Telescopes

Usage of telescopes• Astronomers use telescopes to do 3 things:

– See pages 182-184. Astronomers job: 184-185.

1. Imaging (taking pictures)2. Spectroscopy (like with did with the diffraction

gratings, prisms)• Analyze amount of light at different wavelengths• Two things equivalent to wavelength?

• Color (if using visible light)• Frequency

• How do they do this?• Take pictures of spectrum

3. Timing – watching things change.• How do they do this?• Take many pictures.

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

• take pictures• NOT look through! (Although that’s fun!)

– See Common Misconception, page 178

How this works inYOUR TELESCOPES: your eyes

• Teacher drawing of eye, as we see them– Purpose of pupil, iris.

• Gather light (let light in)• When is pupil most dilated? Contracted?• Wider pupil = more light enters. Need this when …• things are dark or faint.

• See figure 6.1, page 175 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 176 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• Whether you “see” (detect) an object depends on

only one thing:• amount of light detected by a detector

• This depends on several things:

1. Apparent brightness of light source (Why apparent?)

2. Light collecting area

3. Exposure time

4. Efficiency of light detector

5. (Angular) Resolution and focus

• More details about each of these coming up.

Apparent brightness• What affects how bright light sources APPEAR?

1. Distance. Further objects appear ________• (inverse square law for light; see p. 502-503, chap 15)

2. Luminosity (which means …)• We’ve talked about what determines luminosity:

1. Surface temperature

2. Radius (surface area)

• If you forgot, review the tutorials & Stefan’s law (chap 5)

3. Intervening absorption or scattering• 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 180-182 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 177-178.

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. 181, 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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0

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0 1. Primary (first one light hits)

2. Secondary (2nd one light hits)

3. It could be either one

4. Not enough information provided

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Exposure Time• More time that light lands on detector, more light gets

detect.– Human eye detector:

• 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 178-180 (p. 179 – all math)

Which size telescope will produce the sharpest images using visible

light?

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0

0 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 185 “Mathematical Insight”

Which kind of light diffracts the least? (i.e. is EASIEST to focus)?

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0

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0 1. Radio

2. Infrared (IR)

3. Visible

4. Ultraviolet (UV)

5. Gamma rays

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Which kind of light gives the best resolution in diffraction-limited telescopes?

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0

0

0

0 1. Radio

2. Infrared (IR)

3. Visible

4. Ultraviolet (UV)

5. Gamma rays

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Which telescope has the best (smallest) diffraction limit, giving the sharpest pictures?

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0 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

Atmospheric effects• Atmosphere does 3 things to light:

1. Scatters light from Earth lights (= light pollution)• 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 189 now.• 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:• 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!)– 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 188.

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:

X-ray image & visible image superimposed

Sirius binary system

What you’d see through a visible light telescope. Ignore the spikes

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 191.– 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• 5 things affected amount of light detected:1. Apparent brightness of source

• Can’t control with telescope.

2. Light collecting area• C.E.L.T.: 9 million times more light than human eye

3. Exposure time• Film/CCD: 2 million times more light than human eye

4. Efficiency of light detector• CCD: 50-70 times more light than human eye

5. (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. 180-182)

• Details about non-visible light. But you should understand the big picture. (pp. 190-top of 194)

• You SHOULD know the big picture about interferometry (rest of page 194 & 195)