This lab provides an introduction to measurement, measurement error, error estimates, and extrapolation.
The instructor might start by telling the students that the lab is about estimating the number of galaxies in the Universe, but in the process to explore how measurements are made and how they often have significant uncertainties.
Start by having students work in groups to determine the number of chairs in the classroom; however, they only have one minute to do this. After the minute, have the groups present their number and how they determined it. Note the variation in the derived number. Now give each group 3 minutes to do an exact count. After completion, note how the variation is smaller than using the estimation methods: but still, usually, there is some variation. Make sure to mention how working together can help to minimize work, get more accurate results, and get error estimates.
Now move on to a count which requires estimation: the number of chairs in the University. Give students 5-10 minutes, then again collect the answers and descriptions of the techniques. Note whether people took advantage of the more precise knowledge of the number of chairs in the classroom or not, and discuss advantages/disadvantages of various results.
Now move on to the main exercise of the lab: estimating the number of galaxies in the sky based on the Hubble Deep Field. Introducing this picture and the techniques provides a good opportunity to present the concept of angular measurements on the sky to describe the size of the field. At this point, let small groups take over and proceed at their own pace. When they start to figure out the number of pictures needed to cover the whole sky, or the amount of time needed to take this number of pictures, they will run into large numbers on their calculators. By walking around and talking with each group, the instructor can determine whether students are familiar with the concept of scientific notation and can help them understand it and learn how to work with it on their calculators. As students proceed through the lab, you can see whether there is confusion about unit conversion, and address it accordingly.
This labs leads to a very nice homework exercise which is included in the lab summary: an estimation of the number of stars visible to the naked eye in the sky. This gets students to go out and look at the sky and also to use some of the ideas about estimation. Make sure to emphasize that we are interested in naked-eye stars, so the Hubble Deep Field image is not relevant to this. Also make sure to note that there is no correct answer; many students won't believe their numbers because they don't think they are big enough.