1. Astronomical telescopes
How do they work?
Use eyepiece to
watch image
Refractor:
Lens collects light
Use eyepiece to
watch image
Reflector:
Mirror collects light
2”-5” toy
6”-30” amateur
3’-10’ professional
10’-33’ super
Large lens/mirror collects much light -
the diameter of the telescope
is the important indicator!

2. Why do we need telescopes?
To collect much light
- need large diameter
To make small objects look large
- need magnification
Limits to light collection:
Diameter ~ 10 m (30 feet) will be too heavy to hold
Limits to magnification:
“Seeing” - stars jump around (related to twinkling)
- caused by air turbulence
little light
Notice:
all stars twinkle but planets twinkle less - why?
Answer:
planets are disks in a telescope,
but stars are always points
much light

3. Questions coming …

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Question 9
Which of the following is the most important measure of a telescope?
A The magnification.
B The diameter of its mirror (or lens).
C The focal length of its mirror (or lens).
D The length of the telescope.
E The distance of the farthest object it can see.
Next question coming …

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Question 10
The largest telescope we have at Ole Miss
is 15 inches. What category is that?
A Large professional.
B Average professional.
C A decent amateur size.
D A small amateur size.
E Uselessly small for astronomy.
Next question coming …

6. 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

7. 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

8. 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

9. Questions coming …

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Question 11
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 12
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 13
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 14
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.