Optical Assembly

In the Optical Assembly the incident radiance and irradiance are collected and split according to wavelength and focussed on the detector modules of the two optical channels. The Optical Bench consists of a single aluminium structure in which all elements are mounted. Attached to this aluminium structure are the two Detector Modules. The Optical Assembly Thermal Hardware consists of a thermal radiator and heaters. The thermal radiator is thermally connected to the detector modules via cold-fingers and flex-links. The heaters are attached to the Optical Bench structure and the CCD detectors.

Optical Bench (OPB)

The OPB consists of a telescope, an UV channel (264-380 nm), a visible channel (349-504 nm) and calibration systems. The UV channel is split in the UV-1 channel (264-311 nm) and the UV-2 channel (307-383 nm). For switching between Earth observations and the various calibration modes the Optical Bench is equipped with three mechanisms. The figure below shows the optical layout of the telescope, the UV-1 channel, the UV-2 channel and the calibration optics are shown.
Click on the figure for a larger view.

Optical design UV channel of OMI
Optical design UV channel of OMI

Telescope

The telescope is of a wide field reflective telecentric configuration. The telescope consists of two spherical aluminium mirrors (103 and 007). Radiation from the Earth is imaged on the entrance slit (008) of the spectrometer. Between the primary and secondary mirror a polarisation scrambler (005) is positioned close to the aperture stop (006) of the telescope. The large Field Of View of 114° is in the plane perpendicular to the plane of drawing. The secondary mirror has a coating to suppress stray light from 500 nm up towards longer wavelengths. The Field Of View in the flight direction is about 1.0°.

Scrambler

The OMI spectrometer is sensitive to the polarisation of the incident radiation (originating from the polarisation dependence of mirrors, dichroics and gratings). Therefore, Earth radiances with exactly the same intensity but a different polarisation state will yield different detector signals. The purpose of the polarisation scrambler is to make the instrument insensitive to the polarisation of the incident radiation.

In principle the scrambler transforms one polarisation state into a continuum of polarisation states and thus not into real totally unpolarized light. Therefore a scrambler is called a pseudo-depolariser. For OMI a spatial pseudo-polariser is applied, which transforms the polarisation state of the input into a polarisation state that varies with position over the scrambler exit aperture. Polarisation effects originating from the primary uncoated aluminium mirror are compensated for by the first scrambler surface, that is intentionally left uncoated.

UV channel

Behind the entrance slit (008) a dichroic mirror (009) reflects the spectral range of the UV channels to folding mirror (101) and transmits the VIS spectral range to a flat mirror (201), that reflects the light at 90 degrees out of the plane of drawing.

The UV radiation is reflected by mirror (101) to a plano convex fused silica lens (102), that collimates the beam from the entrance slit in the direction of the grating (103). The function of the collimating / imaging lens (102) is twofold: it creates a parallel incoming beam on the grating (103) and forms an intermediate spectrum of the diffracted beams close to the field mirror (104). The grating (103) is used in the first order.

The layout of the UV channel is determined to a large extent by the stray light requirements, especially for the UV-1 spectral area (264 - 311 nm). Within the UV spectral range the variation in radiance from the Earth between the shortest wavelength (< 290 nm) and the longer wavelength (> 320 nm) varies by more than three orders of magnitude. Without proper measures the stray light at wavelengths below 290 nm would exceed the signal itself. This stray light originates from wavelengths between 310 to 380 nm. To avoid this unwanted situation, the UV channel is split into to sub-channels: UV-1 and UV-2.

An intermediate UV spectrum is created, close to a (split) field mirror (104). This mirror has a coating with a wavelength (= position) dependent variable reflectance in order to suppress the stray light of larger wavelengths at the shorter wavelengths. Depending on the bandwidth of reflection coating (at each position), a stray light light suppression of one order of magnitude is obtained.

Moreover, by splitting the spectral range in 2 parts the stray light that is caused by internal reflections in the UV-2 imaging objective, has no effect on the UV-1 spectrum at the detector surface.

The UV-1 image is scaled down by a factor of 2 on the CCD detector as compared to the UV-2 image. This is done in order to improve the signal-to-noise performance of the UV-1 channel by a factor of sqrt(2), at the cost of doubling the groundpixel size in the swath direction.

