6.1 Telescopes
Components of Modern Radiation Measurement Systems
There are three fundamental components in a modern system designed to measure radiation from astronomical sources.
Telescope: Acts as a "bucket" for collecting radiation, whether it's visible light or other wavelengths.
Larger telescopes are more efficient, gathering significantly more light than the human eye, similar to how a garbage can collects more rain than a coffee cup.
Wavelength-Sorting Instrument: Attached to the telescope, this device sorts incoming radiation by wavelength.
Crude sorting: Can separate broad bands, like blue light from red light, to determine a star's temperature.
Fine sorting: Can observe individual spectral lines to identify an object's composition or measure its speed (as detailed in the "Radiation and Spectra" chapter).
Detector: A device that senses the radiation in the selected wavelength regions.
Its primary function is to permanently record the observations made.
Observing the Orion Region at Different Wavelengths
The appearance of the same part of the sky can vary drastically when observed with instruments sensitive to different parts of the electromagnetic spectrum. This demonstrates how various wavelengths reveal different astronomical phenomena.
Visible light (Figure 6.2a): Shows the Orion region as perceived by the human eye, with familiar constellations.
X-rays (Figure 6.2b): Emphasizes point-like X-ray sources.
Images reveal bright, hot stars in Orion, alongside other diverse objects at varying distances, such as other stars, stellar remnants (star corpses), and distant galaxies.
Colors in X-ray images are artificial, typically changing from yellow to white to blue with increasing X-ray energy.
Infrared radiation (Figure 6.2c): Primarily reveals the glowing dust prevalent in the region.
Historical Development of Astronomical Telescopes
The history of astronomical telescope development mirrors technological advancements aimed at improving the efficiency of the three basic components:
The telescopes themselves (light-gathering capability).
The wavelength-sorting devices.
The detectors (recording observations).
Pre-Telescopic Observatories
Ancient civilizations developed sophisticated methods for observing the sky long before the invention of the telescope:
Ancient sites: Many cultures constructed special observatories (e.g., Machu Picchu, Stonehenge, as shown in Figure 6.3).
Purpose: Primarily used to measure the positions of celestial objects to track time and dates.
These sites often also served significant religious and ritual functions.
Limitations: In this era, the human eye was the sole instrument for gathering light.
All colors were observed simultaneously without wavelength separation.
Observations were permanently recorded through human effort, via writing or sketching.
Invention and Early Use of the Telescope
The invention of the telescope occurred around 1608 with several individuals credited:
Inventors: Hans Lippershey, Zaccharias Janssen, and Jacob Metius all applied for patents within weeks of each other.
Galileo Galilei (1610): Though not the inventor, Galileo was the first to widely apply this new instrument (which he called a "spyglass") to astronomical observation.
He used even a small telescope extensively over many nights.
His observations revolutionized prevailing ideas about the nature of planets and Earth's position in the cosmos, marking a pivotal moment in astronomy.
Systems for Measuring Radiation
Three basic components of a modern astronomical measurement system:
Telescope
Acts as a “light bucket” to collect radiation (visible or other wavelengths).
Larger telescopes gather more light than the human eye.
Analogy: a garbage can collects more rain than a coffee cup; larger telescopes gather more light than the eye.
Instrument for wavelength sorting
Separates radiation into different wavelengths or colors.
Can be crude, such as separating blue from red light to estimate a star’s temperature.
Can be detailed, such as studying spectral lines to identify elements or measure speed.
Detector
Device that senses and records radiation at chosen wavelengths.
Early astronomy used the human eye and written notes.
Modern astronomy uses digital sensors (such as CCDs) to store permanent and precise records.
Example: Orion at Different Wavelengths (Figure 6.2)
Visible light: Shows stars and nebula as the eye sees them.
X-rays: Reveals energetic stars, remnants, and distant galaxies.
Infrared: Shows cooler dust and objects hidden from visible light.
Fact: Different wavelengths highlight different processes happening in the same region.
Learning Objective 1 Complete: The three components (telescope, wavelength sorter, detector) have been described.
