5.) Grating section
- 2 (nearly) independent grating spectrometers: SW/LW section
- have to use the same entrance aperture (limits merging!)
- incoming beam is split up in a short and long wavelength part
- fed into grating spectrometers via slits, filters, mirrors
- light path in both grating sections is very similar:
->
lay-out of the SWS grating section
- collimatoring optics:
saddle shaped toroidal mirror
paraboloidal cylindric mirror
=> collimated (= parallel) beam
- scanning device:
each grating has its own scanning mechanism
scanning is achieved by rotation of a flat mirror
reflects collimated beam onto the grating
->
SWS scanner mechanism and light path
- grating:
ruled with 100 and 30 lines/mm for SW/LW section
spectral resolution R = 1000 - 2000 (300 - 150 km/s)
diffracted beam is directed back to the scan mirror
- imaging optics:
after diffraction, the light almost retraces its path
a spectral image of the beam FOV is formed at the
spectrometer exit slit
LW section has two additional exit slits to feed FP
->
first spectral image plane (in the SWS diagram)
Table: SWS grating properties
| section |
grating |
orders |
wavelength |
| |
[l/mm] |
|
[µm] |
| SW |
100 |
1, 2, 3, 4 |
2.38 - 12.0 |
| LW |
30 |
1, 2, 3:FP |
12.0 - 45.2 |
- small-diameter re-imaging correction and relay optics:
concave cylindrical field mirror
conical mirror
toroidal re-imaging mirror
->
lay-out of the SWS grating section
- re-images the spectral image onto the detector block:
relays spectral image to a more easily-accessible location
changes relative opening of the beam to detector dimensions
removes astigmatism from spectral image
enables efficient straylight rejection
- filters are located directly before the detectors
- spectral region of interest is mapped onto a detector arrays
- the arrays cover instantaneously about 8 resolution elements,
with gaps between the individual detectors
- during a measurement, the grating is scanned in small steps
to fully sample the desired wavelength range and provide a
certain redundancy in case of detector failures
- to deal with detector memory effects every scan is performed
twice, as an up and a down scan
(up scan in scanner position is a down scan in wavelength!)
SWS TUTORIAL /
SWS INSTRUMENT : ...
dichroics <
grating >
detectors ...
Last update: 30-Jul-1998 by D. Kunze ( mail to:
isodata@mpe.mpg.de )
Grating spectrometers: