To stay within your budget, you are building a spectrometer with a spectral
resolution of only 100 to work near 0.9 mm. You will be using the CCD
described in exercise 3.4, which you got cheap on eBay. You should have
found in that exercise that the fringe period is about 0.0076 mm. If your
goal is to measure emission line strengths, do you need to increase your
budget to build a higher-resolution spectrometer?