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!
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
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: ~ 0.001 as, Resolution of best telescope: ~ 0.1 as. Much more detail is visible Resolution of a simple telescope is ~ 1 as
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
Questions coming …
sec 8 9 10 12 11 7 6 1 2 3 4 13 30 35 40 45 14 25 16 15 18 17 19 20 5 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 …
sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 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 …
sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 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 …
sec 10 9 11 13 14 8 12 7 2 1 3 4 6 5 15 17 26 25 27 28 30 29 16 24 19 18 23 20 22 21 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.
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
Practical limits on magnification Atmosphere limits resolution to “seeing” (0.5 as to 2 as): • magnification 30 - 120 x already brings out all the detail there is • for convenience, 200 - 300 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 15-30 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
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?
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.
Questions coming …
sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 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 …
sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 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 …
sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 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 …
sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 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 …
sec 10 9 11 13 14 8 12 7 2 1 3 4 6 5 15 17 26 25 27 28 30 29 16 24 19 18 23 20 22 21 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