62
Version 4.2 rev 06 Oct 2020
8.2 x 8.2 mm / 512 x 512 sensor
Lowest Noise Imaging EMCCD
Mechelle
b1.1- g
rating
e
quation
• m
λ
= d (sin
α
+ sin
β
)
→
m
λ
m
= k
→
λ
m
overlap
•
β
> 0 when incident and diffracted rays on the same side of grating normal
•
Max. intensity for wavelengths satisfying the grating equation at angles
α
+
β
0
= 2
θ
→
m
λ
m
= d (sin
α
+ sin
β
0
)
•
Special case
α
=
β
0
=
θ
=
condition for auto-collimation
The design of the echelle system can be optimised for resolution giving
λ
/Δ
λ
~5 x 10
4
, however this would only have a
bandwidth of
∼
60 nm. To simultaneously observe a broad atomic spectrum, from 200 nm, where line density is high, to
~920 nm (main spectral response for sulphur species), the resolution will be in the region of 5 x 10
3
.
Figure 29: Schematic representation of echelle grating order-sorter combination
b1.2 - e
cHelle
l
ayout
The layout of the echelle is shown in
figure 30
below. The patented, dispersion balanced order sorting system provides
a uniform distribution of the spectral orders. The order sorter works like an achromatic lens, correcting the non-uniform
order distribution caused by a single prism order sorter, as used in other échelle spectrographs. This results in efficient
use of the sensor area and allows a large bandwidth to be easily achieved.
Figure 30: Optical layout of the echelle design
Figure 31
Shows the Mechelle layout using a patented combination of prisms and grating to obtain equally spaced
orders at the focal plane of the instrument, as well as simultaneous large bandpass and high spectral resolution.
