Dispersion, Spectral Resolution and Coverage

WYFFOS uses a series of gratings whose tilt angle is computer controlled and can be varied in order to select a specific wavelength region.

In the normal (reflection) mode, WYFFOS uses the ISIS gratings, which permit the following range of spectral reciprocal dispersion, resolution and coverage with the default Red+4 and the WHTWFC (both unbinned):

Grating	Wavelength blaze (ang)	Dispersion with Red+4 (ang/pix)	Dispersion with WHTWFC (ang/pix)	Resolution (ang)	Spectral coverage (ang)	Notes
R158B	3600	3.2	2.9	16	all
R158R	6500	3.2	2.9	16	all
R300B	4000	1.8	1.6	8.8	6000	Actual central wavelength might be ~200 Angstrom larger than set
R316R	6500	1.7	1.5	8.1	5500	Actual central wavelength might be ~200 Angstrom larger than set
R600B	3900	0.89	0.8	4.4	3000
R600R	7000	0.89	0.8	4.4	3000
R1200B	4000	0.43	0.39	2.2	1400
R1200R	7200	0.35	0.32	2.0	1300
H1800V	5500	0.22	0.2	1.3	760
H2400B	4000	0.19	0.17	1.0	600	Max. central wavelength = 4685 Ang

Values listed above are approximate and generally conservative. If you have important spectral features that fall at the extremes of the quoted spectral range, you should take into account that (cf. the detector sketch): i) at the spectrograph entrance, fibres are arranged in three different rows, in such a way that there is a wavelength shift of 60 pixels between three consecutive fibres; ii) the WYFFOS optics produce an overall curvature of the spectra, the selected spectral range shifting to the red for fibres in the centre of the CCD mosaic. The total effect is that spectra of different fibres can be shifted up to about 5% of the total spectral range. The spectral coverage listed above is the one common to all fibres, while is some 5% larger for each individual one.

The quoted spectral resolution is the FWHM of the image of fibres as projected onto the detector, when illuminated from the sky. It amounts to about 5 pixels for Red+4 (or 5.5 pixels for WHTWFC), hence the spectral resolution is oversampled with this detector. However, the FWHM of arc lines is significantly smaller (about 3.5 unbinned pixels) than the actual instrumental resolution. This is because illumination from the AF2/WYFFOS calibration unit, located on the side of the telescope, enters the fibres at a lower f/number than light from the sky, causing a smaller image of the fibres to be produced at the spectrograph entrance. Therefore the FWHM of arc lines does not indicate the resolution of AF2/WYFFOS.

The echelle mode of WYFFOS is set by using a low-order echelle grating and a set of narrowband order sorting ( PDF format ) filters. Click for filter scans ( PDF format ). The dispersion, resolution and coverage obtained in this mode are:

Order	Wavelength blaze (ang)	Dispersion (ang/pix)	Resolution (ang)	Spectral coverage (ang)
3	8830	0.15	0.83	570
4	6623	0.11	0.63	430
5	5298	0.09	0.50	340
6	4415	0.07	0.41	280
7	3784	0.06	0.35	240