Astronomical Spectroscopy - Physics - University of Cincinnati

– 17 –
each order (Equation 9). This is apparent from Figure 5, as the shorter wavelengths (higher
orders) span less of the chip. At lower orders the wavelength coverage actually exceeds the
length of the chip. Note that the same spectral feature may be found on adjacent orders,
but usually the blaze function is so steep that good signal is obtained for a feature in one
particular order. This can be seen for the very strong H and K Ca II lines apparent near the
center of order 16 and to the far right in order 15 in Figure 5.
If a grating is used as the cross disperser, then the separation between orders should
increase for lower order numbers (larger wavelengths) as gratings provide fairly linear dispersion
and the free spectral range is larger for lower order numbers. (There is more of a
difference in the wavelengths between adjacent orders and hence the orders will be more
spread out by a cross-dispersing grating.) However, Figure 5 shows that just the opposite
is true for MagE: the separation between adjacent orders actually decreases towards lower
order numbers. Why MagE uses prisms for cross-dispersing, and (unlike a grating) the
dispersion of a prism is greater in the blue than in the red. In the case of MagE the decrease
in dispersion towards larger wavelength (lower orders) for the cross-dispersing prisms more
than compensates for the increasing separation in wavelength between adjacent orders at
longer wavelengths.
Some echelle spectrographs are listed in Table 3. HIRES, UVES, and the KPNO 4-m
echelle have a variety of gratings and cross-dispersers available; most of the others provide
a fixed format but give nearly full wavelength coverage in the optical in a single exposure.
Table 3. Some Echelle Spectrographs
Instrument Telescope R (1 arcsec slit) Coverage(Å) Comments
HIRES Keck I 39,000 Variable
ESI Keck II 4,000 3900-11000 Fixed format
UVES VLT-UT2 40,000 Variable Two arms
MAESTRO MMT 28,000 3185-9850 Fixed format
MIKE Magellan II 25,000 3350-9500 Two arms
MagE Magellan II 4,100