Methods of Astronomical Observation & Data Analysis
Laboratory Exercise 2
A Simple Long-Slit Grating Spectrometer
Notebooks with Write-ups Due: Wed November 6
During the lab session on October 23, we constructed a simple long-slit grating spectrometer out of slide-mounted transmission gratings, a cardboard tube, some construction paper, card stock, and a whole bunch of electrical and masking tape.
Each of you now has a hand-held, medium dispersion spectrometer you can use to make simple observations of terrestrial light sources. The goal of this lab is to put a simple spectrometer in your hands and get you to make observations of seemingly mundane light sources with it, with the idea of getting you to start thinking about what a spectrum is, training you to look at spectra critically for clues that inform you about the nature of the light source, and to demonstrate, in the best way I know, that all light sources have underlying physics that is only revealed by observing its spectrum. You can poke, and prod, and speculate all you like based on appearance ("looks kind of yellow"), but a spectrum tells the definitive tale.
In your notebook, make a sketch of your spectrometer, including the pertinent dimensions (especially the spectrometer tube length and the width and length of the slit). Make any brief notes of particular features of the spectrometer.
Using a fluorescent lamp as an emission-line source, assess the performance of your spectrometer. In particular, there is a pair of green lines in the middle of the spectrum that should be visible. How well does your spectrometer resolve these two lines? For example, do they appear as two sharply defined lines with darkness between them, or do the lines appear relatively fuzzy, with only a little bit of darkness between them? Or, do you see two lines at all?
Take your spectrometer out in the evening, and make observations of the following light sources. For each, note
Some Light Sources to Observe:
This is probably totally illegal, but it is also extremely useful. Here is a scan of a set of street-lamp spectra that appeared in the September 2002 Sky&Telescope. I've written to ask permission to use this (or for a better copy), but in the meantime don't tell on me:
lamp spectra (3.9Mb JPEG file)Beware, however, the printing process (or the photographic method, I'm not sure which) for this picture tended to saturate very bright colors, so the bright green line of mercury in the spectra of the compact fluorscent, mercury vapor, and metal halide lamps looks yellow instead of green. Oh well...
It is OK to take rough notes while you are observing (it will be hard enough to juggle a notebook, pen/pencil, and spectrometer on the street), and write up a lab summary for your notebook. However, putting the rough notes into your notebook may be useful for future reference, so you don't lose some scrap of paper somewhere when writing the lab up later.
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