Telescopes
Goals & Outcomes
• Appreciating science in general, and astronomy in specific.
• Understanding how knowledge is gained and be critical of what you
see and hear. You will begin to ask “How can we test that?” when
forming hypotheses or “How do we know that?” when reading new
information.
• Learn some simple astronomical nomenclature/terminology.
• Learn about some problems astronomers and astrophysicists are
trying to solve, and understand the methods scientists are using to
try to solve these problems.
• Explain how electromagnetic radiation and astronomical instruments
are used to reveal the properties of stars and galaxies.
Big Picture
• What do telescopes do?
• What must happen for eyes or a telescope to “see”
an object?
• Why are telescopes better than human eyes?
• What are limits of using telescopes, both on Earth
and in space?
• How are telescopes from across the world used
simultaneously to get the best pictures?
Usage of telescopes
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Astronomers use telescopes to do 3 things:
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See pages 189-191. Astronomers job: 190-191.
1. Imaging (taking pictures)
2. Spectroscopy (like with did with the diffraction gratings,
prisms)
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Analyze amount of light at different wavelengths
Two things equivalent to wavelength?
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Color (if using visible light)
Frequency
How do they do this?
Take pictures of spectrum
3. Timing – watching things change.
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How do they do this?
Take many pictures.
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Purposes of telescope
Gather light
Focus light
Detect that light
NOT to magnify images
– That’s only when people look through them.
– Astronomers mostly use telescopes to
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take pictures
NOT look through! (Although that’s fun!)
– See Common Misconception, page 184
Calif. Elementary School Science Standards
for telescopes
– From California Science Standards, grade 3.
(Note: as written, it is misleading and not quite correct!)
a. Students know telescopes magnify the appearance of some distant
objects in the sky, including the Moon and the planets. The number of stars
that can be seen through telescopes is dramatically greater than the
number that can be seen by the unaided eye.
– To correct this standard, they should say “when people look
through telescopes with their eyes, telescopes magnify …”
– Professional astronomers don’t look through telescopes with their
eyes, they take pictures.
How this works in
YOUR TELESCOPES: your eyes
• Teacher drawing of eye, as we see them
– Purpose of pupil, iris.
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Gather light (let light in)
When is pupil most dilated? Contracted?
Wider pupil = more light enters. Need this when …
things are dark or faint.
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See figure 6.1, page 181 for “inside” view
Next, after gathering light, need to
Focus light. What does that for human telescopes?
See figures 6.3 and 6.4, page 182 for
how light is focused by the human eye
• After gathering and focusing light, what must a telescope
do?
• What detects light in the human telescope?
– Retina (rods and cones cells)
Seeing things (with eye or in photos)
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Whether you “see” (detect) an object depends on only
one thing:
amount of light detected by a detector
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5.
This depends on several things:
Apparent brightness of light source (Why apparent?)
Light collecting area
Exposure time
Efficiency of light detector
(Angular) Resolution and focus
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More details about each of these coming up.
Apparent brightness
• What affects how bright light sources APPEAR?
1. Distance. Further objects appear ________
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(inverse square law for light; see p. 519-520, chap 15)
2. Luminosity (which means …)
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We’ve talked about what determines luminosity:
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Surface temperature
Radius (surface area)
If you forgot, review the tutorials & Stefan’s law (chap 5)
3. Intervening absorption or scattering
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What can cause this?
We’ll ignore this for most of the semester
REVIEW: What were 2 processes that emit
light?
• Thermal (continuous) from…
• and emission lines from …
• What happens if you heat up a thermal emitter?
• What happens if you heat up an emission line
source?
• Does it change which emission lines are emitted?
[ask neighbor]
Light collecting area
• English: size of telescope
• For eyes, this is determined by size of ____
• For ‘scopes, determined by size of first mirror
– Astronomers don’t use lenses anymore.
– See pages 187-189 if you want to know why. (Not much
to read there – lots of pics on those pages)
• What matters is the AREA, not the diameter.
– Telescope that has double the diameter collects how
much more light? [hold up right # of fingers]
– Telescopes with 10x diameter collect _________ more
light.
– Book talks about this on pages 183-184.
Light area, continued
• Human eye diameter?
– 1 cm
• Hubble space telescope?
– 2.4 meters [~8 feet]
– 240x diameter of human eye  58000x more light
• Biggest visible light telescope?
– Keck. 10 meters (33 ft – size of this room).
• See p. 188, figure 6.11. What’s orange thing in middle?
– Spain’s GTC is a tiny bit bigger (10.4 m)
– Palomar was biggest (5 m) for about 50 years. Go visit!
It’s only 2-3 hours away, driving.
• Building C.E.L.T., approx 30 meters.
– 9 million times more light than human eye
Telescopes often have 2 mirrors (sometimes more). Which mirror
determines how much light can be detected?
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Primary (first one light hits)
Secondary (2nd one light hits)
It could be either one
Not enough information provided
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Exposure Time
• More time that light lands on detector, more light gets
detect.
– Human eye detector:
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Retina
How long are eye pictures?
How many “frames” each second in a movie?
Exposure time ~0.02 to 0.04 seconds.
– Old cameras detector:
• Film
• How long can film be exposed to light?
– Close shutter, reopen with same film later. “Double exposure.”
– New (digital) cameras detector
• CCD
• How long can CCD be exposed to light?
• 12 hours  1-2 million times more light than human vision
Efficiency of light detector
• Human eye:
– Detects less than 1%
• Film:
– Detects ~10%
• CCD
– Detects 90%
– Extra benefit: results are digital – can be processed!
• Mirrors reduce telescope efficiency to 50-70% overall.
• Telescopes ONLY have 50-70 more light gathering capability
due to efficiency.