VIS channel

In the figure below the optical layout of the visible channel is shown. Note that the orientation of the plane of drawing of this figure is perpendicular to that of the UV channel. The visible part of the spectrum (349 - 504 nm) is reflected by mirror (201) via folding mirror (202) to a collimating mirror (203). The rest of the visible channel consists of two folding mirrors (204 and 206), a plane grating with 1350 grooves/mm (207) and an objective (209 - 213) to image the diffracted beams on the second CCD detector.


Click on the figure for a larger view.

142 kb - Optical design VIS channel of OMI
Optical design VIS channel of OMI

Calibration Devices

OMI has the following on-board calibration/characterisation devices:
A White Light Source (WLS) and Light Emitting Diodes (LEDs).

The white light source (C07 in the figure of the optical bench) is a Tungsten Halogen lamp with a UV-transparant quartz bulb. The main purpose of the WLS is calibration of the detector pixel-to-pixel gain variations.

Two green LED's, emitting at 570 nm, are positioned close to the CCD detectors. The position of the green LED's in the VIS channel is at the aperture 208. In the UV channels two LED's are placed between lenses (110) and (111).
The primary use of the LED's is identification of bad detector pixels, to obtain the relative electronic gain values, and to calibrate the non-linearity of the electronics in flight.

The signals of the read-out register after a draindump (which accompanies every image) can be used to determine the readout noise and the electronic offset values.
The WLS is used once per week. The LED's are used once per day for characterisation of bad and dead detector pixels, once per week to monitor the electronic gain values and once per month to monitor the non-linearity of the electronics.

Mechanisms

In the OPB the following mechanisms can be identified:
  • The Diffuser Mechanism (DifM) and solar irradiance measurements
    The diffusers play a key role in providing the sun-normalised Earth spectrum, i.e. the ratio of the Earth radiance over the sun irradiance. For this reason the diffusers are extensively calibrated during the on-ground calibration. The diffuser mechanism contains one quartz transmission diffuser for the White Light Source (WLS) calibration, two ground aluminium surface reflection diffusers and one quartz volume reflection diffuser. The three reflection diffusers are used for the solar calibration measurement. The transmission diffuser provides homogeneous illumination of the spectrometer slit during WLS measurements. The quartz volume reflection diffuser is used on a daily basis to provide the irradiance spectrum for normalising the Earth radiance spectra. One surface reflection aluminium diffuser will be used on a weekly basis (regular aluminium diffuser) for regular radiometric calibration verification, while the other will be used on a monthly basis (backup aluminium diffuser). Irradiance measurements via all three reflection diffusers provide absolute radiometric irradiance calibration information. The optical beams reflected off the aluminium and the quartz volume reflection diffusers contain spectral and spatial features in the order of 0.01% to 10% originating from white light interference from the diffuser surfaces. These spectral features may interfere with atmospheric trace gas retrievals. The quartz volume reflection diffuser exhibits much smaller spectral and spatial diffuser surface interference features in comparison to the aluminium surface reflection diffusers. This makes the quartz volume reflection diffuser most suitable for normalising the Earth radiance spectra. All on-board diffusers are optimally protected from optical degradation resulting from exposure to space environment.
  • The Folding Mirror Mechanism (FMM) operates a mirror (C03) that can be moved in and out of the Earth radiance optical path between the primary and secondary telescope mirrors. When the mirror blocks the Earth radiance optical path, mirror C03 reflects either sun light from one of the reflection diffusers or White Light Source (WLS) light from the transmission diffuser towards the entrance slit of the spectrometer.
  • The Sun Aperture Mechanism (SAM) allows sunlight to enter the instrument for the solar calibration measurement. The SAM is a shutter that is always closed, except during the solar calibration measurements. In the aperture of the instrument behind the shutter, a mesh is mounted in order to reduce the intensity of the solar radiation. Keeping the shutter closed also prevents space debris and stray light to enter the optical bench.

Optical Path of Earth, solar, WLS and LED measurements.

