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updated: 04/19/2000

# Using a Telescope

## Overview

• Observe astronomical objects through a telescope.
• Discover the results of varying magnification, aperture, etc.

## Introduction

Since Galileo first pointed his spyglass to the heavens, our eyes have been opened to a broader universe. The telescope, even when used with the eye alone, reveals to us stars and other objects too faint to see normally, and the disks of planets and galaxies too small to see normally. With CCDs or film, the telescope acts as a light bucket, collecting photons to uncover the faintest objects. In this lab you will use telescopes to observe various types of objects, and understand the wealth of information attainable with these instruments so vital to modern astronomy.

The basic telescope consists of a main lens or mirror, typically called the objective or primary, which defines the aperture, or diameter of the light collecting area. A second lens, the eyepiece is used to realign the light rays for viewing, resulting in a magnified image. The magnification is the ratio of the focal lengths of the primary and the eyepiece, where the focal length is the distance between the lens and the focal point, where incoming light is concentrated.

Magnification = fo/fe

Note that the image is inverted (appears backwards and upside-down when compared to the object). The figure above is for a refracting telescope, that is one with a lens as the primary. Reflecting telescopes use a mirror as the primary, but the function is the same.

For the following sections, use a separate worksheet to sketch and describe what you are viewing through the telescope. Always include the magnification and specify which telescope is being used. Pay close attention to colors, subtle details and number of objects visible. Notice any differences between the telescopic image and the object's appearance to the naked eye. Vary the magnification, and note the differences you see. For submission, translate your notes and sketches into coherent observations.

## Observations

### Solar System Objects

• Observe the following (if available). Address the posed questions and note any other interesting or puzzling observations.
1. The Moon. Note its phase, the presence of earthshine, and/or color differences between the maria and highlands. Do these areas have the same number of craters? Count craters and carefully sketch the terminator.
2. Planet(s). Note their position in the sky and relative to the Sun and/or Moon. Estimate its phase and position in its orbit relative to the Earth and Sun. Sketch the location of any moons, dark features and/or rings. Does the planet appear spherical? What colors do you see?
3. Other. Specify and describe in detail. Include a sketch.

### Stars, Galaxies and Nebulae

• Observe the following types of objects. Again be descriptive and include sketches.
1. Double star. Note the name of the star(s) and any contrasts in color or brightness. Is this a true binary (i.e., are they gravitationally bound)? Can you see both with the naked eye? Compare the naked eye and telescopic color(s).
2. Cluster. Specify if globular or open, or observe both if possible. Do you see individual stars or a hazy blob? Estimate how many stars are in the cluster. Do you see stars obviously not part of the group? How can you tell? Note any color differences between stars and sketch the cluster. Does it have a clear shape?
3. Galaxy. Specify the galaxy name and type (spiral, elliptical, irregular). Note its color(s), shape, brightness, etc. If it is a spiral, estimate its inclination. Does the brightness peak sharply towards the nucleus? Is it visible to the naked eye?
4. Nebula. Specify the cloud by name. Sketch it carefully noting the location of associated stars, bright spots and colored areas.

### Time-Exposed Observations

• Using the CCD camera on the 16-inch telescope, image a couple of interesting objects observed earlier. Note the object, its location in the sky, the weather conditions, exposure time and magnification. Be sure to get several hardcopies of your images and turn in one for lab.
1. How does the time-exposed image compare to the image you saw through the telescope with your eye? Note shapes, sizes, the number of objects, etc.
2. Do certain objects yield more information than others when exposed?
3. Was any information lost in the long exposure?