History of Observing the Sky
Ancient observatories (pre-telescopic)
Built to track celestial positions for timekeeping, calendars, and rituals.
Examples: Machu Picchu in Peru (15th century) and Stonehenge in England (3000–2000 BCE).
Tools available: human eyes and written or sketched records.
Invention of the telescope (around 1608)
Inventors: Hans Lippershey, Zaccharias Janssen, Jacob Metius.
Galileo (1610): Used the telescope scientifically; called it a “spyglass.”
Impact: Revolutionized astronomy, revealed moons of Jupiter, phases of Venus, surface of the Moon, and challenged Earth-centered cosmology.
How Telescopes Work
Why build larger telescopes?
Celestial objects send much more light than the eye can detect.
Larger aperture means more light gathered, which allows astronomers to see fainter objects.
A telescope captures light that would otherwise be lost.
Main functions of a telescope:
Collect faint light from astronomical sources.
Focus light into an image for study.
Light-gathering power
Determined by the diameter of the aperture (lens or mirror).
Example:
1-meter telescope has a collecting area of 0.79 square meters.
4-meter telescope has a collecting area of 12.6 square meters.
Ratio is 16 to 1, meaning the 4-meter telescope gathers sixteen times more light.
Learning Objective 2 Complete: The two main functions of a telescope (light collection and image focusing) have been described.
Recording Images
Before the nineteenth century
Astronomers used direct observation with eyes.
Records were written descriptions, which were often unreliable.
Nineteenth century
Photography became common, using chemically treated glass plates.
Modern times
Digital detectors such as CCDs replaced photographic plates.
Images are recorded electronically, stored in computers, and analyzed in detail.
Professional astronomers rarely look directly through telescopes today.
Formation of an Image: Lenses and Mirrors
Lenses (Refracting Telescopes)
Lenses bend parallel light rays so they converge at a focus, forming an image.
Focal length: distance from lens to focus.
Eyepiece lens: magnifies image for viewing or redirects it to a detector.
Known as a refracting telescope (refractor).
Limitations:
Large lenses are hard to make without flaws or bubbles.
Chromatic aberration: different wavelengths focus at different points, blurring the image.
Gravity causes large lenses to sag since they can only be supported at edges.
Both sides must be precisely shaped, which is expensive.
Largest refractor still in use: 40-inch Yerkes Observatory telescope.
Mirrors (Reflecting Telescopes)
Concave mirrors reflect light to a focus point.
Advantages:
No chromatic aberration.
Only surface of mirror needs precision, not the whole thickness.
Can be supported from behind to prevent sagging.
Easier and cheaper to construct large mirrors.
Common focus designs:
Prime focus: image forms directly above mirror.
Newtonian focus: secondary mirror reflects light to the side of the telescope.
Cassegrain focus: secondary mirror reflects light through a hole in the main mirror.
Learning Objective 3 Complete: The two types of telescopes (refractors and reflectors) and how they form images have been explained.
Amateur Telescope Use (Choosing a Telescope)
Key considerations:
Portability versus permanent installation.
Visual observing versus astrophotography.
Types of objects to observe: planets, galaxies, comets, clusters.
Stability of mount: essential for steady, clear viewing.
Aperture size: larger apertures allow viewing of fainter objects.
Cost: refractors are more expensive than reflectors of the same aperture.
Magnification: determined by eyepieces, not telescope size. Excessive magnification worsens image quality.
Practical advice:
Start with binoculars, which are portable and effective.
Join astronomy clubs or attend star parties to test telescopes.
Aperture size is the most important specification.
Summary
Three Components of Modern Measurement Systems: Telescope (collects light), wavelength sorter (separates colors), detector (records data).
Main Functions of a Telescope: Collect faint light and focus it into an image.
Two Basic Types of Telescopes:
Refractors (lenses): limited by chromatic aberration and construction challenges.
Reflectors (mirrors): dominant in modern astronomy due to advantages in size and quality.
Imaging evolved from direct observation to photography to modern digital detectors.
Amateur telescope users should focus on aperture size, stability, and realistic observing goals.