The procedures for LonGSp observations are those commonly used in NIR spectroscopy, optimized for the characteristics of the instrument. For compact sources, observations consist of several groups of frames with the object placed at different positions along the slit. In the case of extended sources, observations consist of several pairs of object and sky frames. On-chip integration time is 60 s or less for a background level of roughly 6000 counts/pixel because of ensuing problems with sky line subtraction (see below). At a given position along the slit, several frames can be coadded.
The main steps in the reduction of NIR spectroscopic data are flat-field correction, subtraction of sky emission, wavelength calibration, correction for telluric absorption, and correction for optical system + detector efficency. Data reduction can be performed using the IRSPEC context in MIDAS, the ESO data analysis package, properly modified to take into account the LonGSp instrumental setup. We have found it useful to acquire dark and flat frames at the beginning and the end of the night; we obtain flat-field frames by illuminating the dome with a halogen lamp.
Observations of a reference star are taken for a fixed grating position. An early type, featureless star (preferably an O star) is needed to correct for telluric absorption and differential efficency of the system, and a photometric standard star is needed if one wants to flux calibrate the final spectrum (only one grating position in each band is required). An alternative technique, proposed by Maiolino et al. (1996), consists of using a G star corrected through data of the solar spectrum. Both methods have been succesfully tested.
Flat field frames are first corrected for bad pixels, then dark-current subtracted, and normalized. Dark current is subtracted from all raw frames, and then divided by the normalized flat field.
For compact sources (the frames taken at the different positions
along the slit are denoted by A, B and C),
the sky is subtracted by
considering A-B, B-(A+C)/2, and C-B, and taking a median of the
three differences.
In case of extended objects, if A and B denote object and sky frames,
the sky is subtracted by considering (A1+A2)/2-(B1+B2)/2
(the order of observations is 
).
However, a simple sky subtraction is almost never sufficient to
properly eliminate the bright OH lines whose intensity varies
on time scales comparable with object and sky observations.
Moreover, mechanical instabilities can produce movements of spectra
(usually a few hundreds of a pixel) which are nevertheless enough to
produce residuals which exceed the detector noise.
To correct for these two effects, the sky frames are multiplied by a
correcting factor and shifted along the dispersion direction by
a given amount. These factors and shifts are chosen automatically
by minimizing the standard deviation in selected detector areas
where only sky emission is present.
Because this effect increases with the integration time, it is advisable
not to exceed 60 seconds for each single integration.
Slit images at various wavelengths are tilted as a consequence of the off-axis mount of the grating. Sky subtracted frames are corrected by computing analytically the tilt angle from the instrumental calibration parameters, or by directly measuring it from the data.
Wavelength calibration in LonGSp data is performed using OH sky emission lines. The wavelength dispersion on the array is linear to within a small fraction of the pixel size and is computed analytically once the central wavelength of the frame is known. At the beginning of the data reduction, the nominal central wavelength used in the observations is assigned to a properly chosen sky frame. Then the calibration is refined using the bright OH sky lines (precise wavelengths of OH lines as well as a discussion of their use as calibrators are given in Oliva & Origlia 1992).
The same procedures are applied to the reference stars frames to obtain the calibration spectra, and the spectrum of a photometric standard star can be used to flux calibrate the final frames.