Introduction to modern astronomy 9&10
島袋隼⼠(Hayato Shimabukuro)（云南⼤学、
SWIFAR）
©GETTYIMAGES

5.Telescope(望远镜)

What is telescope?
•Telescopes is device to observe distant stars, galaxies and so on by magnifying them.
•You can use the telescope to observe stars and galaxies at home.
•The telescopes which are used in astronomy are more
powerful, high specification, and large.
•We observe electromagnetic waves from the universe through telescopes

•Remember electromagnetic waves. Depending on their wavelengths, the electromagnetic
waves have different names.
Electromagnetic waves

I(ν, T) =
2hν3
c2
1
ehν/kT − 1
The blackbody spectrum
F = σT4
[W/m2]
•Remember Blackbody spectrum (Planck spectrum)
Total energy
If we measure energy from stars and galaxies by
electromagnetic wave, we can estimate their temperature.

Universe by multi-wavelength
•If we use different wavelength telescopes, our universe seems to be different.

•If we see a galaxy by radio wavelength, we can see cold gas inside the galaxy (~600K)
Universe by multi-wavelength

•If we see a galaxy by radio wavelength, we can see cold gas inside the galaxy (~600K)
Universe by multi-wavelength

•If we see a galaxy by infrared and optical wavelengths, we can see stars inside the galaxy
(~4500-6000K)
Universe by multi-wavelength

•If we see a galaxy by infrared and optical wavelengths, we can see stars inside the galaxy
(~4500-6000K)
Universe by multi-wavelength

•If we see galaxies by ultraviolet wavelength, we can see hot stars inside galaxies (~10,000K)
Universe by multi-wavelength

•If we see galaxies by ultraviolet wavelength, we can see hot stars inside galaxies (~10,000K)
Universe by multi-wavelength

•If we see galaxies by X-ray wavelength, we can see hot gas inside the galaxy (~ K)
107
Universe by multi-wavelength

•If we see galaxies by X-ray wavelength, we can see hot gas inside the galaxy (~ K)
107
Universe by multi-wavelength

Universe by different wavelength
The Milky Way galaxy seen by
multi-wavelength

A kind of telescope
•There are various kinds of telescope corresponding to wavelength.

A kind of telescope
•There are various kinds of telescope corresponding to wavelength.
Subaru telescope
•Optical and infrared
wavelengths

A kind of telescope
•There are various kinds of telescope corresponding to wavelength.
Subaru telescope
•Optical and infrared
wavelengths
ALMA telescope
•Radio wavelength

A kind of telescope
•There are various kinds of telescope corresponding to wavelength.
Subaru telescope
•Optical and infrared
wavelengths
ALMA telescope
•Radio wavelength
Chandra satellite
•X-ray wavelength

Telescopes in China
•China has its own telescope projects.

Telescopes in China
•There are some big telescope projects.
CSST(巡天) FAST
•Radio wavelength
•UV-optical wavelengths
500m!

Telescopes at Yunnan
•Yunnan University (SWIFAR)
leads “Mephisto” telescope
•Yunnan observatory has 40 m radio
telescope

Optical telescope
•Optical telescopes(光学望远镜) are designed to collect the visible light that human can see.
•The history of optical telescope has a long history, back to the days of Galileo in
the 17th century.
Galileo

Optical telescope
•The light is reflected by flat mirror.
•If the angle of incident light is large, the deflection angle is
also large. On the other hand, if the angle of incident light is
small, the deflection angle is also small.
•On the other hand, curved mirrors focus on a single
point all rays of light arriving parallel to the mirror.

Optical telescope
•The light is reflected by flat mirror.
•If the angle of incident light is large, the deflection angle is
also large. On the other hand, if the angle of incident light is
small, the deflection angle is also small.
•On the other hand, curved mirrors focus on a single
point all rays of light arriving parallel to the mirror.

Optical telescope
•The light is reflected by flat mirror.
•If the angle of incident light is large, the deflection angle is
also large. On the other hand, if the angle of incident light is
small, the deflection angle is also small.
•On the other hand, curved mirrors focus on a single
point all rays of light arriving parallel to the mirror.

Optical telescope
•The light is reflected by flat mirror.
•If the angle of incident light is large, the deflection angle is
also large. On the other hand, if the angle of incident light is
small, the deflection angle is also small.
•On the other hand, curved mirrors focus on a single
point all rays of light arriving parallel to the mirror.

Optical telescope
•The light is reflected by flat mirror.
•If the angle of incident light is large, the deflection angle is
also large. On the other hand, if the angle of incident light is
small, the deflection angle is also small.
•On the other hand, curved mirrors focus on a single
point all rays of light arriving parallel to the mirror.

Optical telescope
•There are various kind type of optical telescope

Optical telescope
•There are various kind type of optical telescope

Optical telescope

Optical telescope

Optical telescope
•For example, Keck telescope (and Subaru telescope) is Cassegrain type telescope

Breaktime

Optical telescope
•The important properties of the telescope are sensitivityand resolution
Sensitivity(聚光能⼒) : How fainter images we can observe
The sensitivity depends on the collecting area. A larger diameter telescope has more sensitivity.
＜
This telescope can observe fainter objects.

Optical telescope
•The important properties of the telescope are sensitivityand resolution
Sensitivity(聚光能⼒) : How fainter images we can observe
The sensitivity depends on the collecting area. A larger diameter telescope has more sensitivity.
＜
This telescope can observe fainter objects.

