J/MNRAS/505/5253 A comparison of centering in ISS astrometry (Zhang+, 2021)
A comparison of centering algorithms in astrometry of Cassini imaging science
subsystem Images and Anthe's astrometric reduction.
Zhang Q.F., Zhou X.M., Tan Y., Lainey V., Cooper N.J., Vienne A., Qin W.H.,
Li Z., Peng Q.Y.
<Mon. Not. R. Astron. Soc. 505, 5253-5259 (2021)>
=2021MNRAS.505.5253Z 2021MNRAS.505.5253Z (SIMBAD/NED BibCode)
ADC_Keywords: Solar system ; Planets ; Positional data
Keywords: methods: observational - techniques: image processing - astrometry -
ephemerides - planets and satellites: individual: Anthe
Abstract:
In the caviar software package, a standard tool for astrometry of
images from the Cassini imaging science subsystem (ISS), Gaussian
fitting is used to measure the centre of point-like objects, achieving
a typical precision of about 0.2 pixels. In this work, we consider how
alternative methods may improve on this. We compare three traditional
centroiding methods: two-dimensional Gaussian fitting, median, and
modified moment. Results using 56 selected images show that the
centroiding precision of the modified moment method is significantly
better than the other two methods, with standard deviations for all
residuals in sample and line of 0.065 and 0.063 pixels, respectively,
representing a factor of over 2 improvement compared to Gaussian
fitting. Secondly, a comparison of observations using Cassini ISS
images of Anthe is performed. Anthe results show a similar
improvement. The modified moment method is then used to reduce all ISS
images of Anthe during the period 2008-2017. The
observed-minus-calculated residuals relative to the JPL SAT393
ephemeris are calculated. In terms of αxcos(δ) and
δ in the Cassini-centred international celestial reference
frame, mean values of all residuals are close to 0km, and their
standard deviations are less than 1km for narrow angle camera images,
and about 4km for wide angle camera images.
Description:
We provide the Cassini ISS astrometric reduction of the Saturnian
satellite Anthe. The observations were taken from 2008 to 2017.
Observation residuals were compared to JPL ephemerides SAT393.
Astrometric observations of Cassini's Imaging Science Subsystem (ISS)
Narrow Angle Camera (NAC) and Wide Angle Camera (WAC) of Anthe.
The NAC's field of view is ∼0.35 degree with a resolution of
∼1.2354-arcsec/pixel. Image size is 1024x1024. The WAC's field of view
is ∼3.5 degree with a resolution of ∼12.354-arcsec/pixel.
Image size 1024x1024. 112 NAC and 80 WAC observations of Anthe are
provided. Positions in right ascension and declination, camera's
pointing vector orientation, image positions with phase correction in
sample and line (in pixels) and phase error in sample and line are
given in the table. The methods for astrometric reduction of Anthe are
well described in paper. Anthe's astrometric positions are given in
ICRF reference frame centered on Cassini spacecraft.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table4.dat 124 182 Astrometric observation of Cassini's Imaging
Science Subsystem Narrow Angle Camera (NAC) and
Wide Angle Camera (WAC) images of Anthe
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 13 A13 --- Image Image identification number
15- 25 A11 "YYYY/MMM/DD" Obs.date Date of the observation
27- 38 A12 "h:m:s" Obs.time Observation time (image mid-time)
41- 50 F10.6 deg RAcdeg Camera's right ascension
52- 61 F10.6 deg DEcdeg Camera's declination
63- 72 F10.6 deg Twist Camera's Twist angle
74- 79 F6.2 pix Sample Body's sample position after phase
correction (1)
81- 86 F6.2 pix Line Body's line position after phase
correction (1)
88- 97 F10.6 deg RAdeg Body's right ascension (ICRF)
after phase correction
99-108 F10.6 deg DEdeg Body's declination (ICRF)
after phase correction
110-116 F7.4 pix BiasX Phase error in sample
118-124 F7.4 pix BiasY Phase error in line
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Note (1): we use the Cassini ISS convention of referring to the image pixel
coordinate along the X-axis as "sample" and the Y-axis as "line".
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Acknowledgements:
Qingfeng Zhang, tqfz(at)jnu.edu.cn
References:
Cooper et al., 2008Icar..195..765C 2008Icar..195..765C
(End) Qingfeng Zhang [JNU], Patricia Vannier [CDS] 12-Apr-2021