The Imaging Spectrometer MOS IRS-P3



 MOS IRS
zum Bild MOS IRS

Spectrometer Design

The Modular Optoelectronic Scanner MOS consists of two imaging pushbroom spectrometers MOS-A and MOS-B with similar optical design. It was manufactored by using commercial available components as far as possible. The spectrometers have a plane grating mounting consisting of the spectrometer slit, the collimator, the grating, the imager and the focal plane assembly (FPA). A short focus entrance optics images a swath from the earth surface onto the spectrometer slit and the spectrometer produces spectral images of this scene on the FPA. Each linear CCD sensor on the FPA represents one spectral channel and the CCD-elements define the spatial and spectral resolution. Fig. 3.1 demonstrates this optical principle. To explain the differences between MOS-A and MOS-B a more detailed description of the spectrometers is given below.

 

Optical principle of MOS

 

MOS-A

Because of their different scientific tasks the spectrometers MOS-A and MOS-B have some differences in the design too. MOS-A was built for monitoring atmospheric parameters in 4 spectral channels at the O 2-A-absorption band from 757 nm to 767 nm with 1.4 nm spectral half width. At an orbit altitude of 400 km the imaging spectrometer has a swath width of about 91 km with 140 pixels of 0.7 x 2.4 km 2 . To obtain a nearly quadradic field 5 pixels are superimposed. In tab. 3.1 the performance data are listed and fig. 3.2 shows the optical design of the MOS-A.

MOS-A optical design

Both the grating and the focal plane housing consist of black anodized alumium. The mounting kit for the entrance optics (3), slit (8) and collimator (10) is built of titanium to reduce distance variations between these components due to temperature effects. It is also black anodized. All the objectives are commercial available, but they are modified to meet the launch and mission loads (vibrational, temperature, humidity, shock).

A special high efficient antireflection coating for the MOS-A spectral range covers all the lenses of the objectives (3), (10) and (12). The color filter (1) for suppressing the second order spectrum of the grating, the quarter wave plate (2) for reduction of mainly by the grating produced polarization sensitivity of the instrument, the glass target for the spectrometer slit (8) and the hermetic window (13) of the FPA are also antireflection coated.

The spectrometer slit of 0.15 x 5.97 mm 2 area has a curvature of 80 mm to compensate the spectral line curvature due to the grating. This is necessary because of the requirement of nearly constant central wavelength along the swath rsp. along the CCD-line due to the shape of the O 2 A-absorption band and the small spectral halfwidth of the instrument. Besides the spectrometer slit two additional slits are arranged for the radiation coming from the high stable internal control mini lamps (7) via the color glass prisms (5). This complex multi slit structure is composed of a chromium layer of high optical quality and density ( D > 4) on a 2 mm thick glass target.

In front of the spectrometer slit an electromagnetic shutter (4) is mounted to block the radiation coming from the earth scene during the internal calibration procedure and the dark signal measurement. The internal control lamps serve for checking the radiometric stability of the optoelectronic channel (without the entrance optics (3)) and the wavalength stability too.

The ruled plane grating (11) with 1302 lines/mm has a blaze wavelength of 750 nm and a high efficient alumium coating. It is used at an incident angle of 3.7° and diffraction angles between 67.09 and 68.96°. 

The 4 CCD-lines of the MOS-A FPA are arranged on a plane surface with distances between each other corresponding to the required centre wavelengths. This plane arrangement is suitable with its small working spectral range only because of the small amount of chromatic aberrations due to the lens objectives in this range.

To meet the spectral, radiometric and imaging requirements there are used specially manufactured CCD-lines. They consist of 512 elements of 480 x 23 µm 2 (spectral direction x spatial direction) area. Only 420 elements are used. Then 140 pixels are formed by superimposing the analogue read-outs of 3 CCD-elements. Thermo-electrical cooling with peltier cooler (15) upto 5° C and temperature stabilizing of +0.1° C of the FPA guarantees high S/N-ratio and radiometric stability. The heat transfer element (16) transports the heat to the housing.

Stray light reduction inside the spectrometer is be done by favourable arrangement of different diaphragms, by selection of optical elements with small amount of stray light and black anodizising of all the mounting kits and the housing.

The optical components can be adjusted in different directions. Entrance optics and collimator can be focused to the spectrometer slit only. The imager can´t be adjusted. High accuracy in centreing of the objectives therefore is to be done by manufactoring. The principal ray defines the optical axis which is transferred to the normal of the adjustment mirror (9).This mirror serves as a tool for the adjustment of the optical axes of MOS-A and MOS-B parallel each to other. The quarter wave plate we need rotate around the optical axis for optimising the polarization sensitivity only. The grating we can turn around one axis to meet the correct incidence angle and around the grating normal for adjustment of the spectrum . Most of the degrees of freedom of adjustment are incorporated into the mounting kit of the FPA. It can be moved in direction of the optical axis for focussing, translated perpendicular to this axis for wavelength calibration and can also be rotated around three axis for rectification the spectral lines, for focussing the CCD-lines at the ends of the spectrum and at the ends of the swath.

To prepare the instrument for the orbit mission most of the adjustment tools are removed and the mounting kits and screws are sticked. The focal plane housing is hermetical closed and filled with dry nitrogen to avoid condensation on the cooled focal plane.

 

MOS-B

The MOS-B was designed especially for determination the optical state parameters of the atmosphere-ocean system. This will be done in the 400 -1010 nm spectral range by using 13 channels of a spectral halfwidth of about 10 nm. At an orbit altitude of 400 km MOS-B covers a swath width of about 98 km and the pixel area is about 0.8 x 0.7 km 2 . The performance data are listed in tab. 3.1 and fig. 3.3 shows the MOS-B optical design.

MOS-B optical design

 

Both the grating and the focal plane housing consist of black anodized alumium too. The mounting kit for the entrance optics (3), slit (8) and collimator (10) is built of titanium to reduce distance variations between these components due to temperature effects. It is also black anodized. All the objectives are commercial availabe, but they are modified to meet the launch and mission loads (vibrational, temperature, humidity, shock).

An antireflection coating working from 400 nm to 1010 nm covers all the lenses of the objectives (3), (10) and (12). The quarter wave plate (2) for reduction of mainly by the grating produced polarization sensitivity of the instrument, the protection quartz plate (1) and the glass target of the spectrometer slit (8) are also antireflection coated.

MOS-B uses a straight spectrometer slit of 0.041 x 6.140 mm 2 area. That is possible because the spectral line curvature due to the grating gives a maximum shift of the centre wavelength of 1nm only. It can be neglected concerning the great spectral halfwidth of about 10 nm. Besides the spectrometer slit are arranged two slits for the radiation coming from the high stable internal control mini lamps (7) via the color glass prisms (5). This complex multi slit structure is composed of a chromium layer of high optical quality and density ( D > 4) on a 2 mm thick glass target.

In front of the spectrometer slit an electromagnetic shutter (4) is mounted to block the radiation coming from the earth scene during the internal calibration procedure and the dark signal measurement. The internal control lamps serve for checking the radiometric stability of the optoelectronic channel (without the entrance optics (3)) and the wavalength stability too.

The ruled plane grating with 325 lines/mm has a blaze wavelength of 530 nm and a high efficient alumium coating. It is used at an incident angle of 42° and at diffraction angles between -19.6 and -32.3°.

Because of the very large chromatic aberrations in the wide spectral working range of MOS-B due to the lens objectives we can´t use a plane CCD-line arrangement as it was done in MOS-A. In the MOS-B the 13 CCD-lines were mounted on a stairlike target to fit the curved focal plane.The distances between the CCD-lines correspond to the required centre wavelengths. The filter glass (13) in front of the FPA suppresses the second order spectrum of the grating.

