The top image shows the spectrum of a z=4.19 emission-line galaxy from a 6-hr net exposure. The bottom image shows the night sky emission lines over the same wavelength region.
Observations are taken in a sequence with objects shifted up and down along the slit in successive exposures. Differencing adjacent exposures removes the night sky lines but leaves the spectrum of the shifted object. Below we show night sky subtraction from three shifted 20-minute LRIS exposures, processed using only data in that hour-long sequence to remove night sky lines.
The emission line, which fell close to a strong night sky line, is visible in each spectrum. A faint continuum is also just visible. Eighteen such spectra are combined to produce the final spectrum shown at the top of the page.