The optical paths for the Earth and solar measurements are not the same. For this reason the optical paths could degrade differently during the mission, which may produce an artificial trend in the Earth radiance to solar irradiance ratio (sun-normalised Earth reflectance).

For an Earth measurement the complete optical path of the radiation is shown in the figure for the UV-channel and in the figure for the VIS channel. The radiation enters the instrument by aperture (001), reflects from the primary mirror (103), passes the scrambler (005) and reflects from the secondary mirror (007) before entering the spectrometer by entrance slit (008). Directly behind the entrance slit the radiation is split by a dichroic mirror (009) that reflects the light of shorter wavelengths into the UV channel and transmits the light of higher wavelengths into the VIS channel.

For a solar calibration measurement, the Sun Aperture Mechanism is opened so that the sunlight can enter the instrument throught the solar mesh. This sunlight is then reflected by one of the three reflection diffusers on the Diffuser Mechanism towards the mirror C03 on the Folding Mirror Mechanism, which moves in the optical path of the telescope for this occasion in order to reflect the sunlight via the scrambler (005) towards the secondary telescope mirror (007). The solar calibration measurements therefore use a different optical path than the Earth radiation measurement, adding the mesh, the reflection diffuser and the FMM mirror C03, and missing the primary telescope mirror (103). In first approximation the folding mirror C03 and the primary telescope mirror 003, that are both made of uncoated aluminium, will have a comparable degradation.

During a White Light Source (WLS) measurement the transmission diffuser (C05) in the Diffuser Mechanism is used and the folding mirror C03 is moved into the Earth optical path of the telescope. The WLS light passes lens (C08), is reflected from mirrors (C09) and (C10), passes the transmission diffuser and is reflected by the folding mirror C03 through the polarisation scrambler (005) towards the secondary telescope mirror (007). With the WLS most of the optical path can be monitored for optical degradation.

The LEDs are directly in front of the CCD detectors with only a few optical elements in between and illuminate the CCD detectors directly, that is, without going through the complete optical system.

Detector Modules

The main function of the (DEM) is to convert the optical signal provided by OPB, to an analogue electrical signal. The OMI-EOS system includes two identical DEMs. Each DEM consists of a CCD, control electronics and thermal hardware.

CCDs

The OMI CCD's are thinned, backside illuminated CCD's from EEV (currently E2V) , model EEV 55 with 576 rows and 780 columns. The detectors have a HfO2 anti-reflection coating to obtain a high quantum efficiency, especially in the UV. This coating is optimised to have minimum reflectivity and best quantum efficiency at 270 nm.

OMI CCD description
The OMI detectors are frame transfer CCD's. This means that the CCD's have three sections: an image section, a storage section, and a read-out register. The pixels are 22.5 × 22.5 micron in size and there are no regions in between the pixels that are not sensitive to light, which is important for accurately detecting spectral features of trace gases. Each image part of the CCD consists of 580 (spatial)  ×  780 (spectral) CCD pixels, of which 480 × 750 are used for Earth and sun measurements. The storage region of the CCD is an exact copy of the image part of the CCD. The read-out register has 17 extra pixels on each end.

The CCD uses 480 rows of 750 pixels for the Earth and sun measurements. The wavelength information is distributed along a CCD row, the spatial information (viewing angle) is distributed along a CCD column. The image section is in the focal plane of the spectrograph and after exposure the frame is transferred rapidly to the storage region. This is the snapshot mode of the CCD detectors. During transfer illumination of the detector continues, causing smearing of the detected signals, that will be corrected for in the Level 0 to 1B software. After transfer to the storage region, the rows are transfered to the read-out register and the pixels are read out.

Binning of images
As described above the rows of the storage area are clocked to the read-out register. In this process, 4 or 8 rows are added in the read-out register, before clocking them out over the read-out amplifier. This on-chip binning is needed to reduce the instrument data rate. Binning of 4 rows results in the spatial zoom-in mode pixel size of 13 × 12  km², binning of 8 rows in the global mode pixel size of 13 × 24 km². For the UV-1 these ground-pixels sizes are 13 × 24 km² and 13 × 48 km², respectively.

