Remote Sensing Detectors Modelling
An important activity for instrument performance is software modelling, which allows us to understand the complex interplay of factors that affect performance.
- Prediction of performance of an instrument. This can indicate whether the instrument will achieve its requirements/goals, and which parameters need to be optimised
- Comparison of a software model with actual behaviour. This can help identify problems in an instrument or areas where further improvements can be made, or it can confirm that the instrument is behaving as well as it was designed to.
We use various modelling tools for different instruments and detectors:
- Optical ray-tracing: We use packages (usually Zemax) to calculate the performance of photon illumination systems.
- Radiation/absorption modelling: Packages such as GEANT4 and the MULASSIS (MUlti-LAyered Shielding SImulation Software) front-end allow simulation of the absorption of radiation particles either desirably in detectors, or undesirably as sources of damage for electronic components.
- Bespoke software: In many cases, there is no convenient off-the-shelf package to do exactly what we want. In this case, we write our own code to simulate instument performance, e.g. sensitivity as a function of wavelength, effects of noise on performance. The code can be implemented as spreadsheets, in graphical tools such as LabVIEW, or using more conventional programming languages.
Optical Ray Tracing
Optical Ray Tracing is used at MSSL in the design of the highly complex optical systems (telescopes and calibration standards), often in partnership with other institutes or consultants.
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A first sequential model using STEP models from the true mechanical parts to model the stray light off the trumpet and verify that the output beam is well kept within a predefined solid angle.
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This is the landing pattern of the telescope light inside the sphere. The landing pattern has to be far enought from the sphere exit aperture..
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FOV: 2x1.6 degrees; waveband=847-874 nm; dispersion 265 um per nm. Two off axis aspheric mirrors and one blazed grating arranged in an Offner telescope configuration
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This was designed in collaboration with Kayser-Threde and Richard Bingham for an industrial study of a possible version of a spectrometer on Gaia.
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The various diffraction spots centroids were calculated for multiple star positions and multiple wavelenghts to recalibrate spectra obtained in Space
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This is the full non sequential ray tracing using the real mechanical parts to verify the final output uniformity performance at the sphere exit plane
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This is the non sequential stray light analysis of the optics with the real mechanical parts, in order to verify the extra reflection due to the tube walls. This model was also used to define the relectivity properties of the tube
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Zemax model for the sequential ray tracing of the CAA telescope optics, the light bulb is the right and the sphere aperture is on the left. The optics collimate the bulb illumination onto the two central filters to obtain a colimation within +/- 10 degrees
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Shows the non sequential ray tracing to measure the Radiant intensity performance at the reference location behind the sphere snout
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Click to view image at full size and access the gallery slider.
Shows both the nominal beam delivery through the pipe and with a fibre placed near the sphere, to compare their input pattern inside the sphere
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Click to view image at full size and access the gallery slider.
This is the sequential ray tracing of a flown GRISM assembly that produces a spectrum centred on the wavelength of blue light
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Click to view image at full size and access the gallery slider.
A zoom showing the reflections due to the CCD jig. These unwanted reflections were later attenuated by the addition of a black CCD jig baffle
A first sequential model using STEP models from the true mechanical parts to model the stray light off the trumpet and verify that the output beam is well kept within a predefined solid angle.
This is the landing pattern of the telescope light inside the sphere. The landing pattern has to be far enought from the sphere exit aperture..
FOV: 2x1.6 degrees; waveband=847-874 nm; dispersion 265 um per nm. Two off axis aspheric mirrors and one blazed grating arranged in an Offner telescope configuration
This was designed in collaboration with Kayser-Threde and Richard Bingham for an industrial study of a possible version of a spectrometer on Gaia.
The various diffraction spots centroids were calculated for multiple star positions and multiple wavelenghts to recalibrate spectra obtained in Space
This is the full non sequential ray tracing using the real mechanical parts to verify the final output uniformity performance at the sphere exit plane
This is the non sequential stray light analysis of the optics with the real mechanical parts, in order to verify the extra reflection due to the tube walls. This model was also used to define the relectivity properties of the tube
Zemax model for the sequential ray tracing of the CAA telescope optics, the light bulb is the right and the sphere aperture is on the left. The optics collimate the bulb illumination onto the two central filters to obtain a colimation within +/- 10 degrees
Shows the non sequential ray tracing to measure the Radiant intensity performance at the reference location behind the sphere snout
Shows both the nominal beam delivery through the pipe and with a fibre placed near the sphere, to compare their input pattern inside the sphere
This is the sequential ray tracing of a flown GRISM assembly that produces a spectrum centred on the wavelength of blue light
A zoom showing the reflections due to the CCD jig. These unwanted reflections were later attenuated by the addition of a black CCD jig baffle
It can also be used for mechanical and thermal tolerancing studies, stray light studies, full opto-mechanical system simulations, optical system assembly, alignment, final system performance verification and integration.
Here are some existing designs which were used at various stages, from the initial proposal to the final missions, in flight.
Calibration
An important aspect of any space instrument is calibration. This means knowing and understanding the performance of the instrument in terms of sensitivity, resolution, noise factors. Even when these aspects are known, the work continues, as various parameters change with time, temperature and radiation dose in space. So calibration is a combination of testing the instrument before launch against known standards, and monitoring performance in orbit (e.g. regular specific observations of standard astronomical objects). At MSSL we have calibration experts whose role is to monitor and maintain the performance of instruments using a combination of pre-launch and in-flight testing and monitoring.
Resources:
Remote Sensing Detectors Outreach:
Services to Industry
Remote Sensing Detectors:
Modelling for instrument performance
Head of Group:
Dr Dave Walton
+44 1483 204 190
d.walton@ucl.ac.uk