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.
Reflection Mode
In the normal (reflection) mode, WYFFOS uses the
ISIS gratings, which permit the following range
of spectral reciprocal dispersion, resolution and coverage with the (unbinned)
2-chip EEV
mosaic:
Grating |
Wavelength
blaze (ang) |
Dispersion
(ang/pix) |
Resolution
(ang) |
Spectral
coverage (ang) |
Neon plots |
Helium plots |
Notes |
R158B |
3600 |
2.9 |
16 |
all |
|
|
|
R158R |
6500 |
2.9 |
16 |
all |
|
|
|
R300B |
4000 |
1.6 |
8.8 |
6000 |
|
R300B |
Actual central wavelength might be ~200 Angstrom larger than set |
R316R |
6500 |
1.5 |
8.1 |
5500 |
7500A zoom
BLUE
RED |
|
Actual central wavelength might be ~200 Angstrom larger than set |
R600B |
3900 |
0.8 |
4.4 |
3000 |
|
600B 5100A |
|
R600R |
7000 |
0.8 |
4.4 |
3000 |
|
|
|
R1200B |
4000 |
0.39 |
2.2 |
1400 |
1200B 4900A |
1200B 4900A |
|
R1200R |
7200 |
0.32 |
2.0 |
1300 |
|
|
|
H1800V |
5500 |
0.2 |
1.3 |
760 |
|
|
|
H2400B |
4000 |
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 fibres 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. As it amounts to about 5.5 pixels, the spectral resolution is oversampled
with the present detector, and therefore a 2 x 2 binning of the CCDs
is suggested. Note that 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.
Echelle Mode
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 |