Chapter 6. The Basics Of Telescopes And CCDs

6.1 Introduction

The goals of this module: After completing this exercise, you should be able to:

1. Describe the benefits of observing with a telescope and CCD camera.
2. Explain how astronomers perform photometric observations of objects.

In this module you will explore:

1. The various instruments that astronomers use to observe celestial objects.
2. How a Charge-Coupled Device (CCD) improves data collection.

Why you are doing it: We can observe only so much with just our eyes. We are limited by how much light they can gather and how quickly our brain processes the data. Astronomers have developed a number of tools that allow us to gather and store more data, and therefore detect fainter objects and determine various properties associated with them.

6.2 Background

Have you ever looked at a picture taken by a professional telescope and wondered how astronomers can see such detail, when we can't just by looking up at the night sky? Let's investigate why.

Astronomical imaging took off in the nineteenth century with the introduction of photographic film, but unfortunately, while you can expose film for longer periods of time, it is not very efficient. Today astronomers use a different method of collecting light, with the use of CCDs, or Charge Coupled Devices.

A CCD is a small chip a few square centimeters in size, which is divided into an array of tiny light-sensitive pixels. A typical CCD can contain over a million pixels! With so many pixels, CCDs can record the positions of incoming photons with incredible precision. This is the technology your phone or digital camera uses. In fact, many amateur astronomers simply attach their digital cameras to their telescopes!

The animation above shows light being given off by a galaxy. We can observe this light in a number of different ways. First, place the person (eye) in the path of the light, so that we can "observe" the galaxy. Notice the bar representing how much light we collect from the galaxy before the human brain processes the data, which it does once every 0.2 seconds. Due to the small size of our eyes (roughly 8 millimeters in diameter) and the quick processing time of our brain, we can collect only a small amount of light from the galaxy at one time, meaning that we can't see dim objects with our eyes.

Now select the Eye+Telescope and see what happens. For the moment we will observe the galaxy with a 1-meter telescope (the opening of the telescope is 1 meter in diameter).

6.3 Light-Gathering Power

The ability of a telescope, human eye, or other instrument to collect light in a given amount of time is referred to as its light-gathering power. The light-gathering power of a telescope is directly related to the area of the objective lens, which depends on the square of its diameter:

That squiggly mark instead of an equals sign is shorthand for "is proportional to" In other words: when comparing two telescopes, if one has twice the diameter of the other, then the bigger telescope will collect not twice but four times as much light in the same amount of time.

6.4 Exposure Time

Even using a telescope, we are still limited by the short exposure time of the human brain. What if we replace the person with a CCD camera, what happens then? Try it and find out!

6.5 Photometry

Photometry deals with the measurement of the apparent brightness of an astronomical object as viewed from Earth. It is important in the study of variable stars, supernovae, extrasolar planet detection, and many other fields.

In addition to their ability to gather light for longer periods of time, CCDs also have a higher efficiency than photographic film, being able to collect roughly 70% of the light that falls on them. Because of this, astronomers now use them for photometry measurements of astronomical objects.

So how do astronomers determine the total amount of light received from an object over time, if even a CCD doesn't capture all of it? Astronomers use what they call "standard stars", or stars which have a well-known apparent brightness. When astronomers observe their object in question, they also observe a standard star on the same night. They then compare the amount of light received from both objects.

Assuming that everything else is the same (same telescope, same CCD, on the same night), the difference in the amount of light recorded for each object must be proportional to the difference in the amount of light emitted towards Earth by each object. Since we know the apparent brightness of the standard star, we can then determine the apparent brightness of the object that we are observing.

6.6 Quick Check Quiz

Indepth Activity: The Basics Of Telescopes And CCDs