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Long-slit Spectroscopy

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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 GMOS-S original EEV CCDs, GMOS-N original EEV CCDs, and upgraded GMOS-N e2v DD devices had two ~2.8 arcsec (39 pixel) gaps. The inter-chip gaps between the new Hamamatsu detectors are 4.88 arcsec wide (61 pixels in unbinned mode) for GMOS-S and 5.41 arcsec wide (67 pixels in unbinned mode) for GMOS-N. For both Hamamatsu detectors arrays, the effective chip gap width, not usable for science, is wider than the physical chip gap size due to bright edges at either side of the chip gaps. The equivalent gap in the wavelength coverage depends on the grating used for the observation. More information and recommended wavelength dither step sizes are provided in the Observing strategies section.


The longest wavelengths are at small X pixel coordinates in the first extension (extension [1]), the shortest wavelengths are at large X pixel coordinates in the last extension ([12] in the Hamamatsus, [6] in the e2vDD's, and [3] in the older E2V's). 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.

GMOS longslit data

A typical GMOS observation of a spectrophotometric standard star. Only the central ~75 arcsec of the field of view are read out for this type of observation. The gaps between the detectors are shown.

GMOS longslit data

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.


GMOS longslit data

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.

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