To meet the spectral, radiometric and imaging requirements there are used specially manufactured CCD-lines. They consist of 512 elements of 480 x 23 µm 2 (spectral direction x spatial direction) area. Only 384 elements are used. Then 128 pixels are formed by superimposing the analogue read-outs of 3 CCD-elements. Thermo-electrical cooling with peltier cooler (15) upto 5° C and temperature stabilizing of +0.1° C of the FPA guarantees high S/N-ratio and radiometric stability. Stray light reduction inside the spectrometer is be done by favourable arrangement of different diaphragms, selection of optical elements with small amount of stray light and black anodizising of all the mounting kits and the housing. The heat transfer element (16) transports the heat to the housing.

The optical components can be adjusted in different directions. Entrance optics and collimator can be focused to the spectrometer slit only. The imager can´t be adjusted. High accuracy in centreing of the objectives therefore is to be done by manufactoring. The principal ray defines the optical axis which is transferred to the normal of the adjustment mirror (9).This mirror serves as a tool for the adjustment of the optical axes of MOS-A and MOS-B parallel each to other. The quarter wave plate we need rotate around the optical axis for optimising the polarization sensitivity only. The grating we can turn around one axis to meet the correct incidence angle and around the grating normal for adjustment of the spectrum . Most of the degrees of freedom of adjustment are incorporated into the mounting kit of the FPA. It can be moved in direction of the optical axis for focussing, translated perpendicular to this axis for wavelength calibration and can also be rotated around three axis for rectification the spectral lines, for focussing the CCD-lines at the ends of the spectrum and at the ends of the swath.

To prepare the instrument for the orbit mission most of the adjustment tools are removed and the mounting kits and screws are sticked. The focal plane housing is hermetical closed and filled with dry nitrogen to avoid condensation on the cooled focal plane.


Contact
Dr.rer.nat. Horst Schwarzer
German Aerospace Center

Institute of Optical Sensor Systems
, Optic, Calibration and Validation
Tel: +49 30 67055-584

Fax: +49 30 67055-572

E-Mail: Horst.Schwarzer@dlr.de
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Texte zu diesem Artikel
MMS, MCS, HIRES (MOS-Validierung) (http://www.dlr.de/os/en/desktopdefault.aspx/tabid-3486/5374_read-7981/usetemplate-print/)
MOS - PRIRODA (http://www.dlr.de/os/en/desktopdefault.aspx/tabid-3486/5374_read-7987/usetemplate-print/)
MOS - IRS-P3 (http://www.dlr.de/os/en/desktopdefault.aspx/tabid-3486/5374_read-7983/usetemplate-print/)
Downloads zu diesem Artikel
Spots on Earth as seen by MOS-IRS (http://www.dlr.de/os/en/Portaldata/48/Resources/dokumente/mos_irs_neu.pdf)
MOS (http://www.dlr.de/os/en/Portaldata/48/Resources/dokumente/97_02_mos_text_bild.pdf)
Meeresforschung (http://www.dlr.de/os/en/Portaldata/48/Resources/dokumente/mos_meeresforschung_aus_dem_all.pdf)
Three Years of Experiences (http://www.dlr.de/os/en/Portaldata/48/Resources/dokumente/millenium_artikel.pdf)
8 erfolgreiche Jahre für die Ozeanfernerkundung (http://www.dlr.de/os/en/Portaldata/48/Resources/dokumente/mos_8Jahre.pdf)
Eight Years MOS-IRS – Summary of Calibration Activities (http://www.dlr.de/os/en/Portaldata/48/Resources/dokumente/SummaryCalibration_MOS_8Years.pdf)
Eight Years MOS-IRS - Paper Nordwijk 10-04 (http://www.dlr.de/os/en/Portaldata/48/Resources/dokumente/Paper_Noordwijk10_04.pdf)