Optical telescope
•The important properties of the telescope are sensitivityand resolution
•Resolution(分辨能⼒): How small structure we can observe
Angular resolution ∝
wavelength
diameter（直径）
•If the angular resolution is smaller, we can observe smaller objects. Thus, a larger
telescope has better resolution.
＜

Optical telescope
•The important properties of the telescope are sensitivityand resolution
•Resolution(分辨能⼒): How small structure we can observe
10’
1’
5’’
1’’
1’(arcmin)=1/60°
1’’(arcsec)=1/360°

Optical telescope
In summary….
A larger telescope has more sensitivity
and more angular resolution.
But, of course, we cannot make the unlimited large telescope because of technically and
financial problems.

Optical telescope
(TMT is under construction)
•The comparison of optical telescope.

Current observational frontier
•In 2022, JWST was launched as the successor to Hubble Space Telescope (HST).
•JWST is larger mirror than HST
•Thus, the sensitivity is better than
HST !
JWST
HST
2.4m
6.5m

HST JWST
Images by JWST
•The galaxy cluster observed by JWST is more clear than by past telescopes.

HST JWST
Images by JWST
•The Carina Nebula (star forming region) observed by JWST is more clear than by past
telescopes.

Radio telescope
•In addition to the optical telescope, we can also use a radio telescope to observe radio
wavelength. For example, we use single-dish type radio telescope
FAST(China)
Green Bank telescope (USA)
•This kind of single dish telescope is
similar to optical telescopes

Radio telescope
Angular resolution ∝
wavelength
diameter（直径）
Remember angular resolution.
•Optical wavelength: ∼ 10−7
m
•Radio wavelength: ≳ 1mm
•Because smaller angular resolutions can resolve smaller
scales, the angular resolution of radio wavelength is
times worse!
104
Then, let’s make the diameter larger to obtain good
angular resolution!
Of course, there is a limitation of the diameter of a single dish…
•The angular resolution of a single dish radio telescope
is nog good

Radio telescope
Angular resolution ∝
wavelength
diameter（直径）
Remember angular resolution.
•Optical wavelength: ∼ 10−7
m
•Radio wavelength: ≳ 1mm
•Because smaller angular resolutions can resolve smaller
scales, the angular resolution of radio wavelength is
times worse!
104
Then, let’s make the diameter larger to obtain good
angular resolution!
Of course, there is a limitation of the diameter of a single dish…
•The angular resolution of a single dish radio telescope
is nog good

Radio interferometer
•If we use a single dish, we cannot avoid the limitation of the diameter of a single dish.
But, we have a good idea!
We regard many antenna as “one” dish
•This kind of radio telescope is called a radio
interferometer（射电⼲涉仪）

Radio interferometer
•In radio interferometer, if the distance between 2 antennas is large, the angular
resolution becomes better.
Angular resolution ∝
wavelength
D
[rad]
•Angular resolution in radio interferometer is expressed by
D
1∘
=
π
180
[rad]

Let’s calculate!
Angular resolution =
wavelength
D
1∘
=
π
180
[rad]
•This image is planet-forming disc around young star
(I explain detail next week.)
•The spatial resolution of this image is roughly 30
milli ( ) arcsec. If we use 3mm
wavelength electromagnetic wave, How large size
telescope is required to observe this image?
30 × 10−3
1’’(arcsec)=1/360°
(Answer)

Let’s calculate!
Angular resolution =
wavelength
D
1∘
=
π
180
[rad]
•This image is planet-forming disc around young star
(I explain detail next week.)
•The spatial resolution of this image is roughly 30
milli ( ) arcsec. If we use 3mm
wavelength electromagnetic wave, How large size
telescope is required to observe this image?
30 × 10−3
1’’(arcsec)=1/360°
(Answer) 3 × 10−3
×
(
1
360)
∘
×
π
180
=
3 × 10−3
m
D[m]
D ∼ 2km

ALMA telescope
•ALMA telescope is sub-mm & mm radio interferometer
located at Chili, Atacama.
•ALMA provides us exiting scientific results.

Current observational frontier
•EHT is located all over the world to achieve good angular resolution.
•This is the first image of blackhole seen by human
being !
•Event Horizon Telescope (EHT) is radio interferometer targeting to observe black hole
images
∼ 10−6
arcsec
Angular resolution ∝
wavelength
D
~1000km
•The angular size of blackhole shadow is
arcsec
∼ 10−6

Future telescope
•SKA radio telescope will start observation in 2027.
•Science target: When do first stars and galaxies form? Is there life in the universe?
•China is a membership of SKA.

SKAO

SKAO

Current observational frontier
•Not only electromagnetic waves but also gravitational waves had been detected by LIGO
and Virgo.
LIGO detectors
•We observed the gravitational wave from
merging black holes in 2015.
•Gravitational wave was predicted by
Einstein in 1915.

Current observational frontier
•Not only electromagnetic waves but also gravitational waves had been detected by LIGO
and Virgo.
LIGO detectors
•We observed the gravitational wave from
merging black holes in 2015.
•Gravitational wave was predicted by
Einstein in 1915.

Summary
• In order to observe the universe, we use telescopes.
• There is a kind of telescope corresponding to wavelength.
• We can see the di
ff
erent universe when we use di
ff
erent
wavelengths.
• A gravitational wave is one of the tools to observe the
universe.
• Recently, there has been many surprising observation
results.