1. 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 B
Question # 3: answer A
Setup:
Recall reading assignment:
Chapter 6 (Telescopes), pp. 165 – 189
Please take a moment to mute your cell phone!

2. How large is the Big Dipper?
Angular sizes
How large is the Big Dipper?
250 apparent size
Stretch out your hand:
1 cm = 1o
25o
1o = 60 am (minute of arc)
1 am = 60 as (second of arc)
How large is the Moon?
1/2 0 apparent size
1/2o = 30 am = 1800 as
0.5o

3. Much more detail is visible
Resolution
Resolution of the human eye is 1 am = 60 as
Smallest visible detail is as small as this.
Stars are points in
all telescopes:
Calculated size of a star:
~ as,
Resolution of best telescope:
~ 0.1 as.
Much more detail is visible
Resolution of a simple telescope is ~ 1 as

4. The handle of the Big Dipper: the middle star is a double
Alcor and Mizar
Alcor
25 am
25 am
Mizar A & B
14 as
The handle of the Big Dipper:
the middle star is a double
How far apart are they?
25 am

5. Questions coming …

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Question 4
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 …

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Question 5
How large does the Moon appear in the sky?
A A few inches.
B A few thousand miles.
C A few arc seconds.
D A light year.
E A half a degree.
Next question coming …

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Question 6
How large is the Big Dipper?
A A few miles.
B A few light years.
C Five million light years.
D More than twenty degrees.
E A half a degree.
Next question coming …

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Question 7
The two stars Alcor and Mizar are easily separated by the human eye.
This means that they must be farther apart than …
A a few degrees
B one arc minute.
C a few light years.
D a few million kilometers.
E a million light years.

10. 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

11. 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

12. 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?

13. 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.

14. 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