Download Telescopes & Light: Part 3 All About Telescopes

Telescopes & Light: Part 3
All About Observing
Detectors
• CCDs (charge-coupled devices), much like what is
found in your digital cameras, are used to produce
images with telescopes (CCD is placed where the
eyepiece would normally go).
• CCDs replaced photographic plates as the standard
imaging detector.
• CCD data is manipulated using various computer
software packages.
Image Processing
• Images taken on research quality CCDs are
always monochromatic - the colors are added
in after the fact.
• Say you take data through a blue filter (which
blocks out all but blue light) and then through
a red and green filter. You can assign each
image its color and then add the three images
to produce a “true color” image. (True color
means that’s how the object imaged would
look if you could view it yourself up close.)
Light-Gathering Power
• A telescope’s light-gathering power
increases as the square of its diameter.
• The more light you can gather, the
shorter your exposure times. So you
use a larger telescope (larger mirror
diameter) to observe fainter objects.
Resolving Power
• Resolution - ability to form distinct
separate images of objects lying close
together in the field of view.
• The better the resolution, the more
detail you can see.
• In astronomy, we talk about the
separation of objects on the sky, or the
angular resolution.
Atmospheric Blurring
• Atmospheric turbulence can blur the light from a star (or galaxy,
etc.) as the light passes through the atmosphere.
• Astronomers use the term seeing to describe this blurring effect.
The smaller the seeing, the crisper your image.
• Good seeing means the atmosphere is fairly stable.
(Unfortunately, the best seeing often comes along with clouds!)
New Telescope Design
• Two techniques - both used to improve seeing.
– Active optics - changes the set-up (mirror temperature,
airflow, etc.). See below left.
– Adaptive optics - changes the shape of the mirror (mirror
resembles a “honeycomb” shape with many small mirrors
making up the primary mirror - each mirror can be moved
independently to achieve the best focus). Usually focus
telescope using a laser. See below right.
The Value of Radio Astronomy
• Sun is a weak radio source, so radio
observations can cover nearly the entire sky.
• Observations can be made during daytime to
within a few degrees of the sun.
• While visible light can be blocked by gas and
dust between us and the object, radio waves
usually pass through unaffected.
• One drawback - usually has poor angular
resolution.
Interferometry
• Two or more telescopes are
used in tandem to observe the
same object at the same time
(telescopes combined in such
a way are called an
interferometer).
• The effective diameter is
equivalent to the distance
between the outermost
dishes. The larger diameter
results in much better angular
resolution.
Infrared and Ultraviolet
Astronomy
• Infrared telescopes are often optical telescopes used
with detectors sensitive to longer wavelengths.
There are only a few windows (wavelengths) where
IR radiation is not absorbed by the atmosphere.
• Ultraviolet observations have to be done from space
since UV radiation is mostly blocked by Earth’s
atmosphere.
High Energy Astronomy
• X-ray telescopes are space based.
• Currently, we have the Chandra X-ray Observatory.
• Gamma ray telescopes simply count photons received - no
image is produced.
Image of supernova remnant.
Full Spectrum Coverage
• Full-spectrum coverage is the complete picture, imaging an
object at all possible wavelengths.