5.1. Importance of the slit in the case of surface objects

A prototype of entrance slit for the Spect'Aude.

The effect on spectral resolution of entrance slit fitness is illustrated below for the situation of the observation of comet C/2001 A2 (LINEAR). The Littrow spectrograph is at the focus of a FSQ-106 Takahashi refractor (4.2 inch aperture - F/D=5). For a complete analysis of the observation of this comet click here.

Zero order image of Comet A2 trough a very large slit (slitless mode).

Comet A2 trough an intermediate wide slit (not completely focused).

Comet A2 trough a narrow slit (50 microns in this image).

Slitless spectral image (Castanet-Tolosan - 29 June 2001) - composite of 7 images exposed 120 seconds each (cumulative exposure of 14 minutes). The spectral resolution is dominated by the size of the coma.

Intermediate narrow entrance slit configuration for the spectrograph (250 microns wide). Total exposure time of 28 minutes the 08 July 2001. The sky background is not subtracted (note the presence of the major terrestrial atmospheric airglow in the visible part of the spectrum).

The 11 July 2001 spectra for narrow entrance slit configuration (90 microns wide). Observation made at Pic du Midi Observatory.

Now, for the 13 July 2001 observation, the slit wide is of 50 microns. Note the slight increase in spectral resolution.

Spectral resolution versus entrance slit wide. Date: slitless spectrum: 02 July 2001, medium: 08 July 2001, narrow: 11 July 2001. For this latter spectrum the FWHM is estimated to 40A (90 microns wide slit = 7 arc-second on the sky). Note the detection of [OI] signature at 6300A and fine structure on the 5150 C2 swan band in the high resolution spectrum.

Deconvolution of the 13 July spectrum (Vancittert algorithm). The spectral resolution is about 30 Angstroms (click on the image to enlarge).

5.2. Acquisition and processing procedures


First, the object is centered by using the zero order mode and a large slit (here the nebula M8). Then, the slit is narrowed.

The spectrograph mode is commuted and the spectral image is taken. Stack of 14 images exposed 60 seconds (dispersion of 8 A/pixel in this example).

Geometrical correction of the spectrum. The orientation of the dispersion is corrected by the command ROT of Iris (a command like ROT 160 100 0.75, correct a default angle of 0.75 degrees around the point of coordinates (160,100)). The inclinaison of the spectral lines is corrected by the command SLANT (for example SLANT 100 1.9, rotate image along the line of vertical coordinate 100 of an angle of 1.9 degrees).

Then, the spectrum is binned (command L_ADD for example). A binning of ten wide lines is used in the 2D image.

Finally VisualSpec is used to calibrate the spectral profile (radiometric correction and wavelength determinations). This spectrum show the exact relative intensity of the emission lines in the nebula Messier 8 (Lagoon nebula).

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