TELESCOPIC ASTRONOMY

What is a telescope?

What are telescopes used for? View distant objects Collect light

First Telescope 1608- Hans Lippershey Hans Lippershey was a Dutch lens maker. 1609- Galileo. Built his own telescope, used it for scientific study. Mountains and valleys on Moon Moons of Jupiter Phases of Venus Saturn’s rings

Galileo’s Telescope Refraction telescope: 14x magnification Restricted field of view: only 1/3 of moon at a time Museo di Fisica e Storia Naturale in Florence, Italy has original

Optical telescopes use a lens and light. One type: Refracting telescope uses two large lenses to gather and focus light.

the main lens in a refracting Primary lens: the main lens in a refracting telescope. It is also called an objective lens.

A small lens to magnify the image produced by the Eyepiece: A small lens to magnify the image produced by the objective (primary) lens

Focal length – the distance from the lens or mirror to the image formed of a distant light source In telescopy and most photography, longer focal length or lower optical power is associated with larger magnification of distant objects, and a narrower angle of view. Conversely, shorter focal length or higher optical power is associated with a wider angle of view. In microscopy, on the other hand, a short objective lens focal length leads to higher magnification.

Pros Simple design Minimal maintenance Good for planets and moons Good for photography High contrast

Cons Costly Bulky and large (large focal length) Chromatic aberration

Refraction Limitation When light is refracted through glass, shorter wavelengths bend more than longer wavelengths, and blue light comes to a focus closer to the lens than does red light.

Refraction Limitation If we focus on the blue image, the red image is out of focus and we see a red blur around the image. This color separation is called chromatic aberration.

An achromatic lens is made of two components made of different kinds of glass and brings the two different wavelengths to the same focus. Other wavelengths are still out of focus.

Do we still use these types of telescopes??

Yerkes Refracting Telescope Largest refracting telescope in the world is at Yerkes Observatory in Wisconsin Lens is 1m in diameter ½ tonne The glass sags under its own weight

Reflecting Telescope 1666- Newton found that a prism breaks up white light into a rainbow of colours Telescope lenses do the same Creates haloes of coloured light around objects being viewed

Newton’s Telescope

Primary mirror: the main mirror in a reflecting telescope. It is larger than the secondary mirror. It is also called an objective mirror. NOTE: The primary mirror is in a different spot than the primary lens of a refracting telescope.

Secondary mirror: the smaller mirror in a reflecting telescope. It directs the light through a small hole in the primary mirror to the eyepiece.

Benefits of Reflecting Telescopes Less expensive. Only the front surface of the mirror must be ground. The mirror can be supported to reduce sagging. They do not suffer from chromatic aberration because the light is reflected toward the focus before it can enter the glass.

Cons of Reflecting Telescopes More maintenance

William Hershel (1757-1822) By late 1770s, he was making the best metallic mirrors and telescopes in the world. 1781- Discovered Uranus His telescope had a 125cm mirror and was 40ft in length

Hershel’s Telescope

Large Telescopes Up until mid-1800s telescopes were generally small and not very powerful 1838- Earl of Rosse, Ireland, taught himself art of mirror-making and built a 91cm telescope 1842- attempted to build a 181cm telescope but it broke when moved built another one that couldn’t be moved Rosse is important because he built the largest telescope the world had ever seen and he saw further into space than anyone had before him.

Rosse’s Telescope Rosse is important because he built the largest telescope the world had ever seen and he saw further into space than anyone had before him.

Cassegrain Telescope Specific type of reflecting telescope a wide-angle reflecting telescope the eyepiece or camera is mounted at the back end of the tube developed in 1672

Schmidt-Cassegrain Telescope Specific type of reflecting telescope a wide-angle reflecting telescope (Cassegrain telescope) with a correcting lens that minimizes spherical aberration The correcting plate (a lens) was added in 1930 by the Estonian astronomer and lens-maker Bernard Schmidt (1879-1935).