Exposure times and co-adding of images
The nominal exposure time of the CCD is 0.4 seconds. In order to improve the signal-to-noise and to reduce the downlink data rate, 5 sequentially taken exposures are co-added on-board. Co-adding is done by the electronics in the Electronics Unit (ELU). Co-addition of 5 exposures results in a total 2 seconds interval in which measurements are taken. This results in the 13 km spatial resolution in the flight direction. This 2 seconds interval is called the Master Clock Period (MCP), which is the basic time interval in the ELU.
The extreme rows and columns of the image area are masked and not sensitive to light. These masked rows are used to determine the dark signal (columns) and exposure smear (rows).

Straylight, dark-current, offset and gain-switching information
Between the image areas on the CCD detectors and the masked areas above and below the image areas there are a number of rows that are not masked, but that are not illuminated directly by the optical system. These stray light rows are used to estimate and correct for spatial stray light. The stray light rows above and below the image are binned to achieve better signal-to-noise.

Information on the electronics offset and dark current can also be obtained from the read-out register. The read-out register within the CCD has 17 excess pixels compared to the CCD itself. The offset can be measured via initial register readout preceding all image data. As the register is cleared just before this, it also contains the offset data.

The detector electronics is equipped with a 12 bit Analog to Digital Converter (ADC). The Electronics Unit (ELU) allows for switching the electronic gain as a function of the CCD column number (wavelength) to optimise signal over the available ADC range. Switching as a function of column (wavelength) can be used to obtain more electronic gain for low radiance levels. The highest gain is used for the dark current, smear rows and stray light rows above and below the directly illuminated image areas on the CCD detectors. The register readout uses the same gain switching as the image area. In this way electronic offset values for all gains are obtained. OMI allows to select four gain values: x1, x4, x10 and x40.

It has been observed in flight that high energetic protons (>10 MeV) trapped in the magnetic field of the Earth can cause damage to the individual OMI CCD detector pixels. As a result, the dark current of the individual CCD pixels and the average dark current of the complete CCD detector will increase over the lifetime of OMI. To maintain sufficiently low dark current towards the end of the mission, the nominal detector operating temperature is 265 K. In order to account for the pixel-to-pixel variable increase in dark current of pixels that have been affected by a proton hit, a dynamical background subtraction scheme has been devised. In this scheme the background signals are updated each day. In order to reduce the impact of proton radiation damage in flight a considerable amount of aluminium shielding has been added to minimise detector degradation.

Pixels that have been affected by a high-energetic proton can show two effects: a considerable increase in dark current and so-called Random Telegraph Signal (RTS) behavious. RTS behaviour manifests itself as jumping of the signal level between two or more quasi-stable levels. Pixels that have been hit by a proton can feature modest RTS behavious, but in exceptional cases also severe RTS behaviour. The latter behaviour is detected by the in-flight calibration and monitoring activities and pixels that can no longer be used for science purposes are flagged as bad pixels. With the above precautions the in-flight proton radiation damage is within acceptable limits and does not seriously affect the quality of the OMI level-1b and level-2 data products and the objectives of the OMI mission.

In order to reduce the risk of ice-formation on the surfaces of the detectors or the optical components, the instrument was outgassed at an elevated optical bench and detector temperature of 303 K for a time period of more than 6 weeks immediately after lauch.

Optical distortion
The instrument optical components introduces optical distortion of the image that is focused on the CCD. The optical distortion has two components. Firstly, optical distortion in the swath direction (i.e. curvature of lines of equal viewing direction). Secondly, optical distortion in the spectral direction (i.e. curvature of lines of equal wavelength). The optical distortion in the swath direction introduces spatial aliasing: pixels which are part of the same row (i.e. pixels which are part of the same spectrum) having different viewing angles. As a result of binning of the rows in the read-out register the optical distortion in the spectral direction potentially introduces a decrease in the spectral resolution. However, this has a negligible effect on the spectral resolution. Therefore the optical design has been optimised to minimise to the distortion in the spatial direction. The optical distortion in the spectral direction is more pronounced than the distortion in the spatial direction. It is more pronounced for the UV channel than for the VIS channel. The instrument calibration corrects for these distortion effects.


© OMI -- Last update: Tuesday, 16-Dec-2008 02:21:11 UTC. --