Long slit spectroscopy can be obtained in either normal or Nod-and-Shuffle modes using GMOS standard slits or custom slits.

The dispersion direction of GMOS is along the rows of the image. Thus, the gaps between the detectors cause small gaps in the wavelength interval covered by a single observation. The gaps are approximately 39 unbinned GMOS pixels. The equivalent gap in the wavelength coverage depends on the grating used for the observation. Dealing with this and other observing issues is discussed in the Observing strategies section.

The longest wavelengths are at small X pixel coordinates in extension [1], the shortest wavelengths are at large X pixel coordinates in extension [3]. Thus, when the image has been displayed with gdisplay or mosaiced together with gmosaic the longest wavelengths are to the left and the shortest wavelengths to the right. The figures below show example longslit spectra with the wavelength direction and the gaps marked.

A typical GMOS observation of a spectrophotometric standard star. Only the central 1024 rows of the detector array are read out for this type of observation. The gaps between the detectors are shown.

A GMOS longslit observation of a nearby galaxy. The full detector array is read out. The gaps between the detectors are shown. The longslits have bridges, causing small gaps in the spatial coverage. These bridges are necessary in order to keep the slit stable.

The average of the central 20 rows of the galaxy spectrum shown above. An approximate sky subtraction has been applied. No wavelength calibration or flux calibration have been applied. The gaps between the detectors are shown.