New Generation Telescopes Refraction and Reflection telescopes are OPTICAL telescopes (uses light) New generation telescopes are RADIO telescopes (uses waves) Radio telescopes use dishes to pick up radio waves. Sensors on the dishes collect the waves and turn them into a picture.

Radio Telescopes Objects in space, such as planets and comets, giant clouds of gas and dust, and stars and galaxies, emit light at many different wavelengths. Some of the light they emit has very large wavelengths - sometimes as long as a mile!. These long waves are in the radio region of the electromagnetic spectrum. Because radio waves are larger than optical waves, radio telescopes work differently than telescopes that we use for visible > light (optical telescopes). Radio telescopes are dishes made out of conducting metal that reflect radio waves to a focus point. Because the wavelengths of radio light are so large, a radio telescope must be physically larger than an optical telescope to be able to make images of comparable clarity. For example, the Parkes radio telescope, which has a dish 64 meters wide, cannot give us any clearer an image than a small backyard telescope! In order to make better and more clear (or higher resolution) radio images, radio astronomers often combine several smaller telescopes, or receiving dishes, into an array. Together, the dishes can act as one large telescope whose size equals the total area occupied by the array. Many astronomical objects emit radio waves, but that fact wasn't discovered until 1932. Since then, astronomers have developed sophisticated systems that allow them to make pictures from the radio waves emitted by astronomical objects. Radio telescopes look toward the heavens at planets and comets, giant clouds of gas and dust, and stars and galaxies. By studying the radio waves originating from these sources, astronomers can learn about their composition, structure, and motion. Radio astronomy has the advantage that sunlight, clouds, and rain do not affect observations.

New Generation Telescopes 1993 – Keck telescope 1000cm mirror, made of smaller segments Photographic plates were more sensitive and permitted a permanent record of observations Photographic plates have since been replaced by electronic imaging devices

Keck Telescope

Handicaps to Radio Telescopes 1) Poor resolution: To improve resolution, two or more radio telescopes can be combined to improve the resolving power

Handicaps to Radio Telescopes Low intensity In order to get strong signals focused on the antenna, the radio astronomer must build large collecting dishes. The largest dish is the 300 m dish at Arecibo, Puerto Rico.

Handicaps to Radio Telescopes Interference: This occurs because of poorly designed transmitters in Earth satellites to automobiles with faulty ignition systems.

Light-gathering power- Powers of a Telescope Light-gathering power- This is the ability of a telescope to collect light. The larger the telescope’s lens, the more light it can gather.

Powers of a Telescope Resolving Power – the ability of a telescope to reveal fine detail. The larger the telescope, the better its resolving power. However, optical quality of the lens and atmospheric conditions limit the detail we can see.

Powers of a Telescope Magnifying power – the ability to make the image bigger. Magnification of a telescope can be changed by changing the eyepiece.

Astronomers identify telescopes by diameter because that determines both light-gathering power and resolving power.

Telescopes and Observatories An observatory is a place for viewing the sky through telescopes. Observatories can be small with just a single telescope outside or they can be large buildings housing a number of telescopes, each in their own room.

Hale Observatory, California

The traditional image of an observatory is probably that of a large building with domed rooms housing telescopes. But an observatory can be just a single room with a telescope, or nothing more than an open area with telescopes.

Observatories are built on top of mountains because: air is thin and more transparent the sky is darker stars are brighter 4) wind blows smoothly over some mountaintops there is less pollution

Telescopes and the Scientific Method

The Scientific Method

The Scientific Method’s 4 Steps Observation and description of a phenomenon. The observations are made visually or with the aid of scientific equipment (like a telescope). 2) Formulation of a hypothesis to explain the phenomenon (usually in the form of math or a causal mechanism or a mathematical relation. Test the hypothesis by analyzing the results of observations or by predicting and observing the existence of new phenomena that follow from the hypothesis. If experiments do not confirm the hypothesis, the hypothesis must be rejected or modified (Go back to Step 2). Establish a theory based on repeated verification of the results.