| Midterm Questions |
Q0. You said there would be no math in the course! What was up with all that stuff about "angular resolution" in the first short answer question of the midterm?
A0. By any reasonable definition, there has been no actual *math* in this course. You were not required to do any calculations with the formula that you were asked to write down (unless you felt like trying for some bonus points). In other words, you simply had to know the formula, but you did not even have to use it. Furthermore, that formula was expressing a concept about imaging -- a concept that you were required to know because it was taught in class. That concept is that when a point-like object (e.g. a star) is being imaged, there is a fundamental limit on how sharply it can be imaged. This limit arises because diffraction causes the image of this point-like object to be smeared out into a disc: the diffraction disc. So this fundamental limit on how sharply you can image things, (and therefore on your ability to distinguish two closely-spaced objects as being distinct), which is known as angular resolution, arises from the laws of physics, and not from any technological limitations that you might be facing.
The formula was expressing the relationship between the diameter of your telescope, the wavelength of the light being imaged, and the size of the diffraction disc. **The relationship is that the angular size of the diffraction disc is directly proportional to the wavelength and inversely proportional to the diameter of the telscope aperture.** (This is an expression of the formula in words, emphasizing the underlying concept).
This means that for larger wavelengths, the diffraction disc will be larger, all other things being equal. For larger telescope apertures, the diffraction disc will be smaller, all other things being equal. This is the extent to which you were expected to apply this formula. Some concepts in astronomy are, in fact, relationships between physical quantities (such as the one expressed in double asterisks). These relationships tell you a lot about nature.
Just to complete the thought, obviously, in situations in which we are diffraction-limited, we would like the diffraction disc to be as small possible. This is one of the reasons why larger and larger telescopes are being built -- in order to be able to resolve astronomical sources with ever finer detail. These aspects of imaging are what you were intended to take away from AST 201. Those of you who studied and understood the concepts well would have been well-positioned to solve this problem on the midterm. Furthermore, given that the concepts underlying this problem were an integral part of your first homework assignment, and given that the topic of the second short answer question was revealed to you in your tutorials, we feel that the exam was quite fair. I hope that this explanation helps illustrate our motivation for giving you this problem on your midterm examination.
Q1. What is the format of the midterm?
A1.
The midterm consists of 30 multiple choice and 3 short-answer questions, of which you only have to do 2.
Some of the multiple choice questions will be taken from the quizzes, some from the masteringastronomy.com tutorials, and some will be different.
One of the short-answer questions might be derived from the Assignment #1.
Q2. What material will be covered?
A2. You are responsible for material covered in lectures, assignments, and recommended readings. (Note: The schedule for recommended readings as well as downloadable lecture slides are available on the AST201 webpage.)
Q3. Are there math questions?
A3. No, there are no math questions in the sense that you will not have to do any algebra. That having been said, some concepts in astronomy are, in fact, relationships between physical quantities. These relationships are best expressed using mathematical formulae. It is important that you understand what these formulae *mean* and can answer questions about the concepts underlying them. However, you will never have to use them to calculate anything.
Q4. Can we see the midterm exams from the past years?
A4.
Unfortunately not, but this is an example of a past short answer question:
If you are on the Moon and drop a hammer and a feather at the same time and from the same height, does the hammer hit the ground first, or the feather, or both at the same time? Why? If you do the same on Earth, what would be different about the order of hitting the ground on Earth and in the time it takes relative to what happens on the Moon.
Finally, if you put the feather and hammer in orbit around the Moon, how would their orbital periods compare?