Resolution and focus
• To get sharp images and detailed spectra, light must be
focused. What that means:
– Light from one point on source should be spread out as little as
possible.
• Resolution = how many pixels the equipment spreads light
over
• Mirror focus = similar idea.
– People need glasses because light isn’t focused on the eye
detectors; it is spread out when it lands on the rods and cones.
• What do things look like when you don’t wear your
glasses?
– All the light is there. Hard to “see” the message.
Bad vs. Good focus
Bad vs. Good focus part 2
Bad vs. Good focus part 3
Resolution – diffraction limit
• Previous pics – caused by flaw in mirror. Human mistake.
Very expensive. Required space shuttle visit
• Nature has natural limit caused by diffraction
– Waves bend around corners
– Example: why you can hear people even if you can’t see them.
Sound waves bend around corners.
• All waves bend: amount of bending depends on two things:
– Wavelength. Longer WL bend more. (Spreads out more)
– Size of A) hole light fits through or B) size of object light hits.
Smaller hole/object  bend waves more. (Has harder time
“squeezing” through small hole)
• Smallest amount of bending light (spreading of light) is
called the “diffraction limit.” Can’t focus light better than
this.
• See pages 184-186 (p. 185– all math)
Which size telescope will produce the
sharpest images using visible light?
1. Big
2. Small
3. Both are the same
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For math people only
• Light bends by changing direction.
• Specified by an angle
• Bending angle = (1.22) * WL / D
– D = size of hole or object light hits
– Angle = units of radians (not degrees)
– Book has equivalent formula using different units
on page 186 “Mathematical Insight”
Which kind of light diffracts the least? (i.e.
is EASIEST to focus)?
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Radio
Infrared (IR)
Visible
Ultraviolet (UV)
Gamma rays
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Which kind of light gives the best resolution in
diffraction-limited telescopes?
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Radio
Infrared (IR)
Visible
Ultraviolet (UV)
Gamma rays
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Which telescope has the best (smallest)
diffraction limit, giving the sharpest pictures?
1. Big visible light telescope
2. Small visible light telescope
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Animations to help see diffraction
• From MasteringAstronomy website OR the CD
that came with your book:
• Diffraction Rings
• Effect of wavelength on angular resolution
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Atmospheric effects
Atmosphere does 3 things to light:
1. Scatters light from Earth lights (= light pollution)
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NOT caused by pollution. Caused by AIR.
Why LA skies have few stars – too much city light; stars get
lost in scattered “night sky” light, similar to why we can’t see
many stars/planets during daytime sky.
2. Absorbs most light. See fig 6.22, page 195 now.
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Which forms reach ground? Which forms are absorbed? (Any
guesses what absorbs most of the UV?)
3. First song you ever learned as a kid:
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Stars twinkle. Light gets bumped around, and as a result,
spread out. Technically called “seeing.” Caused by air
speed/temperature differences.
You can see this yourself. Look at air over a hot BBQ or over
hot pavement. Similar to “mirages” and it causes mirages to
be wavy.
Compensating for “seeing”
• “Seeing” limits ALL visible-light ground-based telescopes
• Air (not _______) causes seeing. How to reduce seeing?
– Where are most telescopes built?
– Extra advantages to being on top of mountain?
• Where put a telescope to ELIMINATE seeing?
• Adjust telescope QUICKLY to compensate for air changes.
(100s times per second!)
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Called “adaptive optics.”
Expensive.
Requires fast computers and quickly moving parts
Most new big telescopes use this.
Twinkled vs. compensated
• See also figure 6.20 page 194.
Why we use non-visible telescopes
• Why is it hard to do astronomy using light that
is not visible to the human eye?
• Here’s what you see if you try:
Sirius binary system
What you’d see through a visible light
telescope. Ignore the spikes
X-ray image & visible image
superimposed
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Interferometry
Which form of light DIFFRACTS most?
Would that make images sharp or blurry?
What else determines amount of diffraction?
Astronomers combine telescopes that are really far
apart, act like one HUGE telescope.
• Technique is called interferometry. (2+ telescopes
“interfere” signals.)
• Radio light best for interferometry because radio
wavelength is ________________
– Biggest interferometer is literally size of entire Earth: use
telescopes on opposite sides of Earth
– Big one in New Mexico: VLA (Very Large Array). See fig 6.30. In
many commercials and movie Contact.
• Gives us SHARPEST images, even better than Hubble
SPACE telescope! (but in radio light!)
Big telescopes
• Visible light: Keck. 10 meters.
• Radio light: Arecibo, in Puerto Rico. 305 m
– Not steerable.
– See figure 6.24, page 197.
– Seen in “Golden Eye” (James Bond)
• Other telescopes aren’t big yet.
• X-ray & gamma-rays REALLY hard to focus.
– Best current X-ray telescopes about as good as visible
light scopes on the ground
Putting it all together
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5 things affected amount of light detected:
Apparent brightness of source
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Light collecting area
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C.E.L.T.: 9 million times more light than human eye
Exposure time
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Film/CCD: 2 million times more light than human eye
Efficiency of light detector
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Can’t control with telescope.
CCD: 50-70 times more light than human eye
(Angular) Resolution and focus
Diffraction limit on big visible scope:
600x sharper pictures
Why do we use telescopes? Combining everything above, telescopes get
650,000,000,000,000,000 =
6.5 x 1017 = 650 million billion times more light.
Distances: can see 800 million times further.
Things you DON’T need to know from
chapter 6
• Two basic designs of telescopes (pp. 187-188)
• Details about non-visible light. But you should
understand the big picture. (pp. 196-top of 199)
• You SHOULD know the big picture about
interferometry (rest of page 199 & 200)