Review for Midterm (Astronomy Labs) ----------------------------------- Exercise 6--About Your Eye: Why does red filter look red or why does blue filter look blue? You need to know the definition of a filter. Also, you need to know the Electromagnetic Spectrum from shortest wavelength to longest wavelength: Gamma-rays, X-rays, Ultra-violet (UV), Visible, Infrared (IR), Microwave, and Radio. In other words, Gamma-rays have the shortest wavelengths or highest frequencies while Radio waves have the longest wavelegths or lowest frequencies (Page 1 of Exercise 6). Exercise 1: First of all, you need to know the definition of the following terms: Zenith, Horizon, Meridian, Celestial Equator, North Celestial Pole/South Celestial Pole, Ecliptic, Circumpolar Constellations/ Circumpolar Stars, Right Ascension (RA), Declination (Dec), Sidereal Period, Synodic Period. (I am sure there are more terms that you have to remember but I forget them at this moment!!! If I can't remember those, you don't have to either. What a deal!) Then, using your Star Wheel, you need to read of RA (Right Ascension) in hours and minutes and Dec (Declination) in degrees for stars or some special objects. To do that, just put an object on your meridian and see where it crosses the Celestial Equator (or Equator on your Star Wheel) which has the hours on. For Dec, figure out how far something is either above or below the Celestial Equator using either those vertical lines with number in degrees on them (if a star is next to one of those vertical lines) or use a ruler to determine how far something is either above or below the Celestial Equator in centimeters. Then, place the ruler next to a vertical line and convert your centimeters to degrees for your object. Also, everything rises on the East and sets on the West so just put half a star above the eastern horizon and the other half below for rising (do the same for setting). Then whatever the hour mark is pointing at the date, it is your answer... I will give you the location of the object, rising, setting, or on meridian, and time. Then you have to give me the date or vice versa. Please look over the in class exercises about Star Wheels. Those are some of the basic questions I would ask you on the midterm so make sure you know how to do those again without looking at the answers. Exercise 25: Know the names of all the phases of the Moon: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Third/Last Quarter, Waning Crescent. Also, it takes about a week to go from one major phase to the next: New Moon to First Quarter. To go from New Phase back to New Phase again, it takes 29.50 days--Synodic Period of the Moon--while Sidereal Period of the Moon is only 27.33 days... Phases of the Moon would tell you where the Sun is and once you know where the Sun is, you know what time it is. REMEMBER, we tell time by looking to see where the Sun is, not the Moon! The Moon only gives us the location of the Sun. New Moon is in "front" of the Sun (either above or below the Sun) so wherever the Sun is, that's where the Moon is as well... First Quarter Moon is 6 hours BEHIND the Sun or 90 degrees East of the Sun... Full Moon is opposite of the Sun or 180 degrees (12 hours) from the Sun. So if the Sun is on the western horizon, then Full Moon must be on the eastern horizon--Moon rise! Third/Last Quarter Moon is 6 hours AHEAD of the Sun or 90 degrees West of the Sun... Remember that the motion of the sky over a night is from East to West with stars moving at 15 degrees per hour. Again, we did a lot of examples regarding the Moon in class so please look over those again and if you still can't get the right answers this time, stop by and I will explain them again. The Moon moves how many degrees in one day??? About 13 degrees per day... Also, to get a solar eclipse, the Moon has to be in New Phase and near the node (where the orbit of the Moon crosses the ecliptic--New Moon passes in front of the Sun, blocking out the Sun), and the phase of the Moon has to be in Full Phase to get a lunar eclipse (Full Moon moves into the shadow of the Earth)... Exercise 7: Understand the "ray tracing" technique and be able to indicate where the image is by using that method (Figure 7-1 on Page 3). If the object is farther from the center of the lens than the front focal point, then the image which is real/positive is on the other side of the lens and up-side-down as well. However, if the object is closer than the front focal point, then you will end up with the image on the same side of the object with upright--virtual image/negative. Make sure that you can perform both, real and virtual. Once you have figured out where the image is, then you can calculate the Magnification Power of the lens by using the following equation (Page 2, Equation 2): i h(i) MP = --- = ------ o h(o) where i is the distance from image to center of the lens, o is the distance from object to center of the lens, h(i) is the height of the image, h(o) is the height of the object. Exercise 8: Another way to compute the magnification power, MP, (for any type of telescope) is FL f(o) MP = ---- or ------ fl f(i) where FL is the focal length of the object lens/mirror (big lens), fl is the focal length of the eyepiece (small lens), f(o) is the focal length of the object lens/mirror, f(i) is the focal length of the eyepiece. Also, know what is the "maximum effective magnification" and "focal ratio." 20X MP(max) = ----- D(cm) (cm) where D is the diameter of an objective lens/mirror in centimeters. FL F/ = ---- D where FL is the focal length of the objective lens/mirror, D is the diameter of the objective lens/mirror in the same unit as FL. Furthermore, the "total focal length" of a telescope depends on which eyepiece you use. I. Using biconvex as an eyepiece, then FL(total) = FL + fl II. Using biconcave as an eyepiece, then FL(total) = FL - fl But there is a problem with lenses where all wavelengths don't come to the same focus after refracting through the lens. Blue wavelength comes to focus faster than red wavelength. This is known as Chromatic Aberration and to fix it, just place a second lens making from a different material than the first one behind the first/objective lens. Now, both blue and green wavelengths come to the same focus whereas red and yellow wavelengths come to another focus further out. But because our eyes are sensitive to red so we tend to focus in red and leave the blue out of focus... This is known as Achromatic Aberration. For mirrors, if the shape of the mirror is in spherical, then all the light rays don't come to the same focus so it is a Spherical Aberration. To fix that, just grind or polish the mirror into parabola. Light rays hitting a parabola surface are coming to a same focus. Exercise 9: Be able to identify different type of reflecting telescopes: Primary Focus, Newtonian Focus, Cassegrain Focus, Schmidt-Cassegrain Focus, and Coude Focus. Which type of telescope would give you a long focal length, short focal length, large field of view, small field of view, etc.? Also, know all the parts that make up each type of telescope: primary mirror, secondary mirror, flat mirror, focus, and focal length. (Figures 5 & 6 on Page 5 of Exercise 8) That's all I can think of right now. The best way to study for the midterm is to look over your notes, in class exercises, and quizzes. Again, if you need to talk to me before the test, just give me a call or drop by my office and I am more than happy to discuss about the test (probably won't give you the answers though!). Just go back to my homepage and click on the "Office Hours" icon for office hours and phone number or check them outside my office, 50/1502. GOOD LUCK