the concavecurve that they need.

(A concave mirror has a shallow dip on one side causing light rays tomeet or merge.)Since the ordinary type of concave mirrors that we are going touse (the ones you have in the bathroom) are not of good quality,the resulting telescope is not going to resemble the ones usedby research astronomers. But you will get the idea of how they

work!

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WHAT TO DO:

First we need to know thefocal length or focal

distance of the concavemirror. Go outside on a sunnyday and find a spot with theSun in front of you. Face awall.

Watching the wall ONLY,raise your hand and hold yourmirror until it catches thelight. If you use a piece ofpaper for your wall you willnotice how the reflected heatof the Sun can actually burna hole in the paper. This iswhy we NEVER look at the Sun.

Focus the mirror onto thewall, and adjust it back andforth until you can see theSun like a very fine point. Thedistance between the mirrorand the wall is the focallength.

Ask a friend to measure itwith a metric tape. Writedown the focal length, this isVERY important!

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Find a cardboardtube with adiameter similar tothat of the

concave mirror. Itmust also be atleast as long asthe focal lengthyou have justmeasured.

Fix the concave mirror in one of the tube ends, with itssurface turned inwards.

From theposition of themirror, make a

mark on thetube thatmeasures 10 cmBEFORE that ofthe entire focallength. Make ahole at the 10cm mark with a

diameter equal to the small cardboard tube. This will bewhere you place the ocular or eyepiece.

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From the top of theocular hole, trace aline 90 degrees

downwards and markthe point. Do thesame on the otherside.

Make 2 small holes at these points and poke the thinpiece of wood all the way through.It is important thatthe rod not be smooth but have an edge.

Put some doublesided tape on the back of the flat mirrorand introduce it through the ocular hole. Attach it to thechopstick or dowelso that it forms an

angle of 45degrees with thediameter of the bigtube. This way itwill deflect therays coming fromthe mirror,introducing the

rays in the smallocular.

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Make your ocular or eye-piece in the following way.( this is exactly the same way as you make the eye-piece in arefractory telescope).

Cut along the entire length ofone of the tubes.This is Tube #1.

Hold the tube directly over themagnifying glass.

Grip the tube tightly to makeit smaller in diameter (makethe cut edges overlap) so that

the lens is held securely inplace.

Attach the lens from themagnifying glass securely tothe end of Tube # 1. This willbe your eyepiece the partwhich you will use to observe

with.

Put Tube #1 inside the secondtube and check to see that itslides easily up and down. Onceyou are sure that the lens inTube #1 is secure, and that thetube can move snugly inside the

second tube ( the tube which isnot cut), tape all along the cut border.

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The small tube must be able toslide in and out neatly in orderto focus the final image.

Direct the telescope towards an object and observe it byusing the small tube. Move the ocular back and forthuntil you see the object focused. In the picture below,imagine that the small red lamp is a star or a planet! Its

light is collected by the concave mirror which relects itback onto the flat mirror which in turn reflects it ontothe hand through the ocular.

Source: Ricardo Moreno, UNAWE en espanol

UNAWE is an international programme to inspire young underprivileged children with the beauty

and scale of the universe. Universe Awareness illustrates the multicultural origins of modern

astronomy in an effort to broaden children's minds, awaken their curiosity in science and stimulateglobal citizenship and tolerance.

Universe Awareness is imagination, excitement and fun in the universe for the very young.