1. Recall reading assignment:
Prepare your scantron:
Use
a pencil,
not a pen!
Fill in your name and fill the bubbles under
your name.
LAST NAME FIRST, First name second
Put your 4-digit code instead of “IDENTIFICATION NUMBER”.
--- (The last 4 digits of your OleMiss ID.)
Question # 1: answer A
Question # 2: answer C
Question # 3: answer B
Setup:
Recall reading assignment:
Chapter 6 (Telescopes)
pp. 165 – 189
Test 2 on Wednesday!
Please take a moment to mute your cell phone!

2. Review questions coming …

3. This question counts double!
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Question 4
Which of the following is the most important measure of a telescope?
A The magnification.
B The distance of the farthest object it can see.
C The focal length of its mirror (or lens).
D The length of the telescope.
E The diameter of its mirror (or lens).
Next question coming …

4. This question counts double!
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Question 5
How large is a small research telescope?
A magnification.
B 5-10 meters in length.
C 1-2 meters in diameter.
D 10 centimeters in diameter.
E At least 10 meters in length.
Next question coming …

5. This question counts double!
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Question 6
How do stars look in the largest telescope of the world?
A They look like large disks and we can see a
lot of detail on them.
B They look like large disks but there is not much detail to see,
although telescopes would have been able the resolve the details.
C They look like tiny disks but few or no details can be
seen on them.
D They look like points (no details visible).
E They look like “star-shaped” objects with rays
emanating from them.
Next question coming …

6. This question counts double!
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Question 7
How much is the resolution of a telescope?
A a few degrees.
B ½ to 2 arc seconds.
C a few light years.
D a few million kilometers.
E a million light years.

7. Seeing : air moves  stars jump around and twinkle
• 2 to 4 arc seconds in Mississippi
• 1 arc second in good location
• 0.1 arc second on Mauna Kea, Hawaii
Size of “seeing”
Seeing limits magnification to a few hundred at best
Look at a double star in good and in bad weather:
Good
seeing
Bad
seeing

8. Practical limits on magnification
Atmosphere limits resolution to “seeing” (0.5 as to 2 as):
• magnification x already brings out all the detail there is
• for convenience, x is used in really stable weather
Practical limits
The telescope’s optics limits resolution to 12 as / D[cm] (D is diameter of telescope)
• for very small telescopes max magnification is: 5-10 times D
• example: D=3 cm (small binoculars) - at most x
What is a pair of 6 x 30 binoculars?
• 6 x magnification
• 30 mm = 3 cm diameter
A small but
acceptable telescope
What is a 2-inch diameter telescope with magnification 350 x ?
• 350 x magnification
• 5 cm diameter
A scam!
Resolution: 2.4 as
Blur is 350 x 2.4 as = 840 as = 14 am

9. Magnification computed
M = fobjective/feyepiece
Example:
Meade, diameter = 12 inches, fobj = 3048 mm
Resolution: 0.4 as (optical)
1-2 as atmospheric
With f=26 mm eyepiece, M = 117 x
With f=10 mm eyepiece, M = 305 x
With f = 4 mm eyepiece, M = 762 x
Which ones make sense?

10. The smallest detail we can see on the Moon
Smallest detail Moon
The smallest detail we can see on the Moon
1 mile wide
Resolution limit (atmospheric) 0.5 as at very best
Moon is 0.5o = 30 am = 1800 as in diameter
1800 as/0.5 as = 3600 counts of the smallest detail fits in one line on the Moon
Moon is 3476 km, so 3476 km / 3600  1 km.
Anything smaller than a km is blurred.

11. Questions coming …

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Question 8
What is seeing?
A The amount of air starlight passes through to reach our eyes.
B The diameter of the blur caused by the atmosphere.
C The amount of light passing through the atmosphere.
D The brightness of a star as it appears to us in the sky.
Next question coming …

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Question 9
Which of the following limits the magnification of all but the smallest telescopes?
A Optical effects.
B Quality of the lens/mirror.
C Seeing.
D The shaking of the telescope mount.
E Inaccurate tracking.
Next question coming …

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Question 10
What is an 8 x 50 pair of binoculars?
A Diameter = 8 cm, magnification = 50 x.
B Length = 8 cm, magnification = 50 x.
C Magnification = 8 x, diameter = 50 mm.
D Magnification 8 x, length 50 cm.
E One that cost $50 but is only worth $8.
Next question coming …

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Question 11
How small is the smallest detail we can still discern on the Moon with a very good telescope?
A 100 miles.
B 1 mile.
C 10 feet.
D 1 foot.
E 1 inch.
Next question coming …

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Question 12
The large refractor in Kennon observatory is 15 inches in diameter, and the focal length of the objective is 15 feet.
What is the magnification with a 1-inch (=25 mm) eyepiece?
A 12 x
B 15 x
C 180 x
D 3,000 x
E 30,000x

17. Mounts and observatories
What do you think the most expensive part of an amateur/professional telescope is?
The optics (lens/mirror - needs to be as precisely manufactured as inch!)?
The mount?
The observatory (building and dome)?
The light detectors that register and analyze the light?
Must put the telescope in a dome
to protect from
bad weather
stray light - light pollution
kills astronomy
Must put telescope on a mount that follows
the daily motion of stars (“tracking”):
Equatorial mount (one axis
points at the North Star)

18. Mt. Palomar Observatory
California
Diameter: 200 in
(Length of tube: 16 yards)
Mt. Palomar
Observer in cage
Main mirror

19. How do you fight seeing?
Adaptive optics
(1) Put your observatory on a mountaintop: seeing at best: ~ 0.4 as
(2) Use adaptive optics: seeing at best: ~ 0.05 as
• Deform your mirror to compensate for seeing
• Observe a “guide” star and have a computer tell how to deform mirror
• Do this ~ 100 times each second
No guide star?
Make one!

20. How do you fight seeing? (3) Put your telescope on a spaceship:
The Hubble
(3) Put your telescope on a spaceship:
seeing: 0 as
Hubble Space Telescope is a “small” telescope
Diameter: 2 m; resolution: 0.06 as
More discoveries than all other
telescopes combined.

21. Hubble M101

22. Mauna Kea M A U N K E

23. Questions coming …

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Question 13
What is an equatorial mount?
A A design of the telescope’s mechanics that allows
tracking stars in the sky. One axis points at
Zenith (that is, straight up).
B A design of the telescope’s mechanics that allows tracking stars
in the sky. One axis points at the North Pole.
C A scientific term that explains the motion of stars in the sky.
D A design of a telescope’s optics with a mirror to collect the light.
E A design of a telescope’s optics with a lens to collect the light.
Next question coming …

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Question 14
Why does the Hubble Telescope make very
detailed images?
A Because it has the largest diameter of all telescopes.
B Because it is of exceptionally good quality.
C Because it is in space where the atmosphere does
not blur the image.
D Because it can use infrared radiation that other
telescopes cannot.
E Because it is in space, closer to the stars than other telescopes.
Next question coming …

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Question 15
What is adaptive optics?
A When the telescope follows the stars as they
move in the sky.
B When some of the telescope’s lens or mirror is covered according to changes in the atmosphere.
C When the shape of the optics is changed according to the changes in the atmosphere.
D When appropriate color filters are used as the atmosphere absorbs more or less red or blue light.
E When the magnification of the telescope is chosen according to how turbulent the atmosphere is on the day of the observation.