63
Version 4.2 rev 06 Oct 2020
8.2 x 8.2 mm / 512 x 512 sensor
Lowest Noise Imaging EMCCD
Mechelle
Figure 31: Equally separated spectral orders by using prism and grating design
Figure 32 shows the result of using a single prism. Without this order sorting system, the resolution together with high
bandwidth cannot be achieved:
Figure 32: Single prism used for separating spectral orders compressing the orders towards IR.
A single grating would result in compression of orders in UV.
The bandwidth or free spectral range, F
λ
, for any single order, m, can be dened by the following equation:
Where λ
o
m
is the centre wavelength of any order as shown in gure 33 below. This enables the resolution, R, (equation
below) of the spectrograph to be calculated, where FWHM is the minimum or desired Full Width at Half Maximum on the
CCD or ICCD detector, and CCDWidth is the width of the image area on the CCD.
The overall bandwidth depends on the number of orders that can be placed on the detector multiplied by the bandwidth
of each order as given by F
l
. The number of orders that can be placed on the detector is determined by the distance LT,
(shown in gure 33) between the orders. The design utilised by the Mechelle lead to 80 orders being imaged at the
instrument focal plane.
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