5:15 PM - 6:45 PM
[PPS03-P10] Ground-based Optical Calibration for DESTINY+ on-board Camera of MCAP Engineering Model
Keywords:DESTINY+, (3200) Phaethon, Camera, Optical Calibration
DESTINY+ onboard cameras, TCAP, and MCAP, will observe the asteroid (3200) Phaethon. Optical calibration is necessary to convert raw images obtained by those cameras into scientifically valuable data. Both TCAP and MCAP require calibration data. This manuscript will focus on the calibration data acquisition for the MCAP Engineering Model (EM).
Data acquisition for calibrating the camera in optical calibration is classified into the following categories: (a) radiometric calibration for converting the digital number on the pixels into scientifically valuable data, and (b) geometric calibration for correcting the measurement position captured by the cameras into the accurate position. In addition to calibration data acquisition, (c) meteorite imaging was conducted to confirm the multiband imaging performance.
(a) Radiometric calibration:
Radiometric calibration includes bias images, dark images, data for linearity correction, flat-field images, sensitivity data, which can determine the sensitivity correction coefficients that convert count values of the image sensor to radiance, and data for Broad PSF correction*.
Bias images, dark images, and data for linearity correction of the image sensors were obtained in a thermostatic chamber with temperature control before installation in the optical system. We evaluated the characteristics of the image sensor, such as the bias level, dark current, conversion gain, full-well capacity, and linearity. Each result indicated that the measurement values were almost equivalent to those described in the sensor's catalog.
Flat-field images and sensitivity data were obtained by using an integrating sphere at Tsukuba Space Center, spectral radiance of which is calibrated with high precision. MCAP was aligned with the integrating sphere, and the lamp set was selected to achieve spectral radiance within the expected range of electron generation rates of the image sensor during the Phaethon flyby observation. Although we could not obtain the flat-field images in this test because of strong stray light, its possible cause could be identified, and measures were developed to ensure proper data acquisition in the Proto Flight Model (PFM). We also found that the flat-field images in MCAP are affected by the spectra of the light source. The light source spectrum of an integrating sphere is significantly different from that of sunlight. We have, therefore, established a method to obtain the flat-field images using a light source close to the sunlight spectrum (xenon light source).
To examinine whether the broad PSF phenomenon occurs in MCAP or not, and if it does, building a correction method for it, we have established the measurement system for broad PSF by using a refracting telescope and setting a circular aperture plate at the focal position of it. The measurement system we first constructed enabled us to capture a circular aperture image; however, we could not evaluate the broad PSF due to the strong stray light within the telescope. We refined the system blocking both external and internal scattering light as a source of the stray light between the light source and the telescope, which resulted in the disappearance of almost all the stray light. There is a prospect of obtaining proper Broad PSF data at PFM by the system.
(b) Geometric calibration:
Distortion data were acquired by mounting MCAP on a two-axis gimbal and changing the angle of incidence of the autocollimator light on the MCAP, which enabled us to perform the measurements at various angles of view. The measured distortion was confirmed to be almost the same as the designed values.
(c) Meteorite imaging:
To confirm the multiband imaging performance, we imaged meteorite samples with known spectra using each MCAP band. Meteorite samples include powders and chips of carbonaceous chondrites. We confirmed that the spectra of the samples could be successfully reproduced from the imaging by MCAP.
These MCAP EM calibration data acquisitions help us confirm the measurement procedure, prepare for measurement jig, and find the issues for the PFM optical calibration. In the presentation, we will present their detailed results and the measures taken to address the issue for PFM.
* The Broad PSF phenomenon is characterized by a spread of a certain to several tens of percentages of count values around an object image over a wide range when imaging an object with an area larger than a point-source image.
Data acquisition for calibrating the camera in optical calibration is classified into the following categories: (a) radiometric calibration for converting the digital number on the pixels into scientifically valuable data, and (b) geometric calibration for correcting the measurement position captured by the cameras into the accurate position. In addition to calibration data acquisition, (c) meteorite imaging was conducted to confirm the multiband imaging performance.
(a) Radiometric calibration:
Radiometric calibration includes bias images, dark images, data for linearity correction, flat-field images, sensitivity data, which can determine the sensitivity correction coefficients that convert count values of the image sensor to radiance, and data for Broad PSF correction*.
Bias images, dark images, and data for linearity correction of the image sensors were obtained in a thermostatic chamber with temperature control before installation in the optical system. We evaluated the characteristics of the image sensor, such as the bias level, dark current, conversion gain, full-well capacity, and linearity. Each result indicated that the measurement values were almost equivalent to those described in the sensor's catalog.
Flat-field images and sensitivity data were obtained by using an integrating sphere at Tsukuba Space Center, spectral radiance of which is calibrated with high precision. MCAP was aligned with the integrating sphere, and the lamp set was selected to achieve spectral radiance within the expected range of electron generation rates of the image sensor during the Phaethon flyby observation. Although we could not obtain the flat-field images in this test because of strong stray light, its possible cause could be identified, and measures were developed to ensure proper data acquisition in the Proto Flight Model (PFM). We also found that the flat-field images in MCAP are affected by the spectra of the light source. The light source spectrum of an integrating sphere is significantly different from that of sunlight. We have, therefore, established a method to obtain the flat-field images using a light source close to the sunlight spectrum (xenon light source).
To examinine whether the broad PSF phenomenon occurs in MCAP or not, and if it does, building a correction method for it, we have established the measurement system for broad PSF by using a refracting telescope and setting a circular aperture plate at the focal position of it. The measurement system we first constructed enabled us to capture a circular aperture image; however, we could not evaluate the broad PSF due to the strong stray light within the telescope. We refined the system blocking both external and internal scattering light as a source of the stray light between the light source and the telescope, which resulted in the disappearance of almost all the stray light. There is a prospect of obtaining proper Broad PSF data at PFM by the system.
(b) Geometric calibration:
Distortion data were acquired by mounting MCAP on a two-axis gimbal and changing the angle of incidence of the autocollimator light on the MCAP, which enabled us to perform the measurements at various angles of view. The measured distortion was confirmed to be almost the same as the designed values.
(c) Meteorite imaging:
To confirm the multiband imaging performance, we imaged meteorite samples with known spectra using each MCAP band. Meteorite samples include powders and chips of carbonaceous chondrites. We confirmed that the spectra of the samples could be successfully reproduced from the imaging by MCAP.
These MCAP EM calibration data acquisitions help us confirm the measurement procedure, prepare for measurement jig, and find the issues for the PFM optical calibration. In the presentation, we will present their detailed results and the measures taken to address the issue for PFM.
* The Broad PSF phenomenon is characterized by a spread of a certain to several tens of percentages of count values around an object image over a wide range when imaging an object with an area larger than